Eddy shedding transport from the Asian monsoon as a source of pollution for the lowermost stratosphere

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Transport of polluted air masses from the surface into the upper troposphere and lower stratosphere by the Asian monsoon circulation has crucial effects on stratospheric chemistry and the Earth's radiation budget. The transport of these air masses from the monsoon region into the Northern hemisphere lowermost stratosphere (LMS) is characterised by shedding of smaller scale eddies from the monsoon anticyclone and the separation of filaments (streamers), frequently related to Rossby wave breaking. It is an open question which pathways dominate this eddy transport, their lifetime in the lowermost stratosphere as well as their impact on stratospheric composition. In this project we will study the eddy shedding transport pathway of pollution from the Asian monsoon into the LMS, based on a combination of unprecedented high-resolution in-situ measurements from the PHILEAS HALO-aircraft campaign and a novel model diagnostic approach to identify and characterize this transport pathway. We will perform measurements of C2H6, CO, CH4 and N2O with different chemical life times and complementary source and sink characteristics. The chemically inert N2O allows a precise determination of the chemical tropopause to infer smallest signatures of pollutant tracers in the LMS. Complementary, novel 1Hz C2H6 measurements will be used to diagnose unambiguously tropospheric transport into the LMS, since C2H6 has no chemical stratospheric sources. This will be used to infer a quantitative air mass budget for the monsoon contribution to the LMS. Varying enhancement ratios of pollutant tracers in the LMS will be linked to the new model diagnostics which will be carried out in this project. The novel diagnostic to be developed will allow identification and tracking of shed eddies as low-PV anomalies in the global PV field and will be coupled to 3D chemistry simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS), driven with the newest generation high-resolution ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The outcome of this project will significantly enhance understanding of pollution transport from the monsoon into the global stratosphere and impacts on chemical composition. In particular we will apply a novel combination of transport diagnostics and in-situ tracers to focus on - Export of air masses from the Asian monsoon via eddy shedding by combining measurements of N2O, CH4, CO, and C2H6 with novel Lagrangian methods. - Investigation of the eddy shedding pathways and time scales, and the mixing and erosion process of the filaments combining PV-evolution and tropopause referenced pollutant enhancement ratios. - Quantification of the impact of this eddy shedding transport on the chemical composition of the global LMS using tracer budgets and model diagnostics