ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-11929-2017A growing threat to the ozone layer from short-lived anthropogenic
chlorocarbonsOramDavid E.d.e.oram@uea.ac.ukAshfoldMatthew J.https://orcid.org/0000-0002-2191-1554LaubeJohannes C.GoochLauren J.HumphreyStephenSturgesWilliam T.Leedham-ElvidgeEmmahttps://orcid.org/0000-0002-6993-1271ForsterGrant L.https://orcid.org/0000-0003-1783-9307HarrisNeil R. P.https://orcid.org/0000-0003-1256-3006MeadMohammed Iqbalhttps://orcid.org/0000-0003-0436-4074SamahAzizan AbuPhangSiew MoiOu-YangChang-Fenghttps://orcid.org/0000-0002-8477-3013LinNeng-HueiWangJia-LinBakerAngela K.https://orcid.org/0000-0001-7845-422XBrenninkmeijerCarl A. M.SherryDavidNational Centre for Atmospheric Science, School of Environmental
Sciences, University of East Anglia, Norwich, NR4 7TJ, UKCentre for Ocean and Atmospheric Sciences, School
of Environmental Sciences, University of East Anglia, Norwich, UKSchool of Environmental and Geographical Sciences, University of Nottingham
Malaysia Campus, 43500 Semenyih, MalaysiaCentre for Atmospheric
Informatics and Emissions Technology, School of Energy, Environment and
Agrifood/Environmental Technology, Cranfield University, Cranfield, UKInstitute
of Ocean and Earth Sciences, University of Malaya, Kuala Lumpur, MalaysiaDepartment of Atmospheric Sciences, National Central University,
Taoyuan, TaiwanDepartment of Chemistry, National Central University,
Taoyuan, TaiwanMax Planck Institute for Chemistry, Air Chemistry
Division, Mainz, GermanyNolan Sherry & Associates, Kingston upon
Thames, London, UKDavid E. Oram (d.e.oram@uea.ac.uk)12October20171719119291194125May20172June201712September201714September2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/11929/2017/acp-17-11929-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/11929/2017/acp-17-11929-2017.pdf
Large and effective reductions in emissions of long-lived ozone-depleting
substance (ODS) are being achieved through the Montreal Protocol, the
effectiveness of which can be seen in the declining atmospheric abundances
of many ODSs. An important remaining uncertainty concerns the role of very
short-lived substances (VSLSs) which, owing to their relatively short
atmospheric lifetimes (less than 6 months), are not regulated under the
Montreal Protocol. Recent studies have found an unexplained increase in the
global tropospheric abundance of one VSLS, dichloromethane
(CH2Cl2), which has increased by around 60 % over the past
decade. Here we report dramatic enhancements of several chlorine-containing
VSLSs (Cl-VSLSs), including CH2Cl2 and CH2ClCH2Cl
(1,2-dichloroethane), observed in surface and upper-tropospheric air in East
and South East Asia. Surface observations were, on occasion, an order of
magnitude higher than previously reported in the marine boundary layer,
whilst upper-tropospheric data were up to 3 times higher than expected. In addition, we provide further evidence of an atmospheric transport mechanism
whereby substantial amounts of industrial pollution from East Asia,
including these chlorinated VSLSs, can rapidly, and regularly, be transported
to tropical regions of the western Pacific and subsequently uplifted to the
tropical upper troposphere. This latter region is a major provider of air
entering the stratosphere, and so this mechanism, in conjunction with
increasing emissions of Cl-VSLSs from East Asia, could potentially slow the
expected recovery of stratospheric ozone.
Introduction
Large-scale ozone depletion in the stratosphere is a persisting global
environmental problem. It is predominantly caused by the release of reactive
chlorine and bromine species from halogenated organic compounds. Although
the basic science is well established, there remains significant uncertainty
surrounding the long-term recovery of the ozone layer (Hegglin et al.,
2015). One important issue is the recent, unexplained increase in the global
tropospheric abundance of dichloromethane (CH2Cl2), which has
increased by ∼ 60 % over the past decade (Leedham-Elvidge et
al., 2015; Hossaini et al., 2015a; Carpenter et al., 2015).
CH2Cl2 is one of a large group of halogenated compounds known as
VSLSs (very short-lived substances). Owing to their relatively short
atmospheric lifetimes (typically less than 6 months) and their
correspondingly low ozone depletion potentials (ODPs), VSLSs are not
currently regulated by the Montreal Protocol. It is however estimated that a
significant fraction of VSLSs and their atmospheric degradation products
reach the stratosphere (> 80 % in the case of chlorinated VSLSs;
Carpenter et al., 2015), and, furthermore, halogenated VSLSs have
been shown to have a disproportionately large impact on radiative forcing
and climate due to their atmospheric breakdown and the subsequent depletion
of ozone, occurring at lower, climate-sensitive altitudes (Hossaini et al.,
2015b). According to the most recent Scientific Assessment of Ozone Depletion (Carpenter et al., 2015) over the period
2008–2012, the total chlorine from VSLSs increased at a rate of approximately
1.3 ± 0.2 ppt Cl yr-1, the majority of this increase being due to
CH2Cl2, and this has already begun to offset the decline in total
tropospheric chlorine loading over the same period (13.4 ± 0.9 ppt Cl yr-1) caused by the reduced emissions of substances controlled by the
Montreal Protocol.
In recent years much attention has been focussed on the potential of
bromine-containing VSLSs to contribute to stratospheric ozone depletion (Law
et al., 2007; Montzka et al., 2011). This is primarily due to the
large observed discrepancy between the measured inorganic bromine in the
stratosphere and the amount of bromine available from known, longer-lived
source gases, namely the halons and methyl bromide (Dorf et al., 2006). In
contrast, the role of very short-lived chlorine compounds (Cl-VSLSs) in ozone
depletion has been considered relatively minor because they are believed to
contribute only a few percent to the total chlorine input to the
stratosphere, the majority of which is supplied by long-lived compounds such
as the chlorofluorocarbons (CFCs), methyl chloroform (CH3CCl3), and
carbon tetrachloride (CCl4). Since 1987 the consumption of these
long-lived anthropogenic compounds has been controlled by the Montreal
Protocol, and the sum of total organic chlorine in the troposphere has been
falling since its peak of around 3660 parts per trillion (ppt) in 1993/1994 to
∼ 3300 ppt in 2012 (Carpenter et al., 2015).
Because of its relatively short atmospheric lifetime (∼ 5 years) and its high chlorine content (3 chlorine atoms per molecule), the
main contributor to this decline has been CH3CCl3. However, most
CH3CCl3 has now been removed from the atmosphere with a present-day abundance of less than 5 ppt. Consequently, the rate of decline in total
organic chlorine has fallen to 13.4 ppt yr-1 (2008–2012), which is around
50 % smaller than the maximum seen in the late 1990s (Carpenter et al., 2015).
Owing to their short atmospheric lifetimes and their hitherto low background
concentrations, chlorinated VSLSs have not been considered of major
importance for ozone depletion. Indeed the contribution of VSLSs to the total
chlorine entering the stratosphere is estimated to be only 55 (38–95) ppt
(Carpenter et al., 2015), which is between 1 and 3 % of
the present-day (2012) total (3300 ppt). However, because of their short
lifetimes, the potential impact of VSLSs on stratospheric ozone is highly
dependent on the location of their sources, with emissions close to the
major stratospheric input regions being of far greater significance for
ozone depletion (Brioude et al., 2010; Pisso et al., 2010).
The transport of trace gases and aerosols from the troposphere into the
stratosphere occurs primarily in the tropics, where convective activity and
vertical uplift are most intense. In order to get to the stratosphere, an air
parcel has to pass through the tropical tropopause layer (TTL), the region
of the atmosphere between the level of maximum convective outflow
(∼ 12 km altitude, 345 K potential temperature) and the
cold-point tropopause (∼ 17 km, 380 K) (see Box 1–3, Fig. 1
in Carpenter et al., 2015). The vertical flux into the TTL is
thought to be dominated by two main regional pathways: (1) ascent above the
western Pacific during Northern Hemispheric (NH) winter and (2) the
circulation of the Asian (Indian) Monsoon during NH summer (Fueglistaler et al., 2005; Randel et al., 2010; Bergman et al., 2012; Haines et al., 2014). The latter has been suggested as the most important region for
the transport of anthropogenic pollution (Randel et al., 2010).
Map of the region showing the location of each CARIBIC sample. The
markers have been coloured according to their CH2Cl2 concentration
to highlight the regions where enhanced levels of VSLSs were observed. Also
shown are the approximate locations of the three surface stations (orange
crosses).
Because of their short lifetimes, to be able to accurately determine the
VSLS contribution to total organic halogen loading in the stratosphere, it is
highly desirable to collect data in the TTL. Surface measurements alone,
particularly in regions outside the tropics where most long-term surface
stations are sited, are not sufficient. Furthermore, because of the
distribution and seasonality of stratospheric entry points, it is also
essential to measure in specific locations and at specific times of year,
i.e. in the Indian summer monsoon and over the winter western Pacific.
Unfortunately there are very few available measurements of VSLSs in the TTL
generally as it is above the maximum altitude of most research aircraft,
and, furthermore, there is a paucity of both ground and aircraft data
available in these two key regions of interest. Where recent TTL data are
available it is primarily from different regions and focussed on brominated
VSLSs (e.g. Sala et al., 2014; Navarro et al., 2015).
The focus of the present study is the western Pacific and, in particular,
the region of the South China Sea. During NH winter the region is heavily
influenced by the large anticyclone that forms over Siberia each year, which
gives rise to strong north-easterly winds that impact deep into the tropics
as far south as Malaysia, Singapore, and Indonesia. These north-easterly
winds typically prevail for 4–5 months (November–March) and form part of the
East Asian winter monsoon circulation. Superimposed on this seasonal
synoptic flow are transient disturbances known as cold surges, which are
triggered by a southward shift of the anticyclone and lead to sudden drops
in surface air temperatures and increased wind speeds (Zhang et al., 1997;
Garreaud, 2001). It has been proposed that during these events significant
amounts of pollution from continental East Asia (> 35∘ N) can be transported rapidly to the tropics (Ashfold et al., 2015).
Furthermore, these events, which can last for many days, occur regularly
each winter and are associated with some of the strongest convective
activity in the western Pacific region. Indeed, trajectory calculations show
that it can take less than 10 days for air masses to travel from the East
Asian boundary layer (> 35∘ N) to the upper tropical
troposphere (altitudes > 200 hPa), thereby providing a fast route
by which VSLSs (and many other pollutants) may enter the lower stratosphere,
despite their relatively short atmospheric lifetimes (Ashfold et al., 2015).
Here we provide strong evidence to support this proposed transport mechanism
based on new atmospheric observations in the East and SE Asia region. We will
present new Cl-VSLS measurements from recent ground-based and aircraft
campaigns in the region during which we have observed dramatic enhancements
in a number of Cl-VSLSs, including CH2Cl2, 1,2-dichloroethane
(CH2ClCH2Cl), trichloromethane (CHCl3), and tetrachloroethene
(C2Cl4). Furthermore, we will demonstrate how pollution from China
and the surrounding region can rapidly, and regularly, be transported across
the South China Sea and subsequently uplifted to altitudes of 11–12 km, the
region close to the lower TTL. Using the NAME (Numerical
Atmospheric-dispersion Modelling Environment) particle dispersion model, we
will also investigate the origin of the observed Cl-VSLSs and examine the
frequency and duration of cold surge events. Finally we present some new
estimates of CH2Cl2 emissions from East Asia and use these to
estimate the likely emissions of CH2ClCH2Cl, for which there is
little information in the recent literature.
Methods
Between 2012 and 2014, air samples were collected at various times (1) two coastal sites in Taiwan – Hengchun (22.0547∘ N,
120.6995∘ E) and Fuguei Cape (25.297∘ N,
121.538∘ E); (2) at the Bachok Marine Research Station on the
north-east coast of Peninsular Malaysia (6.009∘ N,
102.425∘ E); and (3) during several flights of the IAGOS-CARIBIC
aircraft between Germany and Thailand or Malaysia. IAGOS-CARIBIC is a European
project making regular measurements from an in-service passenger aircraft
operated by Lufthansa (Airbus A340-600; Brenninkmeijer et al., 2007;
http://www.caribic-atmospheric.com/).
A total of 21 samples were collected at Hengchun between 7 March and
5 April 2013 with a further 22 samples taken at Cape Fuguei between 11 March
and 4 April 2014. Overall, 28 samples were
collected at Bachok between 20 January and 5 February 2014, during the period
of the NE winter monsoon. The approximate location of each surface site is
shown in Fig. 1. The CARIBIC aircraft samples were collected during seven
return flights between (i) Frankfurt (Germany) and Bangkok (Thailand) and
(ii) Bangkok and Kuala Lumpur (Malaysia) during the periods
December 2012–March 2013 (four flights) and November 2013–January 2014
(three flights). All CARIBIC flights in this region between December 2012 and
January 2014 have been included in this analysis. With the exception of three
samples that were taken at altitudes between 8.5 and 9.8 km, the CARIBIC
samples were all (n=179) collected at altitudes between 10 and 12.3 km.
Sample collection
Air samples from Taiwan and Malaysia were collected in 3.2 L
silco-treated stainless steel canisters (Restek) at a pressure of
approximately 2 bar using a battery-powered diaphragm pump (Air Dimensions,
B series). In Taiwan the samples were collected from the surface via a
1 m ×1/4′′ OD Dekabon sampling line, whilst in Bachok the samples
were collected from the top of an 18 m tower via a 5 m ×1/4′′
OD Dekabon sampling line. In both cases the tubing was flushed for at least
5 min prior to sampling. The sampling integrity was confirmed by
sampling high-purity air (BTCA-178, BOC) through the inlet tubing and pump.
Samples were collected within 50 m of the sea and only when the prevailing
winds were from the sea, minimising the impact of any local emissions. The
CARIBIC aircraft samples were collected in 2.7 L glass flasks at a
pressure of 4.5 bar using a two-stage metal bellows pumping system
(Brenninkmeijer et al., 2007; Baker et al., 2010).
Sample analysis
The collected air samples were shipped to UEA (University of East
Anglia) and analysed for their halocarbon
content by gas chromatography–mass spectrometry (GC-MS) following trace gas
enrichment using previously published methods. All samples (i.e. Taiwan,
Bachok, and CARIBIC) were analysed for CH2Cl2, CHCl3, and
C2Cl4 using an Entech-Agilent GC-MS system operating in electron
ionisation (EI) mode, as described in Leedham-Elvidge et al. (2015).
Specifically, 1 L samples were dried and pre-concentrated before injection
onto a 30 m × 0.32 mm GS Gas Pro capillary column (Agilent);
temperature was ramped from -10 to 200 ∘C. Samples were
interspersed with repeated analyses of a working standard (SX-706070), a
high-pressure air sample contained in a 34 L electropolished stainless steel
cylinder (Essex Industries) provided by the Earth System Research Laboratory
of the National Oceanic and Atmospheric Administration (NOAA-ESRL, Boulder,
CO, USA). CH2Cl2, CHCl3, and C2Cl4 were quantified
on ions with a mass-to-charge ratio of 84 (CH235Cl2+), 83
(CH35Cl2+), and 166 (C235Cl337Cl+)
respectively. Mean analytical precisions were ±2 % for CH2Cl2
and C2Cl4 and ±3 % for CHCl3. Instrument blanks,
determined by analysing 1 L aliquots of high-purity nitrogen (BOC, research
grade), were always below the detection limit of the instrument.
Some of the ground-based samples and a subset of the CARIBIC samples were
also analysed for a range of halocarbons, including the newly identified
CH2ClCH2Cl, using a pre-concentration–GC system coupled to a
Waters AutoSpec magnetic sector MS instrument, also operating in EI mode but run at a mass resolution of 1000 at 5 % peak height. Samples (using
between 200 and 250 mL of air) were analysed on an identical GS GasPro
column following a previously described method (Laube et al., 2010, 2012; Leedham-Elvidge et al., 2015). CH2ClCH2Cl was monitored
on the ions with mass-to-charge ratios of 61.99
(C2H335Cl+, qualifier) and 63.99
(C2H337Cl+, quantifier). Mean analytical precision was
1.4 % for CH2ClCH2Cl and the average blank signal was 0.07 ppt
(as quantified using regular measurements of research-grade helium) and was
corrected for on a daily basis.
Calibration and quality assurance
CH2Cl2, CHCl3, and C2Cl4 data are reported on the
latest (2003) calibration scales provided by NOAA-ESRL. As was shown in
Leedham-Elvidge et al. (2015), our CH2Cl2 measurements compare
very well with those of NOAA-ESRL at our mutual long-term sampling site at
Cape Grim, Tasmania, over more than 6 years. As a recognised international
calibration scale for CH2ClCH2Cl is not yet available, this
compound was calibrated at UEA using the established static dilution
technique recently described (Laube et al., 2012). CH2ClCH2Cl was
obtained from Sigma Aldrich with a stated purity of 99.8 %. Three
dilutions were prepared at 7.1, 11.9, and 15.8 ppt. The mixing ratio assigned
to our working standard from these dilutions was 5.67 ppt with a 1σ
standard deviation of 1.8 %. CFC-11 was added to the dilutions as an
internal reference compound, and the CFC-11 mixing ratios assigned to the
working standard through these dilutions agreed with the value assigned by
NOAA-ESRL within 4.3 %. This is well within the estimated uncertainty of
the calibration system of 7 % (Laube et al., 2012). In addition, the
mixing ratios of CH2ClCH2Cl in the working standard were compared
with those in three other high-pressure canisters (internal surface was
either electropolished stainless steel or passivated aluminium) over the
whole measurement period. The ratios between standards did not change within
the 2σ standard deviation of the measurements for any of the
canisters analysed, indicating very good long-term stability for
CH2ClCH2Cl. This was also the case for CHCl3 and
C2Cl4. As noted in Leedham-Elvidge et al. (2015), mixing ratios of
CH2Cl2 were found to change over longer timescales in some of our
standard canisters, but this drift has been successfully quantified and
corrected for as indicated by the very good comparability with NOAA-ESRL
measurements at the Cape Grim site noted above.
Results
Figure 1 shows the location of the three surface observation stations as well
as the location of the CARIBIC samples. The aircraft sampling points have
been coloured by their CH2Cl2 concentration (see later discussion).
Data from the surface stations and from the CARIBIC aircraft flights are
summarised in Table 1, together with a summary of published observations as
reported in the most recent Scientific Assessment of Ozone Depletion
(Carpenter et al., 2015). It should be noted that only selected samples were analysed for CH2ClCH2Cl, and no data are available
from Hengchun 2013 or from CARIBIC flights between Bangkok and Kuala Lumpur.
In addition, only 16 Bachok samples were analysed for CH2ClCH2Cl.
Summary of Cl-VSLS data from the three surface stations and the
seven CARIBIC flights. For comparison, the ranges reported in the most recent
(2014) WMO ozone assessment (Carpenter et al., 2015) for the marine boundary layer
(MBL) and lower tropical tropopause layer (TTL, 12–14 km altitude) are also
shown. All data are reported as mole fractions (ppt). FRA: Frankfurt; BKK: Bangkok; KUL: Kuala Lumpur.
Taiwan 2013 Taiwan 2014 Bachok 2014 MBL (WMO, 2014)bMedianRangeMedianRangeMedian (CS)aMedian (non-CS)RangeMedianRangeCH2Cl2226.668–624227.470–639170.481.964.8–35528.421.8–34.4CH2ClCH2Cl––85.416.7–30962.221.716.4–120c3.70.7–14.5dCHCl333.011.6–23235.113.8–10322.814.712.8–30.57.57.3–7.8C2Cl44.41.7–16.65.51.7–18.64.51.91.5–9.51.30.8–1.7ΣClVSLS––755.8232 -2178546.0243.1207–1078c93.470–134CARIBIC (FRA–BKK, 65–97∘ E) CARIBIC (BKK–KUL, 100–105∘ E) Lower TTL (WMO, 2014)b10–12 km 10–12 km 12–14 km MeanMedianRangeMeanMedianRangeMeanRangeCH2Cl243.231.614.6–12150.446.522.5–10017.17.8–38.1CH2ClCH2Cle9.96.10.4–29.1–––3.60.8–7.0CHCl37.06.02.0–15.69.38.73.7–46.66.85.3–8.2C2Cl40.870.650.1–4.41.61.50.2–5.91.10.7–1.3ΣClVSLSe153.7119.348.4–3306736–103ΣClVSLS*f110.981.435.2–301134.8127.856.6–251––
a CS and non-CS refer to the cold
surge (polluted) and non-cold surge periods at Bachok. b The WMO
data are a compilation of all reported global measurements up to, and
including, the year 2012. The range represents the smallest mean minus 1
standard deviation and the largest mean plus 1 standard deviation of all
considered datasets. Data from the TTL were derived from various aircraft and
balloon campaigns. c CH2ClCH2Cl was only analysed for
in 16 of the 28 samples collected at Bachok. d Note that the
CH2ClCH2Cl MBL data actually date back to the early 2000s. No
recent data were reported. e CH2ClCH2Cl was only
analysed for in selected samples from the Frankfurt–Bangkok flights and not
in any samples collected during the Bangkok–Kuala Lumpur flights. These
statistics are therefore based on a reduced number of samples on the FRA–BKK
route (24 out of 98). fΣClVSLS* is defined as
the sum of Cl-VSLSs excluding the contribution from CH2ClCH2Cl.
Statistics are derived from all samples (98 FRA–BKK; 81 BKK–KUL).
Upper panel (a): Mole fractions (ppt) of the four
chlorinated VSLSs in air samples collected at Cape Fuguei, Taiwan, in
March/April 2014. The error bars are ±1 standard deviation. The black
arrows show the dates of the footprint maps shown below. Lower panel
(b–d): NAME footprint maps indicating the likely origin of the air
sampled at Cape Fuguei. Panel (b), 13 March, and (c), 30
March, show examples where the observed VSLS levels are very high and suggest
a strong influence from continental East Asia. Figure (d) is from
29 March where the influence of the mainland is much lower and the VSLS mole
fractions are much closer to the expected background level. The location of
Cape Fuguei is indicated with a blue circle (see also Fig. 1).
The highest concentrations of chlorinated VSLSs were measured in samples
collected in Taiwan, suggesting that Taiwan is located relatively close to
major emission regions. Figure 2 shows the March/April 2014 data from Cape
Fuguei. The NAME model (see Supplement) can be used to infer the recent
transport history of this pollution. Our NAME analysis (Fig. 2b–d) indicates
that most of the samples that contained high concentrations of Cl-VSLSs had
originated from regions to the north of Taiwan, primarily the East Asian
mainland. The median sum of chlorine from the four VSLSs listed above
(ΣClVSLS) in 22 samples collected at Cape Fuguei in March/April 2014 was
756 ppt (range 232–2178 ppt). Similarly high concentrations and variation
were seen in the 21 samples collected at Hengchun in March/April 2013
(Fig. S1 in the Supplement). To put these concentrations in a global context,
the total organic chlorine derived from all known source gases in the
background troposphere (including CFCs, HCFCs
(hydrochlorofluorocarbons), and longer-lived
chlorocarbons) is currently around 3300 ppt, with a typical Cl-VSLS
contribution in the remote marine boundary layer of approximately 3 %
(Carpenter et al., 2015). Of the four VSLSs measured, the two largest
contributors to ΣClVSLS in Taiwan were CH2Cl2
(55–76 %) and CH2ClCH2Cl (14–30 %).
Panel (a):
ole fractions (ppt) of the four chlorinated
VSLSs in air samples collected at Bachok in January/February 2014. Strongly
enhanced levels of all four compounds were seen for a 7-day period at the
beginning of the campaign (20–26 January). Also shown (dashed line) are the
approximate median background concentrations in the remote marine boundary
layer in 2012 (from Carpenter et al., 2015). Lower panels (b–f):
NAME footprint maps indicating the likely origin of the air sampled at
Bachok. During the pollution episode (b: 21 January; c:
23 January; d: 24 January), the samples would have been heavily
impacted by emissions from the East Asian mainland, whilst this influence is
much reduced during the cleaner, non-polluted periods (e:
3 February; f: 5 February). Note that even after the main pollution
event, the abundance of the VSLSs remain significantly above true background
levels for much of the time, suggesting a widespread influence from
industrial emissions on a regional scale. The location of Bachok is indicated
with a blue circle (see also Fig. 1).
Figure 3 shows the Cl-VSLS data from 28 samples collected at Bachok,
Malaysia, during the winter monsoon season in late January/early
February 2014. During this phase of the East Asian monsoon the prevailing
winds are from the north-east and, as described earlier, are often impacted
by emissions further to the north, including from mainland China. As can be
seen in Fig. 3, there was a 7-day period between 19 and 26 January when
significantly enhanced concentrations of Cl-VSLSs were observed. During this
period NAME back trajectories show air travelling from continental East Asia
and across the South China Sea before arriving at Bachok. Three examples
during this cold surge event are shown in Fig. 3b–d. These trajectories
often pass over Taiwan and, in some instances, also over parts of Indochina
where additional emissions could have been picked up. As in the Taiwan
samples, CH2Cl2 is the largest contributor to
ΣClVSLS (59–66 %), having a mean concentration of
179.9 ± 71.9 ppt (range 94.0–354.9 ppt,
nine samples) during the 7-day period of the pollution event. The
mean concentration of CH2ClCH2Cl was 64.4 ± 23.9 ppt (range
30.2–119.5 ppt), accounting for 19–23 % of ΣClVSLS. These
abundances are substantially higher than those typically found in the marine
boundary layer. For example, the range of ΣClVSLS from the four compounds
listed above in the tropical marine boundary layer reported in WMO (2014) is
70–134 ppt. The range observed at Bachok over the entire sampling period was
207–1078 ppt, with medians of 546 ppt and 243 ppt during the polluted (20–26 January) and less-polluted (27 January–5 February) periods respectively (see Table 1).
It is interesting to note that even in the period after the cold surge event
(Fig. 3e, f), the levels of Cl-VSLSs are still significantly higher than would
be expected, suggesting that this region of the South China Sea is widely
impacted by emissions from E Asia.
The pollution or “cold surge” event observed at Bachok lasted for 6–7 days
and the back trajectories shown in Fig. 3 are typical of those arriving at
Bachok during the winter monsoon period (see NAME animations in Supplement).
To further investigate the frequency and typical duration of these events, a
NAME trajectory analysis using carbon monoxide (CO) as a tracer of industrial
emissions from regions north of 20∘ N was conducted for the entire
winter season (see Supplement for details). Figure 4a
shows a time series of this industrial CO tracer for winter 2013/2014
and suggests that the observed event in January, during which there was a
strong correlation between the industrial CO tracer and CH2Cl2
(Fig. 4b), is likely to be repeated regularly throughout the winter. An
analysis of a further five winters (Fig. 4c) demonstrates that 2013/2014 was not
unusual and that the events depicted in Fig. 3a occur repeatedly every
year (Fig. S2 in the Supplement).
(a) Time series of the modelled carbon monoxide (CO)
anomaly at Bachok (i.e. that due only to industrial emissions from north of
20∘ N in the previous 12 days) for winter 2013/2014. The
CH2Cl2 data (grey squares) from the Bachok sampling period are
overlaid. The dashed lines show the 25 and 50 ppb thresholds referred to in
Fig. 3c (see Supplement for further details). (b) Correlation of the
modelled CO anomaly with the observed CH2Cl2. (c) Average
number of days each month, averaged over six consecutive winters
(2009/2010–2014/2015), where the modelled carbon monoxide anomaly at Bachok
is above a particular threshold (25 and 50 ppb, which, from the regression
in Fig. 3b, correspond to 176 and 315 ppt of CH2Cl2). The
2013/2014 winter is shown separately for comparison with the 6-year average.
The Bachok measurements clearly demonstrate the rapid long-range transport of
highly elevated concentrations of Cl-VSLSs for several thousand kilometres
across the South China Sea, as predicted by Ashfold et al. (2015). However,
to have an impact on stratospheric ozone, it is necessary to demonstrate that
these high concentrations of Cl-VSLSs can be rapidly lifted to the upper
tropical troposphere (lower TTL) or above. Such evidence can be found in
samples from several recent CARIBIC aircraft flights in the South East Asia
region. Figure 1 shows significant enhancements of CH2Cl2 during
flights over northern India and the Bay of Bengal and also between Bangkok
and Kuala Lumpur. The same data are plotted against longitude in Fig. 5a,
which shows that elevated concentrations were observed in the seven CARIBIC
flights in the region during the periods December 2012–March 2013 and
November 2013–January 2014.
The samples were collected in the altitude range 10–12.3 km,
showing that recent industrial emissions can regularly reach the lower
boundary of the TTL. Although CH2ClCH2Cl was only analysed for in a
selection of samples during the flights from Germany to Bangkok, elevated
mixing ratios coinciding with the high levels of CH2Cl2 were
clearly observed (Fig. 5b). CHCl3 and C2Cl4 were also enhanced
during these flights (Table 1), with ΣClVSLS being in the
range of 48–330 ppt (Fig. 5c). This is up to 3.2 times higher than that
previously found in the lower TTL (36–103 ppt; Carpenter et al., 2015). The
highest abundances of Cl-VSLSs were seen in samples collected over the Bay of
Bengal and on flights between Bangkok and Kuala Lumpur (Fig. 5a). NAME back
trajectories (Fig. 5d) indicate that in these cases the sampled air had
almost always been transported from the east and had often been impacted by
emissions from East Asia, with possible contributions from other countries
including the Philippines, Malaysia, and Indochina.
(a) Mole fractions (ppt) of CH2Cl2 in CARIBIC air
samples collected at 10–12 km altitude over northern India, the Bay of
Bengal, and SE Asia. The samples are plotted against longitude and have been
coloured by date. (b) Mole fraction (ppt) of CH2ClCH2Cl in
selected CARIBIC samples (note: CH2ClCH2Cl was not monitored in the
samples collected between Bangkok and Kuala Lumpur and only in a selection of
samples on the Frankfurt–Bangkok route). (c) Total Cl-VSLSs derived
from the four compounds of interest in the CARIBIC samples (note: total
Cl-VSLSs could only be calculated for the samples shown in b above).
(d) NAME footprint maps indicating the likely origin of the air
sampled by the CARIBIC aircraft. NAME footprints at this altitude, and
particularly in regions of strong sub-grid-scale convection not captured
fully in the gridded meteorological input data, may be less reliable than
those at the surface sites. This makes pinpointing particular emission
regions more difficult. The central panel therefore shows a composite
footprint derived from the samples that contained the highest levels of
CH2Cl2 (90th percentile, [CH2Cl2] > 75.6 ppt), with
the composite footprint from the remaining samples
([CH2Cl2] < 75.6 ppt) shown in the left-hand panel. To
emphasise the likely source regions the right-hand panel shows the difference
between the middle and left-hand panels. The geographical location of each
sample included in the composite analysis is shown in blue circles.
Discussion
The high mixing ratios of CH2Cl2 observed in the Taiwan samples are
not entirely unexpected. Previous studies have found very high levels
(> 1 ppb) of CH2Cl2 in various Chinese cities (Barletta et al.,
2006) and in the Pearl River Delta region (Shao et al., 2011). Elevated
levels (several hundred ppt) were also observed in aircraft measurements in
polluted air emanating from China during the TRACE-P campaign in 2001
(Barletta et al., 2006). These studies took place in the early 2000s, and
emissions may be expected to have grown significantly since. CH2Cl2
is predominantly (∼ 90 %) anthropogenic in origin and is widely used
as a chemical solvent, a paint stripper, and as a degreasing agent (McCulloch
and Midgely, 1996; Montzka et al., 2011). Other uses include foam blowing and
agricultural fumigation. A growing use of CH2CL2 is in the
production of HFC-32 (CH2F2), an ozone-friendly replacement for
HCFC-22 (CHF2Cl) in refrigeration applications. Around 10 % of global
CH2Cl2 emissions comes from natural marine and biomass burning
sources (Simmonds et al., 2006; Montzka
et al., 2011).
Whilst the strong enhancements of CH2Cl2 are not entirely
unexpected, the presence of high concentrations of CH2ClCH2Cl most
certainly are. There are very few previously reported measurements of
CH2ClCH2Cl, particularly in recent years. Elevated levels have been
observed in urban environments close to known emission sources (Singh et al.,
1981), and, more recently, Xue et al. (2011) reported elevated levels
(91 ± 79 ppt) in air samples collected in the boundary layer over
north-eastern China in 2007. The few reported measurements of
CH2ClCH2Cl in the remote marine boundary layer are typically in the
low ppt range (see Table 1), but these were mostly made well over a decade
ago. No long-term atmospheric measurements of CH2ClCH2Cl have been
reported, and CH2ClCH2Cl is not reported by the main surface
monitoring networks (AGAGE and NOAA), so current background concentrations
and longer-term trends are unknown. CH2ClCH2Cl is predominantly
anthropogenic in origin, its primary use being in the manufacture of vinyl
chloride, the precursor to polyvinyl chloride (PVC), and a number of
chlorinated solvents. CH2ClCH2Cl also finds use as a solvent and a
dispersant and has historically been added to leaded petrol as a lead
scavenger (EPA, 1984). In common with CH2Cl2, it has also been used
as a cleaning/degreasing agent and as a fumigant. China is the world's
largest producer of PVC, accounting for 27 % of global production in 2009
(DCE, 2017). Production has increased rapidly in recent years (14 % per
year over the period 2000–2009; DCE, 2017), which could potentially have led
to increased atmospheric emissions of CH2ClCH2Cl. Simpson et
al. (2011) observed a small enhancement in CH2ClCH2Cl in Canadian
boreal forest fire plumes (background average, June–July 2008,
9.9 ± 0.3 ppt; plume average 10.6 ± 0.3 ppt) and estimated a
global boreal fire source of 0.23 ± 0.19 kilotonnes (kt) yr-1.
The other Cl-VSLSs presented here are C2Cl4 and CHCl3. In
contrast to CH2ClCH2Cl, long-term atmospheric data records are
available for these compounds, although there are few data from the SE Asia
region. Current trends show that C2Cl4 is declining in the
background troposphere (∼ 6 % yr-1), whilst CHCl3 is
approximately constant (Carpenter et al., 2015). However, both compounds were
elevated in the samples containing high concentrations of CH2Cl2
and CH2ClCH2Cl, suggesting that significant, co-located sources
remain. Like CH2ClCH2Cl, C2Cl4 is almost exclusively
anthropogenic in origin, used primarily as a solvent in the dry-cleaning
industry, as a metal degreasing agent, and as a chemical intermediate, for
example in the manufacture of the hydrofluorocarbons HFC-134a and HFC-125.
CHCl3 is believed to be largely natural in origin (seawater, soils,
macroalgae), but potential anthropogenic sources include the pulp and paper
industry, water treatment facilities, and HFC production (McCulloch, 2003;
Worton et al., 2006; Montzka et al., 2011).
Regional emissions of CH2Cl2 and
CH2ClCH2Cl
China does not report production or emission figures for CH2Cl2.
However, emissions of CH2Cl2 can be estimated from the known
Chinese production of HCFC-22 (CHClF2). This is possible because the
production of HCFC-22 requires CHCl3 as feedstock (1 kg HCFC-22
requires 1.5 kg CHCl3) and because CHCl3 is produced almost
entirely (> 99 %) for HCFC-22 production. Production of chloromethanes
by any manufacturing process leads to the inevitable co-production of
CH2Cl2 and CHCl3, with smaller (3–5 %) co-production of
carbon tetrachloride (CCl4). The production ratios vary by individual
plant but are within the range of 30:70–70:30 (%
CH2Cl2 : CHCl3). Chinese chloromethanes plants, which
together represent some 60 % of global capacity and production, are
generally built to a 40:60–60:40 flexibility ratio. With falling CHCl3
demand due to diminished feedstock demand for HCFC-22 production, and based
on regular discussions with the individual large producers, ratios in China
have been switching in recent years from the traditional 40:60 towards
50:50 (CH2Cl2 : CHCl3; Nolan Sherry associates
Nolan Sherry Associates (NSA) proprietary information: some of the data used
in these calculations are proprietary in nature, being based on direct
information from discussions with the producers, and have been aggregated for
reasons of confidentiality. In the case of the HCFC-22 production data this
is also because there are two uses of HCFC22: as a chemical intermediate, and
as a refrigerant and a foam blowing agent. The latter uses are “emissive”
and are controlled by the Montreal Protocol (http://ozone.unep.org) and
are in the public domain. Information on the controlled uses of HCFC-22 may
be found at http://ozone.unep.org or by access to the Multilateral Fund of the Montreal Protocol
(http://www.multilateralfund.org) and, in the case of China, by private
subscription to the industry magazine China Fluoride Materials
(www.cnchemicals.com).
).
It can be calculated that in 2015 China produced approximately 600 kt of
HCFC-22 for all uses (Nolan Sherry associates), which would require 900 kt of
CHCl3 as feedstock. Subtracting Chinese imports of CHCl3 (40 kt;
Comtrade, 2016) and allowing for some limited emissive solvent use (15 kt) suggests that China produced around 875 kt of CHCl3 in 2015. As
noted above, in the chlorocarbon industry, CH2Cl2 and CHCl3
are produced in the same manufacturing process, and in China this is
currently moving from a historic production ratio of around 40:60 towards
50:50. Using a production ratio of 45:55, it can therefore be estimated
that China produced around 715 kt of CH2Cl2 in 2015. Approximately
90 kt of this was exported (Comtrade, 2016), and another 170 kt was used
for the production of HFC-32 (CH2F2), which is a non-emissive
application (Nolan Sherry associates). This leaves an estimated 455 kt (±10 %)
of CH2Cl2 which is used almost exclusively in emissive applications
such as paint stripping, foam blowing, pharmaceuticals, and solvent use.
Although there is no specific industry-based aggregation of these numbers,
they have been verified in discussion with Chinese and other industry
sources. A similar method has recently been used to assess emissions of
CCl4 (SPARC, 2016).
There is a strong linear correlation between the observed CH2Cl2
and CH2ClCH2Cl data at both Bachok (R2=0.9799) and Cape
Fuguei (R2=0.9189). Combining the datasets yields a slope of
0.4456 ± 0.0194 (R2=0.9228). Using the emissions for
CH2Cl2 derived above (455 kt) and making the assumptions that
(1) all emissions originate in China and (2) there are no significant
relative losses in the two compounds since emission (lifetimes are 144 days
for CH2Cl2 and 65 days for CH2ClCH2Cl), we can estimate
Chinese emissions of CH2ClCH2Cl to be of the order of
203 ± 9 kt yr-1. If accurate, the scale of these emissions is a
major surprise as CH2ClCH2Cl is highly toxic (suggesting that local
emissions would be minimised) and believed to be used almost exclusively in
non-emissive applications.
Concluding remarks
When calculating the VSLS contribution to stratospheric chlorine, it is usual
to assume an average concentration in the region of the TTL known as the
level of zero radiative heating (LZRH). The LZRH is located at the transition
between clear-sky radiative cooling and clear-sky radiative heating. This
occurs at an approximate altitude of 15 km, and it is believed that air
masses above this level will go on to enter the stratosphere (Carpenter et
al., 2015). As noted above there are very few measurements in this region
and, furthermore, many of the available measurements were made over a decade
ago, and assumptions based on surface temporal trends have to be made in
order to estimate present-day values (Carpenter et al., 2015; Hossaini et
al., 2015a). Another key deficiency in this
estimation of VSLS concentrations entering the stratosphere is that most of
the reported measurements have not been made in the two key regions where the
strongest troposphere-to-stratosphere transport occurs. Although we have no
data from the region of the LZRH, the CARIBIC data over northern India and SE
Asia suggest that the contribution of VSLSs to stratospheric chlorine loading
may be significantly higher than is currently estimated (50–95 ppt;
Carpenter et al., 2015). It is also interesting to note that the
much-discussed contribution of VSLS–Br compounds to stratospheric bromine is
approximately 5 ppt, which is equivalent to 300 ppt of chlorine (1 ppt of
bromine is roughly equivalent to 60 ppt chlorine; Sinnhuber et al., 2009).
The CARIBIC measurements suggest that Cl-VSLSs could currently, on occasion,
contribute a similar amount.
These new measurements of Cl-VSLSs in Taiwan, in Malaysia, and from an
aircraft flying above South East Asia show that there are substantial
regional emissions of these compounds; that these emissions can be rapidly
transported long distances into the deep tropics; and that an equally rapid
vertical transport to the upper tropical troposphere is a regular occurrence.
Although the focus of this paper is short-lived chlorinated gases, there are
many other chemical pollutants contained in these air masses which will have
a large impact on such things as regional air quality.
Unlike the bromine-containing VSLSs, which are largely natural in origin, the
Cl-VSLSs reported here are mainly anthropogenic, and consequently it would be
possible to control their production and/or release to the atmosphere. Of
particular concern are the rapidly growing emissions of CH2Cl2, and
potentially CH2ClCH2Cl, especially when considering the
geographical location of these emissions, close to the major uplift regions
of the western Pacific (winter) and the Indian sub-continent (summer).
Without a change in industrial practices, the contribution of Cl-VSLSs to
stratospheric chlorine loading is likely to increase substantially in the
coming years, thereby endangering some of the hard-won gains achieved, and
anticipated, under the Montreal Protocol.
The data form part of a larger halocarbon database that will be submitted to the UK Centre for Environmental Data Analysis
(CEDA, www.ceda.ac.uk) archive in 2018. Until this time, the data are available from the corresponding author upon
request.
The Supplement related to this article is available online at https://doi.org/10.5194/acp-17-11929-2017-supplement.
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank our CARIBIC project partners and the CARIBIC
technical team (in particular Claus Koeppel, Dieter Scharffe). The Malaysia and
Taiwan activities were funded through the UK Natural Environment Research
Council (NERC) International Opportunities Fund (NE/J016012/1, NE/J016047/1,
NE/N006836/1). CARIBIC has become part of IAGOS (www.IAGOS.org)
and is supported by the German Ministry of Education and Science and
Lufthansa. The CARIBIC halocarbon measurements were part-funded by the
European FP7 project SHIVA (226224). UM-BMRS is supported by the Malaysian
Ministry of Higher Education (Grant MOHE-HICoE IOES-2014). The sampling at
Hengchun and Fuguei Cape was operated under the Seven South East Asian
Studies (7-SEAS) program and funded by Taiwan EPA and MOST. Jia-Lin Wang and Lauren J. Gooch
were funded through a NERC fellowship (NE/1021918/1) and studentship
(NE/1210143) respectively. We acknowledge use of the NAME atmospheric
dispersion model and associated NWP meteorological datasets made available
to us by the UK Met Office. We also acknowledge the significant storage
resources and analysis facilities made available to us on JASMIN by STFC
CEDA along with the corresponding support
teams.Edited by: Anne Perring
Reviewed by: Bjoern-Martin Sinnhuber and one anonymous referee
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