Modelling the Impact of Ammonia Emissions on New Particle Formation in the Asian Monsoon Upper Troposphere

Ammonia emissions in south-east Asia are a significant contributor to air pollution. This pollution, through convective transport by the Asian monsoon anticyclone, can initiate new particle formation events in the upper troposphere and the development of the Asian Tropopause Aerosol Layer (ATAL). Despite the acknowledged influence of ammonia emissions and particulate ammonium on upper tropospheric air pollution and cloud formation, a comprehensive understanding of the ATAL remains limited. A substantial knowledge gap persists regarding its origin, maintenance, chemical composition, and the climatic implications of these factors. We use the EMAC chemistry-climate model to study the influence of ammonia emissions on nucleation mechanisms contributing to the development of the ATAL. Through the integration of observational data with model simulations and the application of parameterisations from the CERN CLOUD experiment, we investigate the conditions sustaining the ATAL and explore its climatic implications. The findings suggest that ammonia emissions enhance nucleation rates in the ATAL, resulting in up to 70% increases in cloud condensation nuclei (CCN) concentrations. A diurnal cycle analysis reveals that these new particle formation mechanisms mostly occur during daylight after convection events uplifting precursor gases. Our findings enhance the understanding of the impact of anthropogenic emissions on CCN formation processes and their implications for climate in the regions characterised by high ammonia emissions.