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Abstract
Abstract. Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), plays a key role in atmospheric chemistry, but its sources are still debated. The production of HONO on aerosol surfaces or on ground surfaces in nocturnal atmospheres remains controversial. The vertical profile provides vertical information on HONO and NO2 to understand the nocturnal HONO production and loss. In this study, we report the first high-resolution (<2.5 m) nocturnal vertical profiles of HONO and NO2 measured from in situ instruments on a movable container that was lifted on the side wiring of a 325 m meteorological tower in Beijing, China. High-resolution vertical profiles revealed the negative gradients of HONO and NO2 in nocturnal boundary layers, and a shallow inversion layer affected the vertical distribution of HONO. The vertical distribution of HONO was consistent with stratification and layering in the nocturnal urban atmosphere below 250 m. The increase in the HONO ∕ NO2 ratio was observed throughout the column from the clean episode to the haze episode, and relatively constant HONO∕NO2 ratios in the residual layer were observed during the haze episode. Direct HONO emissions from traffic contributed 29.3 % ± 12.4 % to the ambient HONO concentrations at night. The ground surface dominates HONO production by heterogeneous uptake of NO2 during clean episodes. In contrast, the HONO production on aerosol surfaces (30–300 ppt) explained the observed HONO increases (15–368 ppt) in the residual layer, suggesting that the aerosol surface dominates HONO production aloft during haze episodes, while the surface production of HONO and direct emissions into the overlying air are minor contributors. Average dry deposition rates of 0.74±0.31 and 1.55±0.32 ppb h−1 were estimated during the clean and haze episodes, respectively, implying that significant quantities of HONO could be deposited to the ground surface at night. Our results highlight the ever-changing contributions of aerosol and ground surfaces in nocturnal HONO production at different pollution levels and encourage more vertical gradient observations to evaluate the contributions from varied HONO sources.
Highlights
IntroductionIt is well known that the rapid photolysis of nitrous acid (HONO) (Reaction R1) after sunrise is the most im-
It is well known that the rapid photolysis of nitrous acid (HONO) (Reaction R1) after sunrise is the most im-Published by Copernicus Publications on behalf of the European Geosciences Union.F
The results suggested that a conservative surface reservoir that was formed by the deposition of HONO could be a significant fraction of the unknown daytime source
Summary
It is well known that the rapid photolysis of nitrous acid (HONO) (Reaction R1) after sunrise is the most im-. F. Meng et al.: High-resolution vertical distribution and sources of HONO and NO2 portant hydroxyl radical (OH) source. A total of 25 %–60 % of daytime OH production was accounted for due to HONO photolysis, according to previous reports (Lu et al, 2012; Ma et al, 2017; Tong et al, 2016; Su et al, 2008b; Huang et al, 2017; Spataro et al, 2013). HONO as a nitrosating agent forms carcinogenic nitrosamines (Hanst et al, 1977; Pitts et al, 1978), and its health effects have attracted increasing amounts of concern (Sleiman et al, 2010; Bartolomei et al, 2015; Gómez Alvarez et al, 2014)
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