Interannual variability and formation mechanism of the Asian tropopause aerosol layer and its global impact

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The Asian summer monsoon is associated with strong upward transport by deep convection of tropospheric gases, including pollutants and water vapour. These gases are confined in the anticyclonic circulation in the Upper Troposphere / Lower Stratosphere (UTLS) and contribute to the Asian Tropopause Aerosol Layer (ATAL) measured by satellite instruments in this altitude range. Only sparse in situ ATAL measurements are available in Asia during summer. ATAL is an anthropogenic signal in the Asian monsoon region, one of the most densely populated parts of the world with increasing industrial emissions. Only limited information on the chemical composition of ATAL particles is available from measurements so far. Further, the transport of radiatively active substances such as water vapour and aerosol from the Asian monsoon anticyclone into the northern extra-tropical UTLS is important because changes of the radiative budget in this part of the atmosphere have a significant impact on surface climate.Motivated by the current lack of detailed understanding of ATAL, we propose to combine ATAL in situ measurements with simulations of a Lagrangian transport model (CLaMS). Our Sino-German project will address the following open key questions:1) How is the inter-annual and inter-seasonal variability of ATAL?2) Where are the source regions? Which are key formation processes and transport pathways? What is the chemical composition of ATAL?3) What is the impact of aerosol and H2O from the Asian monsoon to the northern extratropical UTLS?Our Chinese partners have a unique data set of in situ balloon measurements (SWOP) of ATAL from the last 5 years and propose to extend their measurements in time and locations, in particular to the Sichuan Basin one of the most heavily polluted regions in China. Further, we include high-altitude aircraft ERICA (ERc Instrument for the Chemical composition of Aerosols) measurements developed at the University of Mainz, the only available measurements of the chemical composition of aerosol particles within ATAL so far.In our project, we will combine SWOP and ERICA measurements with CLaMS simulations performing both trajectory calculations to infer detailed transport pathways of ATAL particles as well as global 3-dimensional simulations including artificial tracers of air mass origin. CLaMS simulations will be driven by the new ECMWF ERA-5 reanalysis providing a much higher spatial and temporal resolution than the older ERA-Interim product. CLaMS simulations driven by ERA-5 are a unique tool to describe transport processes reliably in the region of the Asian monsoon and its global impact. We emphasise that the results of our proposal can help to develop regulations of anthropogenic surface emissions of ATAL precursors in Asia.