Aerosols from the Asian monsoon in the upper troposphere: sources, evolution, impacts

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Each year, from June till September, a layer of enhanced aerosol loading at 14–18 km altitude resides in a large area extending from the Mediterranean to the western Pacific - the Asian Tropopause Aerosol Layer (ATAL). There are large uncertainties about the composition of these particles as well as their significance for the radiative balance in this sensitive altitude region. The only airborne observations from deep inside this layer have been acquired during the StratoClim campaign based in Kathmandu in 2017. Based on data from the infrared spectrometer GLORIA on the research aircraft Geophysica, we discovered that solid ammonium nitrate (AN) particles account for a considerable part of the aerosol mass. Due to the unique capability of GLORIA to observe simultaneously the precursor gas ammonia (NH3) at these altitudes, we showed that the AN aerosol layer is fed by convection injecting large amounts of NH3 from the surface into the upper troposphere. It has also been demonstrated that solid AN particles count among the most efficient ice nuclei present in the atmosphere. Contributing to the PHILEAS campaign, we propose a common effort of atmospheric modelling and observations to better constrain the composition, origin, impacts and fate of ATAL particles - especially with regard to their evolution as well as their influence on the Northern Hemisphere upper troposphere and lower stratosphere. By acquiring measurements of monsoon-influenced air-masses over the eastern Mediterranean as well as outflow over the northern Pacific we will be able to analyse air with processed aerosol and trace-gas content, thereby complementing the StratoClim observations from deep inside the monsoon. To detect and quantify the likely smaller amounts of aerosol and trace gases in the outflow, we propose to improve the GLORIA data retrieval of NH3 and AN, amongst others, by utilizing spectroscopic data which has become available only recently. Further, we propose to expand the analysis of GLORIA spectra to sulfate aerosols as well as their precursor gas SO2. On the modelling side, we will further develop the ICON-ART global weather and climate model system for simulating the ATAL including various aerosol types (nitrate, ammonium, sulphate, organics, dust), thereby considering the high ice nucleating potential of solid AN. Model runs will be performed to achieve a global overview over the development of the ATAL during the time of the monsoon 2023, as well as detailed cloud resolving views on the aerosol-cloud-radiation interactions tailored to relevant campaign periods. Beyond the direct analysis of the PHILEAS campaign, this work will lay the foundation for improved aerosol retrieval from airborne GLORIA observations of previous and future HALO campaigns as well as satellite observations. Further it will enable ICON-ART, one of the central climate model systems in Germany on simulating aerosol processes as well as aerosol/cloud interactions connected to the ATAL.