The influence of ammonia emissions on the size-resolved global atmospheric aerosol composition and acidity

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. Three NH₃ emission schemes are analyzed: two bottom-up inventories and one derived using an updated top-down method. The results reveal that sulphate-nitrate-ammonium aerosols in two fine mode size ranges (0–1 µm 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 highlights 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. The study emphasizes that pH changes are closely related to NH₃ emission variations, with the highest sensitivity observed in the fine mode size ranges. These results highlight the critical role of NH₃ in shaping aerosol acidity and argue for size-specific approaches to managing particulate matter.