Decoding long-term trends in the wet deposition of sulfate, nitrate, and ammonium after reducing the perturbation from climate anomalies

Abstract

Abstract. Long-term trends of wet deposition of inorganic ions are affected by multiple factors, among which emission changes and climate conditions are dominant ones. To assess the effectiveness of emission reductions on the wet deposition of pollutants of interest, contributions from these factors to the long-term trends of wet deposition must be isolated. For this purpose, a two-step approach for preprocessing wet deposition data is presented herein. This new approach aims to reduce the impact of climate anomalies on the trend analysis so that the impact of emission reductions on the wet deposition can be revealed. This approach is applied to a 2-decade wet deposition dataset of sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) at rural Canadian sites. Analysis results show that the approach allows for statistically identifying inflection points on decreasing trends in the wet deposition fluxes of SO42- and NO3- in northern Ontario and Quebec. The inflection points match well with the three-phase mitigation of SO2 emissions and two-phase mitigation of NOx emissions in Ontario. Improved correlations between the wet deposition of ions and their precursors' emissions were obtained after reducing the impact from climate anomalies. Furthermore, decadal climate anomalies were identified as dominating the decreasing trends in the wet deposition fluxes of SO42- and NO3- at a western coastal site. Long-term variations in NH4+ wet deposition showed no clear trends due to the compensating effects between NH3 emissions, climate anomalies, and chemistry associated with the emission changes of sulfur and nitrogen.