Keywords

1 Introduction

COVID-19, a novel coronavirus, was initially discovered in December 2019 and has since spread around the globe (Prakash et al., 2021; Verma & Kamyotra, 2021). Public isolation and lockdown resulted in a considerable reduction in global air pollution emissions (Bao & Zhang, 2020; Pal et al., 2021; Yadav et al., 2020).

This research looks at how air pollutant levels changed in India's six megacities based on ground and space-derived data during the COVID-19 shutdown compared to the previous years under typical conditions.

2 Research Area and Data Sources

The study area counts six megacities of India and time-averaged maps of Aerosol Optical Depth (AOD) at 500 nm, daily 1° [OMI OMAERUVd v003], and at 55 nm (Deep Blue, Land-only) daily 1° [MODIS-Aqua MYD08_D3 v6.1] were plotted on NASA's Giovanni during 2010–2020. A dataset spanning five primary types of air contaminants, including PM2.5, PM10, NO2, SO2, and O3, makes up the air quality data obtained from Central Pollution Control Board (CPCB). Meteorological data were obtained from www.worldweatheronline.com.

3 Results

3.1 Differences in the Concentrations of Aerosols and Trace Gases

Table 1 shows the difference in AOD concentrations in the megacities, and estimates from the OMI satellite and the MODIS model revealed a considerable decrease in column AOD. A mean reduction in AOD in all the megacities is −0.42%, as estimated by OMI and −0.06% by MODIS. The alteration in pollutants in lockdown was compared to the mean daily pollutants of earlier accessible years to present a comparative measurement of relative improvement in comparison to pre-lockdown circumstances (2018–2019, all through megacities).

Table 1 Difference in AOD concentrations in six megacities of India during lockdown (March 25, 2020–May 17, 2020) and before lockdown during the same period from 2010 to 2019
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Pollutant emissions in the atmosphere such as PM2.5, PM10, NO2, SO2, and O3 changed during the lockdown phase up to −71%, NA, −66%, −35%, −13% in Ahmedabad, −40%, −51%, −72%, 52%, 32% in Bangalore, −56%, −65%, −79%, −34%, 59% in Delhi, −48%, −46%, −44%, 78%, 100% in Hyderabad, −41%, NA, −69%, −28%, 9% in Kanpur, and 28%, −18%, −51%, 51%, −20% in Mumbai, respectively. Figure 1 shows the plots of mean pollutant concentration data during the pandemic year (March 25–May 17) in 2020 and during the same period in 2018–2019. PM2.5 and PM10 showed a reduction in all the megacities except in Mumbai, where PM2.5 levels increased up to 28%.

Fig. 1
figure 1

Impact of lockdown on air pollution levels during two situations, i.e., during lockdown (March 25, 2020–May 17, 2020) and mean concentrations before lockdown during the same period in 2018–2019

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3.2 Influence of Meteorology

An overall 7.2% temperature decrease is observed in megacities during lockdown, which may be due to the minimization of human activities and vehicular movement (Pal et al., 2021). A slight increase in wind speed of up to 6% is noticed in the cities except in Kanpur, and in other towns where wind speed is increased during the lockdown period, which might have helped in the improved scattering of pollutants. The 4% increase in RH in the cities, except Kanpur, is the reason behind the pollutant concentrations’ decrease (Fig. 2).

Fig. 2
figure 2

Percentage changes in meteorological parameters, including temperature, wind speed, and relative humidity in the selected megacities of India

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4 Discussion

The key atmospheric pollutants AOD, PM2.5, PM10, NO2, SO2, and O3 declined during the lockdown phase in the selected megacities compared to previous years’ data. Overall change in mean pollutant concentrations including PM2.5, PM10, NO2, SO2, and O3 including all the cities is −38%, −45%, −64%, 14%, and 35%, respectively. In cities like Delhi, vehicular emissions are one of the emitters of PM2.5; hence, lockdown minimizes the PM2.5 levels (Shrestha et al., 2020). NO2 concentrations were reduced in all the areas, which may be due to a pause in vehicular and transportation activities (Pal et al., 2021). SO2 concentrations increased in Bangalore, Hyderabad, and Mumbai from biomass burning and industrial emissions that may not have been turned down (Bar et al., 2021; Shrestha et al., 2020). O3 emissions often follow the reverse trend as the other trace gases in all regions except in Ahmedabad and Mumbai. The established tight controls have resulted in a significant reduction in vehicle movement and the closure of certain industries, resulting in an improvement in air quality. Such a significant drop in pollution levels is expected to improve public health and prevent health issues and mortality rates linked to air pollution.

5 Conclusions

The research examined six key air contaminants to see how air quality changed after the COVID-19 pandemic curtailed human activity in 6 Indian megacities from March 25, 2020–May 17, 2020, and the same period during 2010–2019. Almost all the contaminants, excluding SO2 and O3, were reduced during the lockdown period. Spatial and ground-based data affirm that general air pollution levels have been greatly influenced following the pandemic. Reduced pollution levels have occurred due to the closure of industrial activities and a decline in vehicular movement during the lockdown phase. According to the study, strict enforcement of air pollution regulations could be an alternate tool for pollution control in industrial and transportation-dominated areas.