To better understand the effects of COVID-19 on air quality in Taiyuan, hourly in situ measurements of PM_2.5(particulate matter with an aerodynamic diameter less than 2.5 mm) and chemical components (water-soluble ions, organic carbon (OC), elemental carbon (EC), and trace elements) were conducted before (P1: 1 January–23 January 2020) and during (P2: 24 January–15 February 2020) the coronavirus disease 2019 (COVID-19) outbreak. The average concentrations of PM_2.5 dropped from 122.0 μg/m³ during P1 to 83.3 μg/m³ during P2. Compared with P1, except for fireworks burning–related chemical components (K⁺, Mg²⁺, K, Cu, Ba), the concentrations of other chemical components of PM_2.5 decreased by14.9–69.8%. Although the large decrease of some emission sources, fireworks burning still resulted in the occurrence of pollution events during P2. The analysis results of positive matrix factorization model suggested that six PM_2.5 sources changed significantly before and during the outbreak of the epidemic. The contributions of vehicle emission, industrial process, and dust to PM_2.5 decreased from 23.1%, 3.5%, and 4.0% during P1 to 7.7%, 3.4%, and 2.3% during P2, respectively, whereas the contributions of secondary inorganic aerosol, fireworks burning, and coal combustion to PM_2.5 increased from 62.0%, 1.8%, and 5.5% to 71.5%, 9.0%, and 6.2%, respectively. The source apportionment results were also affected by air mass transport. The largest reductions of vehicle emission, industrial process, and dust source were distinctly seen for the air masses from northwest.