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This work presents the temporal and spatial characteristics of the major air pollutants and their associated health risks in China from 2019 to 2020, by using the monitoring data from 367 cities. The annual average PM 2.5 , PM 10 , NO 2 , SO 2 , CO, and O 3 concentrations decreased by 10.9%, 13.2%, 9.3%, 10.1%, 9.4%, and 5.5% from 2019 to 2020. National average PM 2.5 concentration in 2020 met the standard of 35 μg/m 3 , and that of O 3 decreased from 2019. COVID-19 lockdown affected NO 2 level dramatically, yet influences on PM 2.5 and O 3 were less clear-cut. Positive correlations between PM 2.5 and O 3 were found, even in winter in all five key regions, e.g., Jing-Jin-Ji (JJJ), FenWei Plain (FWP), Yangtze River Delta (YRD), Pearl River Delta (PRD) and Chengdu-Chongqing Region (CCR), indicating importance of secondary production for both PM 2.5 and O 3 . Large seasonal variability of PM 2.5 -SO 2 correlation indicates a varying role of SO 2 to PM 2.5 pollution in different seasons; and generally weak correlations in winter between PM 2.5 and NO 2 or SO 2 reveal the complexity of secondary formation processes to PM 2.5 pollution in winter. Multilinear regression analysis between PM 2.5 and SO 2 , NO 2 and CO demonstrates that PM 2.5 is more sensitive to the change of NO 2 than SO 2 in JJJ, FWP, PRD and CCR, suggesting a priority of NO x emission control for future PM 2.5 reduction. Furthermore, the new World Health Organization Air Quality Guidelines (WHO AQG2021) were adopted to calculate the excess health risks (ER) as well as the health-risk based air quality index (HAQI WHO ) of the pollutants. Such assessment points out the severity of air pollution associated health risks under strict standards: 40.0% of days had HAQI WHO >100, while only 14.4% days had AQI>100. PM 2.5  ER was generally larger than O 3 ER, but O 3 ER in low PM 2.5 region (PRD) and during summer became more serious. Notably, NO 2 ER became even more important than PM 2.5 due to its strict limit of WHO AQG2021. Overall, our results highlight the increasing importance of O 3 in both air quality evaluation and health risk assessment, and the importance of coordinated mitigation of multiple pollutants (mainly PM 2.5 , O 3 and NO 2 ) in protecting the public health., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

High-intensity emission controls were implemented in Nanjing and in 8 surrounding cities to ensure good air quality during the 2014 summer Youth Olympic Games (YOG). An Aerodyne soot-particle aerosol mass spectrometer (SP-AMS) was deployed at a downwind site of downtown Nanjing to investigate the chemical composition, sources, and size distribution of submicron aerosols (PM 1 ), in response to emission control policies. However, results show that emission controls played a negligible role in reducing PM 1 concentration during the YOG period, yet primary precursors such as NO x and SO 2 were decreased by 10-20%. Low wind speed, high relative humidity, and high ozone (O 3 ) concentration likely play a significant role in the production and accumulation of the oxygenated organic aerosol (OOA) and the secondary inorganic aerosols (SIA) in summer Nanjing. We propose that long-term regional emission reduction could be a solution for future air pollution mitigation strategies in downwind cities of the YRD region, and that seasonal meteorological characteristics in a specific region should be considered before emission control policies are made., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Based on a typical residential area, this paper studies the characteristics of pollutant concentration changes in two rainfall runoffs and the first flush effect of rainfall. In rainfall runoff, the concentrations of seven pollutants (COD Mn , TN, DTN, NH 3 -N, TP, DTP, and PO 4 3- ) increased during the initial rainfall period and decreased in the later period. Rainfall causes the erosion of pollutants on the underlying surface so that water pollution begins when rainfall runoff occurs, and the pollution level drops over time. The seven pollutants all experience this first flush effect, of which, rainfall has the strongest scouring effect on NH 3 -N produced by domestic sewage. The significant excess of pollutants in rainfall runoff should be considered by management departments. In addition, the existence of the first flush effect makes it possible in theory to partially intercept rainfall runoff to control water pollution, thereby reducing the cost of pollution control.

Ambient fine particles (PM 2.5 ) have been shown to have adverse health effects by inducing oxidative stress. Here, dithiothreitol (DTT)-based oxidative potential (OP) was used to assess the capacity of oxidative stress caused by PM 2.5 . In this study, PM 2.5 samples were collected in the Nanjing area in 2016, and physicochemical properties and DTT activity were investigated. The annual mean PM 2.5 mass concentration was 73 μg m -3 and greatly varied among seasons (spring > winter > summer > autumn). Three fluorescent substances were identified by the excitation-emission matrix (EEM) spectrum. The annual mean mass-normalized DTT activity (DTT m ; 0.02 nmol min -1 μg -1 ) was similar to that documented for cities of some developed countries. The annual mean volume-normalized DTT activity (DTT v ) showed a relatively high value of 1.16 nmol min -1 m -3 , and the seasonal mean DTT v was highest in winter, followed by spring, autumn, and summer, whose pattern is different from PM 2.5 mass concentration. Correlation and multiple linear regression analysis suggested that transition metals may have a greater effect on OP in autumn and winter, humic-like substances and UV absorbing aromatic substances may have a strong effect on OP in spring and summer. Generally, this study enhances our understanding of seasonal variation in health effects associated with PM 2.5 .

Air pollution events occurred frequently in China, and tremendous efforts were devoted to the reduction of air pollution in recent years. Here, analysis of ambient monitoring data of six criteria air pollutants from 367 Chinese cities during 2015-2018, showed that PM 2.5 , PM 10 , SO 2 and CO were reduced significantly by 22.1%, 13.5%, 46.4% and 21.5%, respectively, NO 2 reduction was less significant (6.3%) while O 3 level instead increased over China (13.7%). Spatial distribution, seasonal, monthly and diurnal variations of the air pollutants during 2018, implicated of effective control measures, were discussed in details, especially for the five key densely populated regions of Jing-Jin-Ji (JJJ), Fen Wei Plains (FWP), Yangtze River Delta (YRD), Sichuan Basin (SCB) and Pearl River Delta (PRD). Moreover, excess health risks (ERs) of the six pollutants were estimated for 2018, and such risks was two times higher if the World Health Organization (WHO) air quality guideline rather than Chinese guideline was adopted. PM 10 rather than PM 2.5 was the dominant contributor to ERs, and the case with both PM 2.5 and PM 10 exceeding threshold values occupied ~1/3 of total days, yet contributed ~2/3 of total ERs. For 2018, the health-risk based air quality index (HAQI) was further calculated by combining health risks from multiple pollutants, and it was found that high HAQI mostly distributed in North China Plain (NCP). ~15%, ~85% and ~95% people in YRD, FWP and JJJ were exposed to polluted air (HAQI > 100), and population-normalized HAQI further added the inequality, JJJ and a small region of SCB had much higher HAQI (>280). Investigations on HAQI with socioeconomic factors show that total population, population density and built-up area presented an inverted U-shape, suggesting existence of Environmental Kuznets Curve (EKC), while a positive relationship was found between HAQI and share of secondary industry. Multiple regression analysis suggested that built-up area was the most prominent factor to HAQI, followed by the gross domestic product (GDP). The findings here demonstrate in great details the current characteristics of air pollution and its associated health risks in China, therefore providing important implications for effective air pollution control strategies in near future for different regions of China., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The North China Plain and the Yangtze River Delta are the two of the most heavily polluted regions in China. Observational studies revealed that 'explosive' PM 2.5 mass concentration growths frequently occurred in the two regions. This study analyzed all the PM 2.5 mass concentration growth processes from clean condition (i.e., <35 μg m -3 ) to heavy pollution condition (i.e., >150 μg m -3 ) in Beijing (BJ) and Shanghai (SH), two representative cities of the two regions, using hourly monitored PM 2.5 concentrations during 2013-2016. 173 and 76 growth processes were identified in BJ and SH, respectively. PM 2.5 rising rates (PMRR) and dynamic growth durations were calculated to illustrate the characteristics of the growth processes. Hourly particulate chemical composition data and meteorological data in BJ and SH were further analyzed. The 4-year averaged PMRR of PM 2.5 total mass were similarly of 7.11 ± 9.82 μg m -3 h -1 in BJ and 6.71 ± 6.89 μg m -3 h -1 in SH. A decreasing trend was found for the PM 2.5 growth processes in two cities from 2013 to 2016, reflecting the effectiveness of emission controls implemented in the past years. The contributions of particulate components to the PM 2.5 total mass growth were different in BJ and SH. Average PMRR value of PM 1 organic aerosols (OA), SO 4 2- , NO 3 - , and NH 4 + in BJ was 1.90, 0.95, 0.82, and 0.53 μg m -3 h -1 , respectively. Average PMRR of PM 2.5 OA, SO 4 2- , NO 3 mass concentration growth processes in BJ and SH were proposed to be classified into 'other components-dominant growth processes', 'all components-contributing growth processes', 'one or more explosive secondary components-dominant growth processes', and 'mixed-factor growth processes'. Potential source contribution function analysis and the meteorological condition analysis showed that source origins and prevailing wind for the two cities during different categories of growth processes had substantial difference. The important source areas included Hebei and Shandong for BJ, and Jiangsu and Anhui for SH. The dominant wind directions during growth processes were northeast, south and southwest in BJ, and were west to north in SH. The results suggested the contributing components, the prevailing wind conditions, and the formation processes were substantially different in the two cities, despite the similar PMRR of PM - , and NH 4 mass concentration growth processes in the two cities.+ in SH was 1.70, 1.18, 1.99 and 1.14 μg m -3 h -1 , respectively. Based on the contributions of different components, the PM 2.5 mass concentration growth processes in BJ and SH were proposed to be classified into 'other components-dominant growth processes', 'all components-contributing growth processes', 'one or more explosive secondary components-dominant growth processes', and 'mixed-factor growth processes'. Potential source contribution function analysis and the meteorological condition analysis showed that source origins and prevailing wind for the two cities during different categories of growth processes had substantial difference. The important source areas included Hebei and Shandong for BJ, and Jiangsu and Anhui for SH. The dominant wind directions during growth processes were northeast, south and southwest in BJ, and were west to north in SH. The results suggested the contributing components, the prevailing wind conditions, and the formation processes were substantially different in the two cities, despite the similar PMRR of PM 2.5 total mass during the growth processes between BJ and SH. Future research is needed to study the detailed formation mechanisms of the different PM 2.5 mass concentration growth processes in the two cities., (Copyright © 2019 Elsevier Ltd. All rights reserved.)