Exploring the Global Importance of Atmospheric Ammonia Oxidation

Abstract

Ammonia (NH3) is the dominant source of reduced nitrogen in the atmosphere, emitted primarily from agricultural activities. Current representations of NH3 in global chemical transport models (CTMs) largely focus on the thermodynamics governing aerosol formation, ignoring the atmospheric oxidation of NH3 with the hydroxyl (OH) radical since this process is slow and therefore assumed to not be significant. In this study, we incorporate an explicit mechanism to simulate this chemistry using the GEOS-Chem global CTM. While the inclusion of this pathway does not result in a meaningful impact on the global ammonia burden, with an average annual reduction of approximately 3%, the oxidation process leads to small but significant changes in key atmospheric species, particularly over the Indian subcontinent where surface concentrations of ozone (O3), OH, and nitrate aerosol see reductions of over 5%. Our results also suggest that ammonia oxidation accounts for around 8% (and up to 16%) of the global anthropogenic nitrous oxide (N2O) source, with important implications for climate models designed to accurately simulate the impact of changing agricultural emissions. We also conduct a suite of simulations using anthropogenic emission estimates from the representative concentration pathway (RCP) trajectories for 2100, which suggest that the atmospheric oxidation of NH3 will become an increasingly important source of N2O and NOx under future emission scenarios, accounting for up to 21% of future N2O emissions. Given the large uncertainties in the oxidation process, we use a sensitivity analysis to demonstrate the wide range in atmospheric response; our results support the need for further research to better constrain the reaction pathways and associated yields.