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Most previous studies treat regional transport of aerosols as a whole, without distinguishing the transport of secondary aerosols and that of their precursors. A new method of quantifying the transport forms of secondary inorganic aerosols (SIA) using the Nested Air Quality Prediction Modeling System was proposed. The contribution of nonlocal emissions to SIA in the receptor region was divided into three parts: (1) SIA chemically formed by nonlocal emissions in their source regions; (2) SIA chemically formed by nonlocal emissions during transport; and (3) SIA chemically formed by nonlocal emissions in the receptor region, representing transport of precursors. In the North China Plain, the transport of precursors and SIA produced during transport are the two main transport forms. Furthermore, the contribution from transport of precursors increased under polluted conditions in most cities. The results indicate that joint control of precursors is paramount for mitigating air pollution. Plain Language Summary: Uncertainties still exist about the sources of secondary inorganic aerosols (SIA), which are one of the crucial drivers of haze pollution. The spatial distribution of precursors is not exactly consistent with that of aerosols, which is puzzling to policymakers. Most existing studies only treat contribution from regional transport of nonlocal emissions as a whole but do not distinguish the transport of secondary aerosols themselves and that of their precursors. Therefore, it is necessary to quantify the contribution from transport of precursors and secondary aerosols. The Nested Air Quality Prediction Model System with an online tracer‐tagging module was used to investigate the regional sources of SIA in Beijing and surrounding cities. The regional contribution of nonlocal emissions to SIA in the receptor region was divided into three parts: (1) SIA chemically formed by nonlocal emissions in their source region; (2) SIA chemically formed by nonlocal emissions in transport pathway; and (3) SIA chemically formed by nonlocal emissions in the receptor region, indicating transport of precursors. The results showed that transport of precursors and the SIA produced during transport are two main transport forms. The results strongly suggest that the joint control of precursors is important for mitigating air pollution. Key Points: A new method of quantifying the transport forms of secondary inorganic aerosols (SIA) using an air quality model was proposedThe transported SIA in the receptor region is from conversion of nonlocal emissions in the source region, transport path, and receptor regionThe chemical conversion of nonlocal emissions after they leave their source region is crucial in the North China Plain [ABSTRACT FROM AUTHOR]

To investigate the characteristics of particulate matter with an aerodynamic diameter less than 2.5 μm (PM 2.5) and its chemical compositions in the Beijing-Tianjin-Hebei (BTH) region of China during the novel coronavirus disease (COVID-19) lockdown, the ground-based data of PM 2.5 , trace gases, water-soluble inorganic ions, and organic and elemental carbon were analyzed in three typical cities (Beijing, Tianjin, and Baoding) in the BTH region of China from 5-15 February 2020. The PM 2.5 source apportionment was established by combining the weather research and forecasting model and comprehensive air quality model with extensions (WRF-CAMx). The results showed that the maximum daily PM 2.5 concentration reached the heavy pollution level (>150 μg/m3) in the above three cities. The sum concentration of SO 4 2−, NO 3 − and NH 4 + played a dominant position in PM 2.5 chemical compositions of Beijing, Tianjin, and Baoding; secondary transformation of gaseous pollutants contributed significantly to PM 2.5 generation, and the secondary transformation was enhanced as the increased PM 2.5 concentrations. The results of WRF-CAMx showed obviously inter-transport of PM 2.5 in the BTH region; the contribution of transportation source decreased significantly than previous reports in Beijing, Tianjin, and Baoding during the COVID-19 lockdown; but the contribution of industrial and residential emission sources increased significantly with the increase of PM 2.5 concentration, and industry emission sources contributed the most to PM 2.5 concentrations. Therefore, control policies should be devoted to reducing industrial emissions and regional joint control strategies to mitigate haze pollution. [Display omitted] [ABSTRACT FROM AUTHOR]

Air quality in Beijing-Tianjin-Hebei and its surrounding regions has been significantly improved during the Three-Year Action Plan for Winning the Battle Against Air Pollution from 2018 to 2020. The PM 2.5 concentration can be significantly influenced by meteorological conditions, and the effectiveness of anthropogenic emissions reduction on PM 2.5 over the North China Plain remains unclear. In this study, the Nested Air Quality Model System (NAQPMS) and observations were used to quantitatively examine the contribution of meteorological and emission variations to autumn-winter (November to February) PM 2.5 concentrations in Beijing and its surrounding regions from 2018 to 2020. The model reproduced the temporal evolution of atmospheric pollutants and components well. The results showed that the observed mean PM 2.5 concentrations during the winter were reduced by 15% and 29% in 2019 and 2020, respectively, compared with that in 2018. In 2019, the meteorological conditions over the Beijing-Tianjin-Hebei (BTH) region were poor, causing an increase of PM 2.5 by 7.6% in BTH and 11% in Beijing; therefore, the control of air pollutant emission compensated for the unfavorable influence of meteorology and emission reduction was the decisive factor for PM 2.5 reduction; In 2020, the meteorological conditions were relatively favorable, and the changes in meteorological factors and emission reduction led to a decrease in PM 2.5 concentration by 18.6% and 10.5%, respectively, compared to 2018. Emission reduction also contributed to decrease in nitrate, ammonium, organic matter, element carbon and precursors. This work confirmed the obvious environmental benefit of pollution control measures from 2018 to 2020. There was obvious regional heterogeneity in the effect of meteorology and the impact of adverse meteorological conditions should be considered during the formulation of air pollution control measures. The results can serve as a reference for the formulation of air quality objectives or the evaluation of the environmental effect of pollution control schemes. • The effect of winter meteorology and control measures in 2018 to 2020 was assessed. • Observed winter PM 2.5 reduced by 15% and 29% in 2019 and 2020, respectively, compared to 2018. • Unfavorable meteorology in 2019 partially counteracted the emission control effects. • Emission mitigation played an important role in air quality improvement. [ABSTRACT FROM AUTHOR]

A three-dimensional air quality model coupled with an on-line source-tagged module was adopted to investigate the source-receptor relationship in North China during the winter of 2017 and 2018. The total contribution of cities located along transport pathway (the "2 + 26" cities) accounts for most of the fine particulate matter (PM 2.5) across this region both on average and during haze episodes. Regional joint prevention and control strategies are crucial to both routine and emergency environmental governance. Compared to primary PM 2.5 , secondary compounds may be transported across greater distances and impose notable effects in downstream areas. In regard to the secondary inorganic aerosols (SIA) in Beijing during haze episodes, regional transport plays a more important role than do local emissions. The contribution of the transport of particle-phase secondary pollutants is comparable to that of the transport of gas-phase precursors during haze episodes. Control measurements of gas-phase precursors in upstream areas are also important, and they benefit the reduction in the total PM 2.5 level. [Display omitted] • "2+26" cities contribute more than 80% of the PM 2.5 along the transport pathways. • Local contribution dominates the primary PM 2.5 level. • Nonlocal emissions contribute more than local emissions in SIA levels. • Regional transport of particle-phase SIA and gas-phase precursors are comparable. [ABSTRACT FROM AUTHOR]

Haze pollution occurs frequently over northern China Plain (NCP) during winter, resulting in great threats to human health. So far, limited studies have linked between emission sources and health risks from ambient field measurements. In this work, PM 2.5 samples were collected in Beijing in the 2018 winter; 23 metallic elements were determined to assess the health risks induced by PM-bound metals. During the sampling period, metallic elements constituted a minor fraction (5%) in PM 2.5 , but exhibited significant threats to human health. The cancer risk (CR) value induced by PM-bound metals was 1.41 × 10−4 for adults and 2.59 × 10−4 for children while non-cancer risk (NCR) was 2.59 for adults and 3.37 for children. Nickel (Ni), As and Cr(IV) were the major contributing elements to carcinogenic risk. Arsenic (As) contributed a major (57%) of NCR for children, and Mn (30%), Ni (27%) and As (24%) were the major contributing species to NCR for adults. Using positive matrix factorization (PMF) model, traffic-related emission (25% for children and 30% for adults) and heavy oil combustion (39% for children and 28% for adults) were the dominant sources of CR values. Moreover, the CR value induced by local emissions was higher than that by long-range transported air pollution. Local traffic emission and long-range transported heavy oil combustion enhanced CR values, threatening the human health. As results, we highlighted that control of traffic emissions and heavy oil combustion on local and regional scales should be seriously considered by the Chinese government in reduction of health risk in Beijing. Image 1 • The CR value was 1.41 × 10−4 for adults and 2.59 × 10−4 for children while NCR value was 2.59 for adults and 3.37 for children. • Traffic-related emission and heavy oil combustion were the predominant sources of CR in Beijing. • Control of local traffic emission and long-range transported heavy oil combustion is an important way to reduce health risk. [ABSTRACT FROM AUTHOR]