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

<p>The Asian Summer Monsoon (ASM) is a seasonal weather pattern characterized by heavy rains and winds, mainly affecting South and Southeast Asia during the summer months. The 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 <span class="inline-formula">CH₂Cl₂</span> from the PHILEAS (Probing High Latitude Export of Air from the Asian Summer Monsoon) aircraft campaign in late summer 2023 to examine the transport pathways and timescales for polluted air from the ASM to the extratropical upper troposphere and lower stratosphere (UTLS). <span class="inline-formula">CH₂Cl₂</span> mixing ratios of up to 300 <span class="inline-formula">ppt</span> (<span class="inline-formula">≈</span> 500 % of the Northern Hemisphere background) were measured in the upper troposphere in the subarctic region. The three largest observed pollution events were analyzed with the help of the Lagrangian particle dispersion model FLEXPART, both in terms of their origin and their potential entry into the lower stratosphere. The results show that the East Asian Summer Monsoon (EASM) is the key pathway for transporting uncontrolled short-lived chlorinated substances (Cl-VSLSs) 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 shown here suggests that transport to the upper troposphere in the subarctic region did not occur through transport into the Asian Summer Monsoon Anticyclone (ASMA) with subsequent eddy-shedding events but rather by large convective transport contributions from the EASM. The projected entry into the lower stratosphere in the following days amounts to a few percent, indicating that the direct influence of these particular events on the lower stratosphere is probably minor.</p>