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Nitrate (NO 3-) has been the dominant and the least reduced chemical component of fine particulate matter (PM 2.5) since the stringent emission controls implemented in China in 2013. The formation pathways of NO 3- vary seasonally and differ substantially in daytime vs. nighttime. They are affected by precursor emissions, atmospheric oxidation capacity, and meteorological conditions. Understanding NO 3- formation pathways provides insights for the design of effective emission control strategies to mitigate NO 3- pollution. In this study, the Community Multiscale Air Quality (CMAQ) model was applied to investigate the impact of regional transport, predominant physical processes, and different formation pathways to NO 3- and total nitrate (TNO 3 , i.e., HNO 3+ NO 3-) production in the Yangtze River Delta (YRD) region during the four seasons of 2017. NO 3-/ PM 2.5 and NO 3-/ TNO 3 are the highest in the winter, reaching 21 % and 94 %, respectively. The adjusted gas ratio (adjGR = ([NH 3]+ [NO 3- ])/ ([HNO 3]+ [NO 3- ])) in the YRD is generally greater than 2 in the four seasons across most areas in the YRD, indicating that YRD is mostly in the NH 3 -rich regime and that NO 3- is limited by HNO 3 formation. Local emissions and regional transportation contribute to NO 3- concentrations throughout the YRD region by 50 %–62 % and 38 %–50 %, respectively. The majority of the regional transport of NO 3- concentrations is contributed by indirect transport (i.e., NO 3- formed by transported precursors reacting with local precursors). Aerosol (AERO, including condensation, coagulation, new particle formation, and aerosol growth) processes are the dominant source of NO 3- formation. In summer, NO 3- formation is dominated by AERO and total transport (TRAN, sum of horizontal and vertical transport) processes. The OH + NO 2 pathway contributes to 60 %–83 % of the TNO 3 production, and the N 2 O 5 heterogeneous (HET N 2 O 5) pathway contributes to 10 %–36 % in the YRD region. HET N 2 O 5 contribution becomes more important in cold seasons than warm seasons. Within the planetary boundary layer in Shanghai, the TNO 3 production is dominated by the OH + NO 2 pathway during the day (98 %) in the summer and spring and by the HET N 2 O 5 pathway during the night (61 %) in the winter. Local contributions dominate the OH + NO 2 pathway for TNO 3 production during the day, while indirect transport dominates the HET N 2 O 5 pathway at night. [ABSTRACT FROM AUTHOR]

The assessment of premature mortality associated with the dramatic changes in fine particulate matter (PM 2.5) and ozone (O 3) has important scientific significance and provides valuable information for future emission control strategies. Exposure data are particularly vital but may cause great uncertainty in health burden assessments. This study, for the first time, used six methods to generate the concentration data of PM 2.5 and O 3 in China between 2014 and 2018, and then quantified the changes in premature mortality due to PM 2.5 and O 3 using the Environmental Benefits Mapping and Analysis Program-Community Edition (BenMAP-CE) model. The results show that PM 2.5 -related premature mortality in China decreases by 263 (95% confidence interval (CI95): 142–159) to 308 (CI95: 213–241) thousands from 2014 to 2018 by using different concentration data, while O 3 -related premature mortality increases by 67 (CI95: 26–104) to 103 (CI95: 40–163) thousands. The estimated mean changes are up to 40% different for the PM 2.5 -related mortality, and up to 30% for the O 3 -related mortality if different exposure data are chosen. The most significant difference due to the exposure data is found in the areas with a population density of around 103 people/km2, mostly located in Central China, for both PM 2.5 and O 3. Our results demonstrate that the exposure data source significantly affects mortality estimations and should thus be carefully considered in health burden assessments. [Display omitted] • Differences in exposure data led to 40% difference in PM 2.5 -related mortality changes. • Differences in exposure data led to 30% difference in O 3 -related mortality changes. • The difference in mortality changes was up to a factor 2–3 in Central China. • Areas with a population density of 103 people/km2 had the most significant difference. [ABSTRACT FROM AUTHOR]

PM 2.5 and O 3 controls are traditionally considered separately because PM 2.5 is usually high in winter while O 3 is generally high in summer. In this study, we explore the opportunity of controlling the two pollutants simultaneously through a better understanding of their intra-seasonal correlation and chemical-coupling behaviors under different meteorological conditions during the late summer and early fall (August–September) episodes in the San Joaquin Valley (SJV), California. A correlation analysis is first used to identify the temporal correlations between O 3 and PM 2.5 and their underlying physical and chemical drivers. Sensitivity analysis is then applied to determine the chemical coupling between PM 2.5 and O 3 and subsequent multipollutant control opportunities under two contrasting meteorological conditions using the Community Multi-Scale Air Quality (CMAQ) model. We find that O 3 and PM 2.5 are positively correlated on the daily timescale because both are sensitive to atmospheric stagnation. However, O 3 and PM 2.5 are negatively correlated on the hourly timescale determined by the negative correlation between hourly NO 3 − and O 3 , which is mainly due to the opposite effects of T and RH on the diurnal variations of NO 3 − and O 3. Reducing NO x on average lead to O 3 increase, but it can facilitate reducing O 3 at higher O 3 (>75 ppb) locations under the more stagnant conditions. NO x emission control could become beneficial for both O 3 and PM 2.5 when the NO x emissions in 2005 are further reduced by 15% under the more stagnant meteorological conditions and by 30% under the more ventilated meteorological conditions. [Display omitted] • High PM2.5 and O3 tend to occur together in SJV under stagnant conditions in late summer and early fall. • O3 and PM2.5 are positively correlated on the daily timescale, but negatively correlated on the hourly timescale. • Negative correlation between hourly PM2.5 and O3 is mainly driven by the opposite effects of T and RH. • NOx control can effectively reduce PM2.5, but on average has a disbenefit effect to O3 at the high PM locations. • Further 30% NOx reductions from 2005 emission level can become beneficial for both O3 and PM2.5 in SJV. [ABSTRACT FROM AUTHOR]

• 178 nitrate modeling studies in China published during 2007–2021 were reviewed. • Model performance on nitrate and possible reasons for model-observation biases were summarized. • The contribution of N 2 O 5 heterogeneous chemistry to nitrate varies from 21.0% to 51.6% among studies. • Decreased SO 2 emission, enhanced AOC, and weakened deposition account for increasing PM 2.5 nitrate fraction. • Multiple-pollutant control strategies involving NH 3 , NO x , and VOCs are needed to mitigate nitrate pollution. Particulate nitrate (pNO 3) is now becoming the principal component of PM 2.5 during severe winter haze episodes in many cities of China. To gain a comprehensive understanding of the key factors controlling pNO 3 formation and driving its trends, we reviewed the recent pNO 3 modeling studies which mainly focused on the formation mechanism and recent trends of pNO 3 as well as its responses to emission controls in China. The results indicate that although recent chemical transport models (CTMs) can reasonably capture the spatial–temporal variations of pNO 3 , model-observation biases still exist due to large uncertainties in the parameterization of dinitrogen pentoxide (N 2 O 5) uptake and ammonia (NH 3) emissions, insufficient heterogeneous reaction mechanism, and the predicted low sulfate concentrations in current CTMs. The heterogeneous hydrolysis of N 2 O 5 dominates nocturnal pNO 3 formation, however, the contribution to total pNO 3 varies among studies, ranging from 21.0% to 51.6%. Moreover, the continuously increasing PM 2.5 pNO 3 fraction in recent years is mainly due to the decreased sulfur dioxide emissions, the enhanced atmospheric oxidation capacity (AOC), and the weakened nitrate deposition. Reducing NH 3 emissions is found to be the most effective control strategy for mitigating pNO 3 pollution in China. This review suggests that more field measurements are needed to constrain the parameterization of heterogeneous N 2 O 5 and nitrogen dioxide (NO 2) uptake. Future studies are also needed to quantify the relationships of pNO 3 to AOC, O 3 , NOx, and volatile organic compounds (VOCs) in different regions of China under different meteorological conditions. Research on multiple-pollutant control strategies involving NH 3 , NO X, and VOCs is required to mitigate pNO 3 pollution, especially during severe winter haze events. [ABSTRACT FROM AUTHOR]