Intensive photochemical oxidation in the marine atmosphere: evidence from direct radical measurements

Zhang, Guoxian; Hu, Renzhi; Xie, Pinhua; Hu, Changjin; Liu, Xiaoyan; Zhong, Liujun; Cai, Haotian; Zhu, Bo; Xia, Shiyong; Huang, Xiaofeng; Li, Xin; Liu, Wenqing

Comprehensive observations of hydroxyl (OH) and hydroperoxy (HOinline-formula2) radicals were conducted in October 2019 at a coastal continental site in the Pearl River Delta (YMK site, 22.55inline-formula N, 114.60inline-formula E). The daily maximum OH and HOinline-formula2 concentrations were (4.7–9.5) inline-formula× 10inline-formula6 and (4.2–8.1) inline-formula× 10inline-formula8 cminline-formula−3, respectively. The synchronized air mass transport from the northern cities and the South China Sea exerted a time-varying influence on atmospheric oxidation. Under a typical ocean-atmosphere (OCM), reasonable measurement model agreement was achieved for both OH and HOinline-formula2 using a 0-D chemical box model incorporating the regional atmospheric chemistry mechanism version 2-Leuven isoprene mechanism (RACM2-LIM1), with daily averages of 4.5 inline-formula× 10inline-formula6 and 4.9 inline-formula× 10inline-formula8 cminline-formula−3, respectively. Land mass (LAM) influence promoted more active photochemical processes, with daily averages of 7.1 inline-formula× 10inline-formula6 and 5.2 inline-formula× 10inline-formula8 cminline-formula−3 for OH and HOinline-formula2, respectively. Heterogeneous uptake had certain effects on HOinline-formulax chemistry, but the influence of the halogen mechanism was limited by NOinline-formulax level. Intensive photochemistry occurred after precursor accumulation, allowing local net ozone production comparable with surrounding suburban environments (5.52 ppb hinline-formula−1 during the LAM period). The rapid oxidation process was accompanied by a higher diurnal nitrous acid (HONO) concentration (inline-formula> 400 ppt). After a sensitivity test, HONO-related chemistry elevated the ozone production rate by 33 % and 39 % during the LAM and OCM periods, respectively. The nitric acid (inline-formulaP(HNOinline-formula3)) and sulfuric acid (inline-formulaP(Hinline-formula2SOinline-formula4)) formation rates also increased simultaneously (inline-formula∼ 43 % and inline-formula∼ 48 % for LAM and OCM sectors, respectively). In the ozone-prediction test, simulated Oinline-formula3 decreased from inline-formula∼ 75 ppb to a global background (inline-formula∼ 35 ppb) without the HONO constraint, and daytime HONO concentrations were reduced to a low level (inline-formula∼ 70 ppt). For coastal cities, the particularity of the HONO chemistry tends to influence the ozone-sensitive system and eventually magnifies the background ozone. Therefore, the promotion of oxidation by elevated precursors deserves a lot of attention when aiding pollution mitigation policies.

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Zhang, Guoxian / Hu, Renzhi / Xie, Pinhua / et al: Intensive photochemical oxidation in the marine atmosphere: evidence from direct radical measurements. 2024. Copernicus Publications.

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