Abstract. SO2 and NOx are precursors to form sulfate, nitrate and ammonium particles, which account for more than 50 % of PM2.5 mass in the eastern US and Eastern Canada, and are dominant components of PM2.5 during many smog events. H2SO4 and HNO3, formed from oxidation of SO2 and NOx respectively, are the main sources of acid deposition through wet and dry depositions. NOx is also a precursor to the formation of tropospheric O3, which is an important atmospheric oxidant and is also essential for the formation of other atmospheric oxidants, such as OH and H2O2. In the past 26 years from 1990 to 2015, emissions of SO2 and NOx in US were significantly reduced from 23.1 and 25.2 million tons/year in 1990 to 3.7 and 11.5 million tons/year in 2015 respectively. In Canada, SO2 and NOx were reduced by 63 % and 33 % from 1990 to 2014. In response to the significant reduction of SO2 and NOx emissions, air quality in the eastern US and Eastern Canada improved tremendously during 1990–2015. In this study, we analyzed surface air concentrations of SO42−, NO3−, NH4+, HNO3 and SO2 measured weekly by the Clean Air Status and Trends Network (CASTNET) in the US and measured daily from the Canadian Air and Precipitation Monitoring Network (CAPMoN) in Canada to reveal the temporal and spatial changes of each species during the 25-year period. For the whole the eastern US and Eastern Canada, the annual mean concentrations of SO42−, NO3−, NH4+, HNO3, SO2 and TNO3 (NO3− + HNO3, expressed as the mass of equivalent NO3−) were reduced by 73.3 %, 29.1 %, 67.4 %, 65.8 %, 87.6 % and 52.6 % respectively from 1990 to 2015. In terms of percentage, reduction of all species except NO3− was spatially uniform; reduction of SO2 and HNO3 was similar in warm season (May–October) and cold season (November–April), and reduction of SO42−, NO3− and NH4+ was more significant in warm season than in cold season. Reduction of SO42− and SO2 mainly occurred in 1989–1995 and 2007–2015 during warm season, and in 1989–1995 and 2005–2015 during cold season. Reduction of NO3− mainly occurred in the Midwest after 2000. Other than in the Midwest, NO3− had very little change during cold season for the period. The reduction of NH4+ generally followed the reduction trend of SO42−, especially after 2000 the temporal trend of NH4+ was almost identical to that of SO42−. The ratio of S in SO42− to total S in SO42− and SO2, as well as the ratio of NO3− to TNO3 increased by more than 50 % during the period. This indicates that much more percentage of SO2 was oxidized to SO42−, and much more percentage of HNO3 was neutralized to NH4NO3 in the region near the end of the period.
The Unintended Disincentive in the Clean Air Act
