Introduction
Gravimetric methods have long been used to monitor the long-term trends of
the bulk aerosol mass contained in particulate matter (PM) with an
aerodynamic diameter of less than 2.5 or 10 µm (i.e.
PM2.5 and PM10); however, these generally provide no information
on the chemical speciation of the aerosol. Offline chemical analysis after
capture of aerosol on filters by high or low volume filter samplers (e.g.
Partisol Sampler; Thermo Fisher Scientific, Inc.) is quite widely used but
limited to daily or lower frequency measurements and therefore poorly
captures diurnal patterns caused, e.g. by changes in emission and
gas–aerosol partitioning. It is important to understand the composition and
the role of aerosols as they can have a direct and indirect effect on
climate. The current level of scientific understanding for aerosol
properties in terms of their role in the climate system is low and recently
it has been suggested that the major component of the uncertainty globally
is with the biogenic fraction (Carslaw et al.,
2013). As well as having an impact on climate, aerosols affect both
environment and human health. Recent epidemiological research has suggested
that health effects of aerosol may be less closely linked to the total mass
of PM2.5 or PM10 than to the physicochemical characteristic of the
aerosol; however, there is still much to be understood (Fuzzi and
Gilardoni, 2013). Establishment of epidemiological links to individual
aerosol chemical compounds is hampered by a lack of available measurements.
In addition, secondary aerosols and their precursor gases are required to be
monitored to understand atmospheric processes and to validate chemical
transport models, which are used to inform policy such as the UNECE (United Nations Economic Commission for Europe)
Convention for Long-range Transboundary Air Pollution (CLRTAP) and the
revised European Air Quality Framework Directive (Directive 2008/50/EC)
where the measurement of aerosol chemical composition is statutory. In
addition, prior to 2008, EU member countries were fined for exceeding the
total PM2.5 and PM10 limits set in the directive, regardless
whether the exceedance was due to anthropogenic or natural sources. In the
current revised European Air Quality Framework Directive (Directive
2008/50/EC), countries are now allowed to subtract significant “natural”
contributions of aerosol from the total mass, if their contribution can
clearly be quantified. This is important for large “natural” pollution
events, such as the long-range transport of Saharan dust which has been
observed across Southern Europe and on occasions Northern Europe (Ansmann
et al., 2003; Karanasiou et al., 2012). While many member countries have
chosen to make daily filter measurements at very few sites, the UK has
opted for a strategy to combine a large number of sites that make monthly
measurements (Tang et al., 2009) with a couple of “supersites”
that resolve hourly concentrations, as an optimum strategy for capturing the
spatial and temporal variability.
Instrumentation has in the past decade become available for online
monitoring of aerosol chemical composition, at varying levels of complexity.
In particular, the wet chemistry MARGA instrument (Measurement of Aerosols
and Reactive Gases Analyser, Metrohm Applikon B.V., NL) provides hourly
measurements of water-soluble nitrate, chloride, sulfate, sodium, ammonium,
potassium, magnesium and calcium (hereafter NO3-, Cl-,
SO42-, Na+, NH4+, K+, Mg2+ and Ca2+,
respectively) in addition to the gas-phase basic and acid gases: ammonia,
nitric acid, nitrous acid, hydrochloric acid and sulfur dioxide (hereafter
NH3, HNO3, HONO, HCl, SO2, respectively) (Makkonen et
al., 2012; Rumsey et al., 2014; ten Brink et al., 2009), based on aerosol
collection via a steam-jet aerosol collector (SJAC; Khlystov et
al., 1995). Other similar IC (ion chromatography) based systems are available, including the
Ambient Ion Monitor–Ion Chromatograph system (AIM-IC, URG Corp. and
Dionex Inc.) (Markovic et al., 2012) as well as the Particle into
Liquid Sampler with Ion Chromatography (PILS-IC, Metrohm AG, Herisau,
Switzerland), and a range of custom-built wet-chemistry instruments based on
the Particle Into Liquid Sampler (Weber et al., 2001).
In parallel, there has been progress in developing monitoring instruments
based on aerosol mass spectrometry. While there are now some studies using
the standard Aerosol Mass Spectrometers (AMS; Aerodyne Research Inc, USA)
for long-term measurements, a simplified version, the Aerosol Chemical
Speciation Monitor (ACSM, Aerodyne Inc), is being installed at supersites
globally (Ng et al., 2011). Both AMS and
ACSM currently measure an aerosol fraction that is close to PM1
although work is in progress to extend this to PM2.5. While SJAC- and
PILS-based instruments measure water soluble aerosol components, similar to
the filter-pack reference method, the mass spectrometer detects the aerosol
components that volatilise efficiently at ≈ 600 ∘C, which
has the advantage of also characterising the organic fraction of the
aerosol, whereas it does not efficiently quantify the refractory chemical
components such as sea salt and crustal aerosol.
A dual MARGA system measuring both PM10 and PM2.5 has been in
operation at Auchencorth Moss, south-east Scotland since June 2006 as part
of measurements being made by the UK Department for Environment, Food and
Rural Affairs (Defra) air quality monitoring network
(http://uk-air.defra.gov.uk/). To our knowledge the Auchencorth Moss MARGA
is the longest known quasi-continuous operation of a dual MARGA system to
date. Auchencorth Moss has been developed as a Level II/III “supersite”
within the European Monitoring and Evaluation Program (EMEP)
(Aas et al., 2012). EMEP
monitoring sites feed into the EMEP database which serves to underpin the
organisation's modelling and policy role to provide governments information
on the deposition and concentration of air pollutants, and long-range
transport of air pollutants (Tørseth et
al., 2012; UNECE, 2004).
he following study focuses on the first 6.5 years of data (1 June 2006 to
1 January 2013) from Auchencorth Moss, in conjunction with co-located
measurements and air mass back trajectories. Daily, seasonal and annual
variation of inorganic aerosol species and the influences of long-range
transport for this remote rural site are discussed. The trace gases measured
concurrently with the aerosol composition and gas/aerosol partitioning are
described in a companion paper (Twigg et al., 2015).
Methodology
Field site description
Auchencorth Moss, south-east Scotland (55∘47′36′′ N, 3∘
14′41′′ W), is an ombrotrophic mire with an extensive fetch at an elevation
of 270 m, lying 18 km SSW of Edinburgh, and can be categorised as a
transitional lowland raised bog. The moss is extensively grazed by sheep all
year round with < 1 sheep ha-1. Under the European Environment
Agency classification scheme, the site is classed as a rural background site
(Larssen et al., 1999). This has recently been confirmed by
Malley et al. (2014), who demonstrated that the site was
remote in the context of O3 measurements at EMEP stations.
The meteorology is typical of a temperate system in the north of the UK. A
summary of the meteorological conditions from June 2006 to the end of
December 2012 can be found in Table 1. During 2010 atypical low rainfall was
observed; however, an additional 588 mm of precipitation fell as snow. Annual
windroses for 2006–2012 (Fig. 1) show that the field site is dominated by
a SW wind with a secondary NE flow occurring also.
Summary of metrological conditions for the period June 2006–December 2012.
Year
Total
Air temperature
Wind speed at 1 m
RH
St
Rainfall (mm)
(∘C)
(m s-1 )
(%)
(W m-2)
Median
Min
Max
Median
Max
Mean
Mean
2006*
740
11.2
-5.4
28.5
2.8
13.3
88.6
99.52
2007
1124
7.8
-9.7
22.1
3.0
24.8
83.0
88.67
2008
1212
7.3
-8.4
23.2
2.9
24.3
84.4
84.91
2009
989
7.6
-9.0
27.5
3.0
12.2
84.8
92.92
2010
649
6.7
-12.0
24.6
2.7
11.6
83.9
92.39
2011
1101
8.4
-8.9
24.6
3.1
13.5
86.1
88.38
2012
1322
6.6
-8.3
23.3
2.8
16.8
88.1
83.63
* 2006 only includes data from 1 June 2006 onwards. Key: RH –
relative humidity, St – total solar radiation.
Frequency plots of wind direction and wind speed (m s-1)
averaged over an hour at Auchencorth Moss for the years 2007–2012.
Wind speed scale is limited to 30 m s-1. (Graphs produced using
OpeAir; Carslaw and Ropkins, 2012).
Auchencorth Moss has been a long-term monitoring site for a number of trace
gases including NH3, SO2 and CO2 (Famulari et al.,
2010; Drewer et al., 2010; Flechard and Fowler, 1998). The site belongs to a
number of UK national networks including the UK Defra Automatic urban and rural
network (AURN), UK Acid gas and aerosol network (AGANet), UK National
Ammonia Monitoring Network (NAMN), UK Precip-Net (bi-weekly bulk
composition), UK PAH Network, UK Automatic Hydrocarbon Monitoring Network,
UK Black Carbon Network and UK Toxic Organic MicroPollutants (TOMPS)
network. Details of the networks can be found on the Defra website
(http://uk-air.defra.gov.uk/networks/site-info?site_id=ACTH). The site is also a European supersite within the Co-operative
Programme for Monitoring and Evaluation of the Long-range Transmission of
Air Pollutants in Europe (EMEP) (Tørseth et
al., 2012), as well as one of nine sites within the EU FP7 ÉCLAIRE project
(http://www.eclaire-fp7.eu/) and a TransNational Access (TNA) site within
the European FP7 Infrastructure Network ACTRIS (Aerosol, Clouds, and Trace
gases Research InfraStructure Network). It was a Level 3 site within the EU
FP6 NitroEurope IP (Sutton et al., 2007). In 2014, it became a World
Meteorological Organisation Global Atmosphere Watch (WMO GAW) regional site.
MARGA instrument
The MARGA 2S system (Metrohm Applikon B.V. Schiedam, NL) consists of two
sampling boxes and utilises ion chromatography to analyse for a range of
water soluble trace gases and aerosols. The MARGA 2S was set up to measure
both PM10 and PM2.5 aerosol. Air is first drawn through a common
PM10 Teflon-coated inlet (URG Corporation, Chapel Hill, NC, USA) at
3.55 m a.g.l. into a 0.89 m long polyethylene (PE) 14 mm ID (inner diameter)
inlet line, which is housed in the centre of an 11 cm OD (outer diameter) polyvinyl chloride
(PVC) conduit. The conduit has an extractor fan at the base to draw air
through based on the design used by Trebs et al. (2004) aimed at keeping the sample at the temperature of the measurement
height for as long as possible. The PE tubing entering the air conditioned
cabin (21 ∘C) is split into two 1/4 in. PE sample lines (0.4 m). The first line feeds directly into the first sampling box and the second
sampling line goes through a further PM2.5 cyclone (URG Corporation,
Chapel Hill, NC, USA) before the second sampling box. The flow rate in each
sampling box is regulated to a volumetric flow of 1 m3 h-1 using
a mass flow controller downstream of the sampling box. In the sampling box,
air passes through a horizontal annular wet rotating denuder (WRD)
(Keuken et al., 1988). The WRD is continuously coated with a thin
film of solution which strips water soluble gases from the laminar air
stream; the addition of 10 ppm H2O2 acts as a biocide and also
promotes oxidation of SO2 initially trapped as HSO3- through
to SO42-. Water soluble aerosols do not diffuse into the stripping
solution due to their lower diffusion velocity. The air flow then enters a
steam-jet aerosol collector (SJAC). The steam in the SJAC promotes rapid
growth of water soluble aerosols which are then separated out from the air
flow mechanically in a cyclone. Details of the principles of the SJAC are
described by Khlystov et al. (1995). The sampling solutions are
continuously drawn from the WRDs and SJACs to the analyser box at a rate of
25 mL h-1 using syringe pumps. Samples are then analysed online by
anion and cation chromatography (Metrohm AG, Herisau, Switzerland). The
system is continuously calibrated by mixing the sample with a 325 mg L-1
internal standard of LiBr, prior to injection into the IC columns. Anions
are concentrated on a Metrosep A PCC 1 HC IC preconcentration column (2.29 mL) and then separated using a Metrosep A Supp 10–75 column (75 mm × 4.0 mm)
using a 7 mmol L-1 Na2CO3/8 mmol L1 NaHCO3 eluent.
Cations are concentrated on a Metrosep C PCC1 HC IC preconcentration column
(3.21 mL) and separated using a Metrosep C4 (100 mm × 4.0 mm) cation column. A
3.5 mmol L-1 methanesulfonic acid (MSA) eluent was used for the cation
column, rather than the recommended 3.2 mmol L-1 HNO3 eluent
used in other similar systems. This was in order to eliminate a potential
NO3- artefact, which has been reported (ten Brink et
al., 2009; Makkonen et al., 2012). For the anion
column a 1 M H3PO4 solution was used for chemical suppression.
Detection was by conductivity, where concentrations were calculated based on
their specific conductivities relative to the internal standard ions
(Li+ and Br-). These standard set-up conditions and all
significant operational changes over the 6.5 years, which could be
considered to have affected performance or data capture, are summarised in
Table 2. The increase in diameter of the SJACs described in Table 2 reduced
the restriction in maintaining a flow rate, which was mass flow controlled
at 1 m3 h-1 at standard temperature and pressure (STP)
until November 2011, whereas thereafter it was controlled to keep the
volumetric flow rate at ambient temperature and pressure through the size
cuts constant.
Summary of major operational changes which have potentially affected
the MARGA performance or data capture from June 2006 to December 2012.
Date
Operation change
Change to performance
03 Dec 2008
Changed from Metrohm C2 column with 4 mM MSA eluent to Metrohm C4 column with a 3.5 mM MSA.
Better separation of NH4+ and Na+ peaks.
17 Feb 2009
SJACs were replaced with an increased internal diameter. Syringe valves increased from 0.6 mm to 0.8 mm ID.
Improved accuracy in maintaining the cutoff for PM2.5 and PM10. Reduction in downtime due to blockages.
28 Jul 2009
Replaced glass fibre filters to PTFE Whatman ReZist 30 mm filter.
Glass fibre filters had a high Na+ and SO42- background and required rinsing prior to use.
29 Jul 2009
100 ppm H2O2 added to H2O stripping solution.
Prevents loss of NH4+ from bacteria by acting as a biocide. Converts HSO3- to SO42-, resulting in better SO2 recovery in the denuder.
09 Feb 2011
Reduced to 10 ppm H2O2 in stripping solution.
Optimum concentration as a biocide, whilst preserving lifetime of the column.
17 Nov 2011
MARGA hardware and software upgrade.
Calibration of mass flow controllers can now be carried out in situ. Blanks and external standards can be set up remotely.
The performance of the MARGA has been further discussed by Rumsey et al. (2014), Makkonen et al. (2012), ten Brink et al. (2009), and Cowen et al. (2011).
The deployment of pre-concentration columns sets our MARGA instrument aside
from the others, with the exception of Makkonen et al. (2014), allowing
quantitative detection of the low concentrations encountered at this clean
Scottish site.
Quality analysis and quality assurance
As discussed previously, the MARGA used in this work was one of the first to
be field deployed. Processes were developed over the first several years
which could be used to identify potential sources of error or contamination
in the MARGA data. Firstly, periodic field blanks were carried out until
2011 by installing Whatman HEPA (high-efficiency particulate air) filters placed in front of the denuders and
left on for ∼ 24 h. The filters removed aerosols but not
the gas-phase components from the air stream. The resultant change in PM
concentrations allowed a blank value for the PM analytes to be assessed.
Following the upgrade of the instrument in November 2011 (Table 2),
automated monthly blanks were implemented in 2012, where the air pump and
SJAC water supply and heaters were turned off, allowing for blanks for both
aerosol and gas phase to be carried out. The blanks were not used to correct
the data as they were usually below the detection limit (DL) of the
instrument. Instead blanks were used to provide evidence of contamination in
the system and to identify periods to be removed in the data ratification
process. Prior to 2012 verification and instrument maintenance protocols were experimental and the authors are in the process of finalising the
protocols including calibration, which will be published separately. There
was procedural change in 2012, when the initial developmental protocols for
maintenance were replaced by final protocols. The protocols include
quarterly replacement of inlets, cleaning of PM10 head and PM2.5 cyclone, though the frequency increases if there is evidence of pollution
events or visual dirt in the denuder or SJAC, resulting in cleaning of the
glassware too. Monthly calibrations of mass flow controllers (MFCs) have
been implemented, following the upgrade, by carrying out a three-point
calibration using a NIST (National Institute of Standards and Technology) traceable air flow calibrator (Challenger,
Sensidyne, LP. USA). The MFC flows were found to compare well on average with
an independent flowmeter (< 5 % difference on average to the
expected 1 m3 h-1 flow rate). Only on occasions did the flows
differ, after denuders or SJACs had been either moved or cleaned, or as the
result of a faulty air pump or MFC. As a result it is now procedure to
recalibrate flows following any change in the sample boxes. In addition due
to the frequency of the audits, air concentration data are only corrected
when there is evidence of drift of the MFCs calibration. As part of a more
stringent protocol, independent analyses of the internal standard (LiBr) by a
UKAS (United Kingdom Accreditation Service) accredited laboratory (CEH Lancaster) have been carried out since 2012.
The measured LiBr concentrations are on average 10.5 % (Li) and 6.5 %
(Br) lower than the theoretical concentration, when independently analysed
by the UKAS accredited laboratory. The difference between the laboratory and
the MARGA measured values were -0.4±3.4 % and -2.2±3.8 %
for Li and Br, respectively, over 2013 based on monthly measurements. Only on a
few occasions were external solutions analysed by the MARGA as it was
not until 2012, following the instrument upgrade, that external standards
could be successfully run. External standards however have occasionally been
used to confirm peak identification on chromatograms.
In the data ratification process values reported as 0 µg m-3
were replaced with half the DL. The method to determine
the DL has changed over the 6-year period. From 2002 to 2011, the DL was
taken as the average of the reported values below the manufacturer's
published DL. From 2012, the DL was calculated by the analysis of the
logarithmic distribution of the measurements previously described by
Kentisbeer et al. (2014), presented in Table 3. The calculated DLs have been
reported to UKAir (http://uk-air.defra.gov.uk/data/) on a monthly basis
since 2012; only 1.4 % of potential data were filled with one-half DL in
2012. The methodology for analysing the DL and calibrating this type of
on-line IC instrument is an area of research in of itself and we plan to
publish separately on this aspect of the MARGA operation.
Annual average detection limits calculated using a logarithmic profile for
2012.
Component
DL
µg m-3
PM10
NH4+
0.062
Na+
0.123
K+
0.019
Ca2+
0.016
Mg2+
0.015
Cl-
0.086
NO3-
0.105
SO42-
0.349
PM2.5
NH4+
0.069
Na+
0.106
K+
0.014
Ca2+
0.015
Mg2+
0.007
Cl-
0.053
NO3-
0.091
SO42-
0.242
Back trajectories and associated analysis
To relate the aerosol species to air masses, back trajectory analysis was
carried out. Four-day back trajectories at 3 h intervals for Auchencorth
Moss were obtained for the years 2007–2012 through the OpenAir software
package (Carslaw, 2013), which calculates back trajectories with
the HYSPLIT trajectory model (Hybrid Single Model Lagrangian Integrated
Trajectory Model, (Draxler and Hess, 1997) using Global
NOAA-NCEP/NCAR reanalysis data. A cluster analysis was then carried out
using a routine in the OpenAir software, where data were clustered using a
distance matrix, in this case according to the similarity of the angle from
their origin. Further details of the calculations of the cluster analysis
can be found in Carslaw (2013).
Results and discussion
Overview
Annual concentrations of both PM10 and PM2.5 species measured by
the MARGA system at Auchencorth Moss from 1 June 2006 to 1 January 2013.
(* 2006 data coverage: June – December 2006 only) Key: μA –
arithmetic mean, μG – median, Max – maximum, σA –
arithmetic standard deviation, DC – data capture (%).
2006∗
2007
2008
2009
2010
2011
2012
μA
μG
max
σA
DC
μA
μG
max
σA
DC
μA
μG
max
σA
DC
μA
μG
max
σA
DC
μA
μG
max
σA
DC
μA
μG
max
σA
DC
μA
μG
max
σA
DC
PM2.5
µg m-3
µg m-3
µg m-3
µg m-3
%
µg m-3
µg m-3
µg m-3
µg m-3
%
µg m-3
µg m-3
µg m-3
µg m-3
%
µg m-3
µg m-3
µg m-3
µg m-3
%
µg m-3
µg m-3
µg m-3
µg m-3
%
µg m-3
µg m-3
µg m-3
µg m-3
%
µg m-3
µg m-3
µg m-3
µg m-3
%
NH4+
0.93
0.36
6.86
1.23
18.08
0.72
0.21
8.30
1.21
35.99
0.61
0.20
12.35
1.23
36.15
0.77
0.36
12.90
1.24
49.10
0.79
0.36
9.14
1.08
46.02
0.64
0.28
6.81
0.95
25.58
0.89
0.35
14.66
1.46
63.90
Na+
0.26
0.15
3.36
0.36
38.29
0.37
0.26
7.28
0.48
32.39
0.43
0.27
8.55
0.61
34.71
0.70
0.44
7.87
0.91
48.97
0.29
0.18
3.97
0.32
38.28
0.44
0.29
4.02
0.43
24.11
0.48
0.33
2.78
0.43
65.01
K+
0.05
0.04
0.55
0.04
37.59
0.06
0.04
0.50
0.05
27.61
0.10
0.04
1.05
0.13
35.63
0.19
0.06
3.33
0.53
49.11
0.04
0.03
1.79
0.05
46.14
0.05
0.02
0.45
0.09
25.82
0.05
0.04
1.05
0.04
66.75
Ca2+
0.07
0.03
0.73
0.11
37.11
0.09
0.08
1.61
0.07
36.38
0.11
0.09
1.39
0.08
37.42
0.12
0.07
3.70
0.21
49.09
0.06
0.05
1.84
0.06
46.18
0.05
0.03
0.49
0.05
25.91
0.05
0.04
0.54
0.03
66.55
Mg2+
0.04
0.03
0.22
0.02
38.29
0.08
0.06
1.55
0.07
36.38
0.05
0.03
0.61
0.06
37.36
0.06
0.04
0.38
0.05
48.70
0.04
0.03
1.28
0.05
46.18
0.04
0.02
0.41
0.04
25.91
0.04
0.02
0.31
0.04
66.56
Cl-
0.47
0.41
3.53
0.51
38.68
0.59
0.38
9.66
0.75
32.55
0.68
0.38
8.91
0.92
31.55
0.59
0.41
4.56
0.60
18.77
0.56
0.38
6.49
0.64
33.79
0.72
0.38
10.23
0.92
26.85
0.61
0.30
6.18
0.75
53.85
NO3-
1.32
0.79
12.05
1.63
22.81
1.20
0.36
16.12
2.12
37.72
0.98
0.30
20.60
2.11
38.73
1.11
0.35
29.31
2.88
39.93
1.18
0.37
20.18
2.19
38.05
0.91
0.40
15.02
1.61
28.52
1.54
0.43
32.75
3.20
59.23
SO42-
1.46
0.74
15.87
2.05
39.19
0.81
0.44
34.27
1.09
37.73
1.01
0.56
18.63
1.40
38.88
1.22
0.89
15.36
1.31
39.89
0.97
0.66
10.53
1.00
38.11
1.11
0.76
7.75
1.05
28.53
1.21
0.77
11.39
1.25
59.22
PM10
NH4+
0.97
0.43
6.66
1.19
17.76
0.98
0.33
16.88
1.54
47.96
0.57
0.20
14.47
1.11
31.66
0.76
0.38
13.59
1.29
49.32
0.81
0.41
9.33
1.11
44.92
0.64
0.34
8.41
0.90
34.01
0.98
0.35
19.51
1.75
66.80
Na+
0.48
0.32
8.05
0.57
43.71
0.62
0.36
9.58
0.76
45.81
0.62
0.36
13.88
0.95
31.57
0.90
0.64
14.17
0.88
48.90
0.55
0.35
11.14
0.70
39.01
0.57
0.37
4.94
0.62
32.61
0.84
0.63
4.98
0.74
67.96
K+
0.04
0.03
0.36
0.03
43.09
0.06
0.05
0.54
0.05
40.19
0.07
0.04
0.95
0.09
32.70
0.16
0.08
2.67
0.32
49.27
0.05
0.04
2.29
0.06
45.08
0.04
0.02
2.17
0.06
34.09
0.07
0.06
2.61
0.07
70.33
Ca2+
0.06
0.03
0.99
0.09
43.16
0.11
0.08
0.84
0.09
47.93
0.18
0.15
1.06
0.12
32.67
0.18
0.09
3.16
0.42
49.19
0.09
0.07
2.06
0.09
44.84
0.06
0.02
0.80
0.08
33.93
0.06
0.05
1.22
0.07
70.11
Mg2+
0.04
0.03
0.50
0.04
43.71
0.10
0.07
2.14
0.10
48.30
0.07
0.05
0.78
0.07
32.70
0.10
0.07
0.71
0.11
47.77
0.06
0.05
1.59
0.07
44.57
0.05
0.02
0.54
0.07
34.09
0.06
0.04
0.52
0.07
70.14
Cl-
0.70
0.49
7.40
0.86
42.35
0.95
0.55
11.57
1.17
44.14
1.08
0.59
12.65
1.44
33.20
0.97
0.60
7.32
1.06
20.21
1.05
0.67
8.46
1.20
34.51
1.04
0.51
10.25
1.31
35.92
1.11
0.54
8.87
1.38
55.73
NO3-
1.92
1.43
12.58
2.13
21.27
1.55
0.56
31.60
2.48
48.52
1.18
0.38
24.42
2.48
37.17
1.24
0.43
30.65
2.93
41.04
1.31
0.47
21.40
2.32
37.88
1.07
0.49
15.95
1.68
37.42
1.86
0.55
50.15
3.81
60.73
SO42-
1.65
0.91
16.01
2.09
42.79
1.19
0.61
33.98
1.70
48.73
1.01
0.68
11.86
1.07
37.19
1.30
0.92
18.79
1.39
41.22
0.99
0.72
12.57
0.99
37.79
1.03
0.77
8.07
0.88
37.41
1.43
0.89
12.75
1.55
60.74
Table 4 summarises the annual data capture
statistics for each compound. The Auchencorth MARGA was one of the first
long-term field deployments of the MARGA instrument. Through troubleshooting
and instrument improvements the data capture improved over the period
reported with a highest data capture of 64 % (average overall) in 2012.
Though not reported here, data capture for 2013 is on average 83 % for
ratified data (http://uk-air.defra.gov.uk/). As seen in
Table 4, Auchencorth Moss being a rural to remote
site, the aerosol concentrations were low as there are no large local point
pollution sources in the dominant SW wind direction.
Concentration trends from June 2006 to December 2012
The annual average concentrations from June 2006 to December 2012 are
summarised in Table 4. Overall, the concentrations
of individual species were generally low (< 1.5 µg m-3).
When compared with speciated PM2.5 measurements from a background
site in the Midlands, UK (Harrison and Yin, 2010), Cl-
concentrations were higher at Auchencorth Moss based on annual averages
(Table 4) but are in a similar range to other UK
sites (Abdalmogith and Harrison, 2006). The average annual
concentrations of NO3- and SO42- in the PM10, on
the other hand, were larger in other parts of the UK including the other
rural EMEP supersite at Harwell (Harrison and Yin,
2010; Abdalmogith and Harrison, 2006). The maximum concentrations of the
aerosol components, however, show that there were periods where large PM
pollution events took place, which are hypothesised to have taken place due to
long-range transport of polluted air masses. In both PM2.5 and PM10, the largest concentrations of NH4+ and
NO3- were recorded during 2012, SO42- maximum
concentrations were observed in 2007 and other species varied
(Table 4). It is interesting that specific local
events can be picked out from the data record, for example the maximum
K+ concentration in 2012 of 2.61 µg m-3 occurred at 00:00 GMT on 6 November 2012 – Guy Fawkes or “fireworks“ night in the UK.
The meteorological conditions that night were cool, with an average
temperature of -0.1 ∘C at midnight and the wind direction was from
the dominant wind sector (SW). An increase of K+ is not unexpected as
such an increase is reported to occur following firework events
(Vecchi et al., 2008; Drewnick et al., 2006). This example illustrates the
utility of the hourly composition measurements to understand specific
atmospheric events.
(a) Median monthly mass concentrations of
PM2.5 species measured by the MARGA and median wind speed from June 2006 to December 2012. (b) Median monthly mass concentrations of PM10 species measured
by the MARGA and median wind speed from June 2006 to December 2012.
The monthly median concentrations for all 6.5 years are presented in
Fig. 2. There is a clear seasonal variation for
Na+ and Cl- for all years, with the exception of 2009, with lower
concentrations in the summer and higher concentrations in winter. This
seasonality reflects higher average wind speeds in winter leading to more
marine aerosol in the atmosphere, as previously observed at other sites in
the UK, but for Cl- it is also consistent with increased NaCl reaction
with HNO3, which also peaks in summer. Of the secondary inorganic
pollutants, NO3- shows individual peak concentrations only during
the colder months, however not consistently, whereas SO42- and
NH4+ do not have particularly strong annual variation. The
largest monthly median concentration for NH4+ and NO3-
was observed in March 2012. SO42- on the other hand does not show
the same feature; the maximum monthly concentrations were observed in July
2012.
Median seasonal diurnal cycles of molar concentrations of
PM2.5 and PMcoarse, NH4+, NO3-, SO42-, Na+ and Cl- using data from January 2007 to December 2012, with
the shading showing the 95 % confidence level of the median. (Graphs
produced using Open air; Carslaw and Ropkins, 2012).
Comparing the average diurnal cycles for 2007–2012
(Fig. 3), it is apparent that the contribution of
PMcoarse is small compared with PM2.5, where
PMcoarse=PM10-PM2.5.
PMcoarse is dominated by sea salt (Na+ and Cl-). In the fine
fraction (PM2.5) NH4+ aerosol dominates, as it is the major
base in secondary inorganic aerosol (refer to Sect. 3.4). In PM2.5 there is a decrease of
NO3- during the afternoon in all seasons, though this feature is
strongest in winter. This behaviour is consistent with that previously
reported from other north European sites
(Nemitz et al., 2015) including Harwell (UK) (Revuelta et al.,
2012), Cabauw (Netherlands) (Mensah et al., 2012), Melpitz (Germany)
(Poulain et al., 2011) and SMEAR II (Finland)
(Makkonen et al., 2012). It is assumed that the
majority of fine NO3- will be in the form of NH4NO3 and
that the relationship between the gas precursors, temperature, RH and
chemical composition explain the observed cycle (see the discussion on gas
concentrations at this site; Twigg et al., 2015). Timonen et al. (2011), who had also
reported a decrease of daytime NO3- in Helsinki, explained the
decrease to be the result of increased boundary layer mixing as the same
feature was observed in black carbon. At Auchencorth Moss this behaviour of
black carbon is not observed, instead the annual diurnal average shows an
increase of black carbon during the day (Cape et al.,
2012). It is therefore probable that diurnal variation in temperature and
relative humidity exert a stronger influence on the PM2.5
NO3- at this site than the depth of the mixing layer. PM2.5
SO42- at Auchencorth Moss, on the other hand, shows an increase in
concentration during the day, with the feature strongest in summer. The
increase in SO42- is interpreted to be the effect of stronger
insolation in summer, which drives the oxidation of SO2 to form
sulfuric acid and finally SO42-, due to the increase in OH
radicals.
Ion balance
Measured ion balance for the year 2012 in microequivalents
per cubic metre. (a) Neutralisation of PM2.5 NH4+ by PM2.5
nss-SO42- and PM2.5 NO3-, (b) ion balance of
measured PM2.5 anions (Cl-, NO3- and SO42-)
and measured PM2.5 cations (NH4+, Na+, K+,
Ca2+and Mg2+), (c) neutralisation of PMcoarse NH4+
by PMcoarse nss-SO42- and PMcoarse NO3-,
(d) ion balance of measured PMcoarse anions (Cl-, NO3-
and SO42-) and measured PMcoarse cations (NH4+,
Na+, K+, Ca2+and Mg2+).
The ion balance was calculated for PM2.5 and PMcoarse for the year
2012. Figure 4a and b show the ion balance of the
secondary inorganic species, while Fig. 4c and d show the full ion
balance of the measured species. In both PM2.5 and PMcoarse it
is clear that though there is good correlation, there appears to be an
excess of NH4+. This is not the first time excess NH4+ has been observed in aerosol measurements (Mensah et al., 2012). It is
thought that water soluble organic acids such as oxalate may be the missing
species to close the ion balance. Some of the Cl- measured by the MARGA
is likely to represent NH4Cl, which would affect the partial ion balance
of Fig. 4a but not the full ion balance of Fig. 4b. However, Aerosol Mass
Spectrometer (AMS) measurements suggest that this contribution is negligible
in S Scotland
(Nemitz et al., 2015). On the other hand, some of the NO3- in the partial
ion balance is expected to represent NaNO3, even in PM2.5, and the
excess NH4+ may be even larger than suggested by Fig. 4a.
Makkonen et al. (2012) observed that in Finland the
ion balance was seasonal, with acidic aerosol in winter and a basic ion
balance in spring. This seasonal trend was not observed at Auchencorth Moss,
with the average seasonal ion balance always basic (i.e. excess
NH4+) in character, which is consistent with AMS measurements that
have demonstrated that acidic aerosol is only found in the NE, E and S of
Europe, while there is always excess ammonia in the NW and west-central Europe
(Nemitz et al., 2015; Morgan et al., 2010).
Sea salt and sea salt processing
Average composition by mass of the water soluble inorganic aerosol
fraction measured by the MARGA from January 2007 to December 2012 in both
PM2.5 and PMcoarse. Sea salt chloride, sulfate, magnesium,
calcium and potassium were derived based on the known mass ratios to
Na+ in sea water, refer to Eqs. (2)–(5). Key: nd – not detected.
An overview of the average inorganic PM2.5 and PMcoarse
composition based on mass is presented in Fig. 5.
Sea salt is presented as the individual species of measured Na+, sea
salt Cl- (ssCl-), sea salt SO42- (ssSO42-)
sea salt Mg2+ (ssMg2+), sea salt Ca2+ (ssCa2+) and sea
salt K+ (ssK+), which were calculated based on the known mass
ratio to Na+ in sea water (Seinfeld and Pandis, 2006):
[ssCl-]=1.8×[Na+],[ssSO42-]=0.252×[Na+],[ssMg2+]=0.12×[Na+],[ssCa2+]or[ssK+]=0.04×[Na+].
As would be expected, the dominant fraction of the coarse aerosol at this
site is from sea salt (73 %); this is larger than reported at other
European sites such as SMEAR III, near Helsinki
(Makkonen et al., 2012), probably because of
proximity to the ocean in all wind directions. There is also a large
contribution from NO3- in the coarse fraction. This is not the
first time that a large proportion of NO3- has been reported in
the coarse mode; the same was observed in Melpitz, Germany, and has been
explained to be the result of chloride–nitrate exchange that takes place on
coarse aerosol during long-range transport of sea salt (Spindler et al.,
2012; Dasgupta et al., 2007). This was further evident as the average non-sea
salt Cl- mass was -0.17 and -0.08 µg m-3 for PM2.5 and PMcoarse, respectively, where
Non-sea salt Cl-=[Cl-]measured-[ssCl-]calculated .
To investigate the process of sea salt substitution by reaction with
HNO3 further, the ratios of Na+ and Cl- were compared with
NO3- in the coarse mode for the year 2012. In general, larger
NO3- concentrations tended to be observed on occasions where a
depletion of Cl- was observed, though this was not true for all cases
(Fig. 6). It should be noted, however, that the
concentrations of PMcoarse are calculated as differences (Eq. 1) and
therefore subject to considerable uncertainty.
Demonstration of the depletion of Cl- for the year 2012 as a
result of Cl-–NO3- interactions during long-range transport
for coarse aerosol. The black line is the known ratio of Cl- to
Na+ in seawater (Seinfeld and Pandis, 2006). Colour scale is
set to 0–> 0.2 µeq m-3 NO3- to focus in on
the depletion of Cl- at high NO3- concentrations.
PM2.5 on the other hand is dominated by the secondary inorganic
aerosol (SIA) (NH4+, NO3- and SO42-)
(Fig. 5), with a total contribution of 63 % to
the total measured mass by the MARGA, which is to be anticipated.
NO3- is the dominant mass of the SIA at Auchencorth Moss,
accounting for 26 % of the total water soluble species detected by the
MARGA. A similar comparison has been carried out by a MARGA operated at
SMEAR III (near Helsinki, Finland) where SO42- was the dominating
mass responsible for 50.4 % of the total inorganic PM2.5 mass
reported by the MARGA (Makkonen et al., 2012). This
is not surprising as it has also been shown by
Nemitz et al. (2015), from AMS studies, that in Finland PM1 is dominated by
SO42-, whereas in the UK and the rest of NW Europe, NO3-
is the dominant SIA. Sea salt, however, still makes a considerable
contribution (35 %) to the average measured PM2.5 by the MARGA,
bearing in mind that the cutoff might have been somewhat larger than 2.5 µm until November 2011 (see above). In 2012, sea salt still made a
major contribution to the total mass of the PM2.5 (30 %), where there
is the greatest confidence in the cutoff of the cyclone. There was a clear
increase of PM2.5 Na+ with wind speed for 2012
(Fig. 7) in the dominant wind sector (refer to
Fig. 1), suggesting that PM2.5 Na+ was
related to sea salt and its presence at the site is driven by meteorology.
The influence of wind direction and wind speed on the concentration
of PM2.5 Na+ at Auchencorth Moss from 1 January 2007 to 1
January 2013.
Relationship of K+ (left hand figures) and of Mg2+ (right
hand figures) to Na+ for PM2.5 and PMcoarse from 21 March 2012
to 1 January 2013. Black lines show the sea water ratios of K+ and
Mg2+ to Na+ taken from Seinfeld and Pandis (2006). Black
carbon data are provisional data downloaded from the DEFRA UK-Air database
archive (http://uk-air.defra.gov.uk/data/) on 17 March 2014.
Potassium (K+) is present in sea salt and when the available 2012
PM2.5 data was compared to the concentration of Na+ it tended to
follow the known ratio in sea water (Seinfeld and Pandis, 2006);
Fig. 8. The greatest deviation from this curve
appears to be in periods of high black carbon (BC) concentrations.
PMcoarse however had much scatter. High concentrations of black
carbon are often associated with combustion processes, though K+ can
also occur as a product of other anthropogenic sources. There was clear
evidence in the PM2.5 that high concentrations of K+ were
associated with increased BC pointing to a contribution from combustion
sources or biomass burning (Fig. 8). The measured
Mg2+ / Na+ ratio in PM2.5 followed the known ratio in sea
water (Seinfeld and Pandis, 2006). The same comparison was done for
PMcoarse; however, there was much more scatter in the data.
Comparison of total inorganic aerosol with TEOM-FDMS measurements
The total average water-soluble inorganic aerosol mass measured by the MARGA
for the period January 2007 to December 2012 was 3.82 µg m-3
for PM2.5 and 5.04 µg m-3 for PM10,. The measured mass by the MARGA was compared to the tapered
element oscillating microbalance filter dynamic measurement system
(TEOM-FDMS) which measures the total aerosol mass; total mass data were
obtained from the AURN network
(http://uk-air.defra.gov.uk/networks/network-info?view=aurn) for the 6 years of interest (2007–2012). It was found that the PM10 mass
measured by the MARGA accounted for 78 % of total PM10 measured by
the TEOM-FDMS, on average. It is not the first time that inorganic water
soluble aerosols have been found to be major contributors to the total mass
in Europe (Putaud et al., 2010). Aerosol components not resolved by the
MARGA include organic aerosols, BC, water and crustal elements such as
silicate. Organic aerosol often accounts for a larger fraction of the
PM10 mass at central European background sites than the missing mass at
Auchencorth allows for. This is consistent with AMS measurements in
S Scotland that also indicate relatively low contributions from organic
aerosol (Nemitz et al., 2015).
Measured MARGA mass vs. TEOM-FDMS mass and the percentage of time the
TEOM FDMS reported values ≤ 0 µg m-3 for the years
2007–2012.
Year
PM10 unaccounted
PM2.5 unaccounted
PM10 measured by
PM2.5 measured by
(%)
(%)
TEOM-FDMS
TEOM-FDMS
– reported values ≤ 0 µg m-3 (%)
– reported values ≤ 0 µg m-3 (%)
2007
21
-15
14
25
2008
23
-38
10
25
2009
29
-48
7
28
2010
28
-27
6
17
2011
32
-20
9
26
2012
18
-9
3
20
Table 5 summarises the annual mass fraction that is
accounted for by the MARGA instrument when compared with the TEOM-FDMS for
both PM10 and PM2.5. It is very clear that there are
discrepancies between the measured PM2.5 by the MARGA and that by
TEOM-FDMS. Mass closure improved in 2012, probably in response to the
improved flow control implemented in November 2011 on the MARGA (see above).
An alternative explanation is that the PM concentrations at Auchencorth are
close to the detection limits of the TEOM-FDMS, which is indicated by the
large percentage of negative values reported by the instrument over the
period January 2007–December 2012 (26 and 10 %, respectively, for
PM2.5 and PM10), the annual variation of which can be
found in Table 5. During the 6 years presented, the
fraction of negative values for PM10 declined, while it stayed constant
for PM2.5. It therefore can be concluded that the PM2.5
TEOM-FDMS at Auchencorth Moss has an offset, as has previously been
commented by Laxen et al. (2012). It is therefore not possible to
comment on what the true contribution of the measured water soluble
inorganic mass measured by the MARGA is to the total PM2.5.
Influence of air mass on aerosol composition
Mean trajectory associated with each cluster following clustering
of 96 h back trajectories at 3 h intervals calculated for Auchencorth Moss
covering the years 2007–2012 (17 370 back trajectories) (Graphs
produced using Open air; Carslaw and Ropkins, 2012).
Due to the remote location of the site, the origin of air masses at the site
influences the aerosol composition. Back trajectories, run over a 96 h
period, were obtained at 3 h intervals for the years 2007–2012, which
were then clustered (details can be found in Sect. 2.3). Figure 9 displays the
mean trajectory for each of the six clusters assigned. The average
concentration over the 6-year period for each cluster and the percent of species
contribution to the total measured concentration by the MARGA are summarised
in Fig. 10. When calculating the average
associated with each cluster, data were only used when all species were
available. As would be expected, the air masses from the Atlantic Ocean and
the Arctic (clusters 1, 2 and 4) are dominated by Na+ and Cl-
aerosol in PMcoarse. In PM2.5, the same clusters show a large
contribution from Na+ and Cl-, with the largest contribution in
the Atlantic air mass (Cluster 1). Air masses which go over land tend to
have the greatest contribution from secondary inorganic aerosols, as seen in
clusters 5 and 6. Air masses, in particular from continental Europe (Cluster
6), have the largest average molar concentrations of NH4+ and
NO3-, even in the coarse fraction. The dominance of NO3-
compared to SO42- from air trajectories from continental Europe
has previously been highlighted by Abdalmogith and Harrison (2005), who explained this to be the result of high NOx / SO2
emission ratios in western Europe.
Average molar concentrations and average contribution of the
species to the total molar concentration of PM2.5 and PMcoarse
for each back trajectory cluster (refer to Fig. 9) from January 2007 to December 2012. Key: N – number of back
trajectories used to calculate average concentration and percent of contribution
for each trajectory.
Average relative contribution of inorganic water soluble species
to the total PM10 mass measured by the MARGA during 2012 as a
function of total mass concentration. The black line is the number of events
at each mass concentration. Graph produced using the plotting routine of
Crippa et al. (2014).
Aerosol composition during high pollution events
In order to optimise emission controls for the protection of human health
against high concentration episodes, it is important to know which chemical
components dominate when air concentrations are large.
Figure 11 shows the average relative aerosol
contribution as a function of total aerosol concentration for 2012 as an
example, together with the histogram of the frequency with which different
aerosol concentrations occur. The period with the highest concentrations
recorded at the site by the MARGA are dominated by secondary inorganic
aerosols, in particular by NH4+ and NO3-, with a smaller
contribution from SO42-. This is not the first time that
NO3- has been found to be a dominating species during pollution
events in the UK. Vieno et al. (2014) reported
NO3- as a dominating fraction during pollution events at a site
(Bush) approximately 10 km NE from the Auchencorth Moss field site. They
show that the NO3- during pollution events at this site arise from
a combination of emissions from the UK and continental Europe but that the
relative importance depends on synoptic conditions and differs greatly
between episodes with the UK contribution ranging from 35 and 80 %
(Vieno et al., 2014). The Auchencorth measurements
demonstrate the importance of controlling the emissions of NH4NO3
precursor gas concentrations (NH3 and NOx) in both the UK and the
rest of Europe for controlling the high pollution episodes.
The concentration dependence of the relative aerosol composition (Fig. 11)
also shows that sea salt dominates the aerosol composition at moderate
aerosol loading (2–12 µg m-3) while the relative
contributions of K+ and Ca2+ increase at very low concentrations (< 2 µg m-3). Even under very clean conditions there is a basic
concentration of crustal material.
Summary and conclusions
The first 6.5 years of chemically speciated PM2.5 and
PM10 measurements from the MARGA at Auchencorth Moss have been
analysed. This study has provided greater detail in the long-term temporal
variations of inorganic species in the UK background atmosphere and
confirmed the status of the field site as a background site in the European
context, where concentrations of the inorganic species were low over the 6.5 years. The dynamic changes between air masses dominated by anthropogenic and
natural sources is clearly observable on an interannual scale and
continuation of these long-term measurements will be a valuable resource to
understand long-term trends in PM composition in response to climate and
policy drivers.
The average ion balance at this site was biased towards cations, some of
which would probably have been neutralised by organic acids such as oxalic
acid. Additional studies to identify the missing water soluble species would
therefore be beneficial at this site to close the ion balance. Comparison
with the TEOM-FDMS bulk mass method found that the compounds resolved by the
MARGA instrument accounted on average for 78 % of the PM10 mass
measured at Auchencorth Moss, with considerable uncertainty due to changes
in the MARGA configuration over the period and the detection limits of the
TEOM-FDMS and possible difference in the characteristics of the PM10
inlets. One recommendation is to add a continuous measurement of the organic
aerosol mass at Auchencorth Moss to determine its contribution to the total
mass, due to the regional importance of this site.
Based on monthly median concentration, Na+ and Cl- generally were
found to vary seasonally due to the meteorology of the site, with the
highest concentrations in winter when the average wind speed was greatest.
As expected, NH4+ dominated the finer PM2.5 aerosol in terms
of micromoles per cubic metre, as it is the major base for aerosol in the
atmosphere and free ammonia is always available in NW Europe. The influence
of long-range transport at this site is evident, with sea salt dominating
air masses originating from the Arctic and Atlantic Ocean, whereas SIA
dominates air masses that originate over land, with the largest contributions
from continental Europe. It therefore supports the importance of a
transboundary co-operation in controlling precursor gases such as NOx
and NH3 as highlighted in this long-term study, where NH4+
and NO3- tended to be the drivers of the (regional) pollution
events observed at this background site. The dominance of NO3-
compared to SO42- was evident too in the diurnal cycles, with the
exception of summer, and provides evidence of a shift in recent decades from
sulfur to nitrogen driven chemical climate. The air quality implications of
the NH4+ and NO3- predominance during high PM loading
events provide insight for future mitigation of PM impacts. Additional
studies of gas-to-particle conversions at this field site will help to
understand the sulfur–nitrogen budget and atmospheric chemical processing to
form PM (Twigg et al., 2015).