Historical Changes and Drivers of Aerosol Acidity in Switzerland under emission reduction and its implication of regulation policies

Emission controls in Europe have substantially reduced SO_x and NO_x but left NH₃ largely unchanged. This imbalance between acidic and basic species may shift aerosol acidity and its impacts, including toxicity, particulate matter (PM) composition, and the deposition of reactive nitrogen (Nr). The atmospheric deposition of Nr plays a critical role in ecosystem productivity and PM formation, with impacts that vary across spatial and temporal scales.

In this study, long-term observations (2008–2024) of atmospheric gases and aerosols from Swiss monitoring sites ware analyzed to assess changes under current emission reductions. Aerosol pH was estimated using the ISORROPIA-lite thermodynamic model¹ and interpreted using SHapley Additive exPlanations (SHAP) to quantify key drivers. Annual mean aerosol pH shows a slight increasing trend, with consistently lower values in summer than in winter. SHAP results indicate that temperature controls seasonal pH variability at agricultural sites, whereas total ammonia (NH₃^T) dominates at the semi-alpine site.

Dry deposition regimes of HNO₃ and NH₃ were investigated in relation to aerosol liquid water content and acidity following the approach of Nenes et al. (2021).² The findings indicate that NH₃ deposition is rapid across both the lowland and Alpine regions, suggesting localized nitrogen burdens near emission sources. In contrast, PM has become increasingly insensitive to NH₃ and more sensitive to HNO₃, particularly in the agricultural sites. These results highlight that, although HNO₃ precursor controls have effectively reduced PM pollution without the need for NH₃ reductions, evermore significant ecological concerns remain from a lack of NH₃ control. This underscores the need for coordinated reductions in both NO_x and NH₃ emissions.

References:

1 Kakavas, S., Pandis, S. N., and Nenes, A.: ISORROPIA-Lite: A Comprehensive Atmospheric Aerosol Thermodynamics Module for Earth System Models, Tellus B: Chemical and Physical Meteorology, 74, DOI: 10.16993/tellusb.33, 2022.

2 Nenes, A., Pandis, S. N., Kanakidou, M., Russell, A. G., Song, S., Vasilakos, P., and Weber, R. J.: Aerosol acidity and liquid water content regulate the dry deposition of inorganic reactive nitrogen, Atmospheric Chemistry and Physics, 21, 6023–6033 DOI:10.5194/acp-21-6023-2021, 2021.