Triple element stable isotope signatures for constraining the atmospheric chloromethane budget

Background/Summary

The ozone layer in the stratosphere effectively adsorbs solar radiation and is depleted by halogenated compounds such as anthropogenic emitted chlorofluorocarbons (CFCs) but also by CH₃Cl released from natural sources. CH₃Cl is the most abundant chlorine containing trace gas in the Earth atmosphere responsible for around 17% of chlorine-catalysed ozone destruction in the stratosphere and thus, will largely control future levels of stratospheric chlorine. However, current estimates of the CH₃Cl global budget and its distribution between sources and sinks are still highly uncertain. A better understanding of the atmospheric CH₃Cl budget is thus the major objective of this project.
Stable isotope ratio analysis including hydrogen, carbon and chlorine is a potentially powerful
tool for investigations of the atmospheric CH₃Cl budget. An absolute prerequisite, however, for further detailed global budget evaluations will be the determination of the average tropospheric stable hydrogen, carbon and chlorine isotopic compositions of CH₃Cl, a massive analytical challenge due to the relatively low abundance of atmospheric CH₃Cl of ~500 to 600 parts per trillion by volume.

The focus of this project is on the successful development of triple element isotope methods for measurements of atmospheric CH₃Cl and to show - as a proof of concept – its potential for application in numerical modelling approaches. In the first step, a large-volume air sampling system will be constructed, optimized and tested for stable isotope ratio measurements of CH₃Cl. This sampling device will then be used to collect atmospheric air samples at three different locations namely, Heidelberg University (an urban surface site), Hohenpeißenberg (DWD weather station at the boundary layer) and Schneefernerhaus (research station located in the free troposphere) at different times over a period of 1 year to monitor seasonal variations.
The results will be merged and evaluated in order to present the average stable hydrogen, chlorine and carbon isotope values for the different atmospheric locations including their seasonal variations. The novel data will be used to provide a first assessment for their application to better constrain the atmospheric CH₃Cl budget.

Relevant publications

• Keppler, F., Barnes, J.D., Horst, A., Bahlmann, E., Luo, J., Nadalig, T., Greule, M., Hartmann, S.C., Vuilleumier, S. (2020). Chlorine isotope fractionation of the major chloromethane degradation processes in the environment. Environ. Sci. Technol. 54, 1634-1645, DOI: 10.1039/c9em00540d
• Bahlmann, E., Keppler, F., Wittmer, J., Greule, M., Schöler, H.-F., Seifert, R., Zetzsch, C. (2019). Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes. Atmos. Chem. Phys. 19, 1703–1719, DOI: 10.5194/acp-19-1703-2019
• Keppler, F., Bahlmann, E., Greule, M., Schöler, H.F., Wittmer, J., Zetzsch, C. (2018). Mass spectrometric measurement of hydrogen isotope fractionation for the reactions of chloromethane with OH and Cl. Atmos. Chem. Phys. 18, 6625-6635, DOI: 10.5194/acp-18-6625-2018
• Jaeger, N., Besaury, L., Kröber, E., Delort, A.-M., Greule, M., Lenhart, K., Nadalig, T., Vuilleumier, S., Amato, P., Kolb, S., Bringel, F., Keppler, F. (2017). Chloromethane formation and degradation in the fern phyllosphere. Sci. Total. Environ. 634, 1278-1287, DOI: 10.1016/j.scitotenv.2018.03.316
• Jaeger, N., Besaury, L., Kröber, E., Delort, A.-M., Greule, M., Lenhart, K., Nadalig, T., Vuilleumier, S., Amato, P., Kolb, S., Bringel, F., Keppler, F. (2017). Chloromethane degradation in soils - a combined microbial and two-dimensional stable isotope approach. J. Environ. Qual. 47:254-262, DOI: 10.2134/jeq2017.09.0358
• Greule, M., Huber, S.G., Keppler, F. (2012). Stable hydrogen-isotope analysis of methyl chloride emitted from heated halophytic plants. Atmos. Environ. 62: 584-592, DOI: 10.1016/j.atmosenv.2012.09.007