Impact of urbanization on air quality in the Yangtze River Delta during the COVID-19 lockdown in China

Abstract The urban agglomeration of Yangtze River Delta (YRD) is symbol of China's rapid urbanization during the past decades. Urbanization can significantly impact land-cover properties, surface heating, and emissions of air pollutants. To control the spread of COVID-19, China imposed very rigorous restrictions, leading to dramatic reductions in air pollutants (except O3) from satellite and ground-based data. As such, inter-transportation of air pollutants was weak during the lockdown, which was conducive to discuss the impacts of urbanization on the air quality. During the lockdown, the rates of surface PM2.5, PM10, SO2, NO2 and CO reductions in different urban types ranged from 6.6% to 62.4% in the YRD. Urbanization exerted great impacts on the pollutant variations in urban agglomerations despite such large decreases in primary pollution in YRD. Lower values of AOD and tropospheric NO2 columns were noticeably observed over large cities during the lockdown. The extents of surface PM, SO2, NO2 and CO reductions in large cities (first-tier and second-tier) were found to be larger (4.7%-10.6%) than those in small-medium cities (third-tier and fourth-tier) during the lockdown, which was also the case for the extent of the increase (33.0% - 53.0%) in O3. PCA analysis revealed that the PM decreases in large cities made greater improvement in the air quality compared with the small cities during lockdown, while the urbanization had non-obvious influence on the photochemical reactions. It is imperative to adopt policies and programs to mitigate the air pollution in urban agglomerations in the fast urbanization process.