Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions
Chlorofluorocarbons (CFCs) are ozone-depleting substances as well as strong greenhouse gases, and the control of their production and use under the Montreal Protocol has had demonstrable benefits to both mitigation of increasing surface UV radiation and climate forcing. A global ban on consumpti...
| Main Authors: | , , , , , , , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-12-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/15347/2016/acp-16-15347-2016.pdf |
| Summary: | Chlorofluorocarbons (CFCs) are ozone-depleting substances
as well as strong greenhouse gases, and the control of their production and
use under the Montreal Protocol has had demonstrable benefits to both
mitigation of increasing surface UV radiation and climate forcing. A global
ban on consumption came into force in 2010, but there is evidence of
continuing emissions of certain CFCs from a range of sources. One compound
has received little attention in the literature, namely CFC-114
(C<sub>2</sub>Cl<sub>2</sub>F<sub>4</sub>). Of particular interest here is the differentiation
between CFC-114 (CClF<sub>2</sub>CClF<sub>2</sub>) and its asymmetric isomeric form
CFC-114a (CF<sub>3</sub>CCl<sub>2</sub>F) as atmospheric long-term measurements in the
peer-reviewed literature to date have been assumed to represent the sum of
both isomers with a time-invariant isomeric speciation. Here we report the
first long-term measurements of the two isomeric forms separately, and find
that they have different origins and trends in the atmosphere.
<br><br>
Air samples collected at Cape Grim (41° S), Australia, during
atmospheric background conditions since 1978, combined with samples
collected from deep polar snow (firn) enable us to obtain a near-complete
record of both gases since their initial production and release in the
1940s. Both isomers were present in the unpolluted atmosphere in comparably
small amounts before 1960. The mixing ratio of CFC-114 doubled from 7.9 to
14.8 parts per trillion (ppt) between the start of the Cape Grim record in
1978 and the end of our record in 2014, while over the same time CFC-114a
trebled from 0.35 to 1.03 ppt. Mixing ratios of both isomers are slowly
decreasing by the end of this period. This is consistent with measurements
of recent aircraft-based samples showing no significant interhemispheric
mixing ratio gradient.
<br><br>
We also find that the fraction of CFC-114a mixing ratio relative to that of
CFC-114 increased from 4.2 to 6.9 % over the 37-year period. This
contradicts the current tacit assumption used in international climate
change and ozone depletion assessments that both isomers have been largely
co-emitted and that their atmospheric concentration ratio has remained
approximately constant in time. Complementary observations of air collected
in Taiwan indicate a persisting source of CFC-114a in South East Asia which
may have been contributing to the changing balance between the two isomers.
<br><br>
In addition we present top-down global annual emission estimates of CFC-114
and CFC-114a derived from these measurements using a two-dimensional
atmospheric chemistry-transport model. In general, the emissions for both
compounds grew steadily during the 1980s, followed by a substantial
reduction from the late 1980s onwards, which is consistent with the
reduction of emission in response to the Montreal Protocol, and broadly
consistent with bottom-up estimates derived by industry. However, we find
that small but significant emissions of both isomers remain in 2014.
Moreover the inferred changes to the ratio of emissions of the two isomers
since the 1990s also indicate that the sources of the two gases are, in
part, independent. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |