Atmospheric oxidation capacity and its influence on secondary pollution during winter PM_2.5 haze episodes in Urumqi

Atmospheric oxidation capacity (AOC) refers to the oxidation capacity of atmospheric chemical processes for major pollutants. A comprehensive observation campaign was conducted on a typical PM_2.5 pollution event in Urumqi, Northwest China, from January 15 to February 15, 2024. Four typical cases representing PM_2.5 levels from non-polluted to heavily polluted periods were selected to estimate AOC, OH reactivity, their relationship with secondary components, and the HO_x budget in PM_2.5 using the Framework for 0-D Atmospheric Modeling model. Aromatics and alkanes contributed 54 %-79 % and 18 %-46 % of secondary organic aerosols (SOA) formation, respectively. The sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) increased with rising PM_2.5 levels. As AOC increased, SOR showed the same trend, while NOR and SOA decreased slightly, and ammonia oxidation ratio values remained almost unchanged. OH radicals were the primary oxidants during the daytime, whereas NO₃ played a dominant role at nighttime. CO (24 %-51 %), NO₂ (27 %-34%), and alkenes (7 %-12 %) were the dominant contributors to OH reactivity. The HO₂ + NO reaction pathway contributes 67 %-84 % to the production of OH, while Others (including carbonyl compounds photolysis, the permutation reactions of RO₂, etc.), RO₂ + NO and OH + CO reaction pathways were the main pathways for HO₂ production. A comprehensive understanding of the interactions among AOC, SOA, and radical cycling is crucial for formulating effective air quality management strategies aimed at mitigating secondary pollution under diverse atmospheric conditions.