Lockdown measures to contain COVID-19 pandemic has resulted in a considerable change in air pollution worldwide. We estimate the temporal and diurnal changes of the six criteria air pollutants, including particulate matter (PM 2.5 and PM 10 ) and gaseous pollutants (NO 2 , O 3 , CO, and SO 2 ) during lockdown (25 th March – 3 rd May 2020) across regions of India using the observations from 134 real-time monitoring sites of Central Pollution Control Board (CPCB). Significant reduction in PM 2.5 , PM 10 , NO 2, and CO has been found in all the regions during the lockdown. SO 2 showed mixed behavior, with a slight increase at some sites but a comparatively significant decrease at other locations. O 3 also showed a mixed variation with a mild increase in IGP and a decrease in the South. The absolute decrease in PM 2.5 , PM 10, and NO 2 was observed during peak morning traffic hours (08–10 Hrs) and late evening (20–24 Hrs), but the percentage reduction is almost constant throughout the day. A significant decrease in day-time O 3 has been found over Indo Gangetic plain (IGP) and central India, whereas night-time O 3 has increased over IGP due to less O 3 loss. The most significant reduction (∼40–60%) was found in PM 2.5 and PM 10 . The highest decrease in PM was found for the north-west and IGP followed by South and central regions. A considerable reduction (∼30–70%) in NO 2 was found except for a few sites in the central region. A similar pattern was observed for CO having a ∼20–40% reduction. The reduction observed for PM 2.5 , PM 10 , NO 2, and enhancement in O 3 was proportional to the population density. Delhi’s air quality has improved with a significant reduction in primary pollutants, however, an increase in O 3 was observed. The changes reported during the lockdown are combined effect of changes in the emissions, meteorology, and atmospheric chemistry that requires detailed investigations.

The World Health Organization has declared the COVID-19 pandemic a global public health emergency. Many countries of the world, including India, closed their borders and imposed a nationwide lockdown. In India, the lockdown was declared on March 24 for 21 days (March 25-April 14, 2020) and was later extended until May 3, 2020. During the lockdown, all major anthropogenic activities, which contribute to atmospheric pollution (such as industries, vehicles, and businesses), were restricted. The current study examines the impact of the lockdown on tropospheric NO 2 concentrations. Satellite-based ozone monitoring instrument sensor data were analyzed in order to investigate the variations in tropospheric NO 2 concentrations. The results showed that from March 1 to 21, 2020, the average tropospheric NO 2 concentration was 214.4 Â10 13 molecule cm À2 over India, and it subsequently decreased by 12.1% over the next four weeks. An increase of 0.8% in tropospheric NO 2 concentrations was observed for the same period in 2019 and hence, the reduced tropospheric NO 2 concentrations can be attributed to restricted anthropogenic activities during the lockdown. In the absence of significant activities, the contribution of various sources was estimated, and the emissions from biomass burning were identified as a major source of tropospheric NO 2 during the lockdown. The findings of this study provide an opportunity to understand the mechanism of tropospheric NO 2 emissions over India, in order to improve air quality modeling and management strategies.

Fine particulate matter (PM 2.5 ) is the leading environmental risk factor that requires regular monitoring and analysis for effective air quality management. This work presents the variability, trend, and exceedance analysis of PM 2.5 measured at US Embassy and Consulate in five Indian megacities (Chennai, Kolkata, Hyderabad, Mumbai, and New Delhi) for six years (2014–2019). Among all cities, Delhi is found to be the most polluted city followed by Kolkata, Mumbai, Hyderabad, and Chennai. The trend analysis for six years for five megacities suggests a statistically significant decreasing trend ranging from 1.5 to 4.19 μg/m 3 (2%–8%) per year. Distinct diurnal, seasonal, and monthly variations are observed in the five cities due to the different site locations and local meteorology. All cities show the highest and lowest concentrations in the winter and monsoon months respectively except for Chennai which observed the lowest levels in April. All the cities consistently show morning peaks (~08: 00–10:00 h) and the lowest level in late afternoon hours (~15:00–16:00 h). We found that the PM 2.5 levels in the cities exceed WHO standards and Indian NAAQS for 50% and 33% of days in a year except for Chennai. Delhi is found to have more than 200 days of exceedances in a year and experiences an average 15 number of episodes per year when the level exceeds the Indian NAAQS. The trends in the exceedance with a varying threshold (20–380 μg/m 3 ) suggest that not only is the annual mean PM 2.5 decreasing in Delhi but also the number of exceedances is decreasing. This decrease can be attributed to the recent policies and regulations implemented in Delhi and other cities for the abatement of air pollution. However, stricter compliance of the National Clean Air Program (NCAP) policies can further accelerate the reduction of the pollution levels.