Anthropogenic Oxygenated Volatile Organic Compounds Dominate Atmospheric Oxidation Capacity and Ozone Production via Secondary Formation of Formaldehyde in the Urban Atmosphere
Hongtao Qian, Bingye Xu, Zhengning Xu, Qiaoli Zou, Qianxin Zi, Hanfei Zuo, Fei Zhang, Jing Wei, Xiangyu Pei, Wenxin Zhou, Lingling Jin, Xudong Tian, Wenlong Zhao, Zhibin Wang
Abstract
Oxygenated volatile organic compounds (OVOCs) play an important role in the photochemical formation of ozone (O 3 ). Comprehensive measurements and chemical box model analysis conducted in Hangzhou, China reveal that the formation of O 3 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 (CH 3 CHO) accounting for 19 ± 3.8% and 13.8 ± 4.1%, respectively. Various OVOCs (especially CH 3 CHO) act as key precursors for the initial production of the crucial radicals. These include the hydroperoxy radical (HO 2 ), methyl peroxy radical (CH 3 O 2 ), and acetyl peroxy radical (CH 3 CO 3 ), accounting for 53.7 ± 5.0%, 13.9 ± 2.5%, and 10.0 ± 2.4% of the daytime O 3 production (P(O 3 )), respectively. HCHO acted as the key accelerator of HO 2 radical production and subsequently speeded up O 3 production. Model simulation and observational evidence reveal that the OVOC initiated secondary channel dominates the HCHO formation. The radical propagation chain of CH 3 CO 3 → CH 3 O 2 → CH 3 O→ HO 2 accounts for 62.4 ± 6.9% of HCHO production. CH 3 CHO chemistry can account for 14.0% of the production of O 3, and degradation of secondary HCHO derived from CH 3 CHO accounts for 5.2% of total HO 2 production. These findings underscore the significant effect of OVOCs, particularly CH 3 CHO, on AOC and O 3 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 O 3 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.