Introduction
Oxidation controls the fate of many atmospheric trace gases. For example,
increasing the oxidation state of a given species may increase its
deposition velocity
(Nguyen et al.,
2015) or solubility (Carlton et al., 2006) and
reduce its volatility (Carlton et al., 2006),
all of which act to reduce the atmospheric lifetime of that species and can
lead to the formation of secondary material such as secondary organic
aerosol (SOA) or ozone (O3). As the identity of the chemical species
change with oxidation, intrinsic and diverse properties of the chemical
species are altered, influencing their toxicity
(Borduas
et al., 2015) and their impact on the environment, e.g. cloud-particle
nucleating efficiency (Ma et al., 2013) or global
warming potential (Boucher et al., 2009).
The hydroxyl radical (OH) is considered the most important daytime
atmospheric oxidant due to its ubiquity and high reactivity, with an average
tropospheric concentration of 106 molecules cm-3
(Heal et al., 1995). However, rate
coefficients for the reaction of the chlorine radical (Cl) can be 2 orders
of magnitude larger than those for OH (Spicer et al., 1998), indicating that
lower Cl concentrations of 1×104 atoms cm-3 that are estimated to
exist in urban areas (e.g. Bannan et al., 2015) can be just as
significant in their contribution to oxidation.
Cl-initiated oxidation of volatile organic compounds (VOCs) forms
chlorinated analogues of the OH-initiated oxidation products, via addition
Eq. (1) or hydrogen abstraction Eq. (2), forming HCl that may react with OH to
regenerate Cl. Subsequent peroxy radicals formed through Cl oxidation can
take part in the HOx cycle and contribute to the enhanced formation of
O3 and SOA (Wang and Ruiz, 2017). This is represented by the
following;
R+X⟶O2R(X)OO⋅,RH+X⟶O2ROO⋅+HX,
where X is OH or Cl.
Nitryl chloride (ClNO2) is a major reservoir of Cl that is produced by
aqueous reactions between particulate chloride (Cl-) and nitrogen
pentoxide (N2O5), as seen in Eq. (4) and Eq. (6). Gaseous ClNO2 is produced throughout
the night and is typically photolysed at dawn before OH concentrations
reach their peak, as in Eq. (6). This early morning release of Cl induces oxidation
earlier in the day and has been shown to increase maximum 8 h mean
O3 concentrations by up to 7 ppb under moderately elevated NOx
levels (Sarwar et al., 2014). Typical ClNO2 concentrations measured in urban regions range from 10 s of ppt to
1000 s of ppt.
Mielke
et al. (2013) measured a maximum of 3.6 ppb (0.04 Hz) during the summertime in
Los Angeles, with maximum sunrise concentrations of 800 ppt.
Bannan et al. (2015)
measured a maximum concentration of 724 ppt (1 Hz) at an urban background
site in London during summer. They state that in some instances, ClNO2 concentrations increase after sunrise and attribute this to the influx of
air masses with higher ClNO2 concentrations by either advection or
from the collapse of the residual mixing layer. In urban environments where
NOx emission and subsequent N2O5 production is likely,
Cl- may be the limiting reagent in the formation of ClNO2 if
excess NO does not reduce NO3 as seen in Eq. (3) before N2O5 is produced
(e.g. Bannan et al., 2015). Whilst the distance from a marine source of Cl-
may explain low, inland concentrations
Faxon et al., 2015),
the long-range transport of marine air can elevate inland ClNO2
concentrations (Phillips et al., 2012) and
the long-range transport of polluted plumes to a marine location can also
elevate ClNO2 concentrations (e.g. Bannan
et al., 2017). These processes may be represented by the following equations;
NO3+NO⟶2NO2,NO3+NO2⟶N2O5(g)⟶N2O5(aq),Cl-+N2O5⟶ClNO2+NO3-,ClNO2(aq)⟶ClNO2(g)⟶JClNO2Cl+NO2.
The anthropogenic emission of molecular chlorine is identified as another inland
source of Cl- in the US
(e.g.
Thornton et al., 2010; Riedel et al., 2012) and in China
(e.g. Wang et al., 2017; Liu et
al., 2017), where some of the highest concentrations 3.0–4.7 ppb have been
recorded. As well as industrial processes, the suspension of road salt used
to melt ice on roads during the winter has been suggested as a large source
of anthropogenic Cl- Mielke et
al., 2016). This wintertime-only source, combined with reduced nitrate
radical photolysis, is expected to yield greater ClNO2 concentrations
at this time of the year (Mielke et
al., 2016).
The photolysis of molecular chlorine (Cl2) is another potential source
of Cl. Numerous heterogeneous formation mechanisms leading to Cl2
from particles containing Cl- are known. These include the reaction of
Cl- and OH (Vogt et al., 1996), which may
originate from the photolysis of O3(aq) (Oum,
1998) or from the reactive uptake of ClNO2
(Leu et al., 1995), ClONO2
(Deiber et al., 2004) or HOCl
(Eigen and Kustin, 1962) to acidic Cl- containing
particles. Thornton et al. (2010) also suggest that inorganic Cl reservoirs
such as HOCl and ClONO2 may also enhance the Cl concentration,
potentially accounting for the shortfall in the global burden (8–22 Tg yr-1 source from ClNO2 and 25–35 Tg yr-1 as calculated from
methane isotopes). This may be direct through photolysis or indirect
through heterogenous reactions with Cl- on acidic aerosol.
Globally, Cl2 concentrations are highly variable. In the marine
atmosphere, concentrations of up to 35 ppt have been recorded
(Lawler et al., 2011), whereas at urban
costal sites in the US, concentrations on the order of 100s ppt have been
measured (Keene et al., 1993;
Spicer et al.,
1998). Sampling urban outflow,
Riedel et al. (2012) measure a maximum of 200 ppt Cl2 from plumes and mean
concentrations of 10 ppt on a ship in the LA basin. Maximum mixing ratios of
up to 65 ppt have also been observed in the continental US
(Mielke et al., 2011).
More interestingly, these studies
(Keene
et al., 1993; Lawler et al., 2011; Mielke et al., 2011; Spicer et al.,
1998) report maximum Cl2 concentrations at night and minima during the
day. However, there is a growing body of evidence suggesting that daytime
Cl2 may also be observed. Although the primary emission may be one source
of daytime Cl2 (Mielke et al.,
2011), others demonstrate that the diurnal characteristics of the Cl2 time
series have a broader signal suggestive of continuous processes rather than
intermittent signals typically associated with sampling emission sources
under turbulent conditions.
In a clean marine environment
Liao et al. (2014) observe
maximum Cl2 concentrations of 400 ppt attributed to emissions from a
local snow pack source. A maximum was measured during the morning and evening
with a local minimum during midday caused by photolysis. They also describe
negligible night-time concentrations, with significant loss attributed to
deposition. Faxon et al. (2015) measured Cl2 with a time-of-flight chemical ionisation mass
spectrometry (ToF-CIMS) recording a maximum during the
afternoon of 4.8 ppt (0.0016 Hz) and suggesting a local precursor primary
source of Cl2 that is potentially soil emission, with further heterogeneous
chemistry producing Cl2. At a rural site in northern China,
Liu et al. (2017) measured mean
concentrations of Cl2 of 100 ppt and a maximum of 450 ppt, peaking
during the day; they also report 480 ppt observed in an urban environment in
the US during summer. They attribute power-generation facilities burning
coal as the source.
Another potential source of Cl to the atmosphere is the photolysis of
chlorinated organic compounds (ClVOCs, chlorocarbons, organochlorides) that
are emitted from both natural (biomass burning, oceanic and biogenic
emission) (e.g. Yokouchi et al., 2000) and
anthropogenic sources (e.g. Butler, 2000). Whilst many ClVOCs
are only considered chemically important in the stratosphere, those that are
photochemically labile in the troposphere, e.g. methyl hypochlorite
(CH3OCl), whose absorption cross section is non-negligible at
wavelengths as long as 460 nm (Crowley et al., 1994), can act
as a source of Cl and take part in oxidative chemistry.
Photolysis of ClVOCs have been postulated to contribute 0.1-0.5×103 atoms cm-3 globally to the Cl budget of the boundary layer
(Hossaini et al.,
2016), although on much smaller spatial and temporal scales, the variance in
this estimate is likely to be large. Very few data exist on the
concentrations, sources and spatial extent of oxygenated ClVOCs (ClOVOCs)
and their contribution to the Cl budget.
The ToF-CIMS is a highly selective and sensitive instrument with high mass
accuracy and a resolution (m/dm ∼4000) that is capable of
detecting a suite of chlorinated compounds, including HOCl and organic
chlorine (Le Breton et al., 2018) as well
as other oxygenated chlorine species and chloroamines
(Wong et al., 2017). Here we use the
ToF-CIMS with the I- reagent ion to characterise the sources of
chlorine and estimate their contribution to Cl concentrations in the wintertime
in Manchester, UK.
Methodology/experiment
Full experimental details and a description of meteorological and air quality
measurements can be found in Priestley et al. (2018). A time-of-flight
chemical ionisation mass spectrometer (ToF-CIMS) (Lee et al.,
2014b) using iodide reagent ions was used to sample ambient air between
29 October and 11 November 2014 at the University of Manchester's southern campus,
approximately 1.5 km south of Manchester city centre, UK (53.467∘ N, 2.232∘ W)
and 55 km east of the Irish Sea. The sample loss to the 1 m long
3/4′′ Perfluoroalkoxy alkane (PFA) inlet was minimised by using a fast inlet pump
inducing a flow rate of 15 standard litres per minute (slm) which was
subsampled by the ToF-CIMS. Backgrounds were taken every 6 h for 20 min by overflowing dry N2 and were applied consecutively. The
overflowing of dry N2 will have a small effect on the sensitivity of
the instrument to those compounds whose detection is water dependent. Here
we find that due to the low instrumental backgrounds, the absolute error
remains small and is an acceptable limitation in order to measure a vast
suite of different compounds for which no best practice backgrounding method
has been established. Whilst backgrounds were taken infrequently, they are
of a comparable frequency to those used in previous studies where similar
species are measured
(Lawler
et al., 2011; Osthoff et al., 2008; Phillips et al., 2012). The stability of
the background responses, i.e. for Cl2 0.16±0.07 (1σ) ppt, and the stability of the instrument diagnostics with respect to the
measured species suggest that they effectively capture the true instrumental
background.
Formic acid was calibrated throughout the campaign and post campaign. Very
little deviation in the formic acid calibrations was observed. The mean
average sensitivity was 30.66±1.90 (1σ) Hz ppt-1. A number of
chlorinated species were calibrated post campaign using a variety of
different methods, and relative calibration factors were applied based on
measured instrument sensitivity to formic acid as has been performed
previously
(e.g.
Le Breton et al., 2014a, 2017; Bannan et al., 2015). A summary
of calibration procedures and species calibrated are described below. All
data from between 16:30 LT on 5 November and 00:00 LT on
7 November have been removed to prevent the interference of a large-scale anthropogenic biomass burning event (Guy Fawkes Night) on these
analyses.
Calibrations
We calibrate a number of species by overflowing the inlet with various known
concentrations of gas mixtures (Le Breton et
al., 2012), including molecular chlorine (Cl2, 99.5 % purity,
Aldrich), formic acid (98/100 %, Fisher) and acetic acid (glacial, Fisher)
by making known mixtures (in N2) and flowing 0–20 standard cubic
centimetres per minute (sccm) into a 3 slm N2 dilution flow that is
subsampled. The Cl2 calibration factor is 4.6 Hz ppt-1.
As all chlorinated VOCs we observe are oxygenated we assume the same
sensitivity found for 3-chloropropionic acid (10.32 Hz ppt-1) for the rest of the organic chlorine species detected.
Chloropropionic acid (Aldrich) was calibrated following the methodology of
Lee et al. (2014). A known quantity of chloropropionic acid
was dissolved in methanol (Aldrich) and a known volume was doped onto a filter.
The filter was slowly heated to 200 ∘C to ensure the total desorption of the
calibrant whilst 3 slm N2 flowed over it. This was repeated several
times. A blank filter was first used to determine the background.
ClNO2 was calibrated by the method described by
Kercher et al. (2009) with N2O5 synthesised following the methodology described
by Le Breton et al. (2014)a, giving a calibration
factor of 4.6 Hz ppt-1. Excess O3 is generated by flowing 200 sccm
O2 (BOC) through an ozone generator (BMT, 802N) and into a 5 L
glass volume containing NO2 (σ, > 99.5 %). The
outflow from this reaction vessel is cooled in a cold trap held at
-78 ∘C (195 K) by a dry ice and glycerol mixture where N2O5 is
condensed and frozen. The trap is allowed to reach room temperature and the
flow is reversed, where it is then condensed in a second trap held at 195 K.
This process is repeated several times to purify the mixture. The system is
first purged by flowing O3 for 10 min before use. To ascertain the
N2O5 concentration on the line, the flow is diverted through
heated line to decompose the N2O5 and into to a Thermo Scientific
42i NOx analyser, where it is detected as NO2. It is known that the
Thermo Scientific 42i NOx analyser suffers from interferences from
NOy species, indicating that this method could cause an underestimation
of the ClNO2 concentrations reported here. Based on previous studies
(e.g. Le Breton et al., 2014;
Bannan et al., 2017) where comparisons with a broad-beam cavity-enhancement
absorption spectrometer (BBCEAS) have been made, good agreement has been
found between co-located N2O5 measurements. We feel that this
calibration method works well, likely in part due to the high purity of the N2O5 synthesised and because the possible interference of NOy on the
NOx analyser during this calibration is considered negligible. The
N2O5 is passed over a salt slurry where excess chloride may react
to produce ClNO2. The drop in the N2O5 signal is equated to the
rise in ClNO2, as the stoichiometry of the reaction is 1:1. The
conversion efficiency of N2O5 to ClNO2 over wet NaCl is known
to vary by 60 %–100 %
(Hoffman
et al., 2003; Roberts et al., 2008). Here we follow the methodology of
Osthoff et al. (2008) and
Kercher et al. (2009) that ensures that conversion is 100 % efficient, so we assume a 100 %
yield in this study.
We developed a secondary novel method to quantify ClNO2 by cross-calibration with a turbulent flow tube chemical ionisation mass spectrometer
(TF-CIMS) (Leather et al., 2012). Chlorine atoms were produced
by combining a 2.0 slm flow of He with a 0–20 sccm flow of 1 %
Cl2, which was then passed through a microwave discharge produced by a
surfatron (Sairem) cavity operating at 100 W. The Cl atoms were titrated via
a constant flow of 20 sccm NO2 (99.5 % purity NO2 cylinder,
Aldrich) from a diluted (in N2) gas mix to which the TF-CIMS has been
calibrated. This flow is carried in 52 slm N2 that is purified by
flowing through two heated molecular sieve traps. This flow is subsampled by
the ToF-CIMS where the I.ClNO2- adduct is measured. The TF-CIMS is
able to quantify the concentration of ClNO2 generated in the flow tube
as the equivalent drop in NO2- signal. This indirect measurement
of ClNO2 is similar in its methodology to ClNO2 calibration
by quantifying the loss of N2O5 reacted with Cl-
(e.g. Kercher et
al., 2009). We do not detect an increase in I.Cl2 signal from this
calibration and so rule out the formation of Cl2 from inorganic species
in our inlet due to unknown chemistry occurring in the IMR. The TF-CIMS
method gives a calibration factor 58 % greater than that of the
N2O5 synthesis method. The Cl atom titration method assumes a
100 % conversion to ClNO2 and does not take into account any Cl atom
loss, which will lead to a reduced ClNO2 concentration and thus a greater
calibration factor. Also, the method assumes a 100 % sampling efficiency
between the TF-CIMS and ToF-CIMS; again this could possibly lead to an
increased calibration factor. Whilst the new method of calibration is
promising, we assume that the proven method developed by
Kercher et al. (2009) is the correct calibration factor and assign an error of 50 % to
that calibration factor. We feel that the difference between the two methods
is taken into account by our measurement uncertainty.
We calibrate HOCl using the methodology described by
Foster et al. (1999) giving a
calibration factor of 9.22 Hz ppt-1. 100 sccm N2 is flowed through
a fritted bubbler filled with NaOCl solution (min 8 % chlorine, Fisher)
that meets a dry 1.5 slm N2 flow, with the remaining flow made up of
humidified ambient air, generating the HOCl and Cl2 signal measured on
the ToF-CIMS. The flow from the bubbler is diverted through a condensed HCl
(σ) scrubber (condensed HCl on the wall of 20 cm PFA tubing) where HOCl
is titrated to form Cl2. The increase in Cl2 concentration when
the flow is sent through the scrubber is equal to the loss of HOCl signal
and as the calibration factor for Cl2 is known, the relative
calibration factor for HOCl to Cl2 is found.
Additionally, several atmospherically relevant ClVOCs were sampled in the
laboratory to assess their detectability by the ToF-CIMS with I-. The
instrument was able to detect dichloromethane (DCM, VWR), chloroform
(CHCl3, 99.8 %, Aldrich) and methyl chloride (CH3Cl,
synthesised), although the instrument response was poor. The response to
3-chloropropionic acid was orders of magnitude greater than for the ClVOCs
suggesting that the role of the chlorine atom is negligible compared with the
carboxylic acid group in determining the I- sensitivity in this case.
Cl radical budget calculations
Within this system, we designate ClNO2, HOCl and organic chlorine as
sources of Cl. As HCl was not detected, it is not possible to quantify the
contribution of Cl from the reaction of HCl + OH. Loss processes of Cl are
Cl + O3 and Cl + CH4 (7). Photolysis rates for the Cl
sources are taken from the US National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet and Visible TUV
radiation model (Mandronich, 1987) assuming a 100 % quantum yield
at our latitude and longitude with a column overhead O3 measured by
the Brewer spectrophotometer #172 (Smedley
et al., 2012) and assuming zero optical depth. To account for the effective
optical depth of the atmosphere, including clouds and other optical
components, we scale our idealised photolysis rate coefficient (J) by the
observed transmittance values in the UV-A waveband (325 to 400 nm). These
transmittance values are calculated from UV spectral scans of global
irradiance, measured at half-hourly intervals by the Brewer spectrophotometer
and provided as an output of the SHICrivm analysis routine
(Slaper et al., 1995). The Cl rate coefficient for the reaction
with O3 is kCl+O3=1.20×10-11 cm3 molecule-1 s-1 (Atkinson et al., 2006b) and
CH4 is kCl+CH4=1.03×10-13 cm3 molecule-1 s-1 (Atkinson et al., 2006a). The individual kCl+VOC are taken from the NIST chemical kinetics
database. This is represented by the following equation;
ClSS=2JCl2Cl2+JClNO2ClNO2+JHOClHOCl+JClOVOCΣ[ClVOCs]kO3+ClO3+kCH4+ClCH4+∑inkCl+VOCi[VOC]i.
As methane was not measured, an average concentration was taken from the European Centre for Medium-Range Weather Forecasts
(ECMWF) Copernicus atmosphere monitoring service (CAMS). VOC concentrations were
approximated by applying representative VOC : benzene ratios for the UK urban
environment (Derwent et al., 2000) and applying those to a
typical urban UK benzene : CO ratio
(Derwent et al., 1995), where CO was
measured at the Whitworth observatory. The VOC : benzene ratios are scaled to
the year of this study to best approximate ambient levels
(Derwent et al., 2014). The calculated
benzene : CO ratio is in good agreement with a Non-Automatic Hydrocarbon
Network monitoring site (Manchester Piccadilly) approximately 1.5 km from
the measurement location, indicating that the approximation made here is
reasonably accurate. The ratios assume that traffic emissions are the dominant
source of the VOCs, as is assumed here.
The photosensitivity of the ClOVOCs to wavelengths longer than 280 nm
dictates their ability to contribute to the Cl budget in the troposphere. As
many of the identified species here do not have known photolysis rates, we
approximate the photolysis of methyl hypochlorite JCH3OCl for all
ClOVOCs, as it is the only available photolysis rate for an oxygenated
organic compound containing a chlorine atom provided by the TUV model and no
other more suitable photolysis rate could be found elsewhere, e.g. the JPL
kinetics database. The same quantum yield and actinic flux assumptions are
made.
Time series of (a) ClNO2 (ppt), (b) HOCl (ppt) and O3 (ppb), and (c) Cl2
(ppt) and direct solar radiation (Wm-2). (d) NO
(ppb) and NO2 (ppb). (e) Relative humidity (%). Data is removed
during bonfire night (5–6) and HOCl data is discounted
thereafter due to a persistent interference that was not present earlier.
Results
Concentrations of all chlorinated species are higher at the beginning of the
measurement campaign, when air masses originating from continental Europe
were sampled
(Reyes-Villegas
et al., 2018). Toward the end of the measurement campaign, ClNO2 and
ClOVOCs concentrations were low, which is consistent with the pollution
during this period having a high fraction of primary components
(Reyes-Villegas
et al., 2018), see Fig. 1.
Inorganic chlorine
We detect a range of inorganic chlorine species and fragments including
I.Cl-, I.ClO-, I.HOCl-, I.Cl2-,
I.ClNO2-
and I.ClONO2-, however we do not detect I.ClO2-,
I.Cl2O-, I.Cl2O2-, I.ClNO- or I.HCl-.
Laboratory studies have shown that the ToF-CIMS is sensitive to detection of
I.HCl-, however under this configuration, the I.HCl- adduct was
not observed. The statistics of the concentrations reported below do not
take into account the limits of detection (LOD), so for some of the
measurements, values may be reported below the LOD.
ClNO2
ClNO2 (m/z 208) was detected every night of the campaign with an LOD
(3×
standard deviation of the background) of 3.8 ppt. The 1 Hz mean night-time
concentration of ClNO2 was 58 ppt (not accounting for the LOD), and a
maximum of 506 ppt (not accounting for the LOD) was measured as a large
spike on the evening of 30 October. These concentrations are
comparable to other urban UK measured values, although the maximum
concentration reported here is 30 % lower than that measured in London
(Bannan et al., 2015)
but is consistent with high concentrations expected during the winter, as
discussed in the introduction.
The diurnal profile of ClNO2 increases through the evening to a local
morning maximum, with rapid loss after sunrise. Although we observe a rapid
build-up after sunset (ca. 16:30 LT) and loss after sunrise (ca. 07:30 LT), the
maximum concentration measured within a given 24 h period typically peaks
at around 22:00 LT and halves by 03:00 LT, where it is maintained. The reasons for
the early onset in peak concentration and loss throughout the night is
unclear, although on 1 November, a sharp decrease in ClNO2 is a
consequence of a change in wind direction, indicating that the source of
ClNO2 is directional. A minimum concentration of < LOD is
reached by 15:00 LT, indicating that concentrations can persist for much of the day.
On 7 November ClNO2 concentrations grow throughout the morning, even
after photolysis begins, until 11:00 LT. Correlated high wind speeds suggest that
the long-range transport and downward mixing is a likely cause for this daytime
increase.
Typically, elevated concentrations of ClNO2 are measured when the wind
direction is easterly and wind speeds are low (2–4 ms-1), also during periods of southerly winds between 3–9 ms-1. The potential sources of Cl- precursor from these directions are
industrial sites, including waste water treatment facilities (8.5 km east
and 7.0 km south) that may use salt water as part of the chemical
disinfection process (Ghernaout and
Ghernaout, 2010). Another source of the ClNO2 precursor is found from the
southwest at wind speeds of 9 ms-1, indicating a more distant source
that is also likely to be industrial or marine. The correlation between
ClNO2 and Cl2 is poor at most times, apart from the night of the
30 where a strong linear relationship is observed. This is consistent
with polluted continental air masses advecting a variety of trace gases.
Throughout the measurement campaign the relationship between ClNO2 and
Cl2 is poor, so it is unlikely they share the same source.
HOCl
HOCl concentrations average 2.18 ppt (not accounting for the LOD) and reach
a daytime maximum of 9.28 ppt with an LOD of 3.8 ppt. Concentrations peak in
the early afternoon, similarly to Cl2, but remain elevated for longer,
dropping after sunset. The diurnal profile is similar to that for
O3, with a maximum during the day and minima during morning and evening rush
hours when NOx is emitted locally. The strong correlation with O3
(R2=0.67) is expected, as the route to the formation of HOCl is the
oxidation of Cl with O3 to form ClO and then the oxidation by HO2 to
form HOCl. Non-negligible night-time concentrations of a maximum 8.1 ppt are
only measured when concentrations of other inorganic Cl containing species
are high. The HOCl signal is artificially elevated after the night of the
5 due to a persistent interference from a large-scale biomass burning
event (Guy Fawkes Night, Priestley et al., 2018), which cannot be de-convolved
from the dataset due to the small difference in their mass-to-charge ratios
and insufficient instrument resolution. For this reason HOCl data after this
date are discounted from the analysis.
ClO
We detect the I.ClO- adduct at m/z 178, which strongly correlates with
I.ClNO2- and I.Cl- signals, all of which show night-time
maxima. This is inconsistent with the ClO photochemical production pathway
of Cl + O3, suggesting that its maximum concentration should be measured
during the day, as was observed for HOCl. It is not possible to confirm if
the I.ClO- is a fragment of a larger ClO containing molecule, however,
as the fragmentation of multiple larger molecules are detected as a single
adduct, e.g. the I.Cl- cluster is a known fragment from ClNO2 and
HOCl, it is reasonable to suspect that I.ClO- may be a fragment as well.
Cl2
We observe concentrations of Cl2 during the day ranging from 0–16.6 ppt with a mean value of 2.3 ppt (not accounting for the LOD) and night-time concentrations of 0–4.7 ppt with mean concentrations of 0.4 ppt (not
accounting for the LOD), see Fig. 1. The LOD is 0.5 ppt and a calibration factor of 4.5 Hz ppt-1 was found. These
concentrations are of the same order of magnitude as measured at an urban
site in the US but up to 2 orders of magnitude smaller than at US urban
costal sites (Keene et al., 1993;
Spicer et al.,
1998) and a megacity impacted rural site in northern China
(Liu et al., 2017). Although the
maximum measured value here is an order of magnitude greater than that
measured in Houston (Faxon
et al., 2015), the photolysis rate of Cl2 here is 2 orders of
magnitude smaller compared with Houston at that time.
Time series for 1 November 2014, with (a) Cl2, (b) solar
radiation (global, direct and indirect), (c) photochemical marker
C2H4O5, and (d) O3 and NOx, where highlighted boxes
demonstrate that Δ[O3]Δt
is increasing. The increase in concentration of Cl2,
C2H4O5 and O3 production when VOC limited are strongly
coupled with direct solar radiation. Greyed areas are night time.
The diurnal profile of Cl2 exhibits a maximum at midday and a minimum
at night (early morning) consistent with other studies
(Faxon
et al., 2015; Liao et al., 2014; Liu et al., 2017). The days with the
greatest concentration are those where direct shortwave radiation is at its
highest. On 5 November, the incidence of direct shortwave
radiation is unhindered throughout the day and a similarly uniform profile
for Cl2 is also observed. On 1 November, Cl2
concentrations increase unhindered as direct radiation increases but when
cloud cover reduces radiation transmission efficiency, a corresponding drop
in Cl2 is also observed (Fig. 2). Also, when global radiation is low
throughout the day, e.g. 7 November, we observe very low
concentrations of Cl2.
There is the potential that the Cl2 signal detected is an instrumental
artefact generated either by chemistry in the IMR or from displacement
reactions or degassing on the inlet walls. We believe none of these to be
the case. First, the correlation between the signal used for labile chlorine
in the IMR 35Cl (m/z 35) is high with ClNO2 (R2=0.98) yet is
non-existent with Cl2 (R2=0.01) indicating Cl2
concentration is independent of 35Cl concentrations. Second, there is
no correlation between HNO3 and Cl2 (R2=0.07) which
suggests that acid displacement reactions are not occurring on the inlet
walls. Third, there is no correlation between temperature and Cl2
(R2=0.08), indicating that localised ambient inlet heating is also not
a contributing factor to increased Cl2 concentrations. Fourth, we
observe a similar direct radiation dependency for other photochemical
species as we observe for Cl2. For example, the temporal behaviour of
C2H4O5 exhibits a similar diurnal profile and radiation
dependency (Fig. 2). Also, the production of O3 increases and decreases
with direct solar radiation at the same times we observe the enhancements in
concentrations of Cl2 and C2H4O5 (Fig. 2). The changes
in O3 production are observed when NO concentrations are near zero,
indicating that O3 production is VOC limited. Finally, other large organic
molecules e.g. C10H14O4 do not exhibit this strong coupling
with direct solar radiation. This evidence suggests that a local photolytic
daytime mechanism is responsible for the increase in daytime concentrations
as has previously been suggested (e.g. Finley and Saltzman, 2006).
Although peak concentrations of Cl2 are observed in the daytime, high
levels of Cl2 are also observed during the night. At the beginning of
the measurement period, which has previously been characterised using an
aerosol mass spectrometer (AMS) as a period of high secondary activity
(Reyes-Villegas
et al., 2018), there are persistent, non-zero concentrations of Cl2
(≤4 ppt) after sunset. On 4 November, after the period of
high secondary activity, intermittent elevations in night-time Cl2
concentrations when the wind is northerly suggest that a local emission source,
with concentrations reaching a maximum of 4.6 ppt. Two more distinct night-time sources, ranging from the south west through to the east of the
measurement site, indicate a likely origin in industrial areas, some of which
contain chemical production and water treatment facilities.
Organic chlorine
We detected seven C2-C6 ClOVOCs of the forms
CnH2n+1O1Cl, CnH2n+1O2Cl, CnH2n+1O3Cl, CnH2n-1O2Cl,
CnH2n-1O3Cl and CnH2n-3O2Cl (Fig. 3), of which
only C2H3O2Cl has been previously reported (Le Breton et al., 2018). We find no evidence
for the detection of small chlorohydrocarbons, e.g. poly-chloromethanes, such
as methyl chloride, dimethyl chloride and chloroform, or poly-chloroethanes
such as those described by
Huang et al. (2014) in
the ambient data, but qualitative testing and laboratory calibrations show
that the iodide reagent ion can detect CH3Cl (not calibrated),
CH2Cl2 (LOD = 143 ppb) and CHCl3 (LOD = 11 ppb). We
find no discernible evidence for the detection of 4-chlorocrotonaldehyde,
the Cl oxidation product of 1,3-butadiene and unique marker of chlorine
chemistry (Wang and
Finlayson-Pitts, 2001) due to interferences from other CHO compounds. We do
not believe that these species are products of inlet reactions as there is a poor
correlation (R2=-0.039) with labile chlorine 35Cl.
Diurnal profiles of Cl VOCs. (a) Stacked plot showing total Cl VOC
concentration. (b) The first data point of each diurnal trace is mean
normalised to 1.0. Reds show photochemical dominated signals with maxima at
midday, whereas yellow and blue traces show a more typical diurnal
concentration profile associated with changes in boundary layer height,
indicating that these species have longer lifetimes.
The maximum hourly averaged total ClOVOCs concentration is 28 ppt at
12:00 LT and at a minimum of 5 ppt at 07:00 LT, when NOx concentrations are highest
at ∼30 ppb. Concentrations of C2H3O2Cl
(tentatively identified as chloroacetic acid) and C6H13OCl
(tentatively identified as chloro-hexanol) are the highest of any ClOVOCs,
accounting for between 20 % and 30 % of total ClOVOCs
concentrations measured. All concentrations rise towards midday, with
C3H7O2Cl and C2H3O2Cl rising the most by a
factor of 4 and returning to nominal levels by the early evening (red in Fig. 3). C3H7O2Cl and C2H3O2Cl correlate well with
Cl2 (R2 0.77 and 0.75, respectively), which is consistent with a
photochemical formation mechanism identifying these species as secondary
products, potentially chloro-propanediol and chloro-acetic acid.
Whilst the diurnal profiles of C6H13OCl and
C5H7O2Cl (blue in Fig. 3) are similar to those of
C3H7O2Cl and C2H3O2Cl, they do not enhance as
much as those photochemical species or return to nominal levels after the
solar maximum. Instead, they increase again during the night, with
C3H5O3Cl reaching a maximum concentration of 8 ppt at 20:00 LT.
This trend suggests that concentration changes could be a function of boundary
layer height.
C3H7O2Cl and C4H7O2Cl (yellow in Fig. 3) are
the only ClOVOCs that show a positive correlation with NOx
(R2=0.42, R2=0.41) and negative correlation with O3 (R2=-0.58, R2=-0.53). Their correlation is stronger with
NO2 (R2=0.55, R2=0.48), a product of traffic emission.
This suggests that at least some of the time, they accumulate at low wind
speeds, indicating their origins as local, primary emissions or as thermal
degradation products that have a traffic source, e.g. polychlorinated
dibenzo-p-dioxins/dibenzofurans (PCDD/F) and their oxidation products
(Fuentes
et al., 2007; Heeb et al., 2013). The diurnal profile shows maxima during
midday consistent with other photochemical species, which is expected of
secondary formation. It is possible that these compounds are isobaric or
isomeric with the other compounds that interfere with the perceived signals
recorded here.
The diurnal profile of C3H5O3Cl (green in Fig. 3) exhibits a
similar shape to the bimodal distribution observed for NOx.
Cross-correlation indicates that a time lag of -3 h provides the best
correlation with NO2 of R2=0.80. This suggests that local
oxidation chemistry, which takes place over long periods in the day and is
sensitive to traffic emission, is the source of this ClOVOC.
Discussion
Effect of global radiation transmission efficiency on Cl radical
production
Three days are selected based on their different solar short wave
transmission efficiencies to quantify the variation in Cl2 formation
and photolysis and thus the influence of Cl2 on producing Cl. The average
transmission of global radiation on 5 November was high with 84±14 % (1σ), whereas on 7 November it was very
low with 21±14 %, sometimes dropping below 10 % in the middle of the
day. The middle case is represented by 1 November, where the transmission
efficiency in the morning was high with 88±11 %, but in the afternoon
it was highly variable and dropped to 55±20 % (see Fig. 4). These 3
days provide good case studies for the investigation of the effect of global radiation
on molecular chlorine concentrations and therefore the production of Cl.
The reduced transmission efficiency inhibits Cl2 formation, thereby
reducing the contribution of Cl2 to Cl production. The lower
transmission efficiency also reduces the photolysis of Cl2 and so
reduces the production of Cl even further. Figure 5 shows the divergence
between the ideal JCl2 without transmission efficiency correction
(a), the JCl2 value scaled by transmission efficiency (b) and subsequent
Cl formation. Cl production rates are similar until 11:00 LT, when the scaled
production then becomes an average 47 % lower. This is most prominent at
13:00 LT, when the difference between ideal and scaled production is
8.4×104 Cl radicals cm-3 s-1.
Transmission scaled J values for Cl2, ClNO2 and HOCl for
1, 5 and 7 November, where 1 November had high photolysis
rates in the morning that were reduced during the afternoon, 5 November is
the closest to a full day's ideal photolysis and 7 November shows very weak
photolysis.
Contribution of inorganic chlorine to Cl radical production
The contribution of HOCl and ClOVOCs to Cl formation is negligible due to
low photolysis rates and low concentrations, whereas the contributions from
Cl2 and ClNO2 are much greater (Fig. 6). During the morning of 5 November, ClNO2 is the dominant source of Cl, contributing 95 % of
the total Cl concentration, a maximum of 3.0×103 Cl radicals cm-3, to
the steady-state concentration, which is approximately a factor of 3
lower than the estimated maximum concentration of 9.5×103 Cl radicals cm3 produced by ClNO2 photolysis in London during the
summer (Bannan et al.,
2015) and a factor of 22 lower than the maximum concentration of 85.0×103 Cl radicals cm-3 calculated from measurements of
ClNO2 in Houston
(Faxon et al., 2015). In
both instances, this is due to a combination of lower JClNO2 and lower
ClNO2 concentrations.
As the day progresses, concentrations of Cl2 increase and it becomes
the dominant and more sustained source of Cl by contributing 95 % of Cl (12.5×103 Cl radicals cm-3) by the early afternoon, which is
approximately 4× that of the ClNO2 measured in the early morning and
1.3× higher than the maximum estimated concentration calculated from
the ClNO2 photolysis in London
(Bannan et al., 2015).
The maximum Cl concentration produced from Cl2 and ClNO2
photolysis on 5 November reached 14.2×103 Cl radicals cm3 at 11:30 LT which is approximately 16 % of the 85.0×103 Cl,radicals cm3 maximum calculated value from the photolysis
of these two species in Houston in summer
(Faxon et al., 2015). This
is dominated by the contribution of Cl2, indicating that Cl2 can be a
much more significant source of Cl than ClNO2. On this high-flux day,
when hourly mean Cl2 concentrations range between 0–7 ppt, the
source term is calculated between 4–21 ppt Cl2 h-1, which
is slightly lower, although consistent with previous studies
(Faxon
et al., 2015; Finley and Saltzman, 2006; Spicer et al., 1998).
Diurnal profile for 1 November of (a) idealised JCl2 and P(Cl),
(b) scaled JCl2 and P(Cl), and (c) the difference between (a) and (b).
Transmission efficiency scaled photolysis reduce P(Cl) from Cl2
photolysis.
Steady state concentration of Cl from ClNO2, Cl2, HOCl and
total ClOVOC photolysis for (a) 1 November, (b) 5 November, and (c) 7 November.
The importance of ClNO2 during the morning is most
evident on the 5, with a diminishing contribution throughout the day.
On the high-flux days, Cl2 is the most important source of Cl, but on
the low-flux day, ClNO2 is most important.
A day with low photolysis rates and high daytime ClNO2 concentrations has been highlighted as 7 November. On this day, ClNO2 is the
dominant Cl source (95 %) reaching a maximum of 3.4×103 Cl radicals cm-3 at 09:30 LT, which is ∼87 % of that calculated for
London (Bannan et al.,
2015). A mean Cl2 concentration of 0.3 ppt (less than the LOD of 0.5 ppt) on this day is very low, as production of Cl2 at its maximum,
calculated as 0.6 ppt h-1, is also low. This combined with a low
maximum of JCl2=1.13×10-4 h-1 means that maximum Cl production
from Cl2 photolysis on this day is very low, generating 0.9×103 Cl radicals cm-3 at 10:00 LT or a quarter of the maximum contributed by
ClNO2 on this day (see Fig. 6). This is represented by the
following equation;
Cl(aq)-⟶J12Cl2(g)⟶JCl2Cl.
The dependency of Cl formation on Cl2 production and loss highlights
the sensitivity of this reaction channel to the photolysis that is demonstrated on
these 2 days. The production of Cl from ClNO2 is less sensitive, relatively speaking, to the solar flux, as the production of ClNO2 does not
rely on photochemistry but chemical composition cf. Eqs. (6) and (8). This
further highlights the role of photolytic mechanisms in the re-activation of
particulate chloride to gaseous chlorine radicals.
Organic vs. inorganic contribution to Cl radical production
Summing the concentrations of the ClOVOCs and assuming a uniform photolysis
rate JCH3OCl as detailed in the above section, we derive the
contribution of total measured ClOVOC to the Cl budget and compare it to the
contribution from inorganic Cl measured here (Fig. 6). On the high-flux day,
the Cl concentration reaches 4.0×102 Cl radicals cm-3 at
midday, which is 30 % of the contribution by ClNO2, 3.6 % of the
contribution from Cl2 and 2 % of the HOCl contribution for the same
day. On the low-flux day, the ClOVOC contribution is 11.0×102 Cl radicals cm-3, which is ∼2.8 % of the ClNO2
contribution on that day and ∼57 % of the Cl2
contribution. Like Cl2, the production of most ClOVOC requires a
photolytic step to generate concentrations that can then go on to decompose,
providing the Cl. Here it is suggested that the organic contribution to Cl
production is negligible at 15 % on the low radiant-flux day and 3 % on
the high-flux day.
Conclusions
A large suite of inorganic and organic, oxygenated, chlorinated compounds
has been identified in ambient, urban air during the wintertime in the UK.
Of the seven organic chlorinated compounds (ClOVOCs) identified here, only
C2H3O2ClO (tentatively assigned as chloroacetic acid) has
previously been reported. Although the ToF-CIMS with I- is sensitive
towards chlorinated and polychlorinated aliphatic compounds, e.g. methyl
chloride (CH3Cl) dimethyl chloride (CH2Cl2) and chloroform
(CHCl3), their concentrations were below the detection limit. The
sources of ClOVOCs are mostly photochemical with maxima of up to 28 ppt
observed at midday, although C3H7O3Cl and
C4H7O2Cl concentrations correlate with NOx accumulating
at low wind speeds, indicating they are produced locally, potentially as the
thermal breakdown products of higher-mass chlorinated species such as
polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/F) from car exhausts or
the oxidation products thereof. C3H5O3Cl shows a good diurnal
cross-correlation with NO2 with a time lag of 3 h,
suggesting that its production is sensitive to NOx concentrations on that time scale.
Alongside ClOVOCs, daytime concentrations of Cl2 and ClNO2 are
measured, reaching maxima of 17 and 506 ppt, respectively. ClNO2 is a
source of Cl in every daytime period measured. Cl2 shows
strong evidence of a daytime production pathway limited by photolysis as
well as emission sources evident during the evening and night time.
On a day of high radiant flux (84±14 % of idealised values),
Cl2 is the dominant source of Cl, generating a maximum steady state
concentration of 12.5×103 Cl radicals cm-3 or 74 % of the total
Cl produced by the photolysis of Cl2, ClNO2, HOCl and ClOVOC,
with the latter three contributing 19 %, 4 % and 3 %, respectively.
This contrasts with a share of 14 % for Cl2, 83 % for ClNO2
and 3 % for ClOVOCs on a low radiant-flux day (21±14 % of
idealised values). On the low radiance day, not only is the photolysis of all
Cl species inhibited, reducing Cl concentrations, but also the formation of
Cl2 and some ClOVOCs by photochemical mechanisms is inhibited, thus the
variability in contribution between days is highly sensitive to the
incidence of sunlight. This further highlights the importance of
photochemistry in the re-activation of particulate chloride to gaseous
chlorine radicals. Similarly to Cl2, ClOVOCs can be an important source
of Cl, although the behaviour of their contribution is similar to
Cl2,
relying on high rates of photolysis rather than high concentrations as is
the case for ClNO2.
The contribution of the ClOVOCs to the Cl budget would be better determined
if more specific photolysis rates for each compound were available and so
would further improve the accuracy of the contribution they make to the Cl
budget. In addition, future work should aim to identify the processes
leading to the formation of these compounds to better constrain the Cl
budget in the urban atmosphere. Further ambient measurements of a broader
suite of chlorinated species as shown here in different chemical
environments would help to better constrain the contribution that
chlorine-initiated chemistry has on a global scale.