Transport Pathways and Pollution Sources into the extratropical UTLS during PHILEAS (Invited Presentation)

In summer 2023 the HALO mission PHILEAS took place in Alaska, and focused on the impact of the Asian monsoon system and export from the Asian monsoon anticyclone (AMA) into the lowermost stratosphere (LMS). The AMA is known to serve as reservoir for accumulating surface emissions and aerosol precursors due to the PV barrier in the upper UT. During the previous HALO missions TACTS/ESMVal and WISE, a significant impact of the monsoon export on the background composition of the lowermost stratosphere (LMS) could be observed. Recent observations during the research missions StratoClim and ACCLIP show evidence that a ammonium nitrate transported by the AMA constitutes a strong contribution to the UTLS aerosol budget and the Asian Tropopause Aerosol Layer (ATAL), likely relevant for cirrus cloud formation. These missions revealed that the northern central Pacific is a key region for the cross tropopause transport of air masses originating from the AMA and emissions from East Asia and China to cross the tropopause. Particularly, over the northern Pacific dynamical and diabatic forcings lead to a subsequent erosion of these eddies and to mixing into the background lower stratosphere.

We will show key results, highlighting the transport structure and different time scales for pollution export from Asia. Our results show, that transport from east Asia has very different characteristics and time scales on different isentropic levels of the UTLS region. Chemical perturbations far from the sources over North America can be linked to different transport pathways from the Asian monsoon region and east Asia.

We found that cross tropopause transport and mixing of pollution from East Asian strongly contributes to the LMS composition in the extratropics far from the Asian source regions. Thereby the chemical signature below approximately 360K is dominated by rapid transport and mixing within a few days from East Asia to the LMS, whereas above 360K time scales of several weeks dominate. We further found that besides these Asian sources the aerosol as well as the gas phase composition of the lowermost stratosphere was affected by local biomass burning.