Anthropogenic Ammonia's Impact on Upper Tropospheric Aerosol Composition and Climate Forcing

Recent increases in global anthropogenic ammonia emissions have yet to be fully quantified in terms of impacts on atmospheric particle formation, cloud microphysical properties, and climate. We employ the EMAC global climate-chemistry model, including recently published multi-component new particle formation (NPF) parameterisations from the CERN CLOUD experiment, to investigate the impact of anthropogenic ammonia on upper tropospheric processes. Our simulations show that convective transport significantly enhances ammonia-driven NPF and particle growth at these altitudes, leading to an average increase in particle number concentrations by up to 2000 cm⁻³ and a doubling of cloud condensation nuclei (CCN) concentrations over regions with high ammonia emissions. In simulations without anthropogenic ammonia, aerosol composition in the upper troposphere is dominated by sulphate and organic species rather than ammonium nitrate. Furthermore, anthropogenic ammonia emissions contribute to an increase in aerosol optical depth by up to 90%, producing a pronounced radiative forcing pattern: cooling in the Northern Hemisphere and warming in the Southern Hemisphere. These results underscore the critical role of ammonia emissions in aerosol composition in the upper troposphere and the global radiative forcing of climate.