NH 3-promoted hydrolysis of NO 2 induces explosive growth in HONO

Xu, Wanyun; Kuang, Ye; Zhao, Chunsheng; Tao, Jiangchuan; Zhao, Gang; Bian, Yuxuan; Yang, Wen; Yu, Yingli; Shen, Chuanyang; Liang, Linlin; Zhang, Gen; Lin, Weili; Xu, Xiaobin

The study of atmospheric nitrous acid (HONO), which is the primary source of OH radicals, is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes. Heterogeneous inline-formulaNO2 chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain, and also produces sulfate and nitrate. However, controversy exists regarding the various proposed HONO production mechanisms, mainly regarding whether inline-formulaSO2 directly takes part in the HONO production process and what roles inline-formulaNH3 and the pH value play. In this paper, never before seen explosive HONO production was reported and evidence was found – for the first time in field measurements during fog (usually with inline-formula4< pH inline-formula<6) and haze episodes under high relative humidity (pH inline-formula≈4) – that inline-formulaNH3 was the key factor that promoted the hydrolysis of inline-formulaNO2, leading to the explosive growth of HONO and nitrate under both high and relatively lower pH conditions. The results also suggest that inline-formulaSO2 plays a minor or insignificant role in HONO formation during fog and haze events, but was indirectly oxidized upon the photolysis of HONO via subsequent radical mechanisms. Aerosol hygroscopicity significantly increased with rapid inorganic secondary aerosol formation, further promoting HONO production as a positive feedback. For future photochemical and aerosol pollution abatement, it is crucial to introduce effective inline-formulaNH3 emission control measures, as inline-formulaNH3-promoted inline-formulaNO2 hydrolysis is a large daytime HONO source, releasing large amounts of OH radicals upon photolysis, which will contribute largely to both atmospheric photochemistry and secondary aerosol formation.



Xu, Wanyun / Kuang, Ye / Zhao, Chunsheng / et al: NH3-promoted hydrolysis of NO2 induces explosive growth in HONO. 2019. Copernicus Publications.


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