A large-volume air sampling system (LVASS) for isotope determination of atmospheric chloromethane

The global budget of chloromethane (CH₃Cl) remains highly uncertain despite its significant role in catalyzing stratospheric ozone loss. Since conventional, concentration-based methods have limitations in improving current budget estimates including the balance of sources and sinks, stable isotopes offer an alternative approach to address these uncertainties. However, a crucial prerequisite for this concept is the determination of average tropospheric δ²H_CH3Cl, δ¹³C_CH3Cl and δ³⁷Cl_CH3Cl values, which poses a considerable analytical challenge due to the low abundance of atmospheric CH₃Cl of around ~550 parts per trillion by volume. Hence, we constructed a large-volume air sampling system (LVASS) capable of collecting sufficient amounts of CH₃Cl from 50 to 500 L of air at three distinct locations in Germany: Heidelberg University (urban), Hohenpeißenberg (boundary layer) and Schneefernerhaus (free troposphere).

We present preliminary results of δ²H_CH3Cl and δ¹³C_CH3Cl values from the first sampling campaigns applying the LVASS at these three locations. The δ²H_CH3Cl and δ¹³C_CH3Cl values were measured by stable isotope ratio mass spectrometry. We will particularly focus on the reliability of the atmospheric measurements and the evaluation of potential fractionation processes associated with the LVASS, purification system and analysis by IRMS.