<p>Numerous anthropogenic activities were scaled back to control the spread of the Coronavirus Disease 2019 (COVID-19) in Xi’an, providing a good chance to investigate the effects of reducing anthropogenic emissions on chemical composition and source contribution of PM_2.5. An intensive real-time filed observation for PM_2.5 was conducted from January 1, to February 9, 2020. The chemical components of PM_2.5 including organic aerosol (OA), SO₄²⁻, NO₃⁻, NH₄⁺, Cl⁻, BC and trace elements were monitored by a quadrupole aerosol chemical speciation monitor (Q-ACSM), a seven-wavelength aethalometer (AE33) and an ambient metals monitor (Xact625), respectively. The results showed that concentrations of gaseous pollutants (NO₂, SO₂, CO) and PM_2.5 decreased by 10–70% during the lockdown period, while the concentration of O₃ approximately increased 2.4 folds. It noted that the largest decrease of concentration for PM_2.5 chemical components performed in NO₃⁻ and black carbon (BC). As regard to chemical composition on PM_2.5, organic aerosols (OA) became more dominant during the lockdown period, likely due to increased secondary organic aerosol formation. The results of source apportionment revelated that biomass burning and secondary aerosols were the primary contributors to PM_2.5 during the campaign, and industrial and vehicular emissions significantly reduced during the lockdown period. Additionally, the firework emissions during traditional festivals also contributed notably to PM_2.5 levels. The study also investigated the dynamic evolution of pollution episodes, emphasizing the roles of biomass burning, secondary aerosol formation, and firework emissions in haze events. The study highlights the effectiveness of short-term emission controls in reducing PM_2.5 and the complexity of secondary aerosol formation. It implies the need for targeted policies to address biomass burning and fireworks emissions. Those findings provide valuable insights for improving air quality in Xi’an and similar regions in the future.</p>