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Stringent emission controls during the Asia Pacific Economic Cooperation Summit (APEC; November 5–11, 2014) provide a valuable opportunity to examine the impact of such measures on the chemical properties of PM 2.5 and other air pollutants. Here, we measured the water-soluble inorganic ions (WSII) and carbonaceous species in PM 2.5 , NH 3 and NO 2 at multiple sites in Beijing between September and November 2014. Relative to the pre-APEC period (September and October 2014), significant reductions in the average concentrations of WSII (69% for NO 3 −, 68% for SO 4 2−, 78% for NH 4 +, and 29–71% for other species), elemental carbon (EC, 43%) and organic carbon (OC, 45%) in PM 2.5 were found during the APEC period. The contributions of secondary inorganic ions (SIA, including SO 4 2−, NO 3 −, and NH 4 +) to PM 2.5 were significantly lower during the APEC period (9–44%), indicating a combination of lower gaseous precursor emissions and a relative weak secondary aerosol formation. Ion-balance calculations indicated that the PM 2.5 sample in the pre-APEC period was alkaline but was acidic during the APEC period. Relatively lower mean concentrations of EC (1.5 μg m−3), OC (10.5 μg m−3), secondary organic carbon (SOC, 3.3 μg m−3), secondary organic aerosol (SOA, 5.9 μg m−3) and primary organic aerosol (POA, 10.0 μg m−3) appeared during the APEC period. The average concentrations of NH 3 and NO 2 at all road sites were significantly reduced by 48 and 60% during the APEC period, which is consistent with clear reductions in satellite NH 3 columns over Beijing city in the same period. This finding suggests that reducing traffic emissions could be a feasible method to control urban NH 3 pollution. During the APEC period, concentrations of PM 2. 5 , PM 10 , NO 2 , SO 2 and CO from the Beijing city monitoring network showed significant reductions at urban (20–60%) and rural (18–57%) sites, whereas O 3 concentrations increased significantly (by 93% and 53%, respectively). The control measures taken in the APEC period substantially decreased PM 2.5 pollution but can increase ground O 3 , which also merits attention. • Concentrations of PM 2.5 and components reduced significantly during the APEC period. • Major components in PM 2.5 and other pollutants were comparable with other cities. • Vehicle restrictions could lower atmospheric NH 3 concentration over Beijing. • Emission control measures significantly increased ground-level O 3 concentrations. [ABSTRACT FROM AUTHOR]

The Clean Air Action implemented by the Chinese government in 2013 has greatly improved air quality in the North China Plain (NCP). In this work, we report changes in the chemical components of atmospheric fine particulate matter (PM 2.5) at four NCP sampling sites from 2012/2013 to 2017 to investigate the impacts and drivers of the Clean Air Action on aerosol chemistry, especially for secondary inorganic aerosols (SIA). During the observation period, the concentrations of PM 2.5 and its chemical components (especially SIA, organic carbon (OC), and elemental carbon (EC)) and the frequency of polluted days (daily PM 2.5 concentration ≥ 75 μg m−3) in the NCP, declined significantly at all four sites. Asynchronized reduction in SIA components (large decreases in SO 4 2− with stable or even increased NO 3 − and NH 4 +) was observed in urban Beijing, revealing a shift of the primary form of SIA, which suggested the fractions of NO 3 − increased more rapidly than SO 4 2− during PM 2.5 pollution episodes, especially in 2016 and 2017. In addition, unexpected increases in the sulfur oxidation ratio (SOR) and the nitrogen oxidation ratio (NOR) were observed among sites and across years in the substantially decreased PM 2.5 levels. They were largely determined by secondary aerosol precursors (i.e. decreased SO 2 and NO 2), photochemical oxidants (e.g. increased O 3), temperature, and relative humidity via gas-phase and heterogeneous reactions. Our results not only highlight the effectiveness of the Action Plan for improving air quality in the NCP, but also suggest an increasing importance of SIA in determining PM 2.5 concentration and composition. [Display omitted] • PM 2.5 and its chemical components were reduced in North China Plain during 2012–2017. • Reduced air pollutant emissions contributed more to PM 2.5 decline than meteorology. • NO 3 − became increasingly important in the formation of secondary inorganic aerosols. • Both sulfur oxidation ratio and nitrogen oxidation ratio increased unexpectedly. [ABSTRACT FROM AUTHOR]

Atmospheric reactive nitrogen (Nr) emissions and deposition have caused much damage to the global environment and also impacted human health. Determining long-term trends in atmospheric Nr concentrations and deposition is important for evaluating air quality and ecosystem effects and formulating control measures to reduce negative impacts. We measured monthly concentrations of NH 3 , pNH 4 +, HNO 3 , pNO 3 -, and NO 2 and calculated their dry deposition rates using an inferential method from 2011 to 2018 at an urban site in Zhengzhou in the North China Plain. Inorganic N (NH 4 + and NO 3 −) concentrations in precipitation and wet/bulk N deposition were also quantified in 2011, 2016, 2017 and 2018. The results showed high concentrations of atmospheric Nr species, with considerable variation in some species over the monitoring period. Annual mean NH 3 concentrations significantly increased from 2011 to 2018 (p < 0.001) with that in 2018 being twice that in 2011. However, pNO 3 - and pNH 4 + concentrations decreased after 2013. The annual mean NO 2 concentration in 2018 had decreased by 33.4% compared to the peak in 2011. Highest NH 3 concentrations were measured in spring or summer, and lowest concentrations in winter. pNH 4 + and pNO 3 - concentrations peaked mainly in winter. Total annual dry N deposition ranged from 21.9 to 41.2 kg N ha−1. Bulk NH 4 + and NO 3 − deposition was lower in 2016–2018 than in 2011. However, atmospheric Nr concentrations and N deposition remained high in this urban ecosystem. NH 3 emissions urgently need control measures, and Nr impacts on the surrounding ecosystems and human health should be considered. Continuous long-term measurements of atmospheric Nr concentrations and deposition at a regional scale are necessary to help formulate emission control measures and protect human and ecosystem health. • Significant changes in atmospheric reactive nitrogen were observed at an urban site in north China. • NH 3 concentrations increased by 110% from 2011 to 2018. • pNO 3 - and pNH 4 + concentrations in 2018 decreased by 65.1% and 62.8% compared to the maxima in 2012. • As a consequence, N deposition slightly decreased, with some improvement in air quality. [ABSTRACT FROM AUTHOR]

This study detected the long-term trends in NO 2 concentrations in China from 1997 to 2016 based on the NO 2 columns from GOME, SCIAMACHY, and GOME-2A. Both differences in the time-overlapped NO 2 columns from GOME vs. SCIAMACHAY and SCIAMACHAY vs. GOME-2A showed seasonal variations, and the annual NO 2 columns from GOME were 0.9% higher than those from SCIAMACHY, which exceeded that from GOME-2A by 14%. The long-term trends of the NO 2 columns on a provincial scale could be simulated by cubic models (0.60 < R 2  < 0.96, p < 0.05) and presented three shapes: first decreasing then increasing and decreasing again; first decreasing then increasing; and continuously decreasing. The peak years of NO 2 columns in 17 provinces occurred in 2011 and 2012. These trends in NO 2 columns were determined by the economic developments and enacted air quality policies in nearly all the provinces except for Xizang and Qinghai Provinces, where the trends were determined by natural NO x emission sources. In detail, the panel data analysis showed that the simulated model had fixed effects, and the thermal power generation, consumption of diesel oil in agriculture, passenger traffic by highways, and freight traffic by highways significantly increased NO 2 , while the air quality policies in the 12th five-year plan decreased NO 2 columns from 1997 to 2016. The benefits to decreasing NO 2 columns from the air quality policies issued in the 10th and 11th five-year plans were offset by the quickly increasing economies. [ABSTRACT FROM AUTHOR]

In order to cope with heavy haze pollution in Beijing, the Air Pollution Prevention and Control Action Plan was issued in 2013. In this study, we took January observations from 2014 to 2018 as a case study to assess wintertime air quality improvement in Beijing after five years of continuous emission control measures and to investigate the causes of haze formation. SO 2 and NO 2 concentrations in January 2018 decreased by 84% and 41% from January 2014, while NH 3 concentrations remained relatively stable during these years with an average of 7.28 μg m−3. The mean concentrations of PM 2.5 and water-soluble inorganic ions declined over the study period, while compared with 2014, decreased SO 4 2− ratio and increased NO 3 − contribution in PM 2.5 were observed, highlighting the importance of reactive nitrogen (Nr) pollution control. High concentrations of NH 3 , NO 2 , as well as NH 4 + and NO 3 − in PM 2.5 at roadside monitoring sites, indicated that vehicle sources played a vital role in local urban Nr emissions. Finally, we found most NH 3 existed in the gas phase in the Beijing atmosphere, even on polluted winter days, suggesting a considerable risk for additional secondary aerosol formation. Our findings demonstrate the effectiveness of the emissions control in alleviating Beijing's air pollution. Significant reductions of precursor emissions are needed to achieve the goals set by air quality standards in the future, especially for NH 3. It is especially crucial to jointly consider and control multiple pollution sources to effectively reduce the high levels of air pollution that remain in Beijing. • Significant improvements in Beijing air quality were observed during January 2014 to 2018. • The concentrations of SO 2 , NO 2 in air and SO 4 2−, NO 3 −, NH 4 + in PM 2.5 significantly decreased. • NH 3 concentrations remained stable during these 5 years. • Vehicle sources are vital contributors to NH 3 and NO 2 emissions. • The Beijing atmosphere is NH 3 -rich. [ABSTRACT FROM AUTHOR]

Few related studies have been carried out for northwest China's the inland, although the increasing inorganic reactive nitrogen (Nr) deposition and severe air pollution in China have attracted considerable attention. Based on eight-year measurements of the concentrations of NH 3 , HNO 3 , particulate (p) NH 4 +, pNO 3 −, and NO 2 in the air and inorganic nitrogen (NH 4 +, NO 3 −) in the precipitation at a rural site in Wuwei, Gansu Province, we quantified the dry and bulk/wet Nr deposition fluxes and analyzed their temporal trends in combination with air pollution data (2015–2018). The concentrations of the single Nr species in the air did not exhibit significant seasonal variations, except for that of pNO 3 − which were the highest in summer and the lowest in winter. The average annual total Nr (dry + bulk) deposition was 27.2 ± 6.7 kg N ha−1 with a significant (p <.05) annual increase rate of 10%, which was dominated by the dry deposition flux (20.8 ± 7.1 kg N ha−1 yr−1) during the entire period. The NH 4 +-N and NO 3 −-N deposition fluxes in the precipitation were significantly (p <.05) higher in the summer than in the other three seasons owing to the heavy rainfall. The reduced N (NH 3 , pNH 4 +, and NH 4 +) deposition flux was 4.7 times that of the oxidized N (HNO 3 , pNO 3 −, NO 2 , and NO 3 −), which increased with a high annual rate (25%). The annual mean concentrations of the main air pollutants (PM 2.5 , PM 10 , SO 2 , NO 2 , and CO) significantly decreased by 15%–76%; in contrast, increasing trends of the O 3 and NH 3 concentrations were observed. Our results highlight the importance of the control of NH 3 emissions and O 3 pollution to comprehensively mitigate N deposition and improve air quality in the considered region. • The total nitrogen deposition increased at an annual rate of 10% from 2010 to 2018. • Nitrogen deposition has become an important nitrogen source to the agroecosystem. • Air pollutants significantly decreased by 15%–76%, except for O 3 and NH 3. • NH 3 concentrations increased at an annual rate of 25% from 2010 to 2018. • Control of NH 3 emission and O 3 pollution is imperative to improve air quality. [ABSTRACT FROM AUTHOR]

China has the largest reactive nitrogen deposition worldwide. Understanding the change of nitrogen deposition and its socioeconomic drivers in China is crucial for the function and sustainability of regional and global ecosystems. Here we presented satellite-derived nitrogen deposition constrained with the national ground-based measurements over China. We found that oxidized nitrogen deposition peaked at 7.0 kg N ha−1 in 2012, then decreased to 6.2 kg N ha−1 in 2017. In contrast, reduced nitrogen deposition increased continuously, and reached 12.6 kg N ha−1 in 2017. The Chinese Clean Air Act is responsible for the reduction of oxidized nitrogen emissions, while there is no current effective control policy for ammonia emissions in place resulting in reduced nitrogen deposition reaching levels of approximately twice that of oxidized nitrogen in 2017. The ratio of reduced to oxidized nitrogen deposition decreased from 1.30 in 2008 to 1.08 in 2011, while this ratio began to increase from 1.08 in 2011 to 1.56 in 2017. This suggests reduced nitrogen deposition would become more important in the air quality and their ecological consequences in the future. The total nitrogen deposition in China was about 3–4 times that in the US and Europe. The policy of controlling nitrogen pollution is urgent, especially for reducing ammonia emissions, while ensuring the sustainable production of agriculture and maintaining food security. Image 1 • Methods were developed to obtain N deposition using satellite observations. • The ratio of reduced to oxidized N deposition in China increased since 2011. • N deposition in China was about 3–4 times that in the US and Europe. [ABSTRACT FROM AUTHOR]