ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-16293-2018Acid gases and aerosol measurements in the UK (1999–2015): regional
distributions and trendsAcid gases and aerosol measurements in the UK (1999–2015)TangY. Simyst@ceh.ac.ukhttps://orcid.org/0000-0002-7814-3998BrabanChristine F.https://orcid.org/0000-0003-4275-0152DragositsUlrikeSimmonsIvanLeaverDavidvan DijkNettyPoskittJanetThackerSarahPatelManishaCarterHeatherPereiraM. Glóriahttps://orcid.org/0000-0003-3740-0019KeenanPatrick O.LawlorAlanConollyChristopherVincentKeithHealMathew R.https://orcid.org/0000-0001-5539-7293SuttonMark A.CEH, Bush Estate, Penicuik, Midlothian EH26 0QB, UKCEH, Lancaster Environment Centre, Bailrigg, Lancaster LA1 4AP, UKRicardo Energy & Environment, Gemini Building, Fermi Avenue,
Harwell, Oxon, OX11 0QR, UKSchool of Chemistry, University of Edinburgh, David Brewster Road,
Edinburgh EH9 3FJ, UKY. Sim Tang (yst@ceh.ac.uk)16November20181822162931632415May201825May201811October201826October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/16293/2018/acp-18-16293-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/16293/2018/acp-18-16293-2018.pdf
The UK Acid Gases and Aerosol
Monitoring Network (AGANet) was established in 1999 (12 sites, increased to
30 sites from 2006), to provide long-term national monitoring of acid gases
(HNO3, SO2, HCl) and aerosol components (NO3-,
SO42-, Cl-, Na+, Ca2+, Mg2+).
An extension of a low-cost denuder-filter pack system (DELTA) that is used to
measure NH3 and NH4+ in the UK National Ammonia Monitoring
Network (NAMN) provides additional monthly speciated measurements for the
AGANet. A comparison of the monthly DELTA measurement with averaged daily
results from an annular denuder system showed close agreement, while the sum
of HNO3 and NO3- and the sum of NH3 and
NH4+ from the DELTA are also consistent with previous filter pack
determination of total inorganic nitrogen and total inorganic ammonium,
respectively. With the exception of SO2 and SO42-, the
AGANet provides, for the first time, the UK concentration fields and seasonal
cycles for each of the other measured species. The largest concentrations of
HNO3, SO2, and aerosol NO3- and SO42- are
found in southern and eastern England and smallest in western Scotland and Northern
Ireland, whereas HCl are highest in south-eastern, south-western, and central
England, that may be attributed to dual contribution from anthropogenic (coal
combustion) and marine sources (reaction of sea salt with acid gases to form
HCl). Na+ and Cl- are spatially correlated, with largest
concentrations at coastal sites, reflecting a contribution from sea salt.
Temporally, peak concentrations in HNO3 occurred in late winter and
early spring attributed to photochemical processes. NO3- and
SO42- have a spring maxima that coincides with the peak in
concentrations of NH3 and NH4+, and are therefore likely
attributable to formation of NH4NO3 and (NH4)2SO4 from
reaction with higher concentrations of NH3 in spring. By contrast,
peak concentrations of SO2, Na+, and Cl- during winter
are consistent with combustion sources for SO2 and marine sources in
winter for sea salt aerosol. Key pollutant events were captured by the
AGANet. In 2003, a spring episode with elevated concentrations of
HNO3 and NO3- was driven by meteorology and transboundary
transport of NH4NO3 from Europe. A second, but smaller episode
occurred in September 2014, with elevated concentrations of SO2,
HNO3, SO42-, NO3-, and NH4+ that was
shown to be from the Icelandic Holuhraun volcanic eruptions. Since 1999,
AGANet has shown substantial decrease in SO2 concentrations relative
to HNO3 and NH3, consistent with estimated decline in UK
emissions. At the same time, large reductions and changes in the aerosol
components provide evidence of a shift in the particulate phase from
(NH4)2SO4 to NH4NO3. The potential for NH4NO3 to
release NH3 and HNO3 in warm weather, together with the
surfeit of NH3 also means that a larger fraction of the reduced and
oxidized N is remaining in the gas phase as NH3 and HNO3 as
indicated by the increasing trend in ratios of NH3:NH4+ and
HNO3:NO3- over the 16-year period. Due to different removal
rates of the component species by wet and dry deposition, this change is
expected to affect spatial patterns of pollutant deposition with consequences
for sensitive habitats with exceedance of critical loads of acidity and
eutrophication. The changes are also relevant for human health effects
assessment, particularly in urban areas as NH4NO3 constitutes a
significant fraction of fine particulate matter (<2.5µm) that are
linked to increased mortality from respiratory and cardiopulmonary diseases.
Introduction
Monitoring the atmospheric concentrations of acid gases and their aerosol
reaction products is important for assessing their effects on human health,
ecosystems, long-range transboundary transport and global radiative balance.
Concentration data are necessary for quantifying long-term trends and spatial
patterns, understanding gas–aerosol phase interactions, and estimating the
contributions of different pollutants to dry deposition fluxes (ROTAP, 2012;
AQEG, 2013a; Colette et al., 2016), as well as to provide data for testing
the performance of atmospheric models (e.g. Chemel et al., 2010; Vieno et
al., 2014, 2016). Acid gases in the atmosphere include sulfur dioxide
(SO2), nitrogen oxides (NOx), nitric acid
(HNO3), hydrochloric acid (HCl) and nitrous acid (HONO). Secondary
inorganic aerosols (SIA) include sulfate (SO42-), nitrate
(NO3-), chloride (Cl-) and nitrite (NO2-) that are
formed from reactions of SO2 and NOx (and
HNO3, a secondary product of NOx) with ammonia
(NH3) in the atmosphere. These aerosols make an important
contribution to concentrations of particulate matter (PM) in the UK
(15 %–50 % of the mass of atmospheric PM) and constitute a significant fraction
of fine particles that are less than 2.5 µm in diameter
(PM2.5) implicated in harming human health (AQEG, 2012, 2013b). In
addition, base cations in aerosol are also of interest to estimate the extent
to which acidity is neutralized and to estimate the contribution of marine
influences (ROTAP, 2012; Werner et al., 2011).
Anthropogenic emissions of SO2, NOx, HCl, and
NH3 in the UK declined by 81 %, 51 %, 87 %, and 13 %, respectively, over
the period 1999 to 2015 (NAEI, 2018). Despite the success in mitigating
SO2 emissions however, sulfur still remains a pollutant of national
importance, because reduction in sulfur deposition in remote sensitive areas
have been more modest than close to major sources (ROTAP, 2012). HCl was also
recently identified as another important acidifying pollutant for sensitive
habitats (Evans et al., 2011).
Emissions of HCl (from coal burning in power stations) have however declined
to very low levels (from 74 kt in 1999 to 9 kt in 2015), although it could
still pose a threat to habitats close to these sources. For NOx,
the more modest decrease in emissions reflects difficulties in their
abatement, while for NH3, the decrease to date is largely a result of
changes in animal numbers (NAEI, 2018).
With the decline in SO2 emissions and deposition, the large number of
reactive nitrogen compounds in the atmosphere are assuming greater importance
owing to the complexities of the global N cycle and associated challenges in
their abatement. These include the gas phase components NH3, with
over 80 % estimated from agricultural emissions (EEA, 2017) and nitrogen
oxides (NO, NO2) from combustion, the secondary gas phase reaction
products HNO3, HONO and PAN (peroxyacetyl nitrate), and particulate
phase components ((NH4)2SO4, NH4HSO4, and NH4NO3)
formed by the reaction between NH3 and acid gases (AQEG, 2012).
Ammonia and the N-containing aerosols are known to cause nitrogen enrichment
and eutrophication, as well as contributing to acidification processes
(Sutton et al., 2011). Oxidized nitrogen species (NOx) are
precursors to ground-level O3 formation, while the production of
acids (HNO3, HONO) and PAN in the atmosphere affects air quality and
is damaging both to human health and to vegetation (Cowling et al., 1998;
Bobbink et al., 2010).
In Europe, air pollution policies regarding acidification and nitrogen
eutrophication apply the “critical loads approach” (Bull, 1995; Gregor et
al., 2001), which requires that atmospheric deposition inputs be mapped at
an appropriate scale for the assessment of effects. In parallel, the
“critical levels” of concentrations addresses the direct impacts of
concentrations of nitrogen components in the atmosphere (Bull, 1991; Gregor
et al., 2001; Cape et al., 2009). Quantifying the dry deposition of reactive
nitrogen compounds is a major challenge and a key source of uncertainty for
effects assessment (Dentener et al., 2006; Flechard et al., 2011; Schrader
et al., 2018; Sutton et al., 2007). While deposition may be estimated using
atmospheric transport and chemistry models (e.g. Dore et al., 2015; Flechard
et al., 2011; Smith et al., 2000), air concentration data at sufficient
spatial resolution are needed, both to assess the atmospheric models and
provide input data for estimating deposition using inferential models.
In light of policies to reduce atmospheric emissions, e.g. the amended 2012
Gothenburg Protocol (UNECE, 2018) and the revised National Emissions Ceilings
Directive (NECD, EU Directive 2016/2284) (EU, 2016), it is important to
assess long-term trends in the measured pollutants, since this provides the
only independent means to assess the effectiveness of any abatement policies.
Both these international agreements set emissions reduction commitments for
SO2, NOx and NH3, of 59 %, 42 %, and
6 %, respectively, by 2020 (with 2005 as base year) and includes
PM2.5 for the very first time. Under the 2016 NECD, further reduction
commitments of 79 % (SO2), 63 % (NOx), and
19 % (NH3) are also set for the EU 28 countries from 2030. Since
emissions of these gases come from different sources, emissions controls
require very different strategies, making it important to monitor and assess
the relative concentrations and deposition of nitrogen and sulfur
components.
The spatial and temporal patterns of gases and particulate phases of these
pollutants differ substantially. Although it is widely acknowledged that
speciation between reactive gas and aerosol measurement is critical, there
are few national long-term monitoring programmes dedicated to measuring their
concentrations and dry depositions separately at high spatial resolution
(Tørseth et al., 2012). Across Europe, the European Monitoring and
Evaluation Programme (EMEP, 2014) continues to recommend using a daily filter
pack sampling method to measure oxidized nitrogen (total inorganic nitrate,
TIN) and reduced nitrogen (total inorganic ammonia, TIA) (Tørseth et al.,
2012; Colette et al., 2016). The filter pack method is generally considered
as robust for measuring SO2 and SO42- concentrations
(EMEP, 2014; Hayman et al., 2006; Sickles et al., 1999). However, many papers
have shown that there are potential artefacts in filter-pack sampling for
HNO3 and HCl, due to interactions with NH3 and the volatility
of NH4NO3 and NH4Cl aerosol (Pio, 1992; Sickles et al.,
1999; Cheng et al., 2012). Results from EMEP filter pack measurements are
therefore reported as TIN and TIA, due to phase uncertainties in the method
(Tørseth et al., 2012). This has been complemented by daily measurements
of HNO3 and NO3- using annular denuders (Allegrini et al.,
1987; EMEP, 2014) that are made at a restricted number of sites because of
the resources required. In North America, filter pack sampling is also used
in weekly measurements of sulfur and nitrogen species in the CASTnet (Clean
Air Status Trends Network) national monitoring network of 95 sites across the
contiguous USA, Canada, and Alaska (https://www.epa.gov/castnet, last
access: 25 October 2018). At a small number of CASTnet sites, hourly
measurements of water-soluble gases and aerosols are made with the Monitor
for AeRosols and GAses in ambient air (MARGA) system (Rumsey and Walker,
2016).
High time-resolution measurements of gases and aerosols are useful at
selected locations for detailed analysis and model testing, but the high
costs and resources required for these measurements make them unsuitable for
the assessment of long-term trends at many sites, particularly where spatial
patterns are required. To achieve this, a larger number of sites operated at
lower time-resolution is needed. In the UK, the Eutrophying and Acidifying
Atmospheric Pollutants (UKEAP) network provides long-term measurements for
the UK rural atmospheric concentrations and deposition of air pollutants that
contribute to acidification and eutrophication processes (Conolly et al.,
2016). UKEAP is comprised of two EMEP supersites and four component networks:
precipitation network (Precip-net), NO2 diffusion tube network
(NO2-net), National Ammonia Monitoring Network (NAMN), and the Acid
Gases and Aerosol Network (AGANet). At the two EMEP supersites (Auchencorth
and Harwell – relocated to Chilbolton in 2016), semi-continuous hourly
speciated measurements of reactive gases and aerosols are made with the MARGA
system (Twigg et al., 2016). These measurements are contributing to the
validation and improvement of atmospheric models, such as FRAME (Dore et al.,
2015) and EMEP4UK (Vieno et al., 2014, 2016) that are used to develop and
provide the evidence base for air quality policies, both nationally and
internationally.
The long-term dataset of monthly speciated measurements from the AGANet
(1999–2015) are analysed in this paper to provide a comprehensive assessment
of the spatial, temporal, and long-term trends in atmospheric concentrations
of the acid gases HNO3, SO2, HCl and related aerosol
components NO3-, SO42- and Cl- (and also base
cations Na+, Ca2+, and Mg2+) across the UK,
together with an assessment of the DELTA denuder-filter pack sampling method
(Sutton et al., 2001b; Tang et al., 2009) as compared with other sampling
techniques. To aid interpretation of the relative changes and trends in the
acid gases and aerosols, NH3 and particulate NH4+ data from
the NAMN (Tang et al., 2018) are included, since atmospheric NH3 is a
major interacting precursor gas in neutralization reactions with the acid
gases.
List of sites in the UK Acid Gas and Aerosol Network (AGANet) with
details of locations, start dates, and UK-AIR ID
(https://uk-air.defra.gov.uk/networks/network-info?view=aganet, last
access: 23 October 2018).
Site nameUK-AIR IDLatitudeLongitudeStartBarcombe MillsUKA0006950.91910.0486Apr 2000Bush OTCUKA0012855.8623-3.2058Sep 1999CwmystwythUKA0032552.3524-3.8053Sep 1999EskdalemuirUKA0013055.3153-3.2061Sep 1999GlensaughUKA0034856.9072-2.5594Sep 1999High MufflesUKA0016954.3349-0.8086Sep 1999Lough NavarUKA0016654.4395-7.9003Oct 1999RothamstedUKA0027551.8065-0.3604Sep 1999Stoke FerryUKA0031752.55990.5061Sep 1999StrathvaichUKA0016257.7345-4.7766Sep 1999Sutton BoningtonUKA0031252.8366-1.2512Sep 1999Yarner WoodUKA0016850.5976-3.7165Sep 1999New sites added from January 2006 Auchencorth MossUKA0045155.7922-3.2429Jan 2006CaenbyUKA0049253.3979-0.5074Feb 2006CarradaleUKA0038955.5825-5.4962Jan 2006DetlingUKA0048151.30790.5827Feb 2006Edinburgh St LeonardsUKA0045455.9456-3.1822Jan 2006GoonhillyUKA0005650.0506-5.1815Jan 2006HalladaleUKA0031458.4124-3.8758Jan 2006HarwellUKA0004751.5711-1.3253May 2006HillsboroughUKA0029354.4525-6.0833Jan 2006LadybowerUKA0017153.4034-1.7520Feb 2006LagganliaUKA0029057.1110-3.8921Jan 2006LerwickUKA0048660.1392-1.1853Jan 2006London, Cromwell Road 2UKA0037051.4955-0.1787Jan 2006MoorhouseUKA0035754.6901-2.3769Jan 2006NarberthUKA0032351.7818-4.6915Mar 2006Plas y BreninUKA0049353.1018-3.9179May 2006RosemaundUKA0049152.1214-2.6363Jan 2006RumUKA0027657.0100-6.2718Feb 2006MethodsAcid Gases and Aerosol monitoring Network (AGANet)
The UK Acid Gases and Aerosol Network (AGANet), known previously as the
nitric acid monitoring network, was started in September 1999 under the Acid
Deposition Monitoring Network (ADMN, Hayman et al., 2007) to deliver for the
very first time, long-term monthly speciated measurement data on gaseous
HNO3 and particulate NO3- across the UK. Other acid gases
(SO2, HCl) and aerosols (SO42-, Cl-, plus base
cations Na+, Ca2+, Mg2+) are also measured and
reported.
AGANet and NAMN are closely integrated, with AGANet established at a subset
of NAMN sites to provide additional speciated measurements of the acid gases
and aerosol components. To improve on national coverage, the number of sites
in AGANet was increased in 2006 from 12 to 30 (Fig. 1, Table 1). At the same
time, the Rural Sulfur Dioxide Monitoring Program ceased, replaced by
SO2 and SO42- measurements made under the expanded AGANet
(Hayman et al., 2007). A broad spatial coverage of the UK is provided by the
AGANet sites, with a focus on sites providing parallel information on other
air pollutants (e.g. collocation with the Automatic Urban and Rural Network
that provides compliance monitoring against the Ambient Air Quality
Directives (https://uk-air.defra.gov.uk/networks/, last access:
25 October 2018) and ecosystem assessments (e.g. Environmental Change
Network, http://www.ecn.ac.uk/, last access: 25 October 2018) (Monteith
et al., 2016).
Extended DELTA methodology for sampling acid gases and aerosol in
AGANet
A low-cost manual denuder-filter pack method, DELTA (DEnuder for Long-Term
Air sampling) implemented in the NAMN for measurement of NH3 gas and
aerosol NH4+ (Sutton et al., 2001a, b; Tang et al., 2018) is
extended to provide additional simultaneous monthly time-integrated average
concentrations of acid gases (HNO3, SO2, HCl) and particulate
phase NO3-, SO42-, Cl-, Na+,
Ca2+, and Mg2+ for the AGANet (Conolly et al., 2016; Tang
et al., 2015).
Site map of the UK Acid Gases and Aerosol Network (AGANet). The
AGANet was established in September 1999 with 12 sites and expanded to
30 sites from January 2006 to improve national coverage. These sites also
provide measurements of NH3 and NH4+ for the UK National
Ammonia Monitoring Network (NAMN, Tang et al., 2018).
The DELTA method used in AGANet has also been applied in an extensive
European-scale network of 58 sites to deliver 4 years of atmospheric
concentrations and deposition data for reactive trace gas and aerosols from
2006 to 2009 (Tang et al., 2009; Flechard et al., 2011). Detailed
descriptions of the DELTA method are provided by Sutton et al. (2001b) and by
Tang et al. (2009, 2015). In brief, a small air pump is used to provide low
sampling rates of 0.2–0.4 L min-1, and air volumes are measured by a
high-sensitivity diaphragm gas meter. By sampling air slowly, the method is
optimized for monthly measurements, with sufficient sensitivity to resolve
low concentrations at clean background sites (e.g.
LOD = 0.05 µgm-3 for HNO3 for monthly sampling;
see Tables S1, S2). In addition, the power requirement is very
small, and low voltage versions (using 6 and 12 V micro-air pumps) of the
system powered by wind-solar energy operate at some remote sites.
An extended denuder-filter pack sampling train is used to provide speciated
sampling of reactive gases and aerosols (Fig. S1) (Tang et al.,
2009, 2015). A Teflon inlet (2.8 cm long) at the front end ensures
development of a laminar air stream (Table S3), followed by a first pair of
K2CO3 and glycerol coated denuders to collect HNO3,
SO2, and HCl, a second pair of citric acid coated denuders to collect
NH3 and a 2-stage filter pack at the end to collect aerosol
components. Stage 1 of the filter pack is a cellulose filter impregnated with
K2CO3 and glycerol to collect NO3-, SO42-,
Cl-, Na+, Ca2+, Mg2+, with evolved
aerosol NH4+ from this filter collected on the stage 2 citric acid
impregnated filter. The separation of gases and aerosol is achieved by higher
diffusivities of reactive gases to the denuder walls where they react with
the chemical coating and are retained, whereas aerosol components pass
through and are retained by post-denuder filters (Ferm, 1979). In this
approach, potential artefacts caused by phase interactions associated with
filter packs and bubblers are avoided (e.g. Sickles et al., 1999). A particle
size cut-off of around 4.5 µm was estimated for the DELTA air inlet
(Tang et al., 2015). The DELTA will therefore also sample fine mode aerosols
in the PM2.5 fraction, as well as some of the coarse mode aerosols
< PM4.5.
Na2CO3 is reported to be an effective sorbent for acid gases,
allowing simultaneous collection of HNO3, SO2, and HCl on
denuders (e.g. Ferm, 1986), but since the measurement of aerosol Na+
is also of key interest in AGANet, a K2CO3 coating is used instead
to eliminate possible Na+ contamination from Na2CO3.
Glycerol is added to increase adhesion, stabilize the base coating (Ferm,
1986; Finn et al., 2001) and to minimize potential oxidation of nitrite that
is also collected on the denuder to nitrate in the presence of atmospheric
oxidants such as ozone (Allegrini et al., 1987; Perrino et al., 1990). The
lengths of denuders (borosilicate glass tubes 10 and 15 cm long to capture
>95 % of NH3 and acid gases, respectively) in the sampling
train was calculated according to the procedures described by Sutton et
al. (2001b), based on the calculations derived by Gormley and Kennedy (1948)
and Ferm (1979); see Table S3. All sites were set up as “outdoor” systems
sampling directly from the atmosphere, avoiding potential adsorption losses
(in particular HNO3, which is highly surface active) and artefacts in
air inlet lines. The sampling train is installed inside a simple watertight
housing (Fig. S1), which is mounted on a steel post in the desired location.
A low density polyethylene funnel (89 mm aperture) is placed at the inlet as
a rain shelter, and sampling height is approx. 1.5 m.
Chemical analysis
K2CO3/ glycerol-coated denuders and aerosol filters are
extracted into 5 mL of deionized H2O for analysis. Anions
(NO3-, SO42-, and Cl-) in the denuder and filter
extracts are analysed by ion chromatography (IC). Base cations Na+,
Mg2+ and Ca2+ from the filter extracts were analysed by
IC between 1999–June 2008 and by inductively coupled plasma-optical emission
spectroscopy (ICP-OES/ICP-AES) from July 2008. Citric acid coated denuders
and filter papers are also extracted into deionized H2O (3 and 4 mL,
respectively), with analysis of NH4+ performed on a high sensitivity
ammonia flow injection analysis system, as described in Tang et al. (2018).
Up to June 2009, analyses were carried out at Harwell Laboratory (Hayman et
al., 2007) and from July 2009 at CEH Lancaster (Conolly et al., 2016). The
limit of detection (LOD) for the DELTA method for the different components
are calculated by analysing a series of laboratory blanks. The mean and
standard deviation of the results are calculated and the LOD is calculated as
3 times the standard deviation divided by 15 m3, the typical volume
of air sampled over a month by the DELTA system. Details of changes in
laboratory, analytical methods, and LODs for the gases and aerosols are
summarized in Tables S1 and S2, respectively.
Calculation of air concentrations
The air concentration (χa) of a gas or aerosol is calculated
according to Eq. (1) (see Sutton et al., 2001b, Tang et al., 2018):
χa=QV,
where Q is the amount of a gas or aerosol collected on a denuder or aerosol
filter, and V is the volume of air sampled (from gas meter, typically
15 m3 in a month).
The denuder capture efficiency for each of the gas is calculated by comparing
the concentrations of the individual gases in the denuder pairs (Eq. 2) and
are applied in an infinite series correction on the raw data to provide
corrected air concentrations (χa (corrected)) according to
Eq. (3) (see Sutton et al., 2001b; Tang et al., 2018).
Denuder capture efficiency (% CE)=100×Denuder1(Denuder1+Denuder2)χa(corrected)=χa(Denuder1)×11-χaχa(Denuder2)χa(Denuder1)
Sutton et al. (2001b) and Tang et al. (2003) have shown that this procedure
provides an important quality control, flagging up occurrences of poorly
coated denuders and/or sampling issues. With denuder capture efficiency
better than 90 %, the correction represents <1 % of the corrected
air concentration of the gas. Below 60 %, the correction is large (>50 %) and is not applied, and the air concentration is then calculated
as the sum of concentrations of the denuder pair. The amount of correction
for gas not captured that is added to the corrected gas concentration, is
subtracted from the estimated aerosol concentrations of matching anions and
cations (see Tang et al., 2018).
Data quality control
The following data quality checks are applied to the network data, as part
of the network quality management system (Tang and Sutton, 2003; Conolly et
al., 2016).
Air flow rate (0.2–0.4 L min-1) – where this is below the expected range
for a sampling period, the data are flagged as valid but failing the QC
standard.
Denuder capture efficiency – where this is less than 75 % for a sample,
the data are flagged as valid but less certain.
Ion balance checks – close agreement expected between NH4+ and the
sum of NO3- and 2×SO42-, as NH3 is
neutralized by HNO3 and H2SO4 to form NH4NO3 and
(NH4)2SO4, respectively (Conolly et al., 2016), and for Na+
and Cl-, as these are marine (sea salt) in origin.
Screening the whole dataset for sampling anomalies and outliers, e.g. due to
contamination or other issues.
HNO3 measurement artefact and correction
Tang et al. (2009, 2015) have identified that HNO3 concentrations
(NO3- on denuders assumed to be from HNO3) may be
overestimated on carbonate coated denuders, due to partial co-collection of
other oxidized nitrogen components such as nitrous acid (HONO). In the case
of HONO, this collects on the denuder carbonate coating as nitrite
(NO2-), but oxidizes to nitrate (NO3-) in the presence of
oxidants such as ozone (Bytnerowicz et al., 2005) which can result in a
positive interference in HNO3 determination (Tang et al., 2009,
2015). Other oxidized nitrogen species present in the atmosphere such as
peroxyacetyl nitrate (PAN) and nitrogen oxides (NOx) can
also potentially contribute to a further small interference (Allegrini et
al., 1987; Bai et al., 2003). Based on the tests of Tang et al. (2015), raw
HNO3 data are corrected with an empirical factor of 0.45 which is
estimated to be uncertain by ±30 %. Apart from where stated, all
HNO3 data reported in this study have the 0.45 correction factor
applied.
Comparisons of parallel measurement of monthly
(a) atmospheric reactive gases (HNO3, SO2, HCl, and
NH3) and (b) particulate (NO3-, SO42-,
Cl-, and NH4+) concentrations from duplicate DELTA sampling
at the UK Acid Gas and Aerosol Monitoring Network (AGANet) and National
Ammonia Monitoring Network (NAMN) site Bush OTC (UKA00128) in southern
Scotland for the period 1999 to 2015. (c) A summary of the
regression analyses. Each point represents a comparison between the paired
monthly DELTA measurements.
Time series trend analyses
Statistical trend analyses using both parametric linear regression (LR) and
non-parametric Mann–Kendall (MK) (Gilbert, 1987; Chatfield, 2016) tests were
performed on annually averaged data from AGANet, and on a subset of annually
averaged data from NAMN made at the same AGANet sites. The datasets are
considered sufficiently long-term (>10 years) and produced by a consistent
method for effective statistical trend analyses. Both the LR and MK
approaches are widely adopted for trend analyses in long-term atmospheric
data (e.g. Meals et al., 2011; Colette et al., 2016; Jones and Harrison,
2011; Marchetto et al., 2013; Hayman et al., 2007; Conolly et al., 2016), and
were used in a recent trend assessment of atmospheric NH3 and
NH4+ data (1998–2014) from the NAMN (Tang et al., 2018). As
described in Tang et al. (2018), LR tests were performed using R, and MK
tests used the R “Kendall” package (McLeod, 2015), with estimation of the
MK Sen's slope (fitted median slope of a linear regression joining all pairs
of observations) and confidence interval of the fitted trend using the R
“Trend” package (Pohlert, 2016). Results from both tests provides an
indication of uncertainty associated with the choice of approach. Since there
was no difference between either tests, MK results only are presented and
discussed in the paper. A comparison of trend analyses from both approaches
is however provided in supplementary materials (Figs. S6 and S7 and
Tables S4–S6).
Results and discussionPerformance of the DELTA method
This section presents the performance of the DELTA measurements, including a
comparison with other air sampling methods and networks. Replicated sampling
with the DELTA method were also made to assess measurement reproducibility
and this is shown for example for the Bush OTC site in Scotland (UKA00128). A
comparison of the parallel measurements (Fig. 2) showed good reproducibility
in the method, with close agreement for all components (e.g. mean difference
of <±3 % for all components and ±6 % for HCl).
Comparison with daily annular denuder measurements
An assessment of the DELTA method for NH3 has previously been
reported by Sutton et al. (2001b). Following the extension to additionally
sample acid gases and aerosols, the modified system was compared with
independent daily measurements from an annular denuder system (ADS). The ADS
(ChemspecTM model 2500 air sampling system, R&P Co. Inc.) was operated at
Barcombe Mills in southern England (UKA00069) alongside the AGANet DELTA
monthly measurements for a period of 18 months. Due to significant instrument
and local site issues resulting in low data capture with the ADS, only
11 months of data were available for intercomparison. The sampling train used
in the ADS consisted of 2 K2CO3/ glycerol-coated annular
denuders (same coating as AGANet DELTA), 2 citric acid-coated annular
denuders; a cyclone with 2.5 µm cut-off, followed by a 2-stage
filter pack containing a 2 µm PALL Zefluor teflon membrane
(collection of NO3-,
SO42-, Cl-, Na+, Mg2+, Ca2+)
and a 1 µm PALL Nylasorb nylon membrane (collection of evolved
NO3-), with a sampling rate of 10 L min-1. To compare against
the monthly DELTA measurements, daily ADS values were averaged to the
corresponding monthly periods, with results summarized in Fig. 3.
Comparison of HNO3, HONO, sum (HNO3+HONO),
SO2, HCl and aerosol NO3-, SO42-, Cl-,
Na+, Ca2+, Mg2+ concentrations by the Acid Gases
and Aerosol Network (AGANet) DELTA method with available measurements from
the collocated ChemSpec Daily Annular Denuder system (ADS) at Barcombe Mills
(UKA00069). Mean concentrations were derived from the average of daily ADS
data for the corresponding DELTA sampling periods (monthly). HNO3
values shown for DELTA and ADS are as calculated from the amount of
NO3- collected on the denuders and have not been adjusted by a bias
correction factor (see Sect. 2.6). A summary of the regression analyses is
provided in the table below the graphs.
In the measurement of gases, HNO3 determination on DELTA
(mean = 1.56 HNO3µg m-3, n=11) was on
average 23 % higher than the ADS
(mean = 1.31 HNO3µg m-3, n=11). Since both
methods used the same carbonate coating on the denuders to sample acid gases,
the HNO3 data here have not been corrected with the empirical factor
described in Sect. 2.6. Nitrous acid (HONO) was found to be close to or below
limit of detections for most of the DELTA measurements (mean = of
0.03 µg HONO m-3), compared with a significantly higher
concentration (mean = 0.41 HONO µg m-3) from the ADS.
Since the sampling period of the ADS is daily, any HONO collected as nitrite
on the ADS is likely to remain as nitrite and not oxidized to nitrate. The
very low HONO (nitrite on the denuders assumed to be from HONO)
concentrations from the DELTA supports the hypothesis of the retention of
HONO that is subsequently oxidized to nitrate, resulting in an artefact in
HNO3 determination (Possanzini et al., 1983; Allegrini et al., 1987;
Tang et al., 2015). Further corroboration is provided by the improved
agreement between both methods (line of fit closer to the 1 : 1 line) when
comparing the sum of HNO3 and HONO (Fig. 3). Agreement between the
DELTA and ADS was within 19 % for SO2 (mean
DELTA = 1.75 µg m-3 cf. mean
ADS = 2.18 µg m-3) and 4 % for HCl (mean
DELTA = 0.40 µg m-3 cf. mean
ADS = 0.41 µg m-3). Given the limited data available, it
is not clear why SO2 measured on the ADS is higher than the DELTA,
since there was good agreement for HCl.
Comparison of (a) total inorganic nitrate, TIN (sum of
HNO3+NO3-) and (b) total inorganic ammonium, TIA (sum of
NH3+NH4+) concentrations at the Eskdalemuir monitoring station
(EMEP station code = GB0002R; UK-AIR ID = UKA00130) measured under
the EMEP program with concentrations of the corresponding gas and aerosol
from the UK Acid Gases and Aerosol (AGANet, HNO3 and NO3-)
and UK National Ammonia Monitoring Network (NAMN, NH3 and
NH4+). EMEP values (EMEP, 2017a) are means of daily measurements for
TIN and TIA by the EMEP filter pack method, matched to the AGANet and NAMN
sampling periods (monthly). Filter pack measurements at Eskdalemuir
terminated in December 2000. A summary of the regression analyses is provided
in the table below the graphs.
For the particle-phase components, NO3- measured by the DELTA method
(mean = 2.59 µg NO3- m-3) was on average 2-fold
higher than the ADS method
(mean = 1.32 µg NO3- m-3), whereas
SO42- by the DELTA method was on average 23 % lower
(DELTA = 2.10 vs.
ADS = 2.74 µg SO42- m-3) (Fig. 3).
NO3- and SO42- are both present as fine mode
(<1µm) NH4NO3 and (NH4)2SO4 (Putaud et al.,
2010). Some NO3- can also be present in the coarse mode
(>2.5µm), likely as calcium nitrate (Ca(NO3)2) from a
reaction between gas-phase HNO3 (or its precursors) and soil dust
particles (Putaud et al., 2010), while some of the SO42- will be
coarse mode sea salt SO42- (see Sect. 3.5). A particle size
cut-off of 4.5 µm was estimated for the DELTA air inlet) (Tang et
al., 2015), so the DELTA will also sample a small amount of coarse mode
aerosols. An ion balance check of the ratio of µeq. NH4+ to
sum µeq. (NO3-+SO42-) yielded a near unity value,
confirming that NO3- and SO42- collected by the DELTA
aerosol filter are mainly fine mode NH4NO3 and (NH4)2SO4.
In comparison, the ADS has a 2.5 µm cyclone in front of the aerosol
filters to collect aerosols <2.5µm on the aerosol filters.
NH4+ was unfortunately not analysed in these tests, which would have
allowed a similar ion balance check. Na+ and Cl-
concentrations on the DELTA were also on average 331 % and 444 %
higher than on the ADS and the ion balance check of the ratio of
Na+:Cl- was unity for both methods. In the absence of analytical
errors, loss of NO3-, Na+ and Cl- on the surface of
the cyclone, coupled to a small fraction of the aerosols >2.5µm
that is collected (but not analysed) in the cyclone, could partly account for
the observed lower concentrations of the aerosol components. Since
Ca2+ and Mg2+ concentrations by both methods were at or
below detection limits, comparisons of these are not meaningful and have not
been made.
Comparisons with filter pack measurements:
HNO3/NO3- and NH3/NH4+
The EMEP network (http://www.emep.int/, last access: 17 March 2017)
measures atmospheric concentrations and depositions of a wide range of
pollutants at rural background sites across Europe (Aas, 2014; Tørseth et
al., 2012). A daily filter pack method continues to be implemented at 39
sites across Europe for assessment of oxidized and reduced nitrogen species,
with results reported as total inorganic nitrate (TIN: HNO3+NO3-)
and total inorganic ammonia (TIA: NH3+NH4+) (Colette et al.,
2016; Tørseth et al., 2012), as these are considered more reliable than
reporting for the gas and aerosol components separately.
Comparison of gaseous SO2 and particulate SO42-
concentrations at the Eskdalemuir monitoring station (EMEP station
code = GB0002R; UK-AIR ID = UKA00130) measured under the Acid
Deposition Monitoring Program (ADMN, Hayman et al., 2007) with the
corresponding gas and aerosol from the UK Acid Gases and Aerosol network
(AGANet). ADMN values (EMEP, 2017b) are means of daily measurements for
SO2 by the bubbler method and SO42- by the EMEP filter
pack method (Hayman et al., 2007), matched to the AGANet sampling periods
(monthly). Bubbler and filter pack measurements at Eskdalemuir terminated in
December 2001 and April 2009, respectively. A summary of the regression
analyses is provided in the table below the graphs.
At the UK Eskdalemuir site (EMEP station code GB0002R; UKAIR ID UKA00130), a
Scottish rural background site on the border between Scotland and England,
daily filter pack measurements of TIN and TIA were made as part of the EMEP
network from 1989 to 2000 (EMEP, 2017a). Following installation of the DELTA
system in September 1999, both methods were operated in parallel for
14 months at Eskdalemuir, allowing a comparison to be made of TIN and TIA
from both systems. Comparison results are shown in Fig. 4 of parallel data
from the AGANet (sum of HNO3 and NO3-) and NAMN (sum of
NH3 and NH4+), demonstrating close agreement between the two
independent measurements. The EMEP values shown are daily measurements of TIN
and TIA averaged to corresponding monthly means for comparison with the DELTA
data. For TIN, the regression between EMEP TIN and AGANet (sum of uncorrected
HNO3+NO3-) is close to unity (slope = 0.984, R2=0.94),
which provided independent verification and support of the DELTA HNO3
measurements at the start of the network. After applying a bias adjustment
factor of 0.45 to the HNO3 data (see Sect. 2.6), the AGANet values
(sum of corrected HNO3+NO3-) are smaller than the EMEP TIN
(slope = 0.835, R2=0.95). It is possible however, that the filter
pack method may also be subject to similar artefacts in HNO3
determination due to co-collection of other oxidized nitrogen species (Tang
et al., 2015).
Comparisons with bubbler and filter pack measurements:
SO2 and SO42-
Independent measurements of SO2 and SO42- with a daily
bubbler and filter pack method, respectively, are also available for
comparison with the DELTA method at the Eskdalemuir site. Daily SO2
data with a bubbler method (Hayman, 2005) from December 1977 to December 2001
and daily SO42- data with an EMEP filter pack method from
December 1977 to April 2009 (Hayman, 2006) were downloaded from the EMEP
website (EMEP, 2017b). A close agreement is found between the bubbler and
DELTA method for SO2 (slope = 0.86, R2=0.82), while there
is more scatter between the filter pack and DELTA method for SO42-
(slope = 0.67, R2=0.66) (Fig. 5). Concentrations of SO2
for the 26 month overlap period were comparable (mean of bubbler
method = 0.40 µg S m-3 cf mean of DELTA
method = 0.44 µg S m-3), whereas the filter pack
SO42- concentration (mean = 0.44 µg S m-3, n=87) is larger than the corresponding monthly DELTA measurement
(mean = 0.28 µg S m-3, n=87) (Fig. 5). An earlier
detailed assessment of the DELTA system against filter pack with a focus on
SO2 and SO42- in 1999 by Hayman et al. (2006) had shown
close agreement between the methods. It is therefore unclear why the DELTA
gives a reading lower than the filter pack SO42- at Eskdalemuir in
this assessment, since the dataset was a continuation of the original
inter-comparison. Possible explanations include uncertainties associated with
limit of detection of the daily filter pack method at the very low
concentrations encountered at this site, or the sampling of coarser particles
by this method (due to high flow rate and open-face sampling) with higher
concentrations of sea salt sulfate. The DELTA methodology was unchanged for
the duration of the AGANet dataset (1999–2015) in this manuscript, which
allows a consistent assessment of overall trends in the SO42-
data.
AGANet data
Annual data from the AGANet (and also from the NAMN) are submitted to the
Department for Environment, Food & Rural Affairs (Defra) UK-AIR database
(https://uk-air.defra.gov.uk/, last
access: 17 March 2017), in a format consistent with other UK Authority air
quality networks and relevant reporting requirements. Every concentration
value is labelled with a validity flag and an EMEP flag (see
http://www.nilu.no/projects/ccc/flags/index.html,
last access: 23 March 2017). Ratified calendar year data are published from
around June the year following collection. Currently, work is also in
progress for the data to be made available from the EMEP database
(http://ebas.nilu.no/, last access:
23 March 2017). All data used in this paper (up to 2015), except where
specified, are accessed from the UK-AIR website (Tang et al., 2017a, b).
Uncertainties in HNO3 determination
HNO3 data were corrected for sampling artefacts in the DELTA method
with an empirical correction factor of 0.45 (see Sect. 2.6). Interferences in
HNO3 determination arise through the simultaneous collection of
reactive oxidized nitrogen species on the K2CO3 coating that forms
nitrate ions in the aqueous extracts of exposed denuders. Potential
interfering species include HONO, NO2, N2O5 and PAN, as well
as other inorganic and organic nitrogen species. HONO is most likely to
contribute to the interference, since it is collected effectively on a
carbonate coating and concentrations of HONO have been reported to be
comparable to, and in some places exceed HNO3 in the UK (e.g. Kitto
and Harrison, 1992; Conolly et al., 2016). Interference from NO2 on
the other hand should be small, since the reactivity of a carbonate coating
surface towards NO2 is low (Allegrini et al., 1987), with capture of
NO2 on carbonate ranging from 0.5 % to 5 % (Allegrini et al.,
1987; Fitz, 2002) and their concentrations are also small at rural AGANet
sites (<10µg NO2 m-3; Conolly et al., 2016).
Tests by Steinle (2009) on the DELTA K2CO3/ glycerol coated
denuders also confirmed low capture (ca. 3 %) of NO2.
The correction factor was derived from two years of field intercomparison
measurements at five sites across a range of pollutant concentrations across
the UK, from a clean rural background site in southern Scotland (Auchencorth)
to a polluted urban site (London, Cromwell road) in southern England (Tang et
al., 2015). It is recognized that the correction factor to derive the “real
HNO3” signal from the carbonate coated denuders will also be
dependent on the relative concentrations of HNO3 to interfering
species present in the atmosphere and likely to be both site and season
specific. The 2 years of data indeed show this variability between sites and
between seasons. Given the complexities of atmospheric chemistry of the large
family of oxidized nitrogen species, further work is clearly needed to
understand what the carbonate denuders are measuring, before an improved
correction algorithm for the HNO3 data can be developed with any
confidence.
The empirical 0.45 HNO3 correction factor is
therefore at present a best estimate across a range of pollutant
concentrations and seasons encountered in the UK, based on available test
data from 5 sites. At the cleanest rural sites (e.g. Eskdalemuir), where a
much smaller HONO and NO2 interference of the DELTA HNO3
signal is expected, the HNO3 concentrations may be under-estimated
after correction. This may partly explain the slope deviating from unity in
the comparison of corrected DELTA TIN with EMEP filter pack TIN data
(slope = 0.835, R2=0.95) at Eskdalemuir (see Sect. 3.1.2).
Conversely, at more polluted sites such as London that are affected by a
larger interference from HONO and NO2, the HNO3 determination
may be over-estimated after correction. Apart from two urban sites (London
and Edinburgh), all other sites in the AGANet are rural, located away from
traffic, and the 0.45 correction factor should be more representative.
Since January 2016, the DELTA denuder sample train configuration in AGANet
was changed to two NaCl coated denuders (selective for HNO3, e.g.
Allegrini et al., 1987), with a third K2CO3/ glycerol coated
denuder to collect SO2. At three sites (Auchencorth, Bush OTC and
Stoke Ferry), parallel measurements of the old configuration (two
K2CO3/ glycerol coated denuders) and new configuration (two
NaCl coated denuders +K2CO3/ glycerol coated denuder) were
conducted over 12 months in 2016. In the new configuration, nitrates measured
on the NaCl denuders are reported as HNO3, whereas nitrate on the
K2CO3 denuder are assumed to come from other oxidized nitrogen
species and are not reported. Comparing the sum of nitrate concentrations
from the new (2×NaCl+1×K2CO3) with
the old (2×K2CO3) configurations indicated matching capture
of total nitrate by the two parallel systems (new / old nitrate
ratio = 0.95). A comparison of nitrate concentrations on the
2× NaCl denuders only (new configuration) with the
2×K2CO3 denuders (old configuration) yielded an average
ratio of 0.42, lending further support to the 0.45 empirical factor.
Additionally, the new sample train configuration is providing an extensive
dataset which will allow the magnitude of HNO3 interference at each
site to be quantified, by comparing the amount of nitrate measured on the
2× NaCl and K2CO3 coated denuders. Initial analysis of 2016 data
(unpublished data) showed that the mean ratio of nitrate on
NaCl:K2CO3 of all sites was 0.44, ranging from 0.31 (Bush OTC) to
0.59 (Moorhouse). Seasonally, the average monthly ratio (taken as the mean
across all sites for each month) was lowest in winter (0.25 in December and
0.27 in January) and highest between May to June (0.59, 0.56 and 0.57). It
may therefore be possible to derive an improved correction algorithm that is
both site and season specific, and work is ongoing to make this assessment. A
detailed assessment of sampling artefacts and uncertainties in the DELTA
method and the effects of a method change in the AGANet forms the subject of
a next paper that is currently in preparation.
Spatial patterns in relation to pollutant sources and transport
In Fig. 6, the spatial patterns for each of the gas and aerosol components
measured are shown in the annual maps for the example year 2013. A gradient
in the concentrations of acid gases HNO3 and SO2, and related
aerosols NO3- and SO42- can be seen across the UK,
highest in the south and east (combustion or vehicular sources and long-range
transboundary pollutant transport from Europe) and lowest in the north and
west of the UK (fewer sources, furthest from influence of Europe). The ranges
in site-annual mean concentrations (µg molecule m-3) in 2013
for the gases were as follows: HNO3: 0.12–1.2; HCl: 0.15–0.52; SO2:
0.10 –1.08, while those for aerosol were as follows: NO3-: 0.33–3.1;
Cl-: 0.54–3.3; SO42-: 0.35–1.2; Na+: 0.35–1.8;
Ca2+: < lod–0.11; Mg2+: 0.03–0.19.
The largest HNO3 concentrations were measured at the London Cromwell
site (2013 site annual mean = 1.3 µg HNO3 m-3
cf. 2013 mean of 30 sites = 0.40 µg HNO3 m-3).
London and Edinburgh are the only two urban sites in the AGANet, with the
other 28 sites all in rural environments. HNO3 concentrations in
Edinburgh, the capital of Scotland with a population that is 18 times smaller
than London (0.5 million vs. 8.8 million), is about 2 times lower than
London, but larger than the national average (2013 annual
mean = 0.58 µg HNO3 m-3). For SO2, the
highest concentrations were recorded at Sutton Bonington due to close
proximity to the 2000 MW capacity coal-fired Ratcliffe-on-Soar power station
(2 km North). A peak monthly concentration of
10.9 µg SO2 m-3 was recorded in May 2000 at this
site, with an annual mean concentration of
5.9 µg SO2 m-3 for that year that was also 3 times
higher than the national average (mean of 12
sites = 1.9 SO2 m-3 cf. mean of 11 sites (excl. Sutton
Bonington = 1.5 SO2 m-3). At remote sites further away
from sources, concentrations of HNO3 and SO2 are smaller,
e.g. Lough Navar in Northern Ireland (2013 annual mean:
0.15 µg HNO3 m-3 and
0.21 µg SO2 m-3) and Strathvaich Dam in
north-western
Scotland (2013 annual mean = 0.17 µg HNO3 m-3
and 0.18 µg SO2 m-3). NO3- and
SO42- as secondary aerosols have longer residence times in the
atmosphere and are expected to be more spatially homogeneous than their
precursor gases. The spatial distribution in concentrations of particulate
NO3- (0.33–3.1 µg m-3) and SO42-
(0.35–1.2 µg m-3) are however similar to that of
HNO3 (0.12–1.3 µg m-3) and SO2
(0.10–1.1 µg m-3), with no clear differences in the main
regional patterns from only 30 sites.
Annual mean monitored acid gas (HNO3, SO2, HCl) and
aerosol (NO3-, SO42-, Cl-, Na+,
Ca2+, Mg2+) concentrations from the UK Acid Gas and
Aerosol Monitoring Network (AGANet) across the UK from annual averaged
monthly measurements made in 2013. NH3 and NH4+ measured at
the same time from the UK National Ammonia Monitoring Network (NAMN, Tang et
al., 2018) are also shown alongside for comparison.
Correlation coefficients (R2) for different species across the
30 measurement sites.
Significance level: *p<0.05, **p<0.01,
***p<0.001. ns = not significant (p>0.05).
HCl are mostly emitted from coal combustion and the highest concentrations
are in the source areas in the south-east and south-west, and also in central
England (north of Ratcliffe-on-Soar power station). There is also a marine
source for HCl formed by the reaction of sea salt with HNO3 and
H2SO4 (Roth and Okada, 1998; Ianniello et al., 2011) that may
contribute to additional enhancement of local to regional HCl concentrations.
The spatial distributions of Cl- and Na+ were similar, with
largest concentrations at the coastal sites Goonhilly in south-western England
and Lerwick Shetland in the Shetland Isles, highlighting the importance of
marine sources to the sea salt (NaCl) aerosol. Further away from the coast
and influence of marine aerosol, the smallest concentrations of Cl-
and Na+ are measured in the west of the country (Lough Navar in
Northern Ireland and Cwmystwyth in mid-Wales) and most of Scotland (with the
exception of Shetland). Mg2+is also seen to show a similar spatial
distribution to Na+ and Cl-, which suggests that it may be in
the form of MgCl2, although the range of concentrations at sites are
small (0.03–0.19 µg m-3). There is however no clear spatial
pattern for Ca2+, but since concentrations are mostly at or below
LOD, any assessment of this component is highly uncertain.
In the case of NH3, the extensive spatial heterogeneity seen is
related to large variation in emission sources at ground level across the UK
(Tang et al., 2018). Aerosol NH4+, as expected for a secondary
component, show a less variable concentration field. The spatial distribution
of NH4+ is similar to SO42- and NO3- over the UK
(Fig. 6), due to the close coupling between species from the formation of
particle phase (NH4)2SO4 and NH4NO3 (see next section).
Correlations between gas and aerosol species
Correlations plots between the gas and aerosol phases of the different
components are shown in Fig. 7, with a summary of the regression results
provided in Table 2. The comparison of gas phase concentrations show that
gaseous NH3 is poorly correlated with either SO2 or
HNO3, as might be expected since the emission sources of these
pollutants are different. In the case of the acid gases however, the
significant correlations between HNO3:SO2 (R2=0.35),
HNO3:HCl (R2=0.25), and SO2:HCl (R2=0.21) may be
related to similarity in the regional distribution of their emissions. These
comparisons show that there is on average 5 times more NH3 than
SO2 and 13 times more NH3 than HNO3 at the AGANet
sites (on a molar basis), and that SO2 concentration is nearly 3
times larger than HNO3 (on a molar basis).
In the aerosol components, there is very high correlation between
NO3-, SO42-, and NH4+, and between Na+
and Cl-, but no discernible relationship between NH4+ and
Cl- (Fig. 7). The near 1 : 1 relationship in the scatter plot of
the sum of NO3- and SO42- (neq m-3) vs. NH4+
(neq m-3) (slope = 0.91, R2=0.93), in the absence of any
correlation between NH4+ and Cl-, suggests that
H2SO4 and HNO3 in the atmosphere are fully neutralized by
NH3 to form (NH4)2SO4, NH4HSO4 and NH4NO3
(Aneja et al., 2001). For Cl-, the high correlation with Na+
(slope = 1.04, R2=0.8) lends support that the Cl- measured
in the DELTA are derived mainly from sea salt (NaCl). Similar to the relative
concentrations of gases, NH4+ concentrations (on a molar basis) are
larger than SO42- and NO3-, but NO3- is in molar
excess over SO42-. The correlations between NH4+ and sum
(NO3-+2×SO42-), and for Na+ and
Cl- forms the basis of ion balance checks in data quality assessment,
as discussed in Sect. 2.5 and shows that robust data are obtained.
Sea salt aerosol, derived from sea spray, has essentially the same composition
as seawater (Keene et al., 1986). The marine aerosol comprises two distinct
aerosol types: (1) primary sea salt aerosol produced by the mechanical
disruption of the ocean surface and (2) secondary aerosol, primarily in the
form of non-sea salt (nss) sulfate and organic species, formed by
gas-to-particle conversion processes such as binary homogeneous nucleation,
heterogeneous nucleation and condensation (O'Dowd and Leeuw, 2007). It has
been shown that the ratio of the mass concentrations of SO42- and
Cl- to the reference Na+ species in seawater may be used to
estimate mass concentrations of non-sea salt SO42-
(nss_SO4) and non-sea salt Cl- (nss_Cl) in aerosol,
according to Eqs. (4) and (5), respectively (Keene et al., 1986; O'Dowd and
de Leeuw, 2007).
[nss_SO4]=[SO42-]-(0.25×[Na+])[nss_Cl]=[Cl-]-(1.80×[Na+])
Scatter plots between concentrations of (a) gaseous species
HNO3, SO2, and NH3, and (b) particulate
species NO3-, SO42-, NH4+, Cl-, and
Na+ from mean monthly measurements (1999–2015) from the 12 sites in
the UK Acid Gas and Aerosol Monitoring Network (AGANet) that were operational
over the whole period. NH3 and NH4+ data are from the UK
National Ammonia Monitoring Network (NAMN, Tang et al., 2018) made at the
same time. Each data point represents a single monthly DELTA measurement.
Applying Eq. (4) to the SO42- data in Fig. 7, nss_SO4 is
estimated to comprise on average 25 % (range = 3 %–83 %, n=187) of the measured total SO42- aerosol. Regression of
nss_SO4 vs. NH4+ (slope = 0.18, intercept = 0.47,
R2=0.71) (Fig. S3) was not significantly different from the regression
of total SO42- vs. NH4+ (slope = 0.18,
intercept = 2.4, R2=0.73) (Fig. 7). Sources of nss_SO4
are (i) biological oxidation of dimethylsulfide and (ii) oxidation of
SO2 (O'Dowd and de Leeuw, 2007). This analysis demonstrates that sea
salt SO42- aerosol makes up a significant and variable fraction of
the total SO42- measured, consistent with observations of the
contribution by sea salt SO42- to the total SO42- in
precipitation in the UK (ROTAP, 2012). The improved intercept from the
nss_SO4 regression (Fig. S3) suggests that nss_SO4 are
mainly associated with NH4+.
Estimated nss_Cl concentrations according to Eq. (4) was however negligible
(mean =-0.09µg m-3, n=188), compared to the total
Cl- (mean = 1.3 µg m-3, n=188). Studies have
shown that part of the chloride of sea salt can be substituted by
SO42- and NO3- through a reaction with H2SO4 and
HNO3, known as the Cl- deficit (Ayers et al., 1999). The
close coupling between Cl- and Na+ (near 1 : 1
relationship) presented here suggests that the measured Cl- in the
aerosol are mostly sea salt in origin, with no evidence of depletion of
Cl- from sea salt aerosols.
Seasonal variation in acid gases and aerosols
The average seasonal cycles for all gas and aerosol components derived from
the mean of monthly data of all sites for the period 2000 to 2015 are
compared in Fig. 8. Clear differences are observed in these seasonal cycles,
influenced by local to regional emissions, climate, meteorology and
photochemical processes.
Average annual cycles for HNO3, SO2, HCl and aerosol
NO3-, SO42-, Cl-, Na+, Ca2+ and
Mg2+ from the UK Acid Gases and Aerosol Monitoring Network
(AGANet). The NH3 and NH4+ concentrations measured at the
same time in the UK National Ammonia Monitoring Network (NAMN, Tang et al.,
2018) are also shown for comparison. Each data point in the graphs represents
the mean ± SD of monthly measurements of all sites in the network.
HNO3 is a secondary product of NOx, but
NOx emissions are dominated by vehicular sources which are
not expected to show large seasonal variations. Seasonal changes in chemistry
and meteorology are therefore more likely to be a source of the observed
variations in HNO3 and NO3- (Fig. 8). A weak seasonal cycle
is observed in HNO3, with slightly higher concentrations in late
winter and early spring that may be attributed to photochemical processes
with elevated ozone in spring (AQEG, 2009) leading to formation of
HNO3 during this period (Pope et al., 2016). As discussed in
Sect. 3.3, a constant correction factor was applied to all HNO3 data,
which does not take into account seasonal dependency. The concentrations in
HNO3 may therefore be over-estimated in winter (less HNO3
formed from photochemical processes) and under-estimated in summer (larger
HNO3 concentrations due to increased ⋅OH radicals for
reaction with NO2 to form HNO3), masking the true extent in
the seasonal profile.
In contrast, the seasonal cycle for particulate NO3- is more
distinct with a large peak in concentrations that occur every spring,
together with a second smaller peak in autumn (Fig. 8). NH3, the main
neutralizing gas in the atmosphere that reacts with HNO3 to form
NH4NO3, has a correspondingly large peak in concentration in spring,
a second smaller peak in autumn, but with elevated concentrations in summer
and lowest in winter (Fig. 8). Although particulate NO3- formation
is dependent upon the availability of NH3 for reaction with
HNO3, its concentration is also governed by the equilibrium that
exists between gaseous HNO3, NH3, and particulate
NH4NO3, the latter of which is appreciably volatile at ambient
temperatures (Stelson and Seinfeld, 1982). Partitioning between the gas and
aerosol phase is therefore also a key driver for their atmospheric residence
times and concentrations. HNO3 and NH3 that are not removed
by deposition may react together in the atmosphere to form NH4NO3
aerosol, when the concentration product [NH3]⋅[HNO3] exceeds
equilibrium values. Since NH4NO3 is semi-volatile, any that is not
dry or wet deposited can potentially dissociate to release NH3 and
HNO3, effectively increasing their residence times in the atmosphere.
The formation and dissociation in turn are strongly influenced by ambient
temperature and humidity.
Warm, dry conditions in summer promote dissociation, increasing gas-phase
HNO3 relative to particulate-phase NH4NO3. This process
accounts for the minima in NO3- concentrations (Fig. 8) and the
highest ratio of HNO3 to NO3- seen in July (Fig. 9). Cooler
conditions in the spring and autumn sees a larger fraction of the volatile
NH4NO3 remaining in the aerosol phase. The largest peak in
NO3- concentrations (Fig. 8) and the lowest HNO3:NO3-
ratio in spring-time (Fig. 9) is thus a combination of increased
NO3- formation from reaction between higher concentrations of the
precursor gases HNO3 and NH3, and increased partitioning to
the aerosol phase in cooler, more humid climate. Import from long-range
transboundary transport of particulate NO3-, e.g. from continental
Europe into the UK, as discussed in Vieno et al. (2014, 2016) adds to the
elevated NO3- concentrations. In winter, low temperature and high
humidity also shifts the equilibrium to formation of NH4NO3 from the
gas-phase HNO3 and NH3. Since NH3 concentrations are
also lowest in winter, with less NH3 available for reaction,
NH4NO3 concentrations are correspondingly smaller in winter than in
spring or autumn.
Average annual cycles in the ratios of gas:aerosol component
concentrations. HNO3, SO2, HCl and aerosol NO3-,
SO42-, Cl- data (annual mean, µg m-3) are
from the UK Acid Gases and Aerosol Monitoring Network (AGANet). NH3
and NH4+ data (annual mean, µg m-3) that are measured
at the same time for the UK National Ammonia Monitoring Network (NAMN, Tang
et al., 2018) are also shown for comparison. Each data point in the graphs
represents the mean ± 95 % confidence interval (CI) of monthly
measurements of 12 sites operational in the network over the period 2000 to
2015.
Monthly mean concentrations in gaseous HNO3, SO2,
HCl and aerosol NO3-, SO42-, Cl- from the UK Acid
Gases and Aerosol Monitoring Network (AGANet). Monthly mean concentrations of
NH3 and NH4+ that were measured at the same time in the UK
National Ammonia Monitoring Network (NAMN, Tang et al., 2018) are also shown
for comparison. Each data point in the graphs represents the mean of monthly
measurements of 12 sites operational in the network over the period
September 1999 to December 2015. The same plots for the full 30 site network
from 2006 to 2015 are shown in Fig. S6.
SO2, by contrast, are highest in the winter, with concentrations
exceeding summer values on average by a factor of 2 (Fig. 8). Higher
emissions of SO2 from combustion processes (heating) during the
winter months, coupled to stable atmospheric conditions resulting in build-up
of concentrations at ground level contributes to the winter maximum. Since
the reaction of SO2 with NH3 to form (NH4)2SO4 is
effectively irreversible (Bower et al., 1997), the ratio of the
concentrations of SO2 and SO42- (Fig. 9) is largely
governed by the availability of SO2 and NH3 to form
(NH4)2SO4. The temporal profile of SO42- has a peak in
concentrations in spring, although not as pronounced as the NO3-
peak (Fig. 8). This may be attributed to enhanced formation of
(NH4)2SO4, since peaks in concentrations of NH3 and
NH4+ also occur in spring (Fig. 8) and from the import of
particulates from long-range transboundary transport. Unlike SO2,
aerosol SO42- concentrations are higher in summer than in winter,
due to increased photochemical oxidation of SO2 to H2SO4 and
subsequent formation of sulfate aerosols in sunnier and warmer conditions
(Mihalopoulos et al., 2007). In winter, lower SO2 oxidation rates
limits H2SO4 formation and therefore also the formation of
(NH4)2SO4.
Na+ and Cl- also have highest concentrations during winter,
highlighting the importance of marine sources (more stormy weather) in winter
for sea salt aerosol. The seasonal trends in Mg2+ are similar to
Na+, with maxima during winter and minima in summer (Fig. 8). While
sea salt aerosols comprise mainly of NaCl, other chemical ions are also common
in seawater, such as K+, Mg2+, Ca2+ and
SO42- (Keene et al., 1986). Some of the sea salt aerosol may
therefore be in the form of MgCl2. Magnesium is however also a
crustal element, and so it is not as good as sodium as a tracer for sea salt.
Similarly, calcium is also a rock-derived element and its presence in the
atmosphere is thought to come from chemical weathering of carbonate minerals
(Schmitt and Stille, 2005). The seasonal cycle of Ca2+ is similar
to, but less pronounced than, Na+ and Mg2+. Measured
concentrations of Ca2+ were mostly at or below the method LOD which
makes interpretation uncertain, but the higher concentrations of
Ca2+ in the winter months is likely to be both crustal dust and sea
salt in origin.
Long-term time series of (a) oxidized nitrogen
(HNO3 and NO3-) and (b) reduced nitrogen
(NH3 and NH4+) concentrations at Eskdalemuir (EMEP station
code = GB0002R; UK-AIR ID = UKA00130). EMEP values (EMEP, 2017a) are
monthly means of daily measurements for total inorganic nitrogen, TIN (sum of
HNO3 and NO3-) and total inorganic nitrogen, TIA (sum of
NH3 and NH4+) by the EMEP filter pack method
(April 1989–November 2000), matched to the AGANet and NAMN sampling periods
(monthly) where the measurements overlap. The AGANet and NAMN data are for
gaseous HNO3 and NH3 and for the sum of (HNO3+NO3-) and sum of (NH3+NH4+), respectively, by the DELTA
method. The AGANet HNO3 values shown here includes the bias
correction (Sect. 2.6).
Large inter- and intra-annual variability are also observed in the long-term
mean monthly concentrations of gas and aerosol components, as illustrated in
Fig. 10. In 2003, elevated concentrations of HNO3 and NO3-
(and also NH4+) were observed between February to April that were
more pronounced than the normal peak in concentrations that occur in spring.
The large spike in concentrations was of a sufficient magnitude to elevate
the annual mean concentrations for 2003 of HNO3
(0.54 µg m-3 cf. 0.39 and 0.36 µg m-3 for
2002 and 2004, respectively), particulate NO3-
(2.98 µg m-3 cf. 1.99 and 1.93 µg m-3 for
2002 and 2004, respectively) and NH4+ (1.45 µg m-3
cf. 1.06 and 0.97 µg m-3 for 2002 and 2004, respectively). In
comparison, a much smaller spike in elevated SO42- concentrations
resulted in a slight increase in annual average SO42-
(1.79 µg m-3 cf. 1.41 and 1.31 µg m-3 for
2002 and 2004, respectively) (Fig. 10). Meteorological back trajectory
analysis of the period showed air masses coming across the UK from Europe,
and the pollution episode was attributed to the formation and transport of
NH4NO3 from Europe, since other gases (SO2, HCl and
NH3) and particulate Cl- were not affected (Vieno et al.,
2014). At the same time, stable atmospheric conditions due to a persistent
high pressure system over the UK led to an accumulation of pollutant
concentrations from both local and import sources. A similar pollution
episode, of a shorter duration, occurred in spring 2014. At the time, the
observed elevated PM was blamed on a Saharan dust plume, but which in fact
was then shown to be from long-range transport of NH4NO3 (Vieno et
al., 2016). Although the 2014 episode was not sufficiently large to be
captured in the monthly AGANet data, it reaffirms the substantial
contribution of long-range transport into the UK of NH4NO3, with
precursor gas emissions from outside of the UK presenting a major driver
(Vieno et al., 2016).
A second, but smaller pollutant episode that was captured by the AGANet
occurred in September 2014, with elevated concentrations of SO2,
HNO3, SO42-, NO3-, and NH4+ that came
from the Icelandic Holuhraun volcanic eruptions (Twigg et al., 2016). The
elevated SO2 concentration in September 2014 led to a modest increase
in annual concentrations in SO2 for 2014
(0.58 µg m-3, cf. annual mean = 0.54 and
0.27 µg m-3 for 2013 and 2015, respectively). For the other
components (HNO3, particulate SO42, NO3- and
NH4+), the spikes in concentrations were smaller than for
SO2 and did not noticeably elevate their annual mean concentrations
for that year. These pollution events together illustrate very clearly how
short pollutant episodes can have a major influence on the measured annual
concentrations in the UK, and that changes in meteorological conditions,
coupled with long-range transboundary import can have a large effect on the
UK concentration field.
Long-term trends at Eskdalemuir
At the Eskdalemuir rural background site, EMEP filter pack data in TIN (sum
of HNO3 and NO3-) and TIA (sum of NH3 and
NH4+) are available since 1989 (Sect. 3.1.2). In Fig. 11, the EMEP
filter pack TIN and TIA time series (April 1989 to December 2000) is extended
with AGANet (HNO3 and NO3-) and NAMN (NH3 and
NH4+) DELTA data (September 1999 to December 2015), with an
overlapping period of 14 months. The combined time series shows that the
annual concentrations of TIN has halved in 26 years between 1990 to 2015,
from 0.36 to 0.16 µg N m-3, compared with a 3-fold reduction
in NOx emissions (from 928 to 302 kt NO2-N) (NAEI,
2018) over the same period. For TIA, the 52 % decrease between 1990 to
2015 (from 0.93 to 0.45 µg N m-3) is larger than the
corresponding 13 % reduction in NH3 emissions (from 265 to
231 kt NH3-N) (NAEI, 2018). Speciated NH3 and NH4+
data from NAMN over the period 2000–2015 shows that the decrease in TIA is
mainly driven by NH4+, which decreased by 59 % between 2000
(annual mean = 0.62 µg NH4+ m-3) and 2015
(annual mean = 0.25 µg NH4+ m-3), compared with
no change in NH3 (annual mean
0.32 µg NH3 m-3 in 2000, unchanged in 2015). This is
consistent with findings by Tang et al. (2018) that contrary to the reported
decrease in UK NH3 emissions, NH3 concentrations at
background sites (defined by 5 km grid average NH3 emissions <1 kg N ha-1 yr-1) are showing an indicative increasing trend,
while at the same time, a large downward trend in particulate NH4+
is observed. Together, the AGAnet and NAMN are thus providing an important
long-term dataset that distinguishes between the gas and aerosol phase,
allowing gas–aerosol phase interactions to be explored.
An extended time series illustrating the continued decline in SO2 and
SO42- has also been constructed by combining historic SO2
and SO42- measurement data at the Eskdalemuir site going back to
December 1977 (see Sect. 3.1.3) with AGANet SO2 and SO42-
data (September 1999 to December 2015) (Fig. 12). A substantial decline in
SO2 is observed, falling by 98 % from
4.5 µg S m-3 in 1978 to 0.07 µg S m-3 in
2015, in good agreement with similarly large reduction in UK SO2
emissions over the same period of 95 % (from 2570 to
126 kt SO2-S) (NAEI, 2018). The decrease in SO42- is of
a smaller magnitude, declining by 88 % from an annual mean concentration
of 0.89 µg S m-3 in 1978 to 0.11 µg S m-3
in 2015, highlighting the non-linearity in relationship between the
atmospheric gas and aerosol phase of sulfur at this background site.
Long-term time series of SO2 (December 1977–July 1993) and
SO42- (December 1977–December 2001) concentrations measured in
the UK Acid Deposition Monitoring Network (ADMN) (Hayman et al., 2007) and
the AGANet DELTA measurements (October 1999–December 2015) at the
Eskdalemuir monitoring station (EMEP station code = GB0002R; UK-AIR
ID = UKA00130). ADMN values (EMEP, 2017b) are monthly means of daily
measurements for SO2 and SO42- by a daily bubbler and
filter pack method, respectively, matched to the AGANet sampling periods
(monthly) where the measurements overlap.
Assessment of trends in relation to UK emissions
The long-term time series in annually averaged concentrations of the gas and
aerosol components are compared in Fig. 13a and b, respectively. Since there
was a change in the number of sites during the operation of the AGANet,
annually averaged data from the original 12 sites for the period 2000–2015
(1999 data excluded since AGANet started in September 1999) and from the full
network (30 sites) for the period 2006–2015 are plotted alongside each other
for comparison. From 2006–2015, the decreasing trends for all gas and
aerosol components from the expanded 30 sites are seen to be similar to those
from the original 12 sites. The annual mean concentrations in gas and aerosol
components derived from the expanded 30 sites (2006–2015) or from the
original 12 sites over the same period are also in general comparable
(Table 3). The exceptions are Na+ and Cl- that have higher
mean concentrations from the 30 sites than the original 12 sites (Table 3),
due to the addition of two coastal sites (Shetland and Rum), with larger
contribution from sea salt. Larger HNO3 concentrations are due to two
urban sites, London and Edinburgh (higher NOx emissions from
vehicular traffic). The addition of three sites in high NH3 emission
(agricultural) areas (Rosemaund in England, Narberth in Wales and
Hillsborough in Northern Ireland) also elevated measured annual mean
NH3 concentrations. The comparisons here thus illustrate very
clearly the need to consider the effect of site changes in a national network
and the importance of maintaining consistency and site continuity for
assessing long-term trends.
Comparison of mean concentrations from the original 12 Acid Gases
and Aerosol Network (AGANet) sites vs. the expanded 30 AGANet sites for the
different gas and aerosol components. NH3 and NH4+ measured
at the same time in the UK National Ammonia Monitoring Network (NAMN, Tang et
al., 2018) are also included for comparison. Each data point are the mean
± SD of annual mean concentrations over the period 2006 to
2015.
Mean concentration12 sites30 sites(2006–2015),(mean ± SD)(mean ± SD)µg m-3HNO30.31±0.060.34±0.06SO20.66±0.240.70±0.25HCl0.25±0.040.28±0.04NO3-1.40±0.311.41±0.26SO42-0.73±0.250.74±0.23Cl-1.33±0.091.54±0.09Na+0.75±0.070.84±0.08Ca2+0.04±0.030.04±0.02Mg2+0.06±0.010.07±0.01NH31.18±0.161.40±0.16NH4+0.62±0.180.61±0.16
In the gas phase, SO2 decreased 7-fold from an annual mean
concentration of 1.9 µg SO2 m-3 in 2000 to
0.25 µg SO2 m-3 in 2015 (n=12), compared with
more modest reductions in HNO3 (from 0.35 to
0.21 µg HNO3 m-3), NH3 (from 1.4 to
1.0 µg NH3 m-3) and HCl (from 0.31 to
0.20 µg HCl m-3) over the same period (Fig. 13a).
Particulate SO42-, NO3-, and NH4+ also decreased
in concentrations with time, but unlike their gas phase precursors, the
trends of these aerosol components track each other closely, differing only
in the magnitude of concentrations (Fig. 13b), illustrating very clearly the
close coupling between these components. On the other hand, the absence of a
trend in the particulate Cl- is likely to reflect the sea salt origin
of Cl- which is not expected to vary over time.
Long-term trends in (a) acid gases and (b) aerosol
concentrations (µg molecule m-3) from the UK Acid Gases and
Aerosol Network (AGANet). Each data point represents the annually averaged
measurements from either the original 12 AGANet sites for the 16-year period
from 2000 to 2015 or the expanded 30 AGANet sites for the 10-year period from
2006 to 2015. NH3 and particulate NH4+ measured at the same
time in the UK National Ammonia Monitoring Network (NAMN, Tang et al., 2018)
are also included for comparison.
Summary of Mann–Kendall (MK) time series trend analysis on annually
averaged gas and aerosol concentrations from the UK Acid Gases and Aerosol
Monitoring Network (AGANet) for (i) 12 sites that were operational over the
period 2000 to 2015 and (ii) 30 sites that were operational over the period
2006 to 2015. NH3 and NH4+ concentrations data measured at
the same time from the UK National Ammonia Monitoring Network (NAMN, Tang et
al., 2018) are also included for comparison. The 95 % confidence interval
(CI) for the median trend and relative median change (%) are also
estimated.
Significance level: *p<0.05, **p<0.01,
***p<0.001, ns non-significant (p>0.05).a Median annual trend = fitted Sen's slope of Mann–Kendall
linear trend (unit =µg yr-1). b Relative median
change estimated from the annual concentration at the start (y0) and at
the end (yi) of time series computed from the Sen's slope and intercept
(=100×[(yi-y0)/y0]).
Important changes in the chemical climate is captured by the parallel
monitoring of acid gases and aerosols in AGANet and of NH3,
NH4+ in NAMN. It is clear from the long-term data that there is
substantial intra- (Fig. 10) and inter-annual variability in the annual mean
concentrations of both the gas and aerosol phases (Fig. 13), in particular
the spike in concentrations in 2003 (see Sect. 3.6) that buckles the trend.
An interpretation of the direct relationship between emissions and
concentrations in the atmosphere is therefore not straight forward, as the
concentrations are also influenced by other factors such as variations in
meteorological conditions and long-range transboundary import into the UK.
Relative trends in UK emissions (NAEI, 2018) and in annually
averaged gas and particulate concentrations from the UK AGANet and UK
National Ammonia Monitoring Network (NAMN, Tang et al., 2018) for
(a) the original 12 sites for the 16 year period from 2000 to 2015, and
(b) expanded 30 sites compared with the original 12 sites for the 10
year period from 2006 to 2015.
In Fig. 14, the relative trends in UK NOx, SO2, HCl
and NH3 emissions (NAEI, 2018) are compared with the annually
averaged gas and particulate concentrations measured in the AGANet and NAMN
for (i) original 12 sites for the 16 year period from 2000 to 2015,
(ii) original 12 sites for the 15 year period from 2001 to 2015 (because
annual mean concentrations in 2000 for all components were smaller than in
2001–2006), and (iii) expanded 30 sites and also original 12 sites for the
10 year period from 2006 to 2015. All data were normalized to 0 for the
start years in each of the comparison.
The long-term trends in HNO3, SO2, HCl and particulate
NO3-, SO42-, Cl-, based on MK statistical trend
analysis (Sect. 2.7) of annual mean measurement data are compared in Fig. 15
and summarized in Table 4 for the two time series: (i) the original 12 AGANet
sites for the 16 year period from 2000 to 2015, and (ii) the expanded 30
AGANet sites for the 10 year period from 2006 to 2015. This approach avoids
introducing bias as a result of changes in the sites and ensures site
continuity for the long-term trend assessment. NH3 and NH4+
concentrations from the NAMN that were measured at the same time at the
AGANet sites were included for comparison and to aid interpretation of the
acid gas and aerosol data.
To quantify changes in measured concentrations over time, annual trends (e.g.
µg HNO3 m-3 yr-1) are estimated from the
regression results of the MK tests. This is considered as providing a more
reliable estimate of trend than comparing measured annual concentrations at
the beginning and end of the time series, which is subject to bias due to
substantial variability in annual concentrations between years (Tang et al.,
2018). Changes in measured concentrations over time (MK percentage median
change) in the time series are estimated from the MK Sen's slope and
intercept (Eq. 6). MK annual trends and percentage median change are
summarized in Fig. 15 and Table 4.
% median change=100⋅[yi-y0)y0,
where (y0) and (yi) are estimated annual mean concentrations at the
start and end of the selected time period, estimated from the slope and
intercept of the LR or MK tests.
Statistical trend analysis of monthly mean measurement data in the gas and
aerosol components are also shown for comparison in Fig. S3 (mean monthly
data of 12 sites for period 2000–2015) and Fig. S4 (mean monthly data of 12
sites for period 2006–2015). MK annual trends and percentage median change, based
on the monthly data (Tables S7, S8) were similar to the annual test results
(Table 4). While not discussed further here, since assessment of long-term
trends in this paper focuses on trends in annual mean concentrations for
comparison with trends in estimated annual emissions, the monthly plots
serves to illustrate the large intra-annual variability of concentrations in
gases and aerosols.
Time series trend analysis by non-parametric Mann–Kendall Sen slope
on annually averaged gas and aerosol concentration data from the UK Acid
Gases and Aerosol Monitoring Network (AGANet) of (i) 12 sites with complete
time series over the period 2000–2015 and (ii) expanded 30 sites with
complete times series over the period 2006–2015. NH3 and
NH4+ concentrations data measured at the same time in the UK
National Ammonia Monitoring Network (NAMN, Tang et al., 2018) are also
included for comparison.
Trends in HNO3 and NO3- vs.
NOx emissions
The overall downward trends in HNO3 and NO3- are seen to be
broadly consistent with the -49 % fall in estimated
NOx emissions (NAEI, 2018) over the 16 year period between
2000 and 2015 (Fig. 14). Reductions in combustion (power stations and
industrial) and vehicular sources (fitting of catalytic converters), coupled
to tighter regulations are major contributory factors to the decrease in UK
NOx emissions. The rate of reduction however stagnated in
the period 2009 and 2012 (improvement in emissions abatement offset by
proportionate increase from diesel combustion and increase in vehicle
numbers), followed by a 16 % decrease between 2012 and 2015 due to the
closure of a number of coal-fired power stations.
It is notable that the first 6 years (2000–2006) of HNO3 and
NO3- annual data show substantial variability between years and in
particular is dominated by the large 2003 peak in concentrations (see
Sect. 3.6). This highlights the sensitivity of the trend assessment to the
selection of a reference start for the time series, since the annual mean
concentrations of both HNO3 and NO3- in 2000 are in fact
smaller than concentrations in the following 6 years. Re-analysis of the same
annual data normalized against 2001 instead of 2000 takes the relative trend
lines for HNO3 and NO3- much closer to the relative trend
line in NOx emissions. In the later period between 2006 and
2015, the relative trend lines in HNO3 and NO3- derived from
the mean of either 12 or 30 sites were not significantly different, and the
relative trend lines in emission and concentrations followed each other
closely (Fig. 14).
Comparison of percentage change in estimated UK
NOx, SO2, and NH3 emissions reported by the
National Atmospheric Emission Inventory (NAEI, 2018) with % change between
2000 and 2015 (12 sites with complete time series) and between 2006 and 2015
(30 sites with complete time series) in annually averaged HNO3/NO3-
and SO2/SO42- concentrations from the UK Acid Gas and Aerosol
Monitoring Network (AGANet), and annually averaged NH3/NH4+
concentrations from the UK National Ammonia Monitoring Network (NAMN, Tang et
al., 2018).
Components2000–2015 (12 sites) 2006–2015 (30 sites) UK emissionsb % changeMK Sen Slope % relative median changeaUK emissionsb% changeMK Sen slope % relative median changeaGas HNO3-49 (NOx)-45**-40 (NOx)-36*Particulate NO3--52***-43**Gas SO2-80 (SO2)-81***-65 (SO2)-60***Particulate SO42--69***-54**nss_SO42--78***-62**Gas HCl-87 (HCl)-28ns-45 (HCl)-24nsParticulate Cl-+10ns-4nsGas NH3-9 (NH3)-30***-0.7 (NH3)-18nsParticulate NH4+-62***-49**
Significance level: *p<0.05, **p<0.01,
***p<0.001, ns non-significant (p>0.05).a Relative median change calculated based on the estimated annual
concentration at the start (y0) and at the end (yi) of time series
computed from the Sen's slope and intercept (=100×[(yi-y0)/y0]). b UK emissions data from NAEI (2018).
Results of MK tests showed that the reductions in annual HNO3
concentrations are statistically significant for both time series (Fig. 15;
Table 4). The MK percentage median change in annual mean HNO3 was
-45 % (2000–2015, n=12) and -36 % (2006–2015, n=30),
consistent with the -49 % and -40 % fall in estimated
NOx emissions over the corresponding periods (Table 5). The
decrease in HNO3 is accompanied by a larger decrease in particulate
NO3- (2000–2015: MK =-52 % (n=12), 2006–2015: MK =-43 % (n=30)) (Table 4) and an indicative small increasing trend is
observed in the ratio of HNO3 to NO3- with time (Fig. 16),
hinting at an increased partitioning to the gas phase. Since HNO3 is
one of the major oxidation products of NOx, through reaction
with OH. or heterogeneous conversion of N2O5, it provides an
important measure of the fraction of NOx emissions that is
oxidized and signals any long-term changes in the atmospheric processing
timescales of NOx over the country. NO2 is measured
at 24 rural sites across the UK in the UKEAP NO2-net, with 11 sites
collocated with the AGANet (Conolly et al., 2016). The long-term time series
in the data also showed a matching decreasing trend in network averaged
NO2 concentrations with NOx emissions between 2000
and 2015, with annual mean NO2 concentrations across the network
falling 2-fold to 4 µg NO2 m-3 in 2015 (Conolly et
al., 2016). Despite the uncertainty in corrected HNO3 data
(Sect. 3.3), the encouraging agreement between trends in HNO3 and
NO2 concentrations and NOx emissions lends support
to a linear response in HNO3 concentrations to reductions in
NOx emissions.
Trends in SO2 and SO42- vs. SO2
emissions
Unlike NOx, there has been a more significant decline in
SO2, both in emissions and measured concentrations during this period
(Fig. 14). Between 2000 and 2009, SO2 emissions fell substantially by
66 % from 1286 to 432 kt SO2. The reduction reflects mitigation
measures introduced since the 1980s (fitting of flue gas desulfurization to
coal fired power stations) to control S pollution, reductions in energy
production and manufacturing and the switch from coal to gas at the same
time. Similar to trends in NOx emission, the decreasing
trend in SO2 emissions plateaued between 2009 and 2012 and then
decreased again by a further 45 % between 2012 and 2015 following the
closure of a number of coal-fired power stations, as well as conversion of
some coal-fired stations to burn biomass.
Over the same period, the network annual mean concentration decreased from
1.9 µg SO2 m-3 in 2000 to
0.25 µg SO2 m-3 in 2015 (mean of 12 sites),
continuing the long-term decline in SO2 concentrations observed at
the background Eskdalemuir site (Sect. 3.1.3) and across the UK (ROTAP,
2012). The relative trends in SO2 emissions and concentrations
tracked each other closely for all the time periods considered and it is
clear that these decreases are highly correlated (Fig. 14). In the case of
particulate SO42- however, there is an apparent “gap” between
emissions and concentrations in the trend normalized against the year 2000.
Like NO3-, re-analysis of the same annual data normalized against
2001 instead of 2000 takes the relative trend line for SO42-
closer to the trend lines in both SO2 emissions and concentrations
(Fig. 14), thus again highlighting the potential bias in the use of a
measured value at a specific time point in trend assessments when there is
substantial inter-annual variability in the data.
Long-term trends in the gas : aerosol ratio, from a comparison of
the annual mean concentrations of 12 sites with complete time series from
2000 to 2015, and 30 sites with complete time series from 2006 to 2015,
showing indicative differences in direction of trends in this ratio with
time.
From the MK trend analysis, the decrease in annual mean SO2
concentrations of -81 % (2000–2015, n=12), and -60 %
(2006–2015, n=30) (Fig. 15, Table 4) are consistent with the substantial
reduction of -80 % and -64 % in SO2 emissions over the
two overlapping periods, respectively (Table 5). The decrease in both
emissions and concentrations SO2 is also almost twice as large as
HNO3 (Table 5), illustrating the greater success in mitigating
sulfur than nitrogen and the increasing dominance of N components in the
atmosphere compared with S, with larger decline in SO2 than
NOx.
At the same time, the reduction in SO2 emission and measured
concentration is accompanied by a smaller negative trend in particulate
SO42- (2000–2015: -69 % MK; 2006–2015: -54 % MK)
(Table 5), with concentrations falling 3-fold from an annual mean of
1.2 µg SO42- m-3 in 2000 to
0.42 µg SO42- m-3 in 2015. The smaller decrease
in particulate SO42-compared with its gaseous precursor,
SO2, is similar to that observed at Eskdalemuir (Sect. 3.1.3). A
similar picture is also seen in Europe, where atmospheric concentrations of
gas phase SO2 decreased by about 92 % compared with a smaller
reduction of 65 % in particulate SO42- in response to sulfur
emissions abatement over the 1990–2012 period in the EMEP region (EMEP,
2016). The ratio of SO2:SO42- is also seen to show a decreasing
trend over time (Fig. 16), with the largest change occurring between 2000 and
2006 that matches the period of largest decline in SO2 emissions.
Sea salt SO42- (ss_SO4) aerosol, as discussed in
Sect. 3.5, makes up a significant fraction of the total SO42-. It
is possible that the smaller reduction in particulate SO42-,
compared with SO2, may be explained by an underlying increase in the
relative proportion of ss_SO4 to total SO42-. To assess
the contribution of ss_SO4 to the observed trends in total
SO42-, ss_SO4 concentrations (estimated according to
Equation (4) described in Sect. 3.5) and nss_SO4- (= total
SO42--ss_SO4) are compared with the long-term trends in total
SO42- in Fig. 17. Overall, there is no apparent trend in the
long-term annual mean ss_SO4 data, with concentrations in range of
0.16 to 0.21 µg SO42-. Since ss_ SO4 is derived from
an empirical relationship with Na+ (Sect. 3.5), the long-term trend
data for Na+ is also included in the analysis (Fig. 17). Similar to
ss_SO4, there is no overall trend in the Na+ data, with
small inter-annual variability and annual mean concentrations in the range of
0.65–0.85 µg Na+ m-3. ss_SO4 made up just
10 % of the total SO42- in 2000, but by 2015, this had
increased to just over 50 % due to the decrease in nss_SO4 over
that time. MK analysis of the nss_SO4 (Table 4) showed decrease in
concentrations of -78 % (2000–2015) and -62 % (2006–2015),
similar to that observed in SO2 (-81 %: 2000–2015 and
-60 %: 2006–2015), indicating a closer relationship between
nss_SO4 and SO2 than between total SO42- and
SO2.
Comparison of long-term trends in annual mean concentrations of
total sulfate (as determined from the amount of sulfate collected on the
AGANet aerosol filter), nss_sulfate (estimated from the empirical
relationship: [nss_SO4] = [SO42-] - (0.25×[Na+]), ss_sulfate (Total - nss) and sodium. Each data point
represents the annually averaged mean concentration of 12 sites for the
16 year period from 2000 to 2015.
Trends in HCl and Cl- vs. HCl emissions
HCl emissions in the UK also decreased substantially by 89 % between 2000
and 2015, from 82 to 9 kt in 2015 (NAEI, 2018), contrasting with a smaller,
but non-significant decreasing trend in HCl concentrations (Figs. 14, 15,
Table 5). The annual mean monitored concentrations in HCl over this period
decreased from 0.30 µg HCl m-3 in 2000 to
0.19 µg HCl m-3 in 2015. Most of the reduction in HCl
emissions occurred before 2006 (-79 %, from 82 kt in 2000 to 17 kt in
2006), with emissions plateauing since 2006 (NAEI, 2018) (Fig. 14). A
corresponding decrease is not seen in the HCl measurement data, where
concentrations remained fairly stable at between 0.31 µg m-3
HCl in 2000 to 0.33 µg m-3 HCl in 2006. Since 2006 however,
the relative change in HCl emissions is closely tracked by changes in
concentrations of both the annual mean data from the original 12 sites and
from the expanded 30 sites in the AGANet, with the small peak in HCl
emissions in 2013 also captured in the annual mean data. This part of the
time series therefore clearly shows a direct relationship between emissions
and concentrations.
So why is the most significant fall in HCl emissions between 2000 and 2006
not captured by the network? HCl are mainly released as point sources. Coal
burning, particularly from coal-fired power stations, is responsible for the
majority of UK emissions: 92 % in 1990 and 76 % in 2015, and
reductions in HCl emissions in the UK inventory are largely as a result of
declining coal use and the installation of emissions abatement measures at
coal-fired power stations (implemented since 1993) aimed at reducing S that
also coincidentally reduced HCl emissions. It may be that a network of only
12 sites in the early periods failed to capture peak emissions and changes in
source areas. While there is an indicative, but non-significant decreasing
trend in HCl (2000–2015: MK =-28 %, 2006–2015: MK =-24 %),
no detectable trend in particulate Cl- can be seen (Table 4). Since
Cl- is mainly associated with Na+ (sea salt) in the AGANet
measurements (Sect. 3.5), the absence of a trend in Cl- (Fig. 15) and
Na+ (Sect. 3.8.2, Fig. 17) provides evidence of a constant background
in sea salt in the UK atmosphere.
Trends in NH3 and NH4+ vs. NH3
emissions
In comparison to the acid gases, there is a more modest decrease of
-9 % in NH3 emissions, from 254 kt NH3 in 2000 to
231 kt NH3 in 2015 (NAEI, 2018). This is smaller than the decrease
seen in the annually averaged NH3 concentrations at the 12 AGANet
sites (2000–2015: -30 % MK) over the same period (Figs. 14, 15,
Table 5). A recent assessment by Tang et al. (2018) showed that NH3
trends are highly dependent on site selection and categorization of sites in
the analysis. A more comprehensive analysis of a larger number of sites shows
smaller reductions over time, whereas a significant decreasing trend in
NH3 concentrations was observed in the grouped analysis of sites in
areas classed as dominated by pig and poultry emissions, contrasting with an
upward (non-significant) trend for sites in cattle-dominated areas. Therefore
there is a large degree of uncertainty in interpreting the trends in
NH3 concentrations from a subset of just 12 sites, since NH3
emissions are dominated by agricultural emissions (>80 %) that vary
hugely on a local to regional scale across the UK.
At the same time, there is a larger decrease in particulate NH4+
concentrations (-62 % MK), contrasting with the smaller decrease in
NH3 concentrations over the period 2010–2015 (-30 % MK)
(Table 4), with the NH3:NH4+ ratio also increasing with time
(Fig. 16). This provides evidence for a shift in partitioning from the
particulate phase NH4+ to the gaseous phase NH3 in the UK
data, discussed in Tang et al. (2018). The change in partitioning from
particulate NH4+ to gaseous NH3 is also occurring in other
parts of Europe, where decreases in NH3 concentrations have been
smaller than emission trends would suggest, due to large decreases in
SO2 emissions (Bleeker et al., 2009; Horvath et al., 2009).
Changes in UK chemical climate
Atmospheric SO2 concentrations in the UK has declined to very low
levels over the 16 years of measurements in AGANet, with annual mean
concentrations in 2015 (0.25 µg SO2 m-3, n=12)
approaching that of the other acid gases HNO3
(0.21 µg HNO3 m-3, n=12) and HCl
(0.20 µg HCl m-3, n=12). NH3 measured at the
same time at the AGANet sites also decreased, but to a smaller extent, to a
mean concentration of 1.0 µg NH3 m-3 (n=12) in
2015. The changes in measured concentrations of SO2, HNO3,
HCl and NH3 are consistent with the estimated decrease in emissions
of SO2, NOx, HCl, and NH3 since 2000.
SO2 is therefore no longer the dominant acid gas, with HNO3
and HCl together contributing a larger fraction of the total acidity in the
UK atmosphere.
Past studies have shown that the increasing ratio of NH3 to
SO2 in the atmosphere leads to increased dry deposition of
SO2, accelerating the decrease in atmospheric SO2
concentrations than would be achieved by emissions reduction alone (Fowler et
al., 2001, 2009; ROTAP 2012). The dry deposition of SO2 and
NH3, by uptake of the gases in a liquid film on leave surfaces, are
known to be enhanced when both gases are present in a process termed
“co-deposition” (Fowler et al., 2001). Where ambient NH3
concentrations exceed that of SO2, there is enough NH3 to
neutralize acidity in the liquid film and oxidize deposited SO2, and
maintain large rates of deposition of SO2. With changes in the
relative concentrations of acid gases in the UK and across Europe however,
the deposition rates will increasingly be controlled by the
NH3/ combined acidity (sum of SO2, HNO3 and HCl)
molar ratio, rather than based on SO2 alone (Fowler et al., 2009).
Long-term changes between 2000 and 2015 in (a) molar ratio
of NH3 to acid gases (SO2, HNO3, and HCl) and
(b) molar ratio of particulate NH4+ to acid aerosols
(SO42- and NO3-) from measurements made at 12 sites in
AGANet.
To look at the UK situation, an analysis of the molar ratios of NH3
to acid gases is presented in Fig. 18a. The molar ratio of NH3 to
acid gases (sum of SO2, HNO3 and HCl) increased with time,
from 1.9 in 2000 to 4.7 in 2015, confirming that NH3 is increasingly
in molar excess over atmospheric acidity. The ratio of annual mean
concentrations of NH3 (80 nmol m-3) to SO2
(29 nmol m-3) was 2.7 in 2000. By 2015, this ratio had increased to 15
(annual mean concentrations of NH3=58 nmol m-3 cf.
SO2=4 nmol m-3). Molar concentrations of HNO3
(4 nmol m-3) and HCl (6 nmol m-3) were comparable to
SO2 in 2015, highlighting the increasing importance of HNO3
and HCl in contributing to atmospheric acidity. A larger decrease in
SO2 (-81 %) than particulate sulfate (-69 %) in the
AGANet data (Table 4) would appear at first to suggest that the large
NH3:SO2 ratio is contributing to a more rapid decrease in
SO2 concentrations. However, when the sea salt fraction of
SO42- is removed from the sulfate trend (Sect. 3.8.2), the
decrease in nss_SO4 (-78 %) is similar to SO2
(-81 %) (Table 4). Since the decreasing trend in the ratio of
SO2 to SO42- also appeared to stabilize after 2006
(Sect. 3.8.2), this would suggest that maximum deposition rates for
SO2 may have been reached with the smaller SO2 concentrations
since 2006.
The substantial decrease in UK SO2 emissions and concentrations,
while UK NOx emissions and concentrations remain relatively
high in comparison, set against a much smaller decrease in NH3
emissions and concentrations since 2000 is leading to changes in the
respective particulate SO42-, NO3- and NH4+
concentrations. Since the affinity of H2SO4 (oxidation product of
SO2) for NH3 is much larger than that of HNO3 and
HCl, available NH3 is first taken up by H2SO4 to form
ammonium sulfate compounds (NH4HSO4 and (NH4)2SO4), with
any excess NH3 then available to react with HNO3 and HCl to
form NH4NO3 and NH4Cl. Analysis of the different particulate
components in Sect. 3.5 showed that the ammonium aerosols are mainly made up
of (NH4)2SO4 and NH4NO3. With the large reduction in
SO2, more NH3 is available to react with HNO3 to form
NH4NO3 and concentrations of NH4+ and NO3- are now
observed to be in molar excess over SO42-, providing evidence of a
change in the particulate phase from (NH4)2SO4 to NH4NO3
(Fig. 18b).
A change to an NH4NO3 rich atmosphere and the potential for
NH4NO3 to release NH3 and HNO3 in warm weather,
together with the surfeit of NH3 also means that a larger fraction of
the reduced and oxidized N is remaining in the gas phase as NH3 and
HNO3. An increased partitioning to the gas phase may account for the
larger decrease in particulate NH4+ (MK -62 % between 2000
and 2015, n=12) and NO3- (MK -52 % between 2000 and 2015,
n=12) than NH3 (MK -30 % between 2000 and 2015, n=12), HNO3 (MK -45 % between 2000 and 2015, n=12) (Table 4)
and the increase in gas to aerosol ratios (NH3:NH4+ and
HNO3:NO3-) over the 16 year period (Fig. 16). A higher
concentration of the gas-phase NH3 and HNO3 may therefore be
maintained in the atmosphere than expected on the basis of the emissions
trends in NH3 and NOx. Given the larger deposition
velocities of NH3 and HNO3 compared to aerosols, more of the
NH3 and HNO3 emitted will have the potential to deposit more
locally with a smaller footprint within the UK.
Currently, the critical loads of acidity (sulfur and nitrogen) are exceeded
by 44 % of the area of sensitive habitats in the UK (based on mean
deposition data for 2012–2014), whereas the figure for exceedance of
eutrophication (nutrient nitrogen) is even larger, at 62 % (based on
deposition data for 2012–2014) (Hall and Smith, 2016). Air quality policies
have been very successful in abating SO2 emissions (-80 %:
2000–2015) and moderately successful with NOx emissions
(-58 %: 2000–2015), with both on course to meet the emission reduction
targets set out under the 2012 Gothenburg protocol and 2016 NECD.
Difficulties in abating NH3 is reflected in the smaller reduction in
NH3 emissions (-9 %: 2000–2015), with emissions increasing,
rather than decreasing since 2013 and it is likely that abatement measures
may be required to meet emission reduction targets. In recognizing the need
to tackle the ammonia problem, the Code of Good Agricultural Practice (COGAP)
was published under the UK government's Clean Air Strategy (launched in
July 2018) as a step towards reducing NH3 emissions from agriculture.
Based on the current emission trends and evidence from AGANet and NAMN
long-term measurements, atmospheric N deposition from oxidized N
(NOx, HNO3 and NO3-) and from reduced N
(NH3, NH4+) are likely to continue to exceed critical loads
of N deposition over large areas of sensitive habitats, with implications for
UK's commitment to maintain or restore natural habitats (e.g. Natura 2000
sites; Hallsworth et al., 2010) to a favourable conservation status under the
EU Habitats Directive (Council Directive 92/43/EEC) and ecosystem monitoring
under Article 9 and Annex V of Directive 2016/2284 (NECD). The changes are
also relevant for human health effects assessment, since NH4NO3 and
(NH4)2SO4 are mainly in the fine mode and constitute a significant
fraction of PM2.5 that are associated with acute and chronic human
health problems. The change in partitioning from (NH4)2SO4 to
NH4NO3, coupled to import of NH4NO3 from long-range
transport (driven by emissions of NH3 and NOx from
outside the UK) poses policy challenges in protection of human health from
effects of air pollution, particularly in urban areas where concentrations of
the PM2.5 precursor gases NOx, SO2 and
NH3 are higher.
Conclusions
The UK Acid Gases and Aerosol network (AGANet) is delivering, uniquely, a
comprehensive UK long-term dataset of speciated acid gases (HNO3,
SO2, HCl) and aerosol components (NO3-, SO42-,
Cl-, Na+, Ca2+, Mg2+) and also of
NH3 and NH4+ measured within the National Ammonia Monitoring
Network (NAMN). Speciated measurements are made with an established low-cost
DELTA denuder-filter pack methodology, allowing assessment of atmospheric
chemical composition and gas–aerosol phase interactions. Other manual
denuder-filter implementations designed for high time-resolution measurements
are useful at selected locations for detailed analysis and model testing, but
they are resource intensive and expensive. The DELTA monthly measurements on
the other hand are cost-efficient for estimating annual mean concentrations,
providing sufficient resolution for analysis of temporal trends, and which can
be operated at a large number of sites in the network to provide long-term
trends and temporal and spatial patterns.
Large regional patterns in concentrations are observed, with the largest
concentrations of HNO3, SO2, and aerosol NO3- and
SO42- in southern and eastern England, attributed to anthropogenic
(combustion, vehicular) and long-range transboundary sources from Europe, and
smallest in western Scotland and Northern Ireland. HCl concentrations are
also largest in the south-eastern, south-western and central England, attributed to
dual contribution from anthropogenic (coal combustion) and marine sources
(reaction of sea salt with HNO3 and H2SO4 to form HCl). For
Cl-, this has a similar spatial distribution as Na+, with the
highest concentrations at coastal sites, reflecting their origin from
marine sources (sea salt).
Distinctive temporal trends are established for the different components,
with the seasonal variability influenced by local to regional emissions,
climate, meteorology and photochemistry. A weak seasonal cycle is observed in
HNO3, with slightly higher concentrations in late winter and early
spring, due to formation from photochemical production processes. Particulate
NO3- and SO42- have highest concentrations in spring,
coinciding with the peak in concentrations of NH3 and NH4+,
and are therefore likely to be attributed to formation of NH4NO3 and
(NH4)2SO4 from reaction with a surplus of higher concentrations of
NH3 at that time of year. Conversely, peak concentrations of
SO2, Na+, and Cl- occur during winter, likely from
combustion processes (heating) for SO2 and marine sources in winter
(more stormy weather) for sea salt generation. Magnesium is a crustal
element, but which is also present in sea salt aerosols. The seasonal trend
in Mg2+ is similar to Na+, with maxima during winter and
minima in summer; therefore some of the sea salt aerosol may be in the form
of MgCl2.
Enhancement of local to regional concentrations of reactive gases and
aerosols in the UK from long-range transboundary transport of pollutants into
the UK is highlighted by two pollution events, captured in the long-term
AGANet monthly measurements. In 2003, a spring episode with elevated
concentrations of HNO3 and NO3- was driven by meteorology,
with easterly winds transporting NH4NO3 formed in Europe into the UK
and a high pressure system over the UK (February–April) that led to a
build-up of NH4NO3 and HNO3 concentrations from both local
and transboundary sources. A second, but smaller episode of elevated
concentrations of SO2 and HNO3, as well as of particulate
SO42, NO3-, and NH4+, in September 2014 was shown
to be from transport of pollutant plume from the Icelandic Holuhraun volcanic
eruptions at that time.
After more than 16 years of operation, the AGANet is also capturing important
long-term changes in the concentrations and partitioning between gas and
aerosol of the N and S components in the atmosphere. A significant decrease
of -81 % (MK) in annual mean concentrations of SO2 between 2000
and 2015 was in agreement with the estimated -80 % reduction in
SO2 emissions, but larger than the accompanying decline in
particulate SO42- (-69 % MK). A more modest reduction in
HNO3 (-45 % MK) and particulate NO3- (-52 % MK)
are consistent with the estimated 58 % decline in NOx
emissions over this same period. The decrease in particulate NH4+
(-62 % MK) is larger than the precursor gas NH3 (2000–2015 =-30 % MK / LR) and larger than the estimated decline in estimated
NH3 emissions of 9 %. However, it should be noted that
NH3 trends are highly dependent on site selection according to an
earlier assessment made on a more comprehensive dataset from the UK NAMN.
The substantial decrease in UK SO2 emissions and concentrations,
while UK NOx emissions and concentrations (HNO3)
remain relatively high in comparison, set against a much smaller decrease in
NH3 emissions and concentrations since 2000 is leaving more
NH3 available to react with HNO3 to form the semi-volatile
particulate NH4NO3. Particulate NH4+ and NO3- are
now in molar excess over SO42-, providing evidence of a shift in
the particulate phase from (NH4)2SO4 to NH4NO3. A change
to an NH4NO3 rich atmosphere and the potential for NH4NO3
to release NH3 and HNO3 in warm weather, together with the
surfeit of NH3 also means that a larger fraction of the reduced and
oxidized N is remaining in the gas phase as NH3 and HNO3. The
change in partitioning from particulate NH4+ to gaseous NH3
is also occurring in other parts of Europe, where decreases in NH3
concentrations have been smaller than emission trends would suggest, due to successful mitigation in
SO2 emissions. Higher concentrations of the NH3 and
HNO3 in the atmosphere will deposit more locally, exacerbating the
effects of local N deposition loads over large areas of sensitive habitats,
with implications for UK's commitment to maintain or restore natural habitats
(e.g. Natura 2000 sites) to a favourable conservation status under the EU
Habitats Directive (Council Directive 92/43/EEC) and ecosystem monitoring
under Article 9 and Annex V of Directive 2016/2284 (NECD). The changes are
also important in terms of human effects assessment since NH4NO3
constitute a significant fraction of PM2.5 that are implicated in acute
and chronic human health effects and linked to increased mortality from
respiratory and cardiopulmonary diseases.
Ratified data from the Acid Gases and Aerosol Network
(AGANet) and the National Ammonia Monitoring Network (NAMN) are publically
accessible on the Defra UK-AIR website
(https://uk-air.defra.gov.uk/data, last access: 30 March 2018).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-18-16293-2018-supplement.
MAS conceived and proposed the establishment of the NAMN and AGANet.
MAS and YST developed the methods (ALPHA and DELTA) and set up the NAMN and AGANet.
UD assisted in building the DELTA equipment and conducting the DELTA vs. ADS intercomparison during the initial phase.
IS assisted in designing, building, and installing DELTA equipment for the expanded AGANet.
CFB provided resources and coordinated and managed the UKEAP network.
CC and KV coordinated and managed the Precip-net and NO2-net and also the former ADMN networks.
YST, IS, NvD, and CC organized and carried out UKEAP network site servicing.
YST, NvD, JP, ST, MP, POK, and HC performed the sample preparation and chemical analyses.
POK and AL supervised and performed quality checks on chemistry analyses.
MGP provided resources and managed and coordinated chemistry analyses.
DL carried out data curation and designed the CEH AAGA database for NAMN and AGANet data.
KV was involved in producing and maintaining the Defra UK-AIR website.
YST coordinated and managed the NAMN and AGANet (1997–2016), performed the data collection, data analysis, and wrote the paper.
MS, MRH, CFB, UD, NvD, and KV provided input on the paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
The UK AGANet and NAMN are funded by the Department for Environment, Food and
Rural Affairs (Defra) and the devolved administrations, under the UK Acid
Deposition Monitoring Network (ADMN: 1999–2008) and UK Eutrophying and
Acidifying Atmospheric Pollutants network (UKEAP: 2009–present). This work
was also supported by other NERC CEH programmes, including the Natural
Environment Research Council award number NE/R016429/1 as part of the
UK-SCaPE programme delivering National Capability. The authors gratefully
acknowledge assistance and contributions from the following groups: the large
numbers of dedicated local site operators without whom the monitoring work
would not be possible, site owners for provision of facilities, Harwell
Scientifics Laboratory (now Environmental Scientifics Group (ESG) Ltd) for
provision of chemical analysis for the AGANet between 1999 to 2009, the
Centralised Analytical Chemistry facility in Lancaster (in particular Heather
Carter, Darren Sleep and Philip Rowland) for sample preparation and chemical
analysis since 2009, and colleagues at both CEH Edinburgh
(Robert Storeton-West, Linda Love, Sarah Leeson, Matt Jones, Chris Andrews,
Amy Stephens, Margaret Anderson, Ian D. Leith) and Ricardo Energy &
Environment field team (Martin Davies, Tim Bevington, Ben Davies,
Chris Colbeck) for assisting in site and equipment maintenance and data
collection. Edited by: Annmarie
Carlton Reviewed by: Chris Flechard and one anonymous referee
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