A very limited role of tropospheric chlorine as a sink of the greenhouse gas methane
<p>Unexpectedly large seasonal phase differences between CH<sub>4</sub> concentration and its <sup>13</sup>C ∕ <sup>12</sup>C isotopic ratio and their inter-annual variations observed in southern hemispheric time series have been att...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/9831/2018/acp-18-9831-2018.pdf |
| Summary: | <p>Unexpectedly large seasonal phase differences between CH<sub>4</sub>
concentration and its <sup>13</sup>C ∕ <sup>12</sup>C isotopic ratio and
their inter-annual variations observed in southern hemispheric time series
have been attributed to the Cl + CH<sub>4</sub> reaction, in which
<sup>13</sup>CH<sub>4</sub> is discriminated strongly compared to OH + CH<sub>4</sub>,
and have provided the only (indirect) evidence of a hemispheric-scale
presence of oxidative cycle-relevant quantities of tropospheric atomic Cl.
Our analysis of concurrent New Zealand and Antarctic time series of
CH<sub>4</sub> and CO mixing and isotope ratios shows that a corresponding
<span style="" class="text"><sup>13</sup>C ∕ <sup>12</sup>C</span> variability is absent in CO. Using
the AC-GCM EMAC model and isotopic mass balancing for comparing the periods
of presumably high and low Cl, it is shown that variations in extra-tropical
Southern Hemisphere Cl cannot have exceeded
0.9  ×  10<sup>3</sup> atoms cm<sup>−3</sup>. It is demonstrated that the
<sup>13</sup>C ∕ <sup>12</sup>C ratio of CO is a sensitive indicator for
the isotopic composition of reacted CH<sub>4</sub> and therefore for its
sources. Despite ambiguities about the yield of CO from CH<sub>4</sub> oxidation
(with this yield being an important factor in the budget of CO) and
uncertainties about the isotopic composition of sources of CO (in particular
biomass burning), the contribution of Cl to the removal of CH<sub>4</sub> in the
troposphere is probably much lower than currently assumed.</p> |
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| ISSN: | 1680-7316 1680-7324 |