Inorganic chemical components in precipitation in the eastern US and Eastern Canada during 1989-2016: Temporal and regional trends of wet concentration and wet deposition from the NADP and CAPMoN measurements

With the implementation of the Clean Air Act Amendments of 1990 in the U.S. and similar emission reduction measures in Canada, sulfur dioxide (SO2) and nitrogen oxides (NOx = NO + NO2) emissions in the eastern U.S. and Eastern Canada declined significantly during the period 1989-2016. Correspondingly, air quality over the region improved tremendously and sulfate, nitrate and acidity in precipitation decreased significantly. In this study, we analyzed measurement data in the eastern U.S. and Eastern Canada from the two national long-term wet deposition networks, the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) in the U.S. and the Canadian Air and Precipitation Monitoring Network (CAPMoN) in Canada, to reveal the spatial-temporal trends of precipitation concentrations and wet deposition of sulfate, nitrate, ammonium and acid in the eastern part of North America (broken into 4 different regions) for the period of 1989-2016. Comparing 2014-2016 to 1989-1991, the results show that the acid, sulfate and nitrate wet deposition decreased by 78%, 69% and 46% for 1989-2016, and decreased annually at rates of 2.8 similar to-3.6% yr(-1), -2.5 similar to-2.9% yr(-1), and -1.6 similar to-2.0% yr(-1) over different regions, respectively. It also revealed that there were no statistically significant trends for the annual mean wet concentrations of ammonium (NH4+) in precipitation in the Midwest and the Mid-Atlantic of the U.S. (P>0.4 respectively). However, there were statistically significant increasing trends of annual wet deposition of NH4+ over these latter two regions (P = 0.01-0.13) with an annual rate of increase of 0.6% and 0.4%, respectively. The increasing trends of wet deposition of NH4+ in these regions were mostly due to increasing annual precipitation amounts, as indicated by positive correlation coefficients between them (R = 0.67-0.77), and a statistically significant increasing trend of the precipitation amount during the period. The annual mean wet concentration of chloride ion (Cl-) in precipitation in the above two regions had a clear

Automated summary

Implementation of Clean Air Act Amendments of 1990 and similar emission reduction measures in Canada significantly reduced sulfur dioxide and nitrogen oxides emissions in the eastern U.S. and Eastern Canada from 1989-2016, leading to improved air quality and reduced sulfate, nitrate, and acidity in precipitation. Ammonium wet deposition showed varying trends, while chloride ion concentrations in precipitation exhibited clear trends.