Abstract
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 NO(2) and SO(2), 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.