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Aggressive air pollution control in China since 2013 has achieved sharp decreases in fine particulate matter (PM 2.5 ), along with increased ozone (O 3 ) concentrations. Due to the pandemic of coronavirus disease 2019 (COVID-19), China imposed nationwide restriction, leading to large reductions in economic activities and associated emissions. In particular, large decreases were found in nitrogen oxides (NO x ) emissions (>50%) from transportation. However, O 3 increased in the Yangtze River Delta (YRD), which cannot be fully explained by changes in NO x and volatile organic compound (VOCs) emissions. In this study, the Community Multi-scale Air Quality model was used to investigate O 3 increase in the YRD. Our results show a significant increase of atmospheric oxidation capacity (AOC) indicated by enhanced oxidants levels (up to +25%) especially in southern Jiangsu, Shanghai and northern Zhejiang, inducing the elevated O 3 during lockdown. Moreover, net P(HO x ) of 0.4 to 1.6 ppb h -1 during lockdown (Case 2) was larger than the case without lockdown (Case 1), mainly resulting in the enhanced AOC and higher O 3 production rate (+12%). This comprehensive analysis improves our understanding on AOC and associated O 3 formation, which helps to design effective strategies to control O 3 ., Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest., (Copyright © 2020. Published by Elsevier B.V.)

The Guangdong government implemented lockdown measures on January 23, 2020, to ease the spread of the coronavirus disease 2019 (COVID-19). These measures prohibit a series of human activities and lead to a great reduction in anthropogenic emissions. Starting on February 20, all companies resumed work and production, and emissions gradually recovered. To investigate the response of air pollutants in the Pearl River Delta (PRD) to the emission reduction and recovery related to COVID-19 lockdown, we used the Community Multi-scale Air Quality (CMAQ) model to estimate the changes in air pollutants, including three periods: Period I (January 10 to January 22, 2020), Period II (January 23 to February 19, 2020), Period III (February 20 to March 9, 2020). During Period II, under the concurrent influence of emissions and meteorology, air quality improved significantly with PM 2.5 , NO 2 , and SO 2 decreased by 52%, 67%, and 25%, respectively. O 3 had no obvious changes in most cities, which mainly due to the synergetic effects of emissions and meteorology. In Period III, with the recovery of emissions and the changes in meteorology, the increase of secondary components was faster than that of primary PM 2.5 (PPM), which indicated that changes in PPM concentration were more sensitive to emissions reduction. O 3 concentration increased as emission and temperature rising. Our findings elucidate that more effective emission control strategies should be implemented in PRD to alleviate the increasingly serious pollution situation., Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Economic activities and the associated emissions have significantly declined during the 2019 novel coronavirus (COVID‐19) pandemic, which has created a natural experiment to assess the impact of the emitted precursor control policy on ozone (O3) pollution. In this study, we utilized comprehensive satellite, ground‐level observations, and source‐oriented chemical transport modeling to investigate the O3 variations during the COVID‐19 pandemic in China. Here, we found that the significant elevated O3 in the North China Plain (40%) and Yangtze River Delta (35%) were mainly attributed to the enhanced atmospheric oxidation capacity (AOC) in these regions, associated with the meteorology and emission reduction during lockdown. Besides, O3 formation regimes shifted from VOC‐limited regimes to NOx‐limited and transition regimes with the decline of NOx during lockdown. We suggest that future O3 control policies should comprehensively consider the effects of AOC on the O3 elevation and coordinated regulations of the O3 precursor emissions. Plain Language Summary: Severe ozone (O3) pollution has been a public concern in China during the last decade. Due to the 2019 novel coronavirus (COVID‐19) pandemic, China has implemented the strict restrictions, leading to the significant decline of economic activities and the associated emissions. However, the O3 levels have been elevated in vast areas of China during the COVID‐19 lockdown period. In this study, we applied the comprehensive satellite, ground‐level observations, and source‐oriented chemical transport modeling to investigate the O3 elevation during the COVID‐19 lockdown periods. Importantly, our results showed that O3 has increased significantly in North China Plain (40%) and Yangtze River Delta (35%) during lockdown, which attributed to the enhanced atmospheric oxidation capacity (AOC) associated with the meteorology and emission reduction. Besides, we found the O3 formation regimes shifted from VOC‐limited to NOx‐limited due to the lower NOx during lockdown. In the future, we suggest that O3 control policies ought to ensure a balance between emitted NOx and VOCs to maintain stable O3 formation regimes and AOC levels and thereby control O3 emissions. Key Points: O3 formation regime shift from a VOC‐limited to a NOx‐limited regime has been found in most areas of ChinaEnhanced atmospheric oxidation capacity (AOC) contributed to O3 elevation during COVID‐19 lockdownO3 control policies should consider from the comprehensive sights of O3 formation regime and AOC in the future [ABSTRACT FROM AUTHOR]

Within a short time after the outbreak of coronavirus disease 2019 (COVID-19) in Wuhan, Hubei, the Chinese government introduced a nationwide lockdown to prevent the spread of the pandemic. The quarantine measures have significantly decreased the anthropogenic activities, thus improving air quality. To study the impacts caused by the lockdown on specific source sectors and regions in the Yangtze River Delta (YRD), the Community Multiscale Air Quality (CMAQ) model was used to investigate the changes in source contributions to fine particulate matter (PM 2.5) from 23 January to 28 February 2020, based on different emission control cases. Compared to case 1 (without emission reductions), the total PM 2.5 mass for case 2 (with emission reductions) decreased by more than 20 % over the entire YRD, and the reduction ratios of its components were 15 %, 16 %, 20 %, 43 %, 34 %, and 35 % in primary organic aerosol (POA), elemental carbon (EC), sulfate, nitrate, ammonium, and secondary organic aerosol (SOA), respectively. The source apportionment results showed that PM 2.5 concentrations from transportation decreased by 40 %, while PM 2.5 concentrations from the residential and power sectors decreased by less than 10 % due to the lockdown. Although all sources decreased, the relative contribution changed differently. Contributions from the residential sector increased by more than 10 % to 35 %, while those in the industrial sector decreased by 33 %. Considering regional transport, the total PM 2.5 mass of all regions decreased 20 %–30 % in the YRD, with the largest decreased value of 5.0 µgm-3 in Henan, Hebei, Beijing, and Tianjin (Ha-BTH). In Shanghai, the lower contributions from local emissions and regional transmission (mainly Shandong and Ha-BTH) led to the reduced PM 2.5. This study suggests adjustments of control measures for various sources and regions. [ABSTRACT FROM AUTHOR]

To mitigate the impacts of the pandemic of coronavirus disease 2019 (COVID-19), the Indian government implemented lockdown measures on 24 March 2020, which prohibited unnecessary anthropogenic activities, thus leading to a significant reduction in emissions. To investigate the impacts of this lockdown measure on air quality in India, we used the Community Multi-Scale Air Quality (CMAQ) model to estimate the changes of key air pollutants. From pre-lockdown to lockdown periods, improved air quality is observed in India, indicated by the lower key pollutant levels such as PM 2.5 (- 26 %), maximum daily 8 h average ozone (MDA8 O 3) (- 11 %), NO 2 (- 50 %), and SO 2 (- 14 %). In addition, changes in these pollutants show distinct spatial variations with the more important decrease in northern and western India. During the lockdown, our results illustrate that such emission reductions play a positive role in the improvement of air quality. Significant reductions of PM 2.5 concentration and its major components are predicted, especially for secondary inorganic aerosols that are up to 92 %, 57 %, and 79 % for nitrate (NO 3-), sulfate (SO 42-), and ammonium (NH 4+), respectively. On average, the MDA8 O 3 also decreases 15 % during the lockdown period although it increases slightly in some VOC-limited urban locations, which is mainly due to the more significant reduction of NO x than VOCs. More aggressive and localized emission control strategies should be implemented in India to mitigate air pollution in the future. [ABSTRACT FROM AUTHOR]

Due to the pandemic of coronavirus disease 2019 in China, almost all avoidable activities in China are prohibited since Wuhan announced lockdown on January 23, 2020. With reduced activities, severe air pollution events still occurred in the North China Plain, causing discussions regarding why severe air pollution was not avoided. The Community Multi-scale Air Quality model was applied during January 01 to February 12, 2020 to study PM 2.5 changes under emission reduction scenarios. The estimated emission reduction case (Case 3) better reproduced PM 2.5. Compared with the case without emission change (Case 1), Case 3 predicted that PM 2.5 concentrations decreased by up to 20% with absolute decreases of 5.35, 6.37, 9.23, 10.25, 10.30, 12.14, 12.75, 14.41, 18.00 and 30.79 μg/m3 in Guangzhou, Shanghai, Beijing, Shijiazhuang, Tianjin, Jinan, Taiyuan, Xi'an, Zhengzhou, Wuhan, respectively. In high-pollution days with PM 2.5 greater than 75 μg/m3, the reductions of PM 2.5 in Case 3 were 7.78, 9.51, 11.38, 13.42, 13.64, 14.15, 14.42, 16.95 and 22.08 μg/m3 in Shanghai, Jinan, Shijiazhuang, Beijing, Taiyuan, Xi'an, Tianjin, Zhengzhou and Wuhan, respectively. The reductions in emissions of PM 2.5 precursors were ~2 times of that in concentrations, indicating that meteorology was unfavorable during simulation episode. A further analysis shows that benefits of emission reductions were overwhelmed by adverse meteorology and severe air pollution events were not avoided. This study highlights that large emissions reduction in transportation and slight reduction in industrial would not help avoid severe air pollution in China, especially when meteorology is unfavorable. More efforts should be made to completely avoid severe air pollution. Image, graphical abstract [ABSTRACT FROM AUTHOR]