COVID‐19 Impact on the Concentration and Composition of Submicron Particulate Matter in a Typical City of Northwest China.

In this study, we evaluated the variations of air quality in Lanzhou, a typical city in Northwestern China impacted by the COVID‐19 lockdown. The mass concentration and chemical composition of non‐refractory submicron particulate matter (NR‐PM1) were determined by a high‐resolution aerosol mass spectrometer during January‐March 2020. The concentration of NR‐PM1 dropped by 50% from before to during control period. The five aerosol components (sulfate, nitrate, ammonium, chloride, and organic aerosol [OA]) all decreased during the control period with the biggest decrease observed for secondary inorganic species (70% of the total reduction). Though the mass concentration of OA decreased during the control period, its source emissions varied differently. OA from coal and biomass burning remained stable from before to during control period, while traffic and cooking related emissions were reduced by 25% and 50%, respectively. The low concentration during the control period was attributed to the lower production rate for secondary aerosols. Plain Language Summary: At the beginning of 2020, a novel coronavirus disease (COVID‐19) was spreading in China and lasting through the following months. People's outdoor activities due to the coupling effect of this epidemic and the Chinese New Year holiday were greatly reduced and pollutant emissions related with these activities were also reduced during this period. This situation provides us a unique chance to look into the air quality and evaluate the corresponding mitigation measures in the city. We observed a significant drop of the mass concentration of NR‐PM1 by 50% in Lanzhou. The reduction of NR‐PM1 was mainly from secondary inorganic species accounting for 70% of reduced NR‐PM1. This finding is significantly different from that observed in Eastern China where the mass concentration of fine particulate matter was not reduced significantly with the reduction of primary emissions due to enhanced secondary production. In contrast, the production rates for secondary inorganic and organic aerosols showed a decreasing trend from before to during control period. These results revealed the large difference in air pollution chemistry between East and West China. Key Points: The submicron aerosol mass concentration was reduced by 50% during COVID‐19 lockdown in LanzhouThe reduction of aerosol was mainly due to a decline in secondary species; we identify an overall low production rate as the main driverThe result is contrast to those reported recently in East China where the reductions were offset by an increase in secondary species production [ABSTRACT FROM AUTHOR]