First Insights into the Observations of Halogenated Substances during the PHILEAS Measurement Campaign in Late Summer 2023 - Probing High Latitude Export of Air from the Asian Summer Monsoon

The composition of the upper troposphere and lower stratosphere (UTLS) has a relatively large influence on the Earth's climate, and small changes due to the input of polluted air can lead to changes in, for example, surface temperature. Various dynamic features provide pathways for near-ground air masses to be transported up into the UT and eventually further into the LS. A very important feature in northern hemispheric summer and early autumn is the Asian summer monsoon (ASM) with a resulting Asian summer monsoon anticyclone (ASMA) in the UTLS above. This large anticyclone contains confined air masses that are rich in water vapour, aerosol precursors, and pollutants. When this circulation becomes weaker and unstable, these air masses from inside the circulation may break out in the form of filaments and eddies (eddy shedding) resulting in mixing within extra-tropical UT and LS air.

To investigate the relative importance of these processes to the chemical composition of the UTLS, the PHILEAS measurement campaign takes place from mid-August to the end of September 2023. Flights are planned predominantly from Anchorage (USA) to track mainly eastwards transport of polluted air in the form of eddy shedding events. Measurements are done with several in-situ and remote-sensing instruments onboard the HALO aircraft. One of the instruments is the GhOST instrument with two measurement channels. Halogenated substances, including long-lived and short-lived substances, can be sampled in a time resolution of around 6 minutes with the GC/MS channel, operated in electron impact ionisation mode. Further, SF₆ and CFC-12 can be sampled with the GC/ECD channel in a time resolution of around 60 seconds.

Here, we present the first insight into the observations during the PHILEAS campaign regarding the halogenated substances and focus on the very short-lived chlorinated and brominated substances (VSLS). Recently, elevated mixing ratios of the two major Cl-VSLS CH₂Cl₂ and CHCl₃ in the northern hemispheric UTLS could be linked to predominantly anthropogenic emissions in the region of southern and eastern Asia with the transport into the extra- tropical lower stratosphere via the ASMA¹. In addition, the Indian Ocean has shown to be a strong source of the two major Br-VSLS CH₂Br₂ and CHBr₃, with the ASM circulation likely being an important transport pathway for these substances into the ASMA². These four substances, besides others, are measured with the GhOST instrument and will be used to determine the chemical composition and transport pathways out of the monsoon anticyclone.

References
1. Lauther, V., Vogel, B., Wintel, J., Rau, A., Hoor, P., Bense, V., Müller, R., Volk, C. M.: In situ observations of CH₂Cl₂ and CHCl₃ show efficient transport pathways for very short-lived species into the lower stratosphere via the Asian and the North American summer monsoon, Atmos. Chem. Phys., 22, 2049–2077, https://doi.org/10.5194/acp-22-2049-2022, 2022.

2. Fiehn, A., Quack, B., Hepach, H., Fuhlbrügge, S., Tegtmeier, S., Toohey, M., Atlas, E., Krüger, K.: Delivery of halogenated very short-lived substances from the west Indian Ocean to the stratosphere during the Asian summer monsoon, Atmos. Chem. Phys., 17, 6723–6741, https://doi.org/10.5194/acp-17-6723-2017, 2017.