Historical Changes and Drivers of Aerosol Acidity in Switzerland under Emission Reduction and Its Implication of Regulation Policies

High Resolution Image Download MS PowerPoint Slide Aerosol acidity is a key regulator of atmospheric processes, influencing particulate matter (PM) composition, toxicity, and the deposition of reactive nitrogen (Nr) to ecosystems. Emission controls in Europe have strongly reduced SO x and NO x but left NH 3 largely unchanged, creating an imbalance between acidic and basic species that could shift aerosol pH and its impacts. Thermodynamic analysis of long-term observations (2008–2024) from Swiss monitoring sites combined with SHapley Additive exPlanations (SHAP) quantified aerosol pH and its key drivers. Annual mean pH shows a slight increasing trend, with consistently lower values in summer than in winter. SHAP analysis indicates that temperature drives seasonal pH variability at agricultural sites, whereas total ammonia (NH 3 T ) dominates at the semialpine site. The Nr deposition analysis shows that the fast deposition of NH 3 T predominates across both the lowland and Alpine regions, which increases local soil nitrogen burden. In contrast, PM has become increasingly insensitive to NH 3 and more sensitive to HNO 3, particularly in the agricultural sites. These results highlight that, although HNO 3 precursor controls have effectively reduced PM pollution without the need for NH 3 reductions, evermore significant ecological concerns remain from a lack of NH 3 control. This underscores the need for coordinated reductions in both NO x and NH 3 emissions.