title: Observation of nitrous acid (HONO) in Beijing, China: Seasonal variation, nocturnal formation and daytime budget.
abstract: Seasonal characteristics of atmospheric nitrous acid (HONO) were investigated with high time-resolution field measurements at an urban site of Beijing in four select months (representing four different seasons) from September 2015 to July 2016. The HONO concentrations displayed a pronounced seasonal profile with a maximum in autumn (2.27±1.82ppb) and a minimum in winter (1.05±0.89ppb). Significant diurnal cycles were also observed during the whole campaign. We found that the nighttime build-up of HONO was attributed to the heterogeneous conversion of NO2on wet surface. The calculated NO2to HONO conversion frequencies varied from 0.005h-1in spring to 0.010h-1in summer, with an average value of 0.008h-1. The seasonality of these conversion frequencies was closely related to the RH levels in different seasons. During daytime, large additional HONO sources were calculated. The noontime additional source was the highest in autumn 3.82ppbh-1, followed by summer 3.05ppbh-1, spring 2.63ppbh-1and winter 1.30ppbh-1. Correlation studies between the additional HONO source and related parameters demonstrated that the controlling processes responsible for HONO daytime formation varied in different seasons, and that the photo-enhanced formation on wet surface or the photolysis of adsorbed nitric acid and nitrate could be potential HONO sources in Beijing.
authors.affiliation: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Jiaqi
authors.initials: J
authors.lastname: Wang
authors.affiliation: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100049, China. Electronic address: zhangxsh@rcees.ac.cn.
authors.forename: Xiaoshan
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
authors.forename: Jia
authors.initials: J
authors.lastname: Guo
authors.affiliation: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Zhangwei
authors.initials: Z
authors.lastname: Wang
authors.affiliation: University of Chinese Academy of Sciences, Beijing 100049, China; State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, Beijing 100029, PR China.
authors.forename: Meigen
authors.initials: M
authors.lastname: Zhang
keywords: Daytime source; Heterogeneous conversion; Megacity air pollution; Nitrous acid observation