The influence of ammonia emission inventories on size-resolved global atmospheric aerosol composition and acidity

Wang, X., Tsimpidi, A. P., Luo, Z., Steil, B., Pozzer, A., Lelieveld, J., and Karydis, V. A.

Abstract

Ammonia (NH₃) is an abundant alkaline gas in the atmosphere and a key precursor in the formation of particulate matter. While emissions of other aerosol precursors such as SO₂ and NO_x have decreased significantly, global NH₃ emissions are stable or increasing, and this trend is projected to continue. This study investigates the impact of NH₃ emission changes on size-resolved aerosol composition and acidity using the atmospheric chemistry–climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry). Rather than directly perturbing NH₃ emissions, we employ three distinct emission inventories: two bottom-up inventories and one derived using an updated top-down method. The results reveal that sulfate–nitrate–ammonium aerosols in two fine-mode size ranges (0–1 and 1–2.5 µm) show the greatest sensitivity to NH₃ emission changes. Regional responses vary depending on the local chemical environment of secondary inorganic aerosols. In “NH₃-rich” regions (e.g., East Asia and Europe), the abundance of NH₃ partially offsets the effects of reduced NH₃ emissions when NO_x and SO₂ are available, especially for aerosols in the 1–2.5 µm range. This underscores the importance of coordinated control strategies for NH₃, NO_x, and SO₂ emissions. Further, we find that NH₃ has a buffering effect in densely populated areas, maintaining aerosol acidity at moderate levels and mitigating drastic pH shifts. While pH changes correlate strongly with NH₃ variability, they are also influenced by concurrent changes in SO₂ and NO_x emissions. These results highlight the critical role of NH₃ in shaping aerosol acidity, arguing for size-specific approaches to managing particulate matter.