SARS-CoV-2 was discovered in Wuhan (Hubei) in late 2019 and covered the globe by March 2020. To prevent the spread of the SARS-CoV-2 outbreak, China imposed a countrywide lockdown that significantly improved the air quality. To investigate the collective effect of SARS-CoV-2 on air quality, we analyzed the ambient air quality in five provinces of northwest China (NWC): Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX) and Qinghai (QH), from January 2019 to December 2020. For this purpose, fine particulate matter (PM
2.5), coarse particulate matter (PM
10), sulfur dioxide (SO
2), nitrogen dioxide (NO
2), carbon monoxide (CO), and ozone (O
3) were obtained from the China National Environmental Monitoring Center (CNEMC). In 2020, PM
2.5, PM
10, SO
2, NO
2, CO, and O
3 improved by 2.72%, 5.31%, 7.93%, 8.40%, 8.47%, and 2.15%, respectively, as compared with 2019. The PM
2.5 failed to comply in SN and XJ; PM
10 failed to comply in SN, XJ, and NX with CAAQS Grade II standards (35 µg/m
3, 70 µg/m
3, annual mean). In a seasonal variation, all the pollutants experienced significant spatial and temporal distribution, e.g., highest in winter and lowest in summer, except O
3. Moreover, the average air quality index (AQI) improved by 4.70%, with the highest improvement in SN followed by QH, GS, XJ, and NX. AQI improved in all seasons; significant improvement occurred in winter (December to February) and spring (March to May) when lockdowns, industrial closure etc. were at their peak. The proportion of air quality Class I improved by 32.14%, and the number of days with PM
2.5, SO
2, and NO
2 as primary pollutants decreased while they increased for PM
10, CO, and O
3 in 2020. This study indicates a significant association between air quality improvement and the prevalence of SARS-CoV-2 in 2020.
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