Atmos. Chem. Phys., 18, 4737–4751, 2018
https://doi.org/10.5194/acp-18-4737-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Continued increase of CFC-113a (CCl3CF3) mixing ratios in the
global atmosphere: emissions, occurrence and potential sources
Karina E. Adcock1 , Claire E. Reeves1 , Lauren J. Gooch1 , Emma C. Leedham Elvidge1 , Matthew J. Ashfold2 ,
Carl A. M. Brenninkmeijer3 , Charles Chou4 , Paul J. Fraser5 , Ray L. Langenfelds5 , Norfazrin Mohd Hanif1 ,
Simon O’Doherty6 , David E. Oram1,7 , Chang-Feng Ou-Yang8 , Siew Moi Phang9 , Azizan Abu Samah9 ,
Thomas Röckmann10 , William T. Sturges1 , and Johannes C. Laube1
1 Centre
for Ocean and Atmospheric Sciences, School of Environmental Sciences,
University of East Anglia, Norwich, NR4 7TJ, UK
2 School of Environmental and Geographical Sciences, University of Nottingham Malaysia Campus,
43500 Semenyih, Malaysia
3 Air Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany
4 Research Center for Environmental Changes, Academia Sinica, Taipei 11529, Taiwan
5 Oceans and Atmosphere, Climate Science Centre, Commonwealth Scientific and Industrial
Research Organisation, Aspendale, Australia
6 Department of Chemistry, University of Bristol, Bristol, UK
7 National Centre for Atmospheric Science, School of Environmental Sciences,
University of East Anglia, Norwich, NR4 7TJ, UK
8 Department of Atmospheric Sciences, National Central University, Taipei, Taiwan
9 Institute of Ocean and Earth Sciences, University of Malaya, Kuala Lumpur, Malaysia
10 Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands
Correspondence: Karina Adcock (karina.adcock@uea.ac.uk)
Received: 20 October 2017 – Discussion started: 10 November 2017
Revised: 16 February 2018 – Accepted: 9 March 2018 – Published: 9 April 2018
Abstract. Atmospheric measurements of the ozonedepleting substance CFC-113a (CCl3 CF3 ) are reported from
ground-based stations in Australia, Taiwan, Malaysia and
the United Kingdom, together with aircraft-based data for
the upper troposphere and lower stratosphere. Building on
previous work, we find that, since the gas first appeared in
the atmosphere in the 1960s, global CFC-113a mixing ratios
have been increasing monotonically to the present day. Mixing ratios of CFC-113a have increased by 40 % from 0.50 to
0.70 ppt in the Southern Hemisphere between the end of the
previously published record in December 2012 and February
2017. We derive updated global emissions of 1.7 Gg yr−1 on
average between 2012 and 2016 using a two-dimensional
model. We compare the long-term trends and emissions
of CFC-113a to those of its structural isomer, CFC-113
(CClF2 CCl2 F), which still has much higher mixing ratios
than CFC-113a, despite its mixing ratios and emissions de-
creasing since the 1990s. The continued presence of northern
hemispheric emissions of CFC-113a is confirmed by our
measurements of a persistent interhemispheric gradient in
its mixing ratios, with higher mixing ratios in the Northern
Hemisphere. The sources of CFC-113a are still unclear,
but we present evidence that indicates large emissions in
East Asia, most likely due to its use as a chemical involved
in the production of hydrofluorocarbons. Our aircraft data
confirm the interhemispheric gradient as well as showing
mixing ratios consistent with ground-based observations
and the relatively long atmospheric lifetime of CFC-113a.
CFC-113a is the only known CFC for which abundances are
still increasing substantially in the atmosphere.
Published by Copernicus Publications on behalf of the European Geosciences Union.
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K. E. Adcock et al.: Continued increase in CFC-113a mixing ratios
1 Introduction
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 O1 D (Ko et al., 2013).
These decomposition products act as catalysts in the destruction of ozone and have, in combination with other chlorineand bromine-containing gases, led to the formation of the
ozone hole (Farman et al., 1985; Molina and Rowland, 1974).
The discovery of this phenomenon motivated the Montreal
Protocol on Substances that Deplete the Ozone Layer, an
international agreement to phase out the use of CFCs and
other ozone-depleting substances (ODSs) (UNEP, 2016a). It
came into force in 1989 and, other than for a few critical-use
exceptions, there has been a global ban on CFC production
since 2010 (UNEP, 2016a). Due to this, mixing ratios of most
CFCs are now decreasing in the atmosphere, and the ozone
hole shows signs of recovery (Pawson et al., 2014; Solomon
et al., 2016). Continued reductions in CFC mixing ratios are
needed to allow the ozone layer to recover to pre-1970 levels.
Recently, mixing ratios of CFC-113a (CCl3 CF3 ), the
structural isomer of the well-known ozone-depleting substance CFC-113 (CF2 ClCFCl2 ), were found to still be increasing in the atmosphere up until 2012 (Laube et al., 2014).
The previously published evidence for increasing mixing ratios of CFC-113a comes from air samples collected at Cape
Grim, Tasmania (41◦ S), and firn air data collected in Greenland (77◦ N) in 2008 (NEEM project) (Buizert et al., 2012;
Laube et al., 2014). The firn air depth profile data, when
combined with inverse modelling, provide smoothed time
series of compound mixing ratios going back up to a century (Buizert et al., 2012; Laube et al., 2012). CFC-113a became detectable in the atmosphere in the 1960s (Laube et
al., 2014). Cape Grim is a clean-air measurement site located in Tasmania, Australia, with air sampling/analysis activities since 1976, and the CFC-113a record derived from
the Cape Grim Air Archive (1978 onwards) shows mixing
ratios increasing over time with a sharp acceleration starting
around 2010 (Laube et al., 2014). Global annual emissions
of CFC-113a were estimated using a two-dimensional atmospheric chemistry-transport model, showing increases since
the 1960s and more than doubling between 2010 and 2012,
reaching 2.0 Gg yr−1 in 2012 (Laube et al., 2014). In addition, measurements of aircraft samples from the CARIBICIAGOS (Civil Aircraft for the Regular Investigation of the
Atmosphere Based on an Instrument Container–In-service
Aircraft for a Global Observing System) observatory identified an interhemispheric gradient with mixing ratios increasAtmos. Chem. Phys., 18, 4737–4751, 2018
ing from the Southern Hemisphere to the Northern Hemisphere; the atmospheric lifetime of CFC-113a was estimated
at 51 years from stratospheric research aircraft flights in late
2009 and early 2010 (Laube et al., 2014).
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.
2 Methods
2.1
Analytical technique
Air samples from all the campaigns discussed in this
study were collected in electropolished and/or silcotreated 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
∼ 50 m; ID: 0.32 mm) and partly with a KCl-passivated CPPLOT Al2 O3 column (length: 50 m; ID: 0.32 mm), (Laube
et al., 2016). The latter analysis has been slightly modified by the addition of an Ascarite filter to remove carbon
dioxide. Several tests and comparisons ensured that no significant differences in CFC-113 and CFC-113a mixing ratios were obtained regardless of the column or filter used.
A possible interference could arise when measuring CFC113a on the GS-GasPro column using m/z 116.91 if concentrations of the nearby eluding HCFC-123 are high. This
was the case for a small number of samples analysed for this
work, and those measurements were either (a) repeated using the interference-free m/z 120.90, (b) replaced with measurements on the KCl-passivated CP-PLOT Al2 O3 column
or (c) excluded. The KCl-passivated CP-PLOT Al2 O3 column separated CFC-113 and CFC-113a well, no interferences were observed and m/z 116.91 was used for quantification. All the samples are compared to the same NOAA
standard (AAL-071170) and there were routine measurewww.atmos-chem-phys.net/18/4737/2018/
K. E. Adcock et al.: Continued increase in CFC-113a mixing ratios
Figure 1. 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).
ments of multiple standards to exclude the possibility of mixing ratio changes in the standard over time. The samples collected in Taiwan in 2013 were also measured on the Entech–
Agilent GC–MS system operating in electron ionisation (EI)
mode. This consists of a preconcentration unit (Entech model
7100) connected to an Agilent 6890 GC and 5973 quadrupole
MS (Leedham Elvidge et al., 2015). The calibration scale
used for CFC-113a is the UEA calibration scale, and for
CFC-113 it is the NOAA 2002 calibration scale. On a typical day, the working standard is measured five to eight times,
between every two or three samples. The sample peak sizes
are measured relative to the standards measured just before
and after them. The working standard is used to correct for
small changes in instrument response over the course of a
day. The dry-air mole fraction (mixing ratio) is measured,
and the units, parts per trillion (ppt), are used in this study
as an equivalent to picomole per mole. The measurement uncertainties are calculated the same way for all measurements
and represent 1σ standard deviation. They are based on the
square root of the sum of the squared uncertainties from sample repeats and repeated measurements of the air standard on
the same day.
2.2
Sampling
The following new data are presented in this study (see also
Fig. 1 and Table 1):
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
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4739
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 Al2 O3 column (length:
50 m; ID: 0.32 mm). They showed very good agreement
with the previous GasPro column-based measurement
with comparable precisions and no detectable offset.
The Cape Grim air samples were collected under background conditions with winds from the south-west, marine sector, so that sampled air masses were not influenced by nearby terrestrial sources and are representative of the extra-tropical Southern Hemisphere. Details
of the sampling procedure have been reported in previous publications (e.g. Fraser et al., 1999; Laube et al.,
2013).
2. 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.
3. Ground-based samples were collected from Bachok
Marine Research Station on the northeast coast of
Peninsular Malaysia in January and February 2014.
4. During the StratoClim campaign (http://www.
stratoclim.org/), air samples were collected during
two flights by the Geophysica high-altitude research
aircraft, as described in Kaiser et al. (2006), in the
upper troposphere and lower stratosphere (10–20 km)
over the Mediterranean on 1 and 6 September 2016.
5. 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,
www.caribic-atmospheric.com).
6. 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
Atmos. Chem. Phys., 18, 4737–4751, 2018
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K. E. Adcock et al.: Continued increase in CFC-113a mixing ratios
Table 1. Air sampling campaigns from which atmospheric CFC-113a mixing ratios were measured, including the data published in Laube et
al. (2014).
Sampling
campaign
Location
Longitude and
latitude
Dates
No. of
samples
Nature of data
NEEM
Greenland
77.445◦ N,
51.066◦ W
2484 m a.s.l.
14–30-Jul-2008
3 closest to
the surface
Firn air surface data
Cape Grim
Tasmania,
Australia
40.683◦ S,
144.690◦ E
(07-Jul-1978)
14-Mar-2013–
23-Feb-2017
66 total,
20 new
Southern Hemisphere
ground-based site
Taiwan
East Asia
Hengchun,
22.0547◦ N,
120.6995◦ E,
(2013, 2015)
Cape Fuguei,
25.297◦ N,
121.538◦ E,
(2014, 2016)
2013–2016
2013: 19
2014: 24
2015: 23
2016: 33
Northern Hemisphere
ground-based sites
Tacolneston
Tower
Norfolk, United
Kingdom
52.3104◦ N,
1.0820◦ E
13-Jul-2015–
16-Mar-2017
47
Northern Hemisphere
tall tower site
Bachok Marine
Research Station
Bachok,
Malaysia
6.009◦ N,
102.425◦ E
20-Jan-2014–
03-Feb-2014
16
Tropical ground-based
site
Geophysica
flights
2009–2010
North Sea
76–48◦ N,
28–0◦ E
30-Oct-2009–
02-Feb-2010
98
Research aircraft
Geophysica
flights
2016
Mediterranean
Sea
33–41◦ N,
22–32◦ E
01-Sep-2016
06-Sep-2016
23
Research aircraft
CARIBIC flights
Germany to
South Africa
48◦ N–30◦ S,
6–19◦ E
27-Oct-2009
28-Oct-2009
14-Nov-2010
20-Mar-2011
10-Feb-2015
13-Jan-2016
14
7
13
14
15
7
Commercial aircraft
CARIBIC flights
Germany to
Thailand
32–17◦ N,
70–97◦ E
21-Feb-2013
21-Mar-2013
09-Nov-2013
05-Dec-2013
14
7
14
14
Commercial aircraft
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).
2.3
Emission modelling
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 modAtmos. Chem. Phys., 18, 4737–4751, 2018
elled mixing ratios for the latitude band that Cape Grim is located within (35.7–41.8◦ S) were matched as closely as possible to the observations at Cape Grim (40.7◦ S) by iteratively
adjusting the global emissions rate until the differences between the modelled mixing ratios and the observations were
minimised. For more details about the model see Newland et
al. (2013) and Laube et al. (2016).
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
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92
0.8
Tacolneston
87
0.7
82
0.6
77
0.5
0.4
2014
72
2015
Year
2016
2017
CFC-113a modelled mixing ratios
CFC-113a observations
CFC-113 modelled mixing ratios
CFC-113 observations
CFC-113 (ppt)
CFC-113a (ppt)
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, https://agage.mit.edu/data/afeas-data). The upper
and lower emission uncertainties for CFC-113a and CFC113 were determined by first calculating the uncertainty in
matching the modelled mixing ratios with the observed mixing ratios using their recommended atmospheric lifetimes
and secondly considering the uncertainty range in the lifetimes. The “best fit” (minimum–maximum) steady-state lifetimes used in this study are 51 years (30–148 years) for CFC113a and 93 years (82–109 years) for CFC-113 (Ko et al.,
2013; Leedham Elvidge et al., 2018). Further details are provided in the Supplement.
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◦ N agreed with the observations at Tacolneston for the later part of the trend. Tacolneston can be considered to be representative of Northern Hemisphere background mixing ratios of CFC-113a for that latitude as there
are no significant enhancements in mixing ratios (Fig. 2).
The emission distribution used in the CFC-113a model is
the same as CFC-113 for the first 60 years (1934–1993)
and then gradually shifts over the next 10 years from more
northerly latitudes (36–57◦ N) to more southerly latitudes
(19–36◦ N). It then remains at more southerly latitudes until
the end of the run in 2017. This distribution shift is based
on the assumption that CFC-113a emissions are predominantly from Europe and North America at the beginning of
the model run and then shift to be coming predominantly
from East Asia towards the end of the model run. There
are significant enhancements in CFC-113a mixing ratios in
our measurements from Taiwan, indicating continued emissions in this region (Sect. 3.2.1), which is consistent with
emissions in this latitude band in the model. The latter is
also consistent with previous work that has found emissions
of ozone-depleting substances shifted from more northerly
Northern Hemisphere latitudes to more southerly Northern
Hemisphere latitudes (Reeves et al., 2005; Montzka et al.,
2009). This is likely due to developing countries, which are
mostly located further south, having more time to phase out
the use of many ODSs than developed countries (Newland et
al., 2013; CTOC, 2014; Fang et al., 2016). With this emissions distribution, the modelled CFC-113a mixing ratios at
Tacolneston matched closely to the observations (Fig. 2). It
4741
67
Figure 2. CFC-113a and CFC-113 modelled and observed mixing
ratios at Tacolneston. The error bars represent the 1σ standard deviation. The modelled uncertainties are 5 % and are based on the
model reproducing the reported mixing ratios of CFC-11 and CFC12 at Cape Grim to within 5 % uncertainty (Reeves et al., 2005).
should be noted that, while there is evidence that supports
the emission distribution used here, there might be alternative distributions that result in equally good fits to the trends,
particularly in the earlier part of the record.
2.4
Dispersion modelling
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◦ longitude and 0.375◦ latitude (0.25◦
by 0.25◦ in 2016). The trajectories were calculated using
three-dimensional meteorological fields produced by the UK
Met Office’s Numerical Weather Prediction tool, the Unified Model (UM). These fields have a horizontal grid resolution of 0.35◦ longitude by 0.23◦ latitude for the 2013 and
2014 simulations, and 0.23◦ longitude by 0.16◦ latitude for
the 2015 and 2016 simulations. In all cases the meteorological fields have 59 vertical levels below ∼ 30 km. Dates in
the NAME footprint maps are presented in the format yyyymm-dd and use UTC time.
Atmos. Chem. Phys., 18, 4737–4751, 2018
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K. E. Adcock et al.: Continued increase in CFC-113a mixing ratios
0.7
5
CFC-113a
4.5
Modelled mixing ratios
CFC-113a (ppt)
0.6
0.5
0.4
4
Observations - Laube et al. 2014
3.5
Observations - this study
Emissions - Laube et al. 2014
3
Emissions - this study
2.5
2
0.3
1.5
0.2
1
0.1
0
1960
0.5
1965
1970
1975
1980
1985
1990
Year
1995
2000
2005
2010
2015
0
Estimated emissions (Gg yr -1)
0.8
Figure 3. 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 1σ standard deviations as error bars. Data prior to 4 December 2012 are from
Laube et al. (2014). The blue solid line represents the modelled mixing ratios with uncertainties (dashed blue line). The dashed black and
grey lines represent the modelled “best-fit” emissions with uncertainties (short-dashed). The method used for calculating the upper and lower
emission bounds is in the Supplement.
3 Results
3.1
Long-term atmospheric trends and estimated
global annual emissions of CFC-113a and CFC-113
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−1 ; between 2010 and 2017 the rate rose
threefold to about 0.037 ppt yr−1 .
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−1 over the
period July 2015 to March 2017, based on start and end
points (Fig. 2). Furthermore, for the CARIBIC flights the
mean mixing ratios of CFC-113a increased on average by
0.04 ppt yr−1 between 2009 and 2016. Overall, there is a
consistent picture of a continued global increase in background mixing ratios of CFC-113a. Its atmospheric burden
has been increasing since the 1960s (Laube et al., 2014), and
this continued until early 2017, implying that ongoing emissions of CFC-113a exceed its rate of removal. The modelled
global annual CFC-113a emissions began in the 1960s and
increased steadily at an average rate of 0.02 Gg yr−1 yr−1 until they reached 0.9 Gg yr−1 (0.6–1.2 Gg yr−1 ) in 2010 followed by a sharp increase to 0.52 Gg yr−1 yr−1 from 2010
to 2012, when emissions were 1.9 Gg yr−1 (1.5–2.4 Gg yr−1 )
(Fig. 3). We find that between 2012 and 2016 modelled
Atmos. Chem. Phys., 18, 4737–4751, 2018
emissions were on average 1.7 Gg yr−1 . The best model
fit (minimum–maximum) suggests some minor and statistically non-significant variability between 1.6 Gg yr−1 (1.3–
2.0 Gg yr−1 ) in 2015 and 1.9 Gg yr−1 (1.5–2.4 Gg yr−1 ) in
2012. See the Supplement for more details.
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 (https://www.esrl.noaa.gov/
gmd/dv/ftpdata.html) and AGAGE (http://agage.eas.gatech.
edu/data_archive/agage/) are also included in Fig. 4. There is
a small offset of 2 % between the NOAA data and the current
UEA Cape Grim dataset, with the UEA Cape Grim dataset
being slightly higher, similar to the offset reported previously
(Laube et al., 2013).
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4743
&