Tracing elevated abundance of CH₂Cl₂ in the subarctic upper troposphere to the Asian Summer Monsoon

Abstract. The Asian Summer Monsoon (ASM), characterized by heavy rains and winds, mainly affects South and Southeast Asia during the summer months. Deep convection within the ASM is an important transport process for pollutants from the planetary boundary layer up to the tropopause region. This study uses in situ observations of CH₂Cl₂ from the PHILEAS aircraft campaign in 2023 to examine the transport pathways and timescales for polluted air from the ASM to the extratropical upper troposphere and lower stratosphere (UTLS). CH₂Cl₂ mixing ratios of up to 300 ppt (≈500 % of the northern hemisphere background) were measured in the subarctic upper troposphere. The largest pollution events were analysed with the FLEXPART model, both in terms of their origin and their potential entry into the lower stratosphere. The results show that the East Asia Summer Monsoon (EASM) is the key pathway for transporting Cl-VSLS into the tropopause region, which contributes to an increase in tropospheric background levels with the potential to enter the lower stratosphere. The transport analysis of elevated mixing ratios suggests that transport to the subarctic upper troposphere did not occur through the Asian Summer Monsoon Anticyclone (ASMA) with subsequent eddy-shedding events, but by large convective transport contributions from the EASM. The projected entry into the lower stratosphere in the following days (12-day period) amounts to a few percent. However, the analysis covered only a short time frame, suggesting that these elevated CH₂Cl₂ mixing ratios could still have the potential to enter the lower stratosphere at a later time.