Air quality in the eastern United States and Eastern Canada for 1990–2015: 25 years of change in response to emission reductions of SO2 and NOx 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 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 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 NOx were reduced by 63 % and 33 % from 1990 to 2014. In response to the significant reductions 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 SO2− 4 , NO − 3 , NH + 4 , 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 SO2− 4 , NO − 3 , NH + 4 , HNO3, SO2, and TNO3 (NO−3 +HNO3, expressed as the mass of equivalent NO−3 ) 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 NO−3 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 SO2− 4 , NO − 3 , and NH+4 were more significant in the warm season than in the cold season. The reductions of SO2− 4 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 NO−3 mainly occurred in the Midwest after 2000. Other than in the Midwest, NO−3 exhibited very little change during the cold season for the period. The reduction of NH+4 generally followed the reduction trend of SO 2− 4 ; especially after 2000, the temporal trend of NH+4 was almost identical to that of SO2− 4 . The ratio of S in SO 2− 4 to total S in SO2− 4 plus SO2, as well as the ratio of NO − 3 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 SO2− 4 and a higher percentage of HNO3 being neutralized to NH4NO3 near the end of the period. Published by Copernicus Publications on behalf of the European Geosciences Union. 3108 J. Feng et al.: Air quality in the eastern United States and Eastern Canada for 1990–2015


Introduction
Gases and particulate matter released into the air through anthropogenic activities can pollute the air and deteriorate the air quality locally, regionally, and continentally. Air pollution, which can decrease lung function, causing the development of asthma, bronchitis, and lung cancer (Kunzli et al., 2000;Heroux et al., 2015;WHO, 2006), is considered a major environmental risk to human health by the World Health Organization (WHO). Air pollution is also linked to stroke and heart disease, and the improvement of air quality can significantly reduce the PM 2.5 -and O 3 -related mortality burden (Zhang et al., 2018). When emitted gases and particulate matter or secondary pollutants formed in the air from emissions are brought to the Earth's surface through dry and/or wet deposition, they pose a risk to the established ecosystems through acid rain as well as excessive deposition of nitrogen and sulfur. Air pollution also affects long-term climate through scattering and absorption of solar radiation by directly emitted or secondarily formed aerosols in the air (Haywood and Shine, 1995;Yu et al., 2006). In some heavily polluted regions, even local weather can be affected due to the change in energy budgets in the atmosphere and at the Earth's surface (Kajino et al., 2017).
In order to control air pollution, the US passed the Clean Air Act (CAA) of 1963 (Kuklinska et al., 2015). Major amendments to the law were passed in 1970, 1977, and 1990(Kuklinska et al., 2015. The amendments to the CAA of 1990 addressed acid deposition, ozone depletion, and toxic air pollution (CAA, 1990). Specifically, Title IV of the 1990 amendments to the CAA, also known as acid deposition control, targeted emission reductions of two acid deposition precursors, SO 2 and NO x , which along with CO, O 3 , Pb, and particulate matter are among the six species designated as criteria pollutants by United States Environmental Protection Agency (US EPA). SO 2 and NO x in the air can be oxidized to form acid H 2 SO 4 and HNO 3 , which in turn can react with NH 3 to form fine particulate matter (PM 2.5 ) and with crustal material or sea salts to form coarse particles (Yoshizumi and Hoshi, 1985;Zhuang et al., 1999). NO x , together with volatile organic compounds (VOCs), also participates in the formation of tropospheric O 3 , which is another criteria pollutant and an important atmospheric oxidant. Title IV of the Clean Air Act 1990 specifically targets SO 2 and NO x emissions from stationary fuel combustion facilities. The first phase of Title IV of the 1990 CAA amendment, which was implemented on 1 January 1995, requires 110 power plants to reduce SO 2 emissions to a level calculated as the product of an emissions rate of 2.5 lbs (1 lb is equal to 0.4636 kg) of SO 2 per million British thermal units (Btu; 1 Btu is equal to 1055.056 J) multiplied by an average of their 1985-1987 fuel use (Lee, 1991). The second phase, which took effect on 1 January 2000, requires approximately 2000 utilities to reduce SO 2 emissions to a level of 1.2 lbs of SO 2 per million British thermal units multiplied by the aver-age of their 1985-1997 fuel use (Lee, 1991). Since 1990, the national emissions of SO 2 in the US have decreased steadily from 23.1 million tons in 1990 to 21.3 million tons in 1994 and dropped significantly to 18.6 million tons in 1995 due to the first phase implementation of Title IV of the 1990 CAA amendments (EPA, 2016(EPA, , 2019. The SO 2 emissions underwent a small increase during 1996-1998, to 18.9 million tons in 1998, and then continued the steady decrease to 14.5 million tons in 2005. From 2005 to 2012, the decrease in the emissions was accelerated with an annual reduction rate of 1.34 million t yr −1 during the period. The emissions of SO 2 leveled off during . In 1990.9 % of SO 2 emissions was from stationary fuel combustion facilities, 2 % from on-road vehicles, and 2 % from off-road mobile sources. By 2007, SO 2 emissions from on-road vehicles were totally eliminated due to cleaner gasoline. In 2014, of the 4.9 million tons of total SO 2 emissions, stationary fuel combustion, off-road mobile, and industrial and other processes contributed 4.1, 0.1, and 0.7 million tons, respectively (EPA, 2016). NO x forms in the air when nitrogen reacts with oxygen under high temperature. Anthropogenic emissions of NO x are mainly due to stationary fuel combustion, on-road vehicles, and off-road mobile operations. Nationwide in the US, they contributed 10.9, 9.6, and 3.8 million tons of the total 25.2 million tons of NO x in 1990 (EPA, 2016). Changes in NO x emissions during the 1990s were relatively small (Butler et al., 2003). Total NO x emissions remained generally constant from 1990 to 1998. From 1999 there was a decrease in NO x emissions from stationary fuel combustion due to the implementation of Title IV of the 1990 CAA amendment as well as the implementation of the NO x Budget Trading Program (NBP). Title IV of the 1990 CAA amendment not only required the reduction of SO 2 , but also stipulated the reduction of NO x emissions from power plants, and it took effect in 1996. The NBP started in 2003 and was created to reduce NO x emissions from power plants and other large combustion sources in the eastern US during warm months (https:// www.epa.gov/airmarkets/nox-budget-trading-program, last access: 13 November 2019). The NBP was replaced by the ozone season NO x program under the Clean Air Interstate Rule in 2009. The NO x emissions from stationary combustion facilities decreased steadily from 10.4 million tons in 1998 to 3.6 million tons in 2012, then remained relatively unchanged thereafter (EPA, 2016). Emissions of NO x from on-road vehicles declined slowly from 1990 until 2001. After 2002, on-road emissions of NO x decreased continuously and steadily. The trend of NO x emissions from off-road mobile operations generally increased during the period 1990-2002, up from 3.8 to 4.9 t, but after that it was reduced gradually to 2.7 t in 2014. Combining the emissions from stationary fuel combustion, on-road vehicles, and off-road mobile operations, the nationwide emissions of NO x in the US changed little during 1990-1998 and decreased during 1998-2001. After 2002, they decreased steeply up to recent years.
Note that there was a change in NO x measurement methodology from 2001 to 2002, and it caused a sharp increase in the reported NO x emissions in the US from to 2002(EPA, 2019 In Canada, similar measures were adopted to reduce air pollutant emissions. SO 2 emissions in Canada were mainly from three major sectors: ore and mineral industries, the oil and gas industry, and electric utilities. For each sector, the annual SO 2 emissions were reduced from 1. 5, 0.53, and 0.62 million tons in 19905, 0.53, and 0.62 million tons in to 0.47, 0.28, and 0.27 million tons in 20155, 0.53, and 0.62 million tons in (ECCC, 2019. Nationally, annual SO 2 emissions were reduced from 3.1 million tons in 1990 to 1.1 million tons in 2015 (ECCC, 2019). In 1990, the annual emissions of SO 2 from Eastern Canada accounted for 59 % of the national annual emissions. NO x emissions in Canada were mainly from transportation (43%) and oil and gas industries (14 %) (ECCC, 2019). Nationally, annual emissions of NO x were reduced by 25 %, from 2.4 million tons in 1990 to 1.8 million tons in 2015. Specifically, annual emissions of NO x in Eastern Canada were reduced by close to 50 %, from 1.2 million tons to 0.64 million tons.
Air quality trends during the past few decades, especially since 1990, are of great interest for both scientific communities and the general public. For the eastern part of the US and Canada, trends of air quality after 1990 have been reported in previous studies for O 3 (Chan and Vet, 2010), O 3 and nitrate (Butler et al., 2011), particulate SO 2− 4 (Hand et al., 2012), and air quality and atmospheric deposition (Sickles II and Shadwick, 2007;Sickles II and Shadwick, 2015;Cheng and Zhang, 2017). Sickles II andShadwick (2007, 2015) compared the 5-year averages of air quality and atmospheric deposition in the eastern US for 1990-2004and 1990-2009. Cheng and Zhang (2017 reported the temporal trends of the annual concentration of air pollutants from 31 Canadian rural locations, most of which were located in Eastern Canada. Aas et al. (2019) reported global and regional trends of atmospheric sulfur for 1990-2015 and found that North America and East Asia had the largest reductions of sulfur emissions during the late part of the period. In this study, we analyze the surface air concentration data measured weekly by the CASTNET network in the US and measured daily from the CAPMoN network in Canada to reveal the detailed temporal and spatial trends of air quality from 1990 to 2015. These trends are not only important for the assessment of the improvement of air quality due to emissions reductions, but are also essential for the evaluation of chemical transportation models. The analysis will answer the following questions: (1) what are the trends of air pollutants over the eastern US and Eastern Canada following the significant reductions of SO 2 and NO x emissions during 1990-2015? (2) What are the physical and chemical mechanisms responsible for the trends? We will look at the air concentrations of gases SO 2 and HNO 3 , as well as particulates SO 2− 4 , NO − 3 , and NH + 4 , which are either due to direct emissions of SO 2 or due to the oxidation of SO 2 and NO x , as well as the reaction of these oxidants with NH 3 .

Networks of measurement: CASTNET and CAPMoN
The monitoring of background-and regional-level ambient pollutants is essential for assessing regional air quality. In the US and Canada, this long-term monitoring of air quality in rural and remote areas is fulfilled by two monitoring networks: CASTNET and CAPMoN, respectively. CASTNET is a monitoring network managed and operated by the US EPA in cooperation with several other federal, state, and local partners (Clarke et al., 1997;Bloomer et al, 2010). The network was established under the 1990 CAA to assess the trends of acidic deposition due to emission reduction programs. The network makes weekly integrated measurements of gases (SO 2 and HNO 3 ) and particulates (SO 2− 4 , NO − 3 , NH + 4 , Mg 2+ , Ca 2+ , Na + , and Cl − ) using filter pack methods, as well as hourly measurements of O 3 . At selected sites, it also measures hourly concentrations of NO, reactive nitrogen (NO y ), SO 2 , and CO.
CAPMoN is a monitoring network operated by Environment and Climate Change Canada (ECCC). The network began operation in 1983, although one of its two predecessor networks, the Air and Precipitation Network (APN), measured air concentrations as far back as 1978. The network measures 24 h integrated air concentrations of pollutants (from 08:00 to 08:00 LT, local time) through filter pack sampling and 24 h wet deposition by the collection of precipitation samples at the ground level. The daily air concentration measurements by CAPMoN also include gases (SO 2 and HNO 3 ) and particulates (SO 2− 4 , NO − 3 , NH + 4 , Mg 2+ , Ca 2+ , Na + , and Cl − ), similar to CASTNET's weekly measurements. CAPMoN also measures hourly air concentrations of O 3 , NO y , and gaseous Hg at selected sites. More details about the CAPMoN dataset can be found in Cheng and Zhang (2017).

Statistical analysis and methods
As we focus on the long-term trends of air pollutants over the region, annual means for all seasons and seasonal means for the warm and cold seasons were derived for each site from the weekly measurements of CASTNET and daily measurements of CAPMoN. To be precise, the seasonal mean concentrations in this study refer to the mean concentrations calculated for the warm (May-October) and cold (November-April) seasons for each year. In order to avoid the fluctuations of annual or seasonal mean concentrations due to meteorology, 3-year averages were used to represent the mean concentrations at the beginning or the end of a period in calculating changes for that period. The Mann-Kendall test (MKT) is a nonparametric test to detect the trend of a time series and it does not require the variable of the time series to follow a normal distribution (Mann, 1945;Du et al., 2014). In this study, the MKT was used to detect if an increasing or decreasing trend exists when a time series generally looks flat. The p value and tau coefficient are the two statistical parameters of the MKT, indicating the statistical significance and significance of a monotonic trend, respectively.
To assess the changes in air pollutants in response to emission reductions of SO 2 and NO x , we looked at the following for species of SO 2− 4 , SO 2 , NH + 4 , NO − 3 , HNO 3 , and TNO 3 (NO − 3 + HNO 3 , expressed as equivalent NO − 3 ): 1. temporal and spatial trends in the eastern US and Eastern Canada; 2. 10-year and 25-year changes for the periods of 1990-2000 and 1990-2015; 3. differences in trends in cold and warm seasons; 4. time series of the yearly regional means during the warm and cold seasons; and 5. long-term trends derived from polynomial regressions.
We also looked at correlations between SO 2− 4 and SO 2 , the ratio of sulfur (RSO 4 ) in SO 2− 4 to total sulfur in SO 2− 4 plus SO 2 in the air, the ratio of nitrogen (RNO 3 ) in NO − 3 to TNO 3 , and their changes during the period in order to explain the physical and chemical mechanisms responsible for the trends.

Region clustering of CASTNET and CAPMoN sites in the eastern US and Eastern Canada
In the eastern US (EUS) and Eastern Canada (EC), there are significant spatial differences in emissions of SO 2 , NO x , and NH 3 . This results in distinctive regional patterns of air concentrations of SO 2− 4 , NO − 3 , NH + 4 , HNO 3 , and SO 2 . In this study, we used the cold season (November to April) 3-year mean concentrations of NO − 3 and SO 2 of each site, supplemented with the ratio of RNO 3 , as the criteria to cluster the CASTNET and CAPMoN sites into four different regions. The reasons for selecting the cold season are the following: (1) NO − 3 is mainly in the form of NH 4 NO 3 (Zhang et al., 2008), and it is more thermodynamically stable in the cold season than in the warm season; (2) the oxidation rate of SO 2 is much lower in the cold season than in the warm season, and therefore the air concentration of SO 2 more reflects the SO 2 emission rate of the region; (3) because NH 4 NO 3 is much more thermodynamically stable and much less affected by ambient temperature, RNO 3 is mainly determined by the availability of NH 3 over the region, and therefore RNO 3 during the cold season is an indicator of the abundance of NH 3 to form NH 4 NO 3 . The mean concentrations at the beginning of the period were used to cluster the sites as the emission rate of SO 2 was the highest.
Based on the spatial patterns of the mean air concentration of NO − 3 , SO 2 , and RNO 3 during the cold season of 1989-1991, which are shown in Table S1b and Fig. S1b in the Supplement, four regions in the EUS and EC were clustered.
-Region 1: sites located north of latitude 40 • and with a concentration of SO 2 less than 6.4 µg m −3 in the cold season.
-Region 2: sites with a mean concentration of NO − 3 greater than 2.5 µg m −3 . Except for site ARE128 at 2.1 µg m −3 , the highest air concentration of NO − 3 of all other sites was 1.9 µg m −3 . For sites in region 2, RNO 3 was greater than 54 %, which was higher than any CASTNET and CAPMoN sites in other regions.
-Region 3: sites excluded from regions 1 and 2 and with an air concentration of SO 2 greater than 15.0 µg m −3 during the cold season.
-Region 4: all other sites excluded from regions 1, 2, and 3. The highest mean SO 2 of sites in region 4 during the cold season was less than 11.7 µg m −3 .
The clustering of sites is shown in Fig. 1

Air quality in the eastern US and Eastern Canada
at the beginning of the study period: 1989-1991 The 3-year averages of the air concentrations of SO 2− 4 , NO − 3 , NH + 4 , HNO 3 , SO 2 , and TNO 3 , as well as RSO 4 and RNO 3 for 1989-1991 are used to describe the air quality at the beginning of the study period and are shown in Table S1a and b for the warm and cold seasons. Mapping of 3-year average for each species is also provided as Fig. S1. Among  2 NO − 3 > 2.5 g m −3 and RNO 3 > 54.0 % for all sites; AVE_NO − 3 = 4.2 g m −3 ; AVE_SO 2 = 13.6 g m −3 ; AVE_RNO 3 = 68.5 %.

3
NO − 3 < 2.2 g m −3 , RNO 3 < 47 %, and SO 2 > 15.2 g m −3 for all sites; the four regions and both the warm and cold seasons, region 1 had the lowest air concentration of all species, with mean NO − 3 , HNO 3 , and NH + 4 concentrations of less than 1.0 µg m −3 . The mean air concentration of NO − 3 during the warm season was only 0.14 µg m −3 . Mean SO 2− 4 concentrations were 2.9 and 2.3 µg m −3 during the warm and cold seasons, respectively, and SO 2 was 1.6 and 3.6 µg m −3 correspondingly.
For regions 2-4, SO 2− 4 was highest in region 3 and lowest in region 4 for both seasons, varying from 7.6 to 8.2 µg m −3 during the warm season and 3.6 to 4.2 µg m −3 during the cold season. The difference in the regional mean of SO 2− 4 between region 2 and 3 was less than 0.1 µg m −3 during the cold season. Generally, SO 2− 4 in regions 2-4 was spatially uniform. For each region, SO 2− 4 during the warm season was about double that during the cold season. The same as SO 2− 4 , SO 2 was also highest in region 3 and lowest in region 4 for regions 2-4, but SO 2 in region 3 was much higher and was about 2.5 times that in region 4. SO 2 in regions 3 and 2 during the cold season, being 19.2 and 13.7 µg m −3 , respectively, showed the two highest concentrations and the only two concentrations greater than 10.0 µg m −3 among all species in four regions and during the warm and cold seasons. The difference in SO 2 between region 3 and 2 was less than 1.0 µg m −3 during the warm season but was more than 5.0 µg m −3 during the cold season. Despite significant differences in SO 2 for regions 2-4, the corresponding differences in SO 2− 4 were small. As an example, during the cold season, the seasonal mean concentration of SO 2 in region 3 was higher than that in region 4 by 10.0 µg m −3 , but the corresponding difference in SO 2− 4 was only 0.7 µg m −3 . This can be attributed to the fact that the lifetime of SO 2− 4 (∼ 5-7 d) in the air is much longer than that of SO 2 (∼ 2 d) (Penner et al., 2001;Pitari et al., 2016). Lee et al. (2011) estimated the mean SO 2 lifetime in the eastern US to be 19 ± 7 h in summer and 58±20 h in winter. Comparing SO 2 in the cold season to that in the warm season, it was about 2 times higher in regions 3 and 4 and 59 % higher in region 2. In contrast to the pattern of SO 2− 4 , NO − 3 in regions 2-4 was significantly different from region to region. Region 2 had the highest concentration of NO − 3 at 1.5 µg m −3 during the warm season and 3.6 µg m −3 during the cold season; these values were about triple the value in region 3, which was the second highest. Region 4 had the lowest NO − 3 concentrations among regions 2-4, being 0.3 and 0.6 µg m −3 for the warm and cold seasons, respectively. The lowest value for stations in region 2 during the cold season was 2.5 µg m −3 at ALH157, higher than the highest value of 2.1 µg m −3 at BEL116 in region 3. During the warm season, HNO 3 ranged from 1.7 µg m −3 in region 4 to 2.8 and 2.9 µg m −3 in regions 3 and 2. During the cold season, the highest concentration of HNO 3 was in region 3 and the lowest in region 2, with values of 2.3 and 1.8 µg m −3 , respectively. Considering both seasons, region 3 had the highest concentration of HNO 3 among the four regions. Region 2 had the lowest concentration of HNO 3 among regions 2-4 during the cold season due to the fact that a large portion of HNO 3 was neutralized by NH 3 to form NH 4 NO 3 . For TNO 3 , in both seasons, region 2 had the highest concentration, being 4.3 and 5.3 µg m −3 in the warm and cold seasons, mainly because of the significantly higher concentration of NO − 3 than other regions. TNO 3 was 3.2 and 3.5 µg m −3 for region 3 and 2.0 and 2.3 µg m −3 for region 4 during the warm and cold seasons. NH + 4 in regions 2-4 varied from 1.9 to 2.7 µg m −3 during the warm season and 1.1 to 2.3 µg m −3 during the cold season, with the highest concentrations in region 2 and the lowest concentrations in region 4 for both seasons. NH + 4 was higher in the warm season than in the cold season for all regions, as much more (NH 4 ) 2 SO 4 formed in the warm season than in the cold season.
In general, region 1 had the lowest concentration of all species among the four regions, and region 4 had the second lowest except HNO 3 , which was slightly more than in region 2 during the cold season. Regions 2 and 3 were the two most polluted regions in the EUS and ECA. Region 3 had the highest regional concentration of SO 2 in both seasons, more than double that in regions 1 and 4; region 2 had the highest concentration of NO − 3 and TNO 3 . In both seasons, NO − 3 in region 2 was more than 4 times higher than that in regions 1 and 4, and TNO 3 was more than double that in regions 1 and 4. Time series of seasonal mean concentrations of SO 2− 4 , SO 2 , NO − 3 , HNO 3 , TNO 3 , and NH + 4 for each region as well as each site of the region are shown in Fig. S3a for the cold season and Fig. S3b for the warm season. As an example, time series of regional averaged seasonal mean concentrations in regions 2 and 4 are shown in Fig. 2a and b for the cold and warm seasons. Time series of regional averages for regions 1-4 normalized to the year 2000 are presented in Fig. 3a [1988][1989][1990], which was followed by three relatively constant periods of 1990-1993, 1994-2001, and 2002-2007, with the averages of the seasonal mean concentrations of SO 2− 4 during the periods being 3.4, 3.1, and 2.8 µg m −3 , respectively. There were only two major drops between the periods, in 1994 and 2002.  ing period of 1995-2005. The annual reduction rates during the three periods were 0.14, 0.03, and 0.05 µg m −3 yr −1 in region 1; 0.16, 0.04, and 0.13 µg m −3 yr −1 in region 2; 0.15, 0.05, and 0.15 µg m −3 yr −1 in region 3; and 0.10, 0.04, and 0.14 µg m −3 yr −1 in region 4. The decreasing rates in regions 2 and 3 were close. If all sites within regions 1-4 were combined, the corresponding rates would be 0.14, 0.04, and 0.12 µg m −3 yr −1 for the three periods.
SO 2 in regions 2-4 during the cold season had a significant drop in 1989-1995, with a temporary increase in 1993 and 1994. From 1995 to 2005 the decreasing trend was slow, and then there was a very steep reduction from 2005 to 2012. The trend from 2012 to 2016 was relatively flat. The trend of SO 2 in region 1 was similar except that there was no obvious increase in 1993-1994. SO 2 exhibited an annual reduction rate of 0.28, 0.06, and 0.12 µg m −3 yr −1 in region 1; 0.83, 0.09, and 0.73 µg m −3 yr −1 in region 2; 1.13, 0.22, and 1.13 µg m −3 yr −1 in region 3; and 0.32, 0.08, and 0.49 µg m −3 yr −1 in region 4 for the periods of 1989-1995, 1995-2005, and 2005-2016. For regions 1-4 combined, the corresponding reduction rates were 0.72, 0.16, and 0.48 µg m −3 yr −1 . For the third period, if we only consider 2005-2012, the annual reduction rate was 0.61 µg m −3 yr −1 . Although the trend of SO 2 during 2012-2016 was generally flat, the decrease in SO 2− 4 during the period was still significant, especially in terms of percentage. in region 1 had a significant decrease from 1989 to 1996, followed by a leveling-off until 2005 and a decreasing trend from 2005 to 2016. After a significant peak in 1989, SO 2− 4 in region 2 had a large drop from 1989 to 1993. There was a steady decreasing trend for 1994-2004, followed by a significant peak in 2005, which was captured by all measurement sites within the region. This was followed by a smooth decreasing trend for 2005-2016, with major drops in 2005-2006, 2007-2009, 2012, and 2013-2016. Unlike region 2, SO 2− 4 in region 3 had no significant peak in 1989 and had a decreasing trend during 1989-1999. There was a levelingoff for 1999-2005, and it was followed by a significant decreasing trend for 2005-2016 with steep drops in 2005-2006-2009 1989-1995, 1995-2007, and 2007-2016. 3.2.2 NO − 3 , HNO 3 , and TNO 3 NO − 3 , HNO 3 , and TNO 3 during the cold season NO − 3 in EUS and EC during the cold season was dominated by NO − 3 in region 2, which was much higher than NO − 3 in other regions. As the trends of NO − 3 during the cold season were not as obvious as other species, the MKT was applied to detect if a monotonic (increasing or decreasing) trend existed. The MKT indicated the following: (1) NO − 3 in region 2 had no obvious trend in the period of 1989-2001 (p = 0.45, τ = 0.18), but there was a decreasing trend for the period 2001-2016 (p < 0.001, τ = −0.70); the trends of NO − 3 in regions 1, 3, and 4 over the whole study period were pretty flat (p = 0.17-0.40, |τ | < 0.2).
Excluding a peak of 1.9 µg m −3 in 1993, the trend of HNO 3 in region 2 in the 1990s is flat. From 2003 to 2013, there was a decreasing trend, then a slow increasing trend for 2013-2016 in region 2. HNO 3 in region 3 showed a general declining trend by 21.7 % (2004-2005 vs. 1989-1990) during 1989-2005. The declining trend of HNO 3 in the 1990s was consistent with the increasing trend of NO − during the period, as more HNO 3 was neutralized by additional NH 3 made available from decreasing SO 2− 4 . HNO 3 decreased markedly from 2005 to 2009 in region 3, then exhibited a slower decreasing trend for 2009-2016. HNO 3 in region 4 had an initial decrease from 1989 to 1991, then an increasing trend of 21.1 % (1999-2000 vs. 1991-1992) for HNO 3 dominated over NO − 3 in TNO 3 during the warm season for all regions, especially in regions 3 and 4 where the ratio of NO − 3 to TNO 3 was usually less than 20 %. Therefore, the trend of TNO 3 generally followed that of HNO 3 during the warm season. In the 1990s (1990-1999) the trend of TNO 3 in the warm season over regions 2 and 3 was very flat, and there was a very weakly increasing trend over region 4. The turning point of the trend was in 1999. For the period 1999-2009, all three regions showed significant de-creasing trends. For 2009-2016, the trends in the three regions were generally flat. As mentioned in Sect. 1, emissions of NO x changed little during the 1990s in the EUS. Correspondingly, the time series of TNO 3 in regions 2-4 during the 1990s did not decrease or even increased, as shown in Sect. 3.2. Also, some important metrics, such as RSO 4 during the cold season and RNO 3 during the warm and cold seasons, only started to have significant changes after the first 10 years. In order to capture how ambient air pollutants responded to emission reductions of SO 2 and NO x temporarily, 10 and 25 years of change in ambient SO 2− 4 , SO 2 , NH + 4 , NO − 3 , HNO 3 , and TNO 3 during 1990-2015 are presented in Tables 2 and 3 and are sum-

10 years of changes for the period 1990-2000
During the 10-year period of 1990-2015, air quality in the EUS and EC underwent a number of major changes, which are summarized as follows.
1. During the first 10 years, SO 2 concentrations declined in all regions and seasons by more than 25.0 % except for region 4, which had a reduction of 15.5 % during the warm season and 23.8 % during the cold season.
2. SO 2− 4 showed a similar but less significant decreasing trend as SO 2 . The reduction was more than 20 % in all regions except for region 4 during the cold season. Region 4 during the warm season had a similar reduction rate as region 3 despite the significant difference in the reduction rates of SO 2 in the two regions.
3. NO − 3 increased between 6.6 % and 40.0 % during the cold season for regions 1-4. Changes in NO − 3 during the warm season in regions 3 and 4 were very small and only had a significant reduction of 9.6 % in region 2. 4. TNO 3 increased little in region 1, by 0.09 and 0.02 µg m −3 for the cold and warm seasons, respectively. TNO 3 in regions 2 and 3 changed very little during the cold season and had a 9.4 % and 11.8 % reduction during the warm season. TNO 3 in region 4 increased by 3.9 % during the warm season and by 14.2 % during the cold season.
5. Except for a negligible change in region 4, NH + 4 decreased by 13.5 % to 22.8 % for regions 1-3 during the cold season; during warm season, it decreased by 12.0 % to 29.8 % for regions 1-4.

25 years of changes for the period 1990-2015
During the 25-year period of 1990-2015, air quality in the EUS and EC changed significantly and is summarized as follows.
1. Among all species, the most significant reduction during the period was for SO 2 . The reduction of SO 2 in regions 2-4 was similar in percentage, from 83.9 % in the warm season for region 4 to 91.2 % in the warm season for region 3. There were no major differences between the warm and cold seasons in terms of percentage reduction. In terms of absolute value, the biggest reduction was for SO 2 in region 3 during the cold season, and the 3-year-averaged seasonal mean concentration was reduced from 19.2 to 2.2 µg m −3 .
2. The reduction in SO 2− 4 concentrations during the cold season was relatively uniform in terms of percentage, ranging from 60.1 % in region 2 to 62.5 % in region 3. The reduction was more significant during the warm season than during the cold season, ranging from 72.7 % in region 1 to 78.7 % in region 4. The reductions in regions 2, 3, and 4 were similar in terms of values in both seasons. The reduction of SO 2− 4 in terms of percentage was much smaller than SO 2 in all regions during both seasons except for SO 2− 4 during the warm season in region 1.
3. During the warm season, the reduction of NO − 3 was seen in all four regions, ranging from 14.3 % and 15.6 % in regions 1 and 4 to 36.2 % and 57.5 % in regions 3 and 2. The reduction of NO − 3 during the cold season was only observed in region 2 (30.4 %). Although TNO 3 was reduced during the cold season in regions 3 and 4, a higher percentage of HNO 3 was converted to NO − 3 as more excess NH 3 was available to form NH 4 NO 3 due to the reduction of SO 2− 4 . As a result, the trend of NO − 3 in the two regions during the cold season changed very little. Unlike regions 3 and 4, region 2 did experience a significant reduction of NO − 3 in the cold season, following a 38.3 % reduction of TNO 3 . This can be explained as region 2 is an NH 3 -rich region. The formation of NH 4 NO 3 during the cold season in the region is less sensitive to the excess NH 3 made available from SO 2− 4 reduction than in regions 3 and 4. This can also be demonstrated by the least reduction of HNO 3 (in terms of percentage) in region 2 during the cold season as well as the correlations of RNO 3 vs. SO 2− 4 shown in Sect. 5.4.

The reduction of HNO 3 was similar in all four regions
during the warm season, ranging from 63.1 % to 68.8 %.
During the cold season, region 2 had the lowest percentage reduction at 56.0 %, and region 1 had the highest at 63.5 %. The reduction of HNO 3 can be through two paths: a reduction of NO x emissions and an increased neutralization of HNO 3 by more excess NH 3 due to less formation of (NH 4 ) 2 SO 4 and NH 4 HSO 4 . In terms of percentage, the reduction of HNO 3 was more significant than TNO 3 during the cold season, ranging from 14.4 % more in region 4 to 28.0 % more in region 1. 5. TNO 3 had a reduction rate ranging from 35.5 % for the cold season in region 1 to 64 % during the warm season in region 3. The reduction during the warm season was much greater than in the cold season, ranging from 11.4 % higher in region 4 to 23.9 % higher in region 3. The difference was partially due to extra reductions of NO x emissions from power plants and other large combustion sources during the ozone season (May-September) required by the NBP that began in 2003 and the Clean Air Interstate Rule that started in 2009(Napolitano et al., 2007Butler et al., 2011;Sickles II and Shadwick, 2015).
6. The reduction of NH + 4 was similar in regions 2, 3, and 4, ranging from 48.9 % to 53.2 % in the cold season and from 74.0 % to 75.7 % in the warm season. The reduction of NH + 4 during the warm season was more significant than in the cold season, over 20 % more in regions 2-4. The reduction of NH + 4 generally followed the trends of SO 2− 4 , but the reduction rate was much lower than that of SO 2− 4 during the cold season because a certain percentage of NH + 4 was associated with NO − 3 and the reduction of NO − 3 was not as significant as SO 2− 4 during the cold season. Region 2 exhibited the largest reduction of NH + 4 (75.7 %) during the warm season, contributed by a 76.8 % reduction of SO 2− 4 as well as a 57.5 % reduction of NO − 3 .
7. RSO 4 increased the most in region 3 during the cold season at 166.3 % and the least in region 1 during the warm season at 0.6 %. During the warm season, RSO 4 increased by 54.9 % and 58.4 % in regions 2 and 3, respectively. The increase in RSO 4 during the cold season was much higher than in the warm season in terms of percentage, ranging from 48.6 % in region 1 to 166.3 % in region 3. 8. RNO 3 increased significantly in regions 1, 3, and 4 in both seasons, ranging from 73.9 % to 94.9 %, but RNO 3 only increased by 8.0 % and 12.8 % in the warm and cold seasons for region 2, which was rich in NH 3 . Table 3, the following points apply for the whole region: (1) among the five species of SO 2− 4 , NO − 3 , NH + 4 , HNO 3 , and SO 2 , only SO 2− 4 and SO 2 still had regionally averaged annual mean concentrations exceeding 1.0 µg m −3 at the end of the study period.

As presented in
(2) SO 2− 4 was reduced by 73.3 % for the whole region during the study period, and it was reduced about 15 % more in the warm season than in the cold season in terms of percentage; (3) NH + 4 was reduced more in the warm season than in the cold season in terms of both percentage and absolute value; (4) NO − 3 was reduced by 29.1 % for the whole region. The reduction during the cold season occurred only in region 2, and the reduction during the warm season mainly occurred in regions 2 and 3. The reduction of NO − 3 for the whole region was mainly due to the reductions in region 2 during the warm and cold seasons; (5) RSO 4 increased by 97.7 % in the cold season, much higher than 26.2 % in the warm season. RSO 4 increased the most in region 3 during the cold season in terms of both absolute value and percentage.

Air quality at the end of the study period: 2014-2016
The 3-year-averaged air concentrations for 2014-2016 are used to describe the air quality at the end of the study period and are presented in Table S2 and Fig. S2. The air concentration mentioned in this section refers to the 3-year-averaged air concentration for each species, either for the warm or the cold season.
As at the beginning of the period, region 1 had the cleanest air among all regions, with the lowest air concentrations of less than 1.0 µg m −3 for all species and for both the warm and cold seasons. Unlike at the beginning of the period when SO 2− 4 during the warm season was about double that during the cold season in regions 2-4, SO 2− 4 at the end of the period had no significant differences between the two seasons. The air concentrations of SO 2− 4 were less than 2.0 µg m −3 in all regions and both seasons. For regions 2-4 the regional averages ranged from 1.6 to 1.8 µg m −3 during the warm season and from 1.4 to 1.7 µg m −3 during the cold season. SO 2 during the warm season was only from 0.6 to 1.0 µg m −3 for regions 2-4. In the cold season, SO 2 in regions 2 and 3 was the same at 2.2 µg m −3 and was only 1.1 µg m −3 in region 4. NH + 4 during the warm season varied from 0.5 to 0.7 µg m −3 for regions 2-4. During the cold season, it was 0.5 and 0.8 µg m −3 in regions 4 and 3, respectively, and it was much higher in region 2 with the value of 1.2 µg m −3 . The air concentration of NO − 3 during the warm season was very low in regions 3 and 4, with values of 0.3 µg m −3 , and it was doubled in region 2, being 0.6 µg m −3 . During the cold season, NO − 3 was much higher than during the warm season, being 2.5, 1.3, and 0.5 µg m −3 for regions 2, 3, and 4, respectively. HNO 3 in regions 2-4 varied from 0.6 µg m −3 in region 4 to 1.1 µg m −3 in region 2 during the warm season and from 0.7 µg m −3 in region 4 to 0.9 µg m −3 in region 3 during the cold season. There was little difference between the warm and cold seasons in regions 3 and 4. TNO 3 was the highest in region 2 in both seasons, being 1.7 and 3.2 µg m −3 for the warm and cold seasons, respectively. Region 3 had the second-highest TNO 3 with values of 1.2 and 2.1 µg m −3 for the warm and cold seasons, and the corresponding values for region 4 were 0.9 and 1.3 µg m −3 .
In summary, for species of SO 2− 4 , NO − 3 , NH + 4 , HNO 3 , and SO 2 , region 1 had air concentrations of less than 1.0 µg m −3 for all species in both seasons. For regions 2-4, NO − 3 was less than 1.0 µg m −3 for all regions and both seasons except regions 2 and 3 during the cold season, for which the air concentrations of NO − 3 were 2.5 and 1.3 µg m −3 , respectively; HNO 3 was less than 1.0 µg m −3 except region 2 during the warm season with a value of 1.1 µg m −3 ; NH + 4 was less than 1.0 µg m −3 for all regions except region 2 during the cold season at 1.2 µg m −3 ; SO 2− 4 was greater than 1.0 but less than 2.0 µg m −3 for regions 2-4 and both seasons; SO 2 was greater than 1.0 but less than 2.5 µg m −3 for regions 2-4 and both seasons, except regions 3 and 4 during the warm season, being 0.8 and 0.6 µg m −3 . Among the four regions, region 2 had the highest air concentration for all species except HNO 3 during the cold season. NO − 3 in region 2 was especially high, double the second-highest value in region 3 in both seasons. Also, NO − 3 in region 2 had the highest value (at 2.5 µg m −3 ) among all species in four regions and both seasons, although it significantly decreased from 3.6 µg m −3 at the beginning of the study period.

The long-term trends derived with polynomial regressions
Through trial and error, we found that polynomial regressions can reasonably describe the long-term trends of species at which it increased by 18.7 %. During the warm season, RSO 4 increased at all sites except for VPI120, ASH135, and WST109, at which it decreased by 25.0 %, 12.5 %, and 3.9 %, respectively. The most significant increase in RSO 4 was in region 3 during the cold season, with a regional average of 166.3 %. Figure 7 shows that RSO 4 increased linearly with the year for region 1 and quadratically for regions 2-4 for both sea-sons. RSO 4 increased significantly after 2005 in regions 2-4. Figure 8 shows the correlations of RSO 4 vs. SO 2 for regions 2-4, and it is clear that RSO 4 increased with the decrease in SO 2 . The increase in RSO 4 was relatively slow when the concentration of SO 2 was greater than 5 µg m −3 in the cold season and 7.5 µg m −3 in the warm season. RSO 4 soared when SO 2 was less than 5 µg m −3 in the cold season in regions 2-4 and less than 3 µg m −3 in the warm season  in regions 2 and 3. The increase in RSO 4 with the decrease in SO 2 can be explained as follows: (1)  (2) NH 3 was relatively unchanged during the period and even increased in some regions (Yao and Zhang, 2016). The decrease in SO 2 caused the decrease in H 2 SO 4 formation. Together this made cloud or rain droplets or snow particles less acidic, which was beneficial for the ox-idation of SO 2 by H 2 O 2 in the aqueous phase (Makar et al., 2009;Jones and Harrison, 2011). The disparity in the reduction of SO 2 and SO 2− 4 in response to emission reductions of SO 2 , namely the fact that the reduction rate of SO 2 was faster than SO 2− 4 , has been reported and discussed in some previous studies (Lövblad et al., 2004;Reid et al., 2001;Sickles II and Shadwick, 2015;Shah et al., 2018;Aas et al., 2019). The time series of the normalized regional concentrations of SO 2− 4 and SO 2 in Fig. 3 clearly show the disparity during the period of 1990-2015. The significant increase in RSO 4 during the period, especially during the cold season, explains why the reduction rate of SO 2 was much higher than that of SO 2− 4 . The reduction of SO 2 was not only due to the emission reductions, but also to the fact that a higher fraction of SO 2 was converted to SO 2− 4 . A faster reduction of SO 2 was observed for all four regions during the cold season, both before and after the year 2000, and it was more significant after 2000. This can be explained by the fact that the increase in RSO 4 with time was nonlinear. As shown in Table 2, in the first 10 years of the study period, the increase in RSO 4 was relatively limited. During the cold season, it only increased by 7.3 % in region 4 to 16.5 % in region 3. It was in the last 10 years from 2005 to 2015 when SO 2 was further reduced so that RSO 4 increased dramatically. As shown in Tables 2 and 3, during the cold season in region 3, RSO 4 only increased by 16.5 % in the first 10 years, 1990-2000, but it increased by 149.8 % for the last 15 years of 2000-2015. During the warm season, the disparity in the reduction between SO 2 and SO 2− 4 was much lower, as clearly shown in Fig. 3. This is because the increase in RSO 4 during the warm season was much less significant than during the cold season (Table 4). In the first 10 years, RSO 4 changed from −4.1 % in region 4 to 7.7 % in region 1. For the period of 1990-2015, RSO 4 only increased by 0.6 % and 12.4 % in regions 1 and 4. The disparity in the reduction rate of SO 2 vs. SO 2− 4 for these two regions was only 1.7 % and 5.2 % during the warm season, respectively. This is expected and can be explained as follows: (1) in the warm season more atmospheric oxidants are produced due to more solar photons being available than in the cold season, so the oxidation of SO 2 is less limited by the availability of atmospheric oxidants in the warm season; (2) in the cold season, limited atmospheric oxidants are available for the oxidation of SO 2 . The reduction of SO 2 emissions to the air will make more atmospheric oxidants available to each SO 2 molecule, increase the oxidation rate of SO 2 , and result in an increase in RSO 4 ; (3) furthermore, in the EUS, the seasonal mean O 3 concentration decreased in the warm season and increased in the cold season for the study period (Sickles II and Shadwick;. This made the overall oxidation capacity of the lower atmosphere in the EUS higher in the cold season and lower in the warm season. Figure 8. Correlations of the seasonal means during the cold and warm seasons: RSO 4 vs. SO 2 for regions 2-4. Fourth-order polynomial and quadratic regressions were applied for the cold and warm seasons, respectively. R2, 3, and 4 refer to regions 2-4. The dots with circles represent the seasonal means in the first and last 3 years.

Correlations of SO 2− 4 vs. SO 2
Correlations between SO 2− 4 and SO 2 are presented in Fig. 9 for regions 1-4 and for the warm and cold seasons. The SO 2− 4 -SO 2 relationships for the period of 1990-2010 can be described by linear regressions (not shown in the graph), with R = 0.87-0.98 during the warm season and R = 0.96-0.99 during the cold season. During the cold season, region 1 had the highest slope, and it was followed by regions 4, 2, and 3. During the warm season, the slopes for regions 1 and 4 were similar and were higher than the slopes for regions 2 and 3. A linear relationship between the seasonal mean concentrations of SO 2− 4 and SO 2 indicates that there is a linear relationship between the concentration of SO 2− 4 and the emission Figure 9. Correlations of the seasonal means during the cold and warm seasons: SO 2− 4 vs. SO 2 for regions 1-4. R1, 2, 3, and 4 refer to regions 1-4. of SO 2 . This is consistent with the relationship of the SO 2− 4 concentration and the SO 2 emission rate from the early 1990s through 2010 revealed in the study of Hand et al. (2012). As RSO 4 significantly increased when SO 2 was further reduced during 2010-2016, as seen in Fig. 8, the slopes of the linear regression for 2010-2016 were much higher than those for 1990-2010. A power-law regression, which bends a linear regression with a gentle slope to a linear regression with a steep slope, described the SO 2− 4 -SO 2 relationships very well, with R = 0.97-0.98 during the cold season and R = 0.94-0.99 during the warm season, as shown in Fig. 9. In some previous studies (e.g., Jones and Harrison, 2011), nonlinear power-law relationships have been found for observations at different sites for different seasons and periods. Our results indicate that a linear relationship between SO 2− 4 and SO 2 ex-ists for a subperiod of a long-term period, but generally the correlation of SO 2− 4 vs. SO 2 is a power-law relationship.

RNO 3
Similar to RSO 4 being a gas-particle partition indicator for sulfur in the air, RNO 3 is a metric indicating the fraction of gas HNO 3 that is aerosolized (Sickles II and Shadwick, 2015). In the air, the emitted NO x is oxidized to gas HNO 3 , which can be aerosolized through two paths: (1) reaction with NH 3 to form NH 4 NO 3 and (2) reaction with existing aerosols such as sea salts and crustal materials to form NaNO 3 , Ca(NO 3 ) 2 , and Mg(NO 3 ) 2 . The ratio is significantly sensitive to the air temperature, as NH 4 NO 3 , NH 3 , and HNO 3 in the air are in equilibrium and temperature changes can affect the partitioning between gas and particle phases (Doyle et al., 1979;Harrison and Pio, 1982). RNO 3 for 1989-1991 and 2014-2016, as well as the change in RNO 3 between the two periods are shown in Fig. S5. At the beginning of the period, (1) RNO 3 in the cold season was much higher than the warm season for all regions. RNO 3 in the cold season in regions 2 and 3 was more than double that for the warm season; (2) RNO 3 in region 2 was much higher than other regions and was more than double that in regions 3 and 4. For the 25-year period of 1990-2015, RNO 3 significantly increased by more than 70 % in regions 1, 3, and 4 during both seasons. In region 2, RNO 3 only increased by 12.8 % and 8.0 % during the cold and warm seasons, respectively. The significant increase in RNO 3 in regions 1, 3, and 4 can be attributed to the significant reduction of SO 2− 4 during the period, as is explained in Sect. 4.4. Figure 7 shows that RNO 3 had an increasing trend with the year for all regions and both seasons except for region 2 during the warm season. The trends can be described well by linear regressions in regions 1 and 2 and by quadratic regressions in regions 3 and 4. The linear regression shows that RNO 3 in region 2 had a decreasing trend for 1990-2010 during the warm season. The exact reason for this is unknown. One hypothesis is that due to the global warming trend in recent years and the significant reductions of sulfate and nitrate aerosols (which cool the atmosphere by reflecting more solar radiation back to space), the near-surface temperature in the Midwest had an increasing trend during the period of 1990-2010 (National Climate Assessment, 2014). As region 2 is rich in NH 3 , RNO 3 is more sensitive to the air temperature than to the availability of NH 3 . An increasing trend of air temperature in the warm season can cause a decreasing trend of RNO 3 .

Correlations of RNO 3 vs. SO 2− 4
Correlations between the seasonal mean RNO 3 and the seasonal mean concentration of SO 2− 4 for regions 2-4 and for the warm and cold seasons are presented in Fig. 10. For Figure 10. Correlations of the seasonal means during the cold and warm seasons: RNO 3 vs. SO 2− 4 for regions 2-4. The dots with circles represent the seasonal means in the first and last 3 years. NH 3 -rich region 2, RNO 3 increased slightly with the decrease in SO 2− 4 during the cold season, and there was no obvious trend during the warm season. For regions 3 and 4, which were NH 3 -limited, RNO 3 increased with the decrease in SO 2− 4 . RNO 3 increased steeply when the seasonal mean concentration of SO 2− 4 was less than 4 µg m −3 during the warm season and less than 3 µg m −3 during the cold season. The increase in RNO 3 with the decrease in SO 2− 4 can be explained as follows: (1) in regions 3 and 4, the formation of NH 4 NO 3 was limited by the availability of NH 3 ; (2) as SO 2− 4 decreased, some of the NH 3 previously forming (NH 4 ) 2 SO 4 / NH 4 HSO 4 was released and was available to react with HNO 3 to form NH 4 NO 3 . In contrast, RNO 3 was much less sensitive to the SO 2− 4 reduction in region 2 as the emissions of NH 3 there were much higher than in regions 3 and 4, as seen in Fig. S6. Thus, in general there was always

Summary and conclusion
With the implementation of the Title IV of the 1990 amendments to the CAA in the US in the 1990s, the emissions of SO 2 and NO x in the US were reduced from 23.1 million to 3.7 million t yr −1 for SO 2 and from 25.2 million to 11.5 mil-lion t yr −1 for NO x from 1990 to 2015. In Canada, compared to the emission level in 1990, SO 2 and NO x emissions in 2015 were reduced by 65 % and 25 %, respectively. In both the US and Canada, the reduction of emissions was mainly in the eastern regions of the countries. The air concentrations of gases SO 2 and HNO 3 , as well as particles SO 2− 4 , NO − 3 , and