Long-Range Transport of Polluted Asian Summer Monsoon Air to High Latitudes during the PHILEAS Campaign in the Boreal Summer 2023

The Asian summer monsoon establishes a strong connection between near-surface pollution in Southeast Asia and the global atmosphere by linking local emission sources with the large-scale circulation. There is a strong impact on the extratropical lower stratosphere, which is thought to occur mainly via quasi-horizontal export of polluted and moist air from the upper-level Asian monsoon anticyclone (AMA). The recent Probing High Latitude Export of Air from the Asian Summer Monsoon (PHILEAS) campaign focused on investigating this eddy transport and the associated mixing of monsoon-influenced air into the extratropical lower stratosphere through dedicated High Altitude and Long-Range Aircraft (HALO) observations from Oberpfaffenhofen, Germany, and Anchorage, Alaska, in the late summer and early autumn 2023. We summarize the mission’s motivation and objectives, place the Asian monsoon season 2023 into a climatological context, and present some representative observations. The observations during flights from Oberpfaffenhofen demonstrate the significant spatial and temporal AMA variability, which allowed HALO to investigate the displaced lower AMA boundary over the eastern Mediterranean, Israel, and Jordan. The observations during flights from Anchorage highlight the influence of long-range transport of moist and polluted air from the region of the Asian summer monsoon on the composition of the extratropical upper troposphere and lower stratosphere (UTLS), which impacts both ozone chemistry and the climate-relevant radiation budget. Significance Statement This study examines the Asian summer monsoon’s role in linking Southeast Asia’s near-surface atmosphere to the global upper troposphere and lower stratosphere. We find that the Asian summer monsoon plays a key role in transporting moisture, aerosols, and pollutants (including very-short-lived chlorinated compounds) to high-latitude regions. This, in turn, impacts atmospheric chemistry including stratospheric ozone depletion and alters the climate-relevant atmospheric radiation budget. Additionally, long-range transport of aerosols, such as ammonium nitrate, has the potential to influence cloud formation, further affecting climate and weather patterns.