Air quality in the eastern United States and Eastern Canada for 1990–2015: 25 years of change in response to emission reductions of SO 2 and NO x in the region

SO2 and NOx are precursors to form sulfate, nitrate, and ammonium particles in the air, which account for more than 50 % of PM2.5 mass in the eastern US (Bell et al., 2007) and are dominant components of PM2.5 during many smog events (Dabek-Zlotorzynska et al., 2011). H2SO4 and HNO3, formed from the 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 O-3, 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 the US were significantly reduced from 23.1 and 25.2 million t yr(-1) in 1990 to 3.7 and 11.5 million t yr(-1) in 2015, respectively. In Canada, SO2 and NOT were reduced by 63 % and 33 % from 1990 to 2014. In response to the significant reductions of SO2 and NO, 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-, NOT, 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 in each species during the 25 -year period. For the whole eastern US and Eastern Canada, the annual mean concentrations of SO42-, NOT, 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, the reductions of all species except NOT were spatially uniform. The reductions of SO2 and HNO3- were similar in the warm season (May October) and the cold season (November April), and the reductions of SO42, NOT, and NH4 were more significant in the warm season than in the cold season. The reductions of SO42 and SO2 mainly occurred in 1990-1995 and 2007-2015 during the warm season and in 1990-1995 and 2005-2015 during the cold season. The reduction of NOT mainly occurred in the Midwest after 2000. Other than in the Midwest, NOT exhibited very little change during the 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 plus SO2, as well as the ratio of NOT to TNO3 increased by more than 50 % during the period. This indicates that a notable change in regional chemistry took place from the beginning to the end of the period, with a higher percentage of SO2 being oxidized to SO42 and a higher percentage of HNO3 being neutralized to NH4NO3 near the end of the period.