Meteorologically normalised long-term trends of atmospheric ammonia (NH3) in Switzerland/Liechtenstein and the explanatory role of gas-aerosol partitioning.

Ammonia (NH3) is an important atmospheric pollutant and despite significant management efforts, trends of NH3 concentrations have not shown progressive decreases over the last few decades across much of Europe. To investigate this issue, long-term NH3 concentrations from passive sampling tubes were analysed at 32 locations across Switzerland and Liechtenstein. A trend analysis controlling for changes in meteorology employing generalised additive models (GAMs) between 2000 and 2021 showed that 29 of the 32 (91 %) sites experienced no significant change or increasing NH3 concentrations with the greatest trend being 0.17 μgm-3y-1. These results conflict with an indicated 13 % reduction in NH3 emissions from the Swiss emission inventory. The sensitivity of the NH3 -ammonium (NH4+) system to reductions of NH3 's acidic sinks (mostly in the form of nitric and sulfuric acids) was investigated with thermodynamic equilibrium modelling to explain this disconnect. The simulations indicated that the reductions in NH3 's acidic sinks resulted in less NH4+ transformation, thus increasing the NH3/NHx ratio and this process has compensated for the reduction in NH3 emissions. The average effect of the sink reductions was an increase of 0.9 μgm-3 in NH3 between 2004 and 2021. Increases in the NH3/NHx ratio have likely occurred in many European countries due to reductions of acidic precursor emissions and will have consequences for reactive nitrogen deposition and alter import-export budgets among neighbouring regions and countries.