Anthropogenic Oxygenated Volatile Organic Compounds Dominate Atmospheric Oxidation Capacity and Ozone Production via Secondary Formation of Formaldehyde in the Urban Atmosphere

Oxygenated volatile organic compounds (OVOCs) play an important role in the photochemical formation of ozone (O3). Comprehensive measurements and chemical box model analysis conducted in Hangzhou, China reveal that the formation of O3 is strongly attributed to the increased atmospheric oxidation capacity (AOC). The observed OVOCs contribute 46.5 ± 8.3% to the AOC, especially during the pollution periods, with formaldehyde (HCHO) and acetaldehyde (CH3CHO) accounting for 19 ± 3.8% and 13.8 ± 4.1%, respectively. Various OVOCs (especially CH3CHO) act as key precursors for the initial production of the crucial radicals. These include the hydroperoxy radical (HO2), methyl peroxy radical (CH3O2), and acetyl peroxy radical (CH3CO3), accounting for 53.7 ± 5.0%, 13.9 ± 2.5%, and 10.0 ± 2.4% of the daytime O3 production (P(O3)), respectively. HCHO acted as the key accelerator of HO2 radical production and subsequently speeded up O3 production. Model simulation and observational evidence reveal that the OVOC initiated secondary channel dominates the HCHO formation. The radical propagation chain of CH3CO3 → CH3O2→ CH3O→ HO2 accounts for 62.4 ± 6.9% of HCHO production. CH3CHO chemistry can account for 14.0% of the production of O3, and degradation of secondary HCHO derived from CH3CHO accounts for 5.2% of total HO2 production. These findings underscore the significant effect of OVOCs, particularly CH3CHO, on AOC and O3 production in the urban atmosphere in eastern China. Given the decreasing trend of NOX concentration, model simulations under different NOX levels further imply that reducing anthropogenic emission sources of OVOCs is a crucial strategy for current O3 control, and excessive reduction of NOX emissions (the daytime average concentration of NOX decreases to below ∼ 2 ppb) may affect the effectiveness of reducing VOCs and promote a radical self-reaction.