The strategies to contain the spread of COVID-19 pandemic, including restricted human movement and economic activities, have shown positive impacts on the environment. Present research analysed the effects of COVID-19 led lockdown on air quality with special reference to major pollutants, namely nitrogen dioxide (NO₂), carbon monoxide (CO), sulphur dioxide (SO₂) and aerosol optical depth (AOD). The assessment has been conducted for megacities of India (Delhi, Mumbai, Bengaluru, Chennai and Kolkata) for four months, that is, March and April in 2019 and 2020 using Sentinel 5P and MCD19A2 data. A decrease in concentrations of air pollutants, specifically NO₂ and SO_2, has been observed during the lockdown period in all the cities; whereas CO and AOD have exhibited discrete pattern of spatio-temporal variation. Four megacities except Kolkata have revealed a positive correlation between NO₂ concentration and population density. The results conclude overall improvement in air quality during COVID-19 led lockdown. The current situation provides a unique opportunity to implement a structural economic change that could help us move towards a city with low emission economy. Realizing the achievable improvement of air quality, the study suggests further in-depth research on source attribution of individual pollutants to assess the prospect of emission reduction actions.

遏制 COVID-19 大流行蔓延的战略，包括限制人员流动和经济活动，已显示出对环境的积极影响。目前的研究分析了 COVID-19 导致的封锁对空气质量的影响，特别参考了主要污染物，即二氧化氮 (NO ₂ )、一氧化碳 (CO)、二氧化硫 (SO ₂ ) 和气溶胶光学深度 (AOD)。该评估已使用 Sentinel 5P 和 MCD19A2 数据对印度特大城市（德里、孟买、班加罗尔、钦奈和加尔各答）进行了为期四个月的评估，即 2019 年 3 月和 2020 年的 3 月和 4 月。空气污染物浓度降低，特别是 NO ₂和 SO _2，在所有城市的封锁期间都已观察到；而 CO 和 AOD 表现出离散的时空变化模式。_{除加尔各答外，四个特大城市揭示了 NO 2}浓度与人口密度之间的正相关关系。结果得出结论，在 COVID-19 导致的封锁期间空气质量总体改善。当前形势为实施结构性经济变革提供了独特的机会，这可以帮助我们迈向低排放经济的城市。鉴于空气质量可实现改善，该研究建议进一步深入研究个别污染物的来源归属，以评估减排行动的前景。