The effect of atmospheric acidity on the reactive nitrogen cycle in Switzerland

Anthropogenic activity has caused a dramatic decrease in biodiversity with an estimated 100-1000 fold larger extinction rate compared to the natural background. This biodiversity loss has profound effects on the functioning and stability of ecosystems and consequent adverse societal and economic impacts. Increased deposition of reactive nitrogen (Nr) is one of the major drivers of biodiversity loss, alongside land-use change, biotic exchange, and climate change. The large increase in the input of anthropogenic Nr to the environment is mainly due to combustion, industrial and agricultural processes. The Nr excess is also responsible for degraded soil quality and groundwater pollution.

Emissions of nitrogen oxides (NO_x) and ammonia (NH₃) are the primary sources of atmospheric Nr, while nitric acid (HNO₃) and NH₃ drive most of Nr deposition (more than 90% globally). The cycling of N species in the atmosphere is modulated by aerosol acidity, as it drives the partitioning of each species between the gas and particle phase. In particular, the fraction of HNO₃ partitioning to the condensed phase decreases with acidity, whereas NH₃ has the opposite behavior. Changes in environmental conditions, namely temperature (T) and relative humidity (RH), directly affect aerosol acidity (e.g., a higher RH increases the aerosol water content and decreases the overall acidity – while T has a strong effect in the partitioning constant). Given the large difference in deposition velocity between gas and particles, it is essential to constrain atmospheric acidity to characterize the atmospheric deposition of Nr accurately.

In our study, we used data from the Swiss National Air Pollution Monitoring Network and the ISORROPIA-Lite model to examine aerosol acidity trends over several years, focusing on two Swiss sites: Payerne (agricultural) and Rigi (prealpine). In Payerne, winter brings increased nitrate but reduced acidity due to higher liquid water content. At Rigi, ammonia and sulfate concentrations peak in late spring and summer, about three times higher than in winter, influenced by nearby farming and atmospheric conditions. Despite these variations, aerosol pH at Rigi remains consistently around 3 to 3.5, balanced by the parallel seasonality of ammonia and sulfate. Over the past 14 years, sulfate levels have halved at both sites, as a result of successful emission reduction policies. However, aerosol acidity has remained largely unaffected due to the buffering capacity of ammonia.

Furthermore, we assessed aerosol sensitivity to changes in ammonia and nitrate, along with their deposition patterns. We find that aerosols remain sensitive to both ammonia and nitrate levels throughout the year, although their deposition regimes vary. For instance, in Payerne, nitrate deposition is rapid in summer but slows down in winter. At Rigi, similar patterns are observed for nitrate, with deposition slowing down on about 50% of winter days. Ammonia deposition is consistently fast at both sites, but it slows down for 10-20% of winter days in Rigi.

We conclude by exploring the consequences of these trends for nitrogen emission control strategies and the impact of energy transitions and future climate scenarios on Switzerland's nitrogen cycle, air quality and policy effectiveness.