Aerosol from the Asian monsoon ubiquitous throughout the extratropical stratosphere

Franziska Köllner, Matthias Kohl, Fatih Ekinci, Oliver Eppers, Johannes Schneider, Antonis Dragoneas, Sergej Molleker, Sören Johansson, Jörn Ungermann, Erik Kretschmer, Michael Höpfner, Valentin Lauther, C. Michael Volk, Oliver Appel, Philipp Brauner, Daniel Kunkel, Hans-Christoph Lachnitt, Heiko Bozem, Franziska Weyland, Nicolas Emig, Linda Ort, Andreas Zahn, Laura Tomsche, Andrea Pozzer, Jos Lelieveld, Yafang Cheng, Martin Riese, Peter Hoor, Stephan Borrmann

Asian summer monsoon (ASM) convection efficiently transports surface emissions into the upper troposphere, leading to the formation of the Asian tropopause aerosol layer (ATAL). The ATAL can affect Earth’s climate directly by scattering solar radiation and indirectly by cloud formation. Little is known about the global distribution of aerosol originating in the ASM, particularly in the extratropical tropopause region, where the climate is sensitive to anthropogenic perturbations. Here, we present airborne aerosol composition measurements in the ASM outflow region over the North Pacific. Our analysis demonstrates the important seasonal aerosol transport from Asia into the northern lower stratosphere. Particulate ammonium nitrate and organic compounds are ubiquitous in the stratosphere with a mass concentration of up to 1 μg m−3. This is the first detection of
ammonium nitrate in the stratosphere outside the monsoon region. Together with the results from a global chemistry-climate model, we find that this northward transport of Asian aerosol is persistent from July to September, recurring yearly. The simulations suggest that particulate ammonium persists for several months, significantly affecting aerosol acidity in the lower stratosphere. This highlights the widespread impact of Asian emissions on stratospheric aerosol, with implications for ozone chemistry and the atmospheric radiation budget.