Chemical characteristics of wintertime PM2.5 in background region of northwest China during urban emission reduction

To investigate the impact of urban emission reduction on particulate matter (PM_2.5) pollution characteristics, field observations were conducted near a megacity in the Guanzhong Plain, China, during three periods: COVID-19 lockdown (“lockdown”), pre-lockdown (“normal”), and post-lockdown (“festival”). The observation showed that despite reduced NO₂ and SO₂, PM_2.5 increased significantly during “lockdown.” Molecular characteristics and PMF source apportionment revealed that biomass burning contributions to PM_2.5 increased by over 70% compared with “normal.” Meanwhile, secondary aerosol formation (primarily through liquid-phase oxidation) accounted for more than 50% of PM_2.5 levels during “lockdown.” Additionally, metal ions released by fireworks burning accelerated the liquid-phase formation of sulfate, resulting in secondary sulfate-related sources contributing about 33% of PM_2.5 during “festival.” The study demonstrates synergistic effects between biomass/fireworks burning and liquid-phase oxidation, indicating that unbalanced emission reductions may exacerbate pollution through atmospheric aging and regional transport. Effective air quality management requires coordinated multi-pollutant control strategies.