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 O1D
(Ko et al., 2013). These decomposition products act as
catalysts in the destruction of ozone and have, in combination with
other chlorine- and 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 (CCl3CF3), the structural
isomer of the well-known ozone-depleting substance CFC-113
(CF2ClCFCl2), 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 CARIBIC-IAGOS (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 increasing 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.
Methods
Analytical technique
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
∼ 50 m; ID: 0.32 mm) and partly with a KCl-passivated
CP-PLOT Al2O3
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 CFC-113a 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 Al2O3 column or (c) excluded. The KCl-passivated
CP-PLOT Al2O3 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 measurements 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.
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).
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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
Sampling
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 Al2O3 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).
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 (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.
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).
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).
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 modelled 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 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 CFC-113 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 CFC-113a 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 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.
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 CFC-12 at Cape Grim to within 5 %
uncertainty (Reeves et al., 2005).
![]()
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.
![]()
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 yyyy-mm-dd and use UTC time.
Results
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 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.
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 1σ standard
deviations as error bars. Also for comparison are the NOAA and AGAGE CFC-113
mixing ratios at Cape Grim and previous emissions estimates from AFEAS and
Rigby et al. (2013) (based on AGAGE in situ data) with “likely”
uncertainties (green lines). The dashed black line shows the modelled
“best-fit” emissions with uncertainties (grey lines). The method used for
calculating the upper and lower emission bounds is in the Supplement.
![]()
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).
The CFC-113 model-derived emissions begin in the 1940s and rapidly increase
until they peak in 1989 at 252 Gg yr-1, after which they decrease to
2.4 Gg yr-1 in 2016 (Fig. 4). This sharp decline bears witness to the
success of the Montreal Protocol, which came into force in 1989 and phased
out the production of CFCs by 1996 in developed countries and 2010 in
developing countries (UNEP, 2016a). The total
cumulative emissions of CFC-113, up to the end of 2016, are 3164 Gg, while
the cumulative emissions of CFC-113a are 29 Gg, making the total cumulative
emissions of CFC-113a less than 1 % of its isomer, CFC-113. Alternatively,
in the last decade, 2007–2016, cumulative emissions of CFC-113 are 38 Gg,
while for CFC-113a they are 13 Gg, or a third of the CFC-113 cumulative
emissions. Current CFC-113a emissions are similar to those of CFC-113 and
could even surpass them if the trends continue (Fig. 5).
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 1σ standard
deviation.
![]()
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 110±7 years. For comparison, we
also calculated the maximum lifetime from the observed rate of decrease in
CFC-113 mixing ratios at Cape Grim between 2003 and 2017 using the
continuity equation and assuming no sources of CFC-113 (Supplement, Sect. 2).
The agreement was good, giving a maximum lifetime of
113±5 years. It should be noted that CFC-113 is not the focus of
this study, but we do find that emissions of it persist until 2017, which
leaves room for the possibility that some of the recent emissions of
CFC-113a are related to CFC-113 emissions, possibly through HFC production
or agrochemical production (see Sect. 4) similar to findings for other
isomeric CFCs (Laube et al., 2016).
Global distributions of CFC-113a
Enhancement above background mixing ratios
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. (a), (c), (d) and (g) are examples of one enhanced CFC-113a mixing
ratio in each year. (f) is the sample taken just before (g) when the air was
coming from a different direction and the mixing ratio of CFC-113a was much
lower. (b) and (e) are also examples of samples with lower CFC-113a mixing
ratios. Arrows in Fig. 8 show the mixing ratios of CFC-113a for these NAME
footprints. For the rest of the NAME footprints see the Supplement.
![]()
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
(R2>0.750) in multiple years with those of CFC-113 and
HCFC-133a (CH2ClCF3), indicating a possible link between the
sources of these compounds (Table 2). There is a great deal of variability
in mixing ratios in the Taiwan samples. CFC-113a correlates well with
CFC-113 in 2013 and 2014 but shows almost no correlation in 2015 and a
slightly decreased correlation coefficient in 2016 (Table 2, Fig. 8). In
contrast, HCFC-133a strongly correlates with CFC-113a in the first 3
years (Table 2). The tropospheric lifetime of HCFC-133a is 4–5 years
(McGillen et al., 2015), and its mixing ratios have varied
in recent years. They increased in 2012/2013 and decreased in 2014/2015
(Vollmer et al., 2015). According to our latest data from
Cape Grim, in 2016 they began increasing again. Large changes in emissions
are needed to produce such a variable trend, but the causes of these changes
are still unclear (Vollmer et al., 2015).
Squared Pearson correlations (R2) of CFC-113a mixing ratios
with other compounds in Taiwan 2013–2016.
2013
2014
2015
2016
CFC-113
0.866
0.909
0.013
0.429
HCFC-133a
0.923
0.923
0.891
0.637
HFC-134a
0.001
0.055
0.010
–
HFC-125
0.319
0.219
0.016
0.850
CFC-114a
–
–
0.754
0.386
HCFC-123
–
0.013
0.217
0.202
HCFC-124
–
0.537
0.833
0.078
No. of data points
19
24
23
33
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 1σ
standard deviation.
![]()
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.
Interhemispheric gradient of CFC-113a
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 0.055±0.024 ppt lower than Tacolneston mixing
ratios. This shows that there is an interhemispheric gradient with higher
CFC-113a mixing ratios in the Northern Hemisphere as would be expected for a
compound emitted primarily in the Northern Hemisphere. This gradient is
further supported by data from the six CARIBIC flights between Germany and
South Africa for 2009–2016. The CARIBIC samples (purple circles, Fig. 6)
from the 2016 flight coincide temporally with the Tacolneston and the Cape
Grim samples in Fig. 6 and confirm the observation of higher mixing ratios
in the Northern Hemisphere (filled purple circles) and lower mixing ratios
in the Southern Hemisphere (unfilled purple circles). Also see Fig. S1a in
the Supplement.
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 0.44±0.01 ppt.
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.
Measurements of CFC-113a in the stratosphere
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.
Possible sources of CFC-113a
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 (CH2FCF3) and HFC-125 (CF3CHF2), as both may
involve CFC-113a in their production process. One of the pathways for
production of HFC-134a begins with CFC-113 being isomerised to form
CFC-113a, which is then fluorinated to produce CFC-114a
(CF3CCl2F); the latter is then hydrogenated to produce HFC-134a
(Manzer, 1990; Rao et
al., 1992; Bozorgzadeh et al., 2001; Maranion et al., 2017). Another
production method involves the reaction of hydrogen fluoride with
trichloroethylene to form HCFC-133a and HFC-134a
(Manzer,
1990; McCulloch and Lindley, 2003; Shanthan Rao et al., 2015). The process
for manufacturing HFC-125 involves the starting materials of either HCFC-123
or HCFC-124. CFC-113a, CFC-113 and HCFC-133a can be formed as by-products
when HCFC-123 and HCFC-124 are fluorinated and recycled during the process
that forms HFC-125 (Kono et al.,
2002; Takahashi et al., 2002).
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 (R2=0.791). This strong correlation with HCFC-124
points to HFC-125 production being the dominant source in 2015.
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-1
of HFC-134a and 78 Gg yr-1 of HFC-125 were reported (Fang et al.,
2016). Most industry in China is located on the eastern coast, and the
majority of HFC manufacturers are in the three eastern provinces of
Shanghai, Zhejiang and Jiangsu. There are also HFC-134a and HFC-125
production plants in Japan, South Korea and Taiwan, but the majority are
located in China. The HFC production plants located in Taiwan could
influence the mixing ratios at both the sites in Taiwan, which introduces an
additional uncertainty.
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.
Conclusions
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-1 (1.3–2.4 Gg yr-1) between 2012 and 2016.
Additionally, CFC-113a mixing ratios vary globally, and our findings confirm
an interhemispheric gradient with mixing ratios decreasing from the Northern
Hemisphere to the Southern Hemisphere. No significant emissions of CFC-113a
occur in the UK, but strong sources exist in East Asia. There are multiple
possible sources of CFC-113a emissions, and correlation analysis suggests the
emissions might be associated with the production of HFC-134a and HFC-125.
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 ∼ 510 ppt in 2017
(NOAA, 2017). Therefore, the contribution of
CFC-113a to stratospheric ozone depletion is comparably small and is not a
cause for concern. While its increase in recent years has been considerable
in percentage terms, it would have to continue increasing at this rate for
several centuries before it reaches the atmospheric mixing ratios of the
major CFCs in the 1990s. For example, a constant emission of 2 Gg yr-1
for CFC-113a yields a steady-state global mixing ratio of about 3.2 ppt. In
2016, HFCs were added to the Montreal Protocol, and under the new amendment
HFC consumption will be phased down in the coming decades
(UNEP, 2016b). Therefore, if this phase-down schedule
is successful and the main source of CFC-113a is indeed from HFC production,
then CFC-113a atmospheric mixing ratios should stop increasing in the
future. However, while it seems likely, it is still not clear whether HFC
production is actually the main source of global CFC-113a emissions, and
while CFC-113a emissions have appeared to be stable in recent years, this
does not mean that they will not increase in the future. Further
investigation and continued monitoring are needed to assess future changes
and ensure the continued effectiveness of the Montreal Protocol. When
continuous measurements of CFC-113a in the East Asia region become available,
the magnitude and origins of East Asian CFC-113a emissions can be
quantified.
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 (CF3CCl2F) and CFC-114 (CClF2CClF2)
were also found to have different trends and sources
(Laube et al., 2016). If
policymakers wish to limit the impacts of individual isomers, then
atmospheric observational data on individual CFC isomers should be reported
to UNEP wherever possible. In addition, the increase in CFC-113a
demonstrates that the use of ODSs as chemical feedstock or intermediates is
becoming relatively more important as the use of ODSs for direct
applications decreases. If policymakers target zero emissions of CFCs, then
they might consider regulating these uses of ODSs.