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