Spatial-temporal variations and influencing factors of air quality in China's major cities during COVID-19 lockdown.

To control the spread of COVID-19, the Chinese government announced a "lockdown" policy, and the citizens' activities were restricted. This study selected three standard air quality indexes, AQI, PM2.5, and PM10, of 2017-2021 in 40 major cities in six regions in China to analyze their changes, spatial-temporal distributions, and socioeconomic influencing factors. Compared with 2019, the values of AQI, PM2.5, and PM10 decreased, and the days with AQI levels "AQI ≤ 100" increased during the "lockdown" in 2020. Due to different degrees of industrialization, the concentration of air pollutants shows significant regional characteristics. The AQI values before and after the "lockdown" in 2020 show significant spatial autocorrelation, and the cities' AQI values in the north present high autocorrelation, and the cities in the south are in low autocorrelation. From the data at the national level, carbon emission intensity (CEI), per capita energy consumption (PEC), per capita GDP (PCG), industrialization rate (IR), and proportion of construction value added (PCVA) have the greatest impact on AQI. This study gives regulators confidence that if the government implements regionalized air quality improvement policies according to the characteristics of each region in China and reasonably plans socioeconomic activities, it is expected to improve China's air quality sustainably.