Triple element stable isotope signatures for constraining the atmospheric chloromethane budget

• Applicant: Dr. Markus Greule
• Subject Area: Atmospheric Science
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 491076619

The ozone layer in the stratosphere effectively adsorbs solar radiation (UV-C and UV-B) that could injure or kill most living organisms and is depleted by halogenated compounds such as anthropogenic emitted chlorofluorocarbons (CFCs). As CFCs have been strongly reduced since the Montreal protocol halogenated compounds released from natural sources, such as chloromethane (CH3Cl) have become increasingly relevant in stratospheric ozone depletion. 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. The general underlying concept is that the atmospheric isotope ratio of a compound such as CH3Cl may be considered to equal the sum of isotopic fluxes from all sources, corrected for the weighted average kinetic isotope effect of all degradation processes, thereby enabling the possibility to deconvolute distinct sources and sinks of known isotopic signatures. 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 proposal 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 and optimized for stable isotope ratio measurements of CH3Cl. This sampling device will be first tested in the laboratory and then 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. Samples will be measured for their isotopic composition using state of the art mass spectrometric techniques. 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.

• DFG Programme: Research Grants
• Co-Investigators: Dr. Matthias Gehre; Professor Dr. Frank Keppler