Comparing the Change in Air Quality during the COVID-19 Lockdown between Dry and Wet Seasons in Nepal

In Nepal, a South Asian country located in the central Himalayan region, a countrywide lockdown was imposed from 24 March to 20 July 2020 to contain the spread of the SARS-CoV-2 virus during the first wave of the novel coronavirus disease (COVID-19) pandemic. This study used the rare incidence of countrywide lockdown to investigate the air quality change in Nepal and its topographically unique urban center, the Kathmandu Valley, during the lockdown period, segregated by dry (pre-monsoon) and wet (monsoon) seasons, based on satellite remote sensing and ground-based air quality monitoring data. Our analysis showed a remarkable improvement in air quality during the lockdown in the dry season over the country. The mean aerosol optical depth (AOD), nitrogen dioxide (NO2), and carbon monoxide (CO) levels over the entire country decreased by 27.7%, 12.7%, and 5.12%, respectively, compared to the pre-pandemic levels. Likewise, in the Kathmandu Valley, PM2.5 (particulate matter with diameter ??? 2.5 ??m), AOD, NO2, and CO levels decreased by 38.1%, 38.0%, 16.5%, and 6.03%, respectively, during the dry season segment of the lockdown. It is worth noting that the change in AOD and NO2 levels was notably higher in the Kathmandu Valley than in the entire country. However, during the wet season segment of the lockdown, relatively subtle changes in AOD (???7.41%), NO2 (???6.87%), and CO (???2.80%) levels were observed over the country. Since the lockdown restricted people's mobility and operation of many industries, it might have reduced emissions from transport and industrial sectors. Therefore, our findings provide insights into the potential improvement in air quality that could be achieved by controlling emissions from those sectors and can be useful in formulating urban air quality management strategies.