Abstract: 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 from January to March 2020. The concentration of NR‐PM1 dropped by 50% from pre‐ 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 from secondary inorganic species (70% of the total reduction). Though the mass concentration of OA decreased during control period, its source emissions varied differently. OA from coal and biomass burning remained stable from pre‐ to during control period, while traffic and cooking related emissions were reduced by 25% and 50%, respectively. The low concentration during control period was attributed to the lower production rate for secondary aerosols.