# Global impact of anthropogenic NH3 emissions on upper tropospheric aerosol formation

## TL;DR

Human-caused ammonia emissions significantly boost aerosol formation in the upper atmosphere, affecting clouds and climate.

## Contribution

Quantifies anthropogenic NH3's global impact on upper tropospheric aerosol formation and cloud condensation nuclei.

## Key findings

- Anthropogenic NH3 increases upper tropospheric cloud condensation nuclei concentrations by 2.5 times.
- Aerosol optical depth rises by up to 80% due to anthropogenic NH3 emissions.
- UTLS aerosol mass concentration declines by up to 50% without anthropogenic NH3.

## Abstract

Ammonia (NH3) emissions from human activities can significantly influence aerosol processes in the upper troposphere and lower stratosphere (UTLS). Using an Earth system model, we show that anthropogenic NH3 strongly enhances new particle formation and growth, leading to substantial changes in UTLS aerosol composition and abundance. These changes can enhance cloud condensation nuclei concentrations by a factor of 2.5 in the upper troposphere over high-emission regions. In addition, aerosol optical depth can increase by up to 80%, potentially affecting climate. Our findings underscore the need to account for UTLS NH3-driven aerosol processes in Earth system models to improve predictions of atmospheric composition and cloud effects in climate scenarios.

Anthropogenic ammonia (NH3) emissions have significantly increased in recent decades due to enhanced agricultural activities, contributing to global air pollution. While the effects of NH3 on surface air quality are well documented, its influence on particle dynamics in the upper troposphere-lower stratosphere (UTLS) and related aerosol impacts remain unquantified. NH3 reaches the UTLS through convective transport and can enhance new particle formation (NPF). This modeling study evaluates the global impact of anthropogenic NH3 on UTLS particle formation and quantifies its effects on aerosol loading and cloud condensation nuclei (CCN) abundance. We use the EMAC Earth system model, incorporating multicomponent NPF parameterizations from the CERN CLOUD experiment. Our simulations reveal that convective transport increases NH3-driven NPF in the UTLS by one to three orders of magnitude compared to a baseline scenario without anthropogenic NH3, causing a doubling of aerosol numbers over high-emission regions. These aerosol changes induce a 2.5-fold increase in upper tropospheric CCN concentrations. Anthropogenic NH3 emissions increase the relative contribution of water-soluble inorganic ions to the UTLS aerosol optical depth (AOD) by 20% and increase total column AOD by up to 80%. In simulations without anthropogenic NH3, UTLS aerosol composition is dominated by sulfate and organic species, with a marked reduction in ammonium nitrate and aerosol water content. This results in a decline of aerosol mass concentration by up to 50%. These findings underscore the profound global influence of anthropogenic NH3 emissions on UTLS particle formation, AOD, and CCN production, with important implications for cloud formation and climate.