Atmospheric measurements of the ozone-depleting substance CFC-113a
(CCl
The ozone layer in the stratosphere blocks most of the harmful solar
ultraviolet radiation from reaching the Earth's surface and therefore
protects human health and the environment. Chlorofluorocarbons (CFCs) are
industrially produced chemicals that were commonly used as refrigerants,
aerosol propellants, solvents and foam-blowing agents. CFCs have negligible
loss mechanisms in the troposphere and only break down when they reach the
stratosphere, where they are exposed to strong ultraviolet light and
decompose mostly through photolysis and reaction with O
Recently, mixing ratios of CFC-113a (CCl
The origin of the emissions that cause this increase in CFC-113a mixing ratios is as yet undetermined. Some evidence of a potential connection with hydrofluorocarbon (HFC) production has been found (Laube et al., 2014), and here we use additional data to investigate this possibility further. Laube et al. (2014) reported data until 2012. This study uses data that have become available since 2012 to provide an update on its global trend and emissions and to assess these in terms of our understanding of the sources of this gas and its potential impact on ozone.
Air samples from all the campaigns discussed in this study were collected in
electropolished and/or silco-treated stainless-steel gas canisters (Restek), except
for the CARIBIC observatory, for which samples were collected using
glass-bottle-based samplers (Brenninkmeijer et al.,
2007). Various pumps were used for the different sampling activities, all of
which have been thoroughly tested for a large range of trace gases
(Brenninkmeijer
et al., 2007; Laube et al., 2010a; Allin et al., 2015 and Oram et al.,
2017). After collection, the samples were transported to the University of
East Anglia (UEA) to be analysed on a high-sensitivity gas chromatograph
coupled to a Waters AutoSpec magnetic sector mass spectrometer (GC–MS). The
trace gases were cryogenically extracted and pre-concentrated. A full
description of this system can be found in Laube et al. (2010b).
Analysis was partly carried out using a GS-GasPro column (length
Sampling locations used in this study. Those locations that have been added since Laube et al. (2014) are in white. Those shaded orange featured in, or have been extended since, Laube et al. (2014).
Air sampling campaigns from which atmospheric CFC-113a mixing ratios were measured, including the data published in Laube et al. (2014).
The following new data are presented in this study (see also Fig. 1 and
Table 1):
Laube et al. (2014) reported CFC-113a measurements from Cape Grim,
Tasmania, from 1978 to 2012. We now report 4 more years of CFC-113a measurements
from Cape Grim, up to February 2017. From 2013 to 2017, 20 samples were
collected at Cape Grim at irregular intervals of between 1 and 5 months
apart. The CFC-113 mixing ratios (1978–2017) from analyses of archived air
samples collected at Cape Grim, Tasmania, and analysed at the UEA, together
with NOAA flask data, and Advanced Global Atmospheric Gases Experiment (AGAGE) in situ data are also included to compare the two
isomers. CFC-113 stability in the Cape Grim Air Archive has been
demonstrated in the AGAGE program for periods up to 15 years and longer
(Fraser et al., 1996; CSIRO, unpublished data). Most of the CFC-113 UEA Cape
Grim dataset was previously published in
Laube et al. (2013).
Some of the earlier samples from Laube et al. (2013) and Laube et al. (2014)
were reanalysed on the KCl-passivated CP-PLOT Al Tacolneston tower is a measurement site in Norfolk
(Ganesan et al., 2015) and is part of the UK network of tall towers. Air samples were collected approximately every 2 weeks between July 2015 and March 2017 using an air inlet
at 185 m. Ground-based samples were collected from Bachok Marine Research Station on
the northeast coast of Peninsular Malaysia in January and February 2014. During the StratoClim campaign ( Air samples were collected at regular intervals at altitudes of 10–12 km
during long-distance flights on a commercial Lufthansa aircraft from 2009 to
2016 (Brenninkmeijer et
al., 2007) on four flights between Frankfurt, Germany, and Bangkok, Thailand;
five flights between Frankfurt, Germany, and Cape Town, South Africa; and one
flight between Frankfurt, Germany, and Johannesburg, South Africa; including
the four flights referred to in Laube et al. (2014) (CARIBIC project,
Four ground-based air sampling campaigns took place in Taiwan from 2013 to
2016. Between 19 and 33 air samples were collected in March and April each
year. In 2013 and 2015 samples were collected from a site on the southern
coast of Taiwan (Hengchun), and in 2014 and 2016 samples were collected from
a site on the northern coast of Taiwan (Cape Fuguei). See also Vollmer et
al. (2015), Laube et al. (2016) and Oram et al. (2017).
A two-dimensional atmospheric chemistry-transport model was used to
estimate, top-down, global annual emissions of CFC-113a and CFC-113 for the
purpose of comparing the emissions of the two isomers. The model contains 12
horizontal layers each representing 2 km of the atmosphere and 24 equal-area
zonally averaged latitudinal bands. The modelled mixing ratios for the
latitude band that Cape Grim is located within (35.7–41.8
This model was previously used to estimate the global annual emissions of
CFC-113a (Laube et al., 2014). We now
update the CFC-113a emission estimates using an additional 4 years of
Cape Grim measurements. The CFC-113 emissions are estimated using CFC-113
mixing ratios at Cape Grim for 1978–2017 from the UEA Cape Grim dataset and
compared with bottom-up emissions estimates from the Alternative
Fluorocarbons Environmental Acceptability Study (AFEAS,
A latitudinal distribution of emissions, with 95 % of emissions
originating in the Northern Hemisphere, was assumed for both compounds. As
Cape Grim is a remote southern hemispheric site, the emission distribution
within the Northern Hemisphere has almost no effect on the modelled mixing
ratios in the latitudinal band of Cape Grim. The emission distribution used
for CFC-113 was assumed to be constant for the whole of the model run and
has been used in previous studies for similar compounds
(McCulloch et al., 1994; Reeves et al., 2005; Laube et al., 2014, 2016). For
CFC-113a we decided to select an emission distribution based on how well the
modelled mixing ratios in the latitude band 48.6–56.4
CFC-113a and CFC-113 modelled and observed mixing ratios at
Tacolneston. The error bars represent the 1
CFC-113a modelled and observed mixing ratios at Cape Grim
1960–2017 and estimated global annual emissions of CFC-113a. The
observations are from July 1978–February 2017 with 1
The UK Met Office's Numerical Atmospheric Modelling Environment (NAME;
Jones et al., 2007), a Lagrangian particle dispersion
model, was used to produce footprints of where the air sampled during the
Taiwan and Malaysia campaigns (Table 1) had previously been close to the
Earth's surface. The model setup related to samples collected in Taiwan in
2016 was slightly different to the setup for simulations in 2013–2015;
hereafter those differences are noted in parentheses, though they have no
practical implications for our findings. The footprints were calculated over
12 days by releasing batches of 60 000 (30 000 in 2016) inert backward
trajectories over a 3 h period encompassing each sample. Over the course
of the 12-day travel time the location of all trajectories within the lowest
100 m of the model atmosphere was recorded every 15 min on a grid with a
resolution of 0.5625
CFC-113a mixing ratios at Cape Grim were previously found to have been
increasing from 1978 to 2012 (Laube et al., 2014, Fig. 3). Since 2012, they
have continued to increase from 0.50 ppt in December 2012 to 0.70 ppt in
February 2017 (Fig. 3). Between 1978 and 2009 the average rate of increase
was 0.012 ppt yr
Although measurements at Tacolneston were made for a shorter time period (20 months),
it also experienced an increase in CFC-113a mixing ratios of 0.03 ppt yr
CFC-113 modelled and observed mixing ratios at Cape Grim 1960–2017
and estimated global annual emissions of CFC-113. The observations are from
Cape Grim, Tasmania, July 1978–February 2017 with 1
It is instructive to look at CFC-113 to learn more about CFC-113a. The
atmospheric trends of CFC-113 at Cape Grim (Fig. 4) and estimated
emissions are very different from those of CFC-113a. Mixing ratios of both
compounds increased at the beginning of the record, but then the CFC-113
mixing ratios stabilised in the early 1990s and started to decrease (Fig. 4),
consistent with previous observations
(Fraser
et al., 1996; Montzka et al., 1999; Rigby et al., 2013; Carpenter et al., 2014).
This trend is similar to those of many other CFCs in the
atmosphere (for example CFC-11 and CFC-12; Rigby et al., 2013) but in
contrast to the increasing mixing ratios of CFC-113a. Note that CFC-113a
mixing ratios are still much lower than those of CFC-113 even at the end of
our current record in early 2017. CFC-113 is the third-most-abundant CFC in
the atmosphere (Carpenter et al., 2014), and mixing ratios of CFC-113a are only about 1 % of
CFC-113 mixing ratios in 2017. CFC-113 mixing ratios at Cape Grim measured
by NOAA (
The CFC-113 model-derived emissions begin in the 1940s and rapidly increase
until they peak in 1989 at 252 Gg yr
CFC-113 emissions from this study, AFEAS and Rigby et al. (2013), and CFC-113a emissions from this study 1995–2016 with uncertainties.
Up until 1992, the CFC-113 emissions used in the model are the bottom-up emissions estimates from AFEAS. In the model, these emissions lead to a best-fit match to the CFC-113 observations. This shows that, in the first part of the record, AFEAS report data accurately reflecting global CFC-113 emissions. However, after 1992 the AFEAS emissions lead to lower modelled mixing ratios than the observations, indicating that AFEAS was missing some emissions after 1992. Therefore, the emissions used in our study here are the AFEAS emissions up until 1992. From 1992 onwards they are based on the best model fit to the UEA Cape Grim observations. CFC-113 emissions were also derived in another study using a range of emission inventories and estimates (Rigby et al., 2013). Those emissions mostly agree with ours within the uncertainties. Differences are likely due to this study using different lifetimes than Rigby et al. (2013).
CFC-113a mixing ratios 2008–2017 from all the sources presented in
this study with an inset of the period 2015–2017 to give an enlarged view of
the Tacolneston data. The error bars represent the 1
The upper and lower bounds of the CFC-113 emissions in this study are
derived using the “likely” range in the CFC-113 lifetime given by SPARC of
82–109 years (Ko et al., 2013). The “possible” range of
69–138 years was also estimated by Ko et al. (2013);
however when using a lifetime of 138 years, the modelled mixing ratios did not
decrease sufficiently rapidly after 1990 to match the observed downwards
trend in CFC-113 even in the absence of emissions. We can use the observed
decrease in CFC-113 mixing ratios from 2003 onwards to calculate a decay
time (lifetime at zero emissions). For long-lived gases with stratospheric
sinks, such as CFC-113, the decay time and steady-state lifetime are very
similar, differing by no more than 2 % (Ko et al., 2013). When setting the
emissions to zero from 2003 onwards and adjusting the lifetime so that the
model reproduces the CFC-113 mixing ratios at Cape Grim, the
lifetime for CFC-113 is 110 years. Assuming zero emissions, this lifetime
is a maximum value, since any source of CFC-113 would have to be balanced by
a shorter lifetime. Combining the measurement and model errors as described
in the Supplement gives an error of 5.7 %. Accounting for the
2 % error introduced by assuming the decay time is the same as the
steady-state lifetime gives are overall error of 6 %. Applying this to the
lifetime gives a maximum lifetime of
Many of the CFC-113a mixing ratios observed in Taiwan (light blue stars, Fig. 6) are significantly higher than at the other locations considered in this study. The background mixing ratios consistently increase through this period from about 0.4 to about 0.7 ppt, whereas the highest Taiwan samples have mixing ratios of up to 3 ppt. These enhancements in mixing ratios in all 4 years of the Taiwan campaigns indicate continued emissions in this region, most likely continental East Asia.
NAME footprints derived from 12-day backward simulations and
showing the time-integrated density of particles below 100 m altitude for
the approximate times when samples were collected during the Taiwan
campaign.
To determine the region(s) of emissions more accurately, NAME footprints were used (Fig. 7a–g). In general, when there are enhancements in CFC-113a mixing ratios, the NAME footprints usually show that the air most likely came from the boundary layer over eastern China or the Korean Peninsula as shown in (a), (c), (d) and (g) for example. In contrast, the footprints in (b), (e) and (f) are examples of samples with lower CFC-113a mixing ratios, and correspondingly there is very little influence from eastern China or the Korean Peninsula. However, we recognise the limitations of our relatively sparse dataset which prevents us from pinpointing the source region(s) further.
The mixing ratios in Taiwan are very variable, indicating nearby source
region(s), whereas Cape Grim and Tacolneston mixing ratios are much less
variable. Therefore, the Taiwan measurements are better suited to
investigate correlations that might shed further light on potential sources.
After investigating correlations of CFC-113a with over 50 other halocarbons
in samples from Taiwan, we found CFC-113a mixing ratios correlate well
(
Squared Pearson correlations (
CFC-113a and CFC-113 mixing ratios observed in Taiwan in March and
April 2013–2016. Arrows show the mixing ratios of CFC-113a that relate to
the NAME footprints shown in Fig. 7. The error bars represent the 1
CFC-113a mixing ratios in many of the samples collected at Bachok, Malaysia (grey crosses, Fig. 6), are also enhanced above background levels, although not to the same degree as the Taiwan samples; they range from 0.68 to 1.00 ppt. The higher mixing ratios also have their origin in East Asian air masses being transported rapidly to the tropics by the East Asian winter monsoon circulation (Ashfold et al., 2015; Oram et al., 2017). Figure 9 shows an example NAME footprint from a sample collected in January 2014 that is representative for many other events.
NAME footprint derived from 12-day backward simulation and showing the time-integrated density of particles below 100 m altitude on 22 January 2014 during a period of elevated CFC-113a mixing ratios at Bachok, Malaysia.
The Tacolneston samples (yellow diamonds, Fig. 6) show no significant enhancements in CFC-113a mixing ratios. This indicates the absence of regional sources in this part of the UK. Due to this and the relatively long lifetime of CFC-113a Tacolneston can be considered to be representative of Northern Hemisphere background mixing ratios of CFC-113a for that latitude. Both sites in Taiwan and also Tacolneston are Northern Hemisphere sites, and although the Taiwan sites have many enhancements in CFC-113a mixing ratios there are some samples with background mixing ratios. For example, in spring 2016, the only period for which these datasets overlap, the lowest CFC-113a mixing ratio in Taiwan is 0.70 ppt on 24 March 2016 (Fig. 7e). The closest Tacolneston sample to this is on 4 April 2016 with a CFC-113a mixing ratio of 0.71 ppt. This shows that Taiwan can encounter mixing ratios at background levels of CFC-113a. However, many of the air samples collected in Taiwan show mixing ratios of CFC-113a above background levels, indicating that enhanced levels of CFC-113a are generally widespread across this region.
For the period when measurements were made at both Cape Grim and Tacolneston
(from July 2015 to February 2017), the Tacolneston mixing ratios were almost
exclusively higher (though often indistinguishable within uncertainties)
than the Cape Grim mixing ratios (Fig. 6 – inset). On average Cape Grim
mixing ratios are
Laube et al. (2014) already found an interhemispheric gradient in CFC-113a
using four of these CARIBIC flights (2009–2011) and furthermore discovered
that the increasing trend of CFC-113a at Cape Grim lagged behind the
increasing trend inferred from the firn air samples, collected to a depth of
76 m, from Greenland, in the Northern Hemisphere. As the firn air
measurements in the Laube et al. (2014) study were collected in Greenland
between 14 and 30 July 2008, the surface measurements will be representative of
atmospheric mixing ratios at that time. They will also be representative of
background northern hemispheric CFC-113a mixing ratios for that latitude as
the Greenland firn air location was isolated from any large industrial areas
with potential sources of CFC-113a. Figure 6 includes the three measurements
closest to the surface (brown crosses), although they are so close together
that they appear to be one cross in the figure, and the average mixing ratio
of the three samples is
Overall, these measurements demonstrate that there is an interhemispheric gradient in CFC-113a with higher mixing ratios in the Northern Hemisphere. This persistent interhemispheric difference indicates ongoing emissions of CFC-113a in the Northern Hemisphere with higher emissions in the Northern Hemisphere than the Southern Hemisphere. Similar interhemispheric gradients have been found for other CFCs (Liang et al., 2008), as CFCs are almost exclusively produced by industrial processes and most industrial production (and consumption) takes place in the Northern Hemisphere.
Nearly all air samples collected during CARIBIC flights represent cruising altitudes of 10–12 km, which for samples over northern India, during four flights going from Germany to Thailand (green diamonds, Fig. 6), would be near the tropopause. Their mixing ratios should be representative for air masses prior to entering the tropical tropopause region, which is the main entrance region to the stratosphere (Fueglistaler et al., 2009). For the flight on 9 November 2013, there is some enhancement above background mixing ratios over South East Asia (Figs. 6, S1b). We speculate that this is likely due to air being transported from East Asia into the tropics via cold surges and then being transported up into the upper troposphere via convection (Oram et al., 2017). This means that the uplift mechanism in this region could potentially enhance concentrations of long-lived ODSs entering the stratosphere as compared to the “background” clean-air ground-based abundances that are normally used to derive such inputs (Carpenter et al., 2014). The mechanism has already been proven to exist for shorter-lived gases (Oram et al., 2017), and we see very similar patterns transporting elevated mixing ratios of CFC-113a to the tropics very rapidly (within days) during a time of increased convective uplift.
The Geophysica flights reach altitudes of 20 km and so sample lower stratospheric air. The Geophysica 2009–2010 flights (pink squares) and the Geophysica 2016 flights (orange squares) begin at background mixing ratios and then decrease (Fig. 6). During the 2016 flights, for example, measurements start at 10 km altitude, where mixing ratios are 0.71 ppt, and go up to 20 km, where the mixing ratios are 0.36 ppt. In comparison to this, ground level measurements made at the Northern Hemisphere site, Tacolneston, had an average CFC-113a mixing ratio in 2016 of 0.72 ppt. In general, mixing ratios decrease as the aircraft ascends, mainly because air at higher altitudes will have taken longer to travel there and therefore is older, and CFC-113a at higher altitudes has experienced photolytic decomposition. For more information about the Geophysica flights see the Supplement.
CFCs are entirely anthropogenic in origin. This means that there are processes either producing or involving CFC-113a that lead to continuing emissions of substantial amounts of this compound, especially in East Asia. While the Montreal Protocol has banned the production and consumption of CFCs, there are exemptions including the use of ODSs as chemical feedstocks, chemical intermediates and fugitive emissions (UNEP, 2016a). As the Montreal Protocol does not require isomers to be reported separately, CFC-113 and CFC-113a may be reported together.
The strong correlations of CFC-113a with CFC-113 and HCFC-133a in Taiwan (Sect. 3.2.1) suggest that they are involved in the same production pathways or that their production facilities are co-located. There is an absence of a correlation between CFC-113a and CFC-113 in 2015 in Taiwan; in addition, the overall mixing ratios in 2015 appear to be lower than in the other years and have fewer large enhancements (Fig. 8). This could be because in general less air was arriving from China/Korea in 2015, which is indicated by the NAME footprints (Supplement, Sect. 5). Regions in China and Korea we found to be the most likely locations of CFC-113a emissions. Alternatively, the varying correlations in different years between CFC-113a and CFC-113 could be an indication of two or more independent sources of CFC-113a. CFC-113 feedstock use decreased by over 50 % in 2015 due to one producer, which is also a user choosing not to produce CFC-113 in 2015 and reducing in-house inventories instead (Maranion et al., 2017). If this were the process leading to correlated emissions of CFC-113a and CFC-113, it may explain their lack of correlation in 2015.
One possible source of CFC-113a is from HFC production, specifically, of
HFC-134a (CH
If there were leaks in the system or venting of gases was practiced during these processes, this could lead to enhanced mixing ratios of CFC-113a and strong correlations with its isomer, CFC-113, and HCFC-133a. HFC production should be contained and not involve fugitive emissions to the atmosphere. However, the Chemicals Technical Options Committee (CTOC) 2014 report suggests there may be small leaks, depending on the quality of the system, ranging between 0.1 and 5 % of the feedstock used. The CTOC reported that a leak rate of about 1.6 % would be needed if all CFC-113a and HCFC-133a in the atmosphere had come from their use as feedstock in the production of HFC-134a, HFC-125 and HFC-143a, which is within the previous range (CTOC, 2014). HFC-143a is produced using HCFC-133a, so it was included in the CTOC estimate, but CFC-113a is not involved in its production, so it is not included in this study (CTOC, 2014).
HFC-134a and HFC-125 mixing ratios are not well correlated with those of
CFC-113a, CFC-113 or HCFC-133a, except for HFC-125 in 2016, which has a good
correlation with CFC-113a (Table 2). We would not necessarily expect them to
be well correlated as most of the emissions of the HFCs are usually related
to their uses rather than their production. CFC-114a is also part of the
production process of HFC-134a (Manzer, 1990) and can be another by-product
during HFC-125 production (Kono et
al., 2002; Takahashi et al., 2002). CFC-114a was only measured in 2015 and
2016 in Taiwan and was strongly correlated with CFC-113a in 2015 but not in
2016. This inconsistent correlation does not help to define further the
source of CFC-113a. Furthermore HCFC-123 mixing ratios are not well
correlated with CFC-113a, CFC-113 or HCFC-133a in any year in Taiwan, but
HCFC-124 mixing ratios are well correlated in 2015 with CFC-113a (Table 2)
and with HCFC-133a (
As discussed above, eastern China and the Korean Peninsula are the most
likely source regions for the elevated mixing ratios of CFC-113a observed in
Taiwan, and the HFC industry in China has been growing rapidly in recent
years (Fang et al., 2016). In China in 2013, production rates of 118 Gg yr
Alternatively, there is an official exemption in the Montreal Protocol for the use of CFC-113a as an “agrochemical intermediate for the manufacture of synthetic pyrethroids” (UNEP, 2003), probably because it is used to make the insecticides cyhalothrin and tefluthrin (Brown et al., 1994; Jackson et al., 2001; Cuzzato and Bragante, 2002). In addition CFC-113 is a feedstock used to make trifluoroacetic acid (TFA) and pesticides (Maranion et al., 2017). CFC-113a is an intermediate in this process, and these production processes are used in India and China, so this could also be a source in this region (Maranion et al., 2017). Furthermore HCFC-133a is also used to manufacture TFA and agrochemicals, although the process involving HCFC-133a is not related to the process involving CFC-113a (Rüdiger et al., 2002; Maranion et al., 2017).
Furthermore, CFC-113a is potentially present as an impurity in CFC-113, and the emissions of CFC-113a could be from CFC-113 banks. We saw in Sect. 3.2 that estimated emissions of CFC-113a began in the 1960s and HFC production did not become a large-scale industry until much later, so there must have been another source of CFC-113a during that earlier part of the record. In Sect. 3.1 we concluded that there was possibly a small amount of continued emissions of CFC-113 to maintain the observed atmospheric mixing ratios. This would be consistent with a source from banks and/or release in conjunction with CFC-113a.
To summarise, we have identified four possible sources of CFC-113a: agrochemical production, HFC-134a production, HFC-125 production and an impurity in CFC-113. The correlations indicate that HFC production is the dominant source in the East Asian region; however, there is currently insufficient data available to conclude this with high confidence. Overall, the sources of CFC-113a emissions are still uncertain, and further evidence is needed to quantify and pinpoint them. However, the likely sources we have found do not necessarily indicate a breach of the treaty as the use of CFCs as intermediates in the production of other compounds is permitted under the Montreal Protocol.
There is a continued global increasing trend in CFC-113a mixing ratios based
on a number of globally distributed sampling activities giving a consistent
picture. CFC-113a mixing ratios at Cape Grim, Australia, increased since the
previous study from 0.50 ppt in December 2012 to 0.70 ppt in February 2017.
The derived emissions were still significantly above 2010 levels and were on
average 1.7 Gg yr
The background abundances of CFC-113a reported here are currently small
(< 1.0 ppt) in comparison to the most common CFC, CFC-12, which has
declining atmospheric mixing ratios of
In the past, it was assumed that isomers of CFCs had similar uses, sources
and trends, and therefore it was not necessary to report them separately.
However, in this study, we have found that the isomers CFC-113a and CFC-113
continue to have different trends in the atmosphere and in their emissions.
Recently CFC-114a (CF
All data have been made publicly available in the Supplement.
The authors declare that they have no conflict of interest.
We are grateful for the work of the Geophysica team, the CARIBIC team
(CARIBIC-IAGOS), the staff at the Cape Grim station, the NOAA Global
Monitoring Division and the AGAGE network. The StratoClim flights were
funded by the European Commission (FP7 project Stratoclim-603557,