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

Abstract

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 NO_X concentration, model simulations under different NO_X levels further imply that reducing anthropogenic emission sources of OVOCs is a crucial strategy for current O₃ control, and excessive reduction of NO_X emissions (the daytime average concentration of NO_X decreases to below ∼ 2 ppb) may affect the effectiveness of reducing VOCs and promote a radical self-reaction.