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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.

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 PM2.5 (−26 %), maximum daily 8 h average ozone (MDA8 O3) (−11 %), NO2 (−50 %), and SO2 (−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 PM2.5 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 O3 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 NOx than VOCs. More aggressive and localized emission control strategies should be implemented in India to mitigate air pollution in the future.

To mitigate the impacts of the pandemic of coronavirus disease 2019 (COVID-19), the Indian government implemented lockdown measures on March 24, 2020, which prohibit unnecessary anthropogenic activities and thus leading to a significant reduction in emissions. To investigate the impacts of this lockdown measures 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 and its major components are observed especially for secondary inorganic aerosols with the decreasing rates up to 92 %, 57 %, and 79 % for nitrate (NO 3 − ), sulfate (SO 4 2− ), ammonium (NH 4 + ), respectively. On average, the MDA8 O 3 also decreases 15 % during the lockdown period although it increases sparsely in some urban locations, which is mainly due to the lower NO x and VOCs emissions. More aggressive and localized emissions control strategies should be implemented in India to mitigate air pollutions in the future.

Beijing is one of the most important Chinese megacities with extremely serious air pollution problems and human health impacts. In response to the air pollution the central and municipal governments of China have implemented a series of actions; one of which is the "Action Plan for Comprehensive Prevention and Control of Autumn and Winter Air Pollution in Beijing-Tianjin-Hebei and Surrounding Areas 2017–2018" (the Action Plan) issued in 2017. The morphology, composition and mixing state of individual particles collected after the Action Plan was implemented were analyzed by high resolution-transmission electron microscopy coupled with an energy-dispersive X-ray spectrometer (TEM-EDX). The relative percentages of different individual particle types and the main sources of the particulate pollution before and after the Action Plan were compared. The results showed that sulfur was most frequently detected in the individual particles, and the particle types were mainly composed of soot aggregates, mineral particles, organic particles, metal particles, coal fly ashes, sulfate particles, and mixture particles. The mixture and sulfate particles dominated in the autumn samples, both for the haze and non-haze days. In winter the mineral and mixture particles dominated in samples for the non-haze days, while mixture particles and sulfate dominated in the samples for the haze days. The mixture particles in autumn were mainly the soot aggregates internally mixed with sulfate (S-soot type), while the mixture particles in winter were mainly the S-soot type and the mineral particles internally mixed with sulfate (S-mineral type). After the Action Plan, the relative percentages of sulfate particles, organic particles, and soot aggregates increased, while the relative percentages of mineral particles, metal particles and fly ashes decreased. The contribution from coal-fired sources was reduced significantly as evidenced by the decrease in the fly ash particles. The vehicle emissions and secondary reaction of particulate matter became the main sources of atmospheric particulate matters as evidenced by increase in sulfate particles, organic particles, and soot aggregates. The results presented in this study will assist in other plans to improve air quality and human health in megacities around the world.