Widespread influence of aerosol from the Asian summer monsoon throughout the extratropical lower stratosphere

Motivated by the limited knowledge of how much the extratropical UTLS region is influenced by the Asian summer monsoon (ASM) outflow, we conducted the aircraft-based mission PHILEAS (Probing High Latitude Export of air from the Asian Summer Monsoon). The mission took place in August/September 2023, operating from Oberpfaffenhofen (Germany) and Anchorage (Alaska). The research aircraft HALO was equipped with a comprehensive suite of in-situ and remote-sensing instruments for aerosol and gas analysis. 

We performed vertically-resolved submicron aerosol composition measurements up to a potential temperature of 400 K (~14.5 km altitude) by using the ERICA aerosol mass spectrometer. Our analysis identifies particulate ammonium nitrate and organic compounds as prevalent throughout the extratropical lower stratosphere. The mass concentrations are comparable to those in remote continental regions. The presence of this pollution aerosol is linked to the ASM convection, followed by subsequent isentropic northward transport in the stratosphere. Global simulations with the ECHAM/MESSy Atmospheric Chemistry model (EMAC) suggest that this transport pathway persists from July to September and recurs annually. Even after the ASM dissipates in September, the particulate ammonium remains widespread in the stratosphere, with broad implications for stratospheric aerosol composition and heterogeneous chemistry.