Contribution of nitrous acid to the atmospheric oxidation capacity in an industrial zone in the Yangtze River Delta region of China
Authors: Zheng Jun , Shi Xiaowen , Ma Yan , Ren Xinrong , Jabbour Halim , Diao Yiwei , Wang Weiwei , Ge Yifeng , Zhang Yuchan , Zhu Wenhui 
Published: May 2020

Abstract. A suite of instruments was deployed to simultaneously measure nitrous acid
(HONO), nitrogen oxides (NOx = NO + NO2), carbon monoxide
(CO), ozone (O3), volatile organic compounds (VOCs – including
formaldehyde, HCHO) and meteorological parameters near a typical industrial
zone in Nanjing in the Yangtze River Delta (YRD) region of China from 1 to 31 December 2015. High levels of HONO were detected using a wet-chemistry-based
method. HONO ranged from 0.03 to 7.04 ppbv with an average of 1.32±0.92 ppbv. Elevated daytime HONO was frequently observed with a minimum of
several hundred parts per trillion by volume (pptv) on average, which cannot be explained by the
homogeneous OH + NO reaction (POH+NO) and primary emissions
(Pemission), especially during periods with high particulate matter (PM2.5) loadings. HONO chemistry and its impact on the
atmospheric oxidation capacity in the study area were further investigated
using a Master Chemical Mechanism (MCM) box model. The results show that the
average hydroxyl radical (OH) production rate was dominated by the
photolysis of HONO (7.13×106 molec. cm−3 s−1),
followed by the ozonolysis of alkenes (3.94×106 molec. cm−3 s−1), the photolysis of O3 (2.46×106 molec. cm−3 s−1) and the photolysis of HCHO (1.60×106 molec. cm−3 s−1) during the campaign period, especially within plumes that
originated from the industrial zone. Model simulations indicated that
heterogeneous chemistry played an important role in HONO formation. The
average nighttime NO2 to HONO conversion rate was determined to
be ∼0.8 % h−1. A good correlation between nocturnal
HONO∕NO2 and the product of particle surface area density (S∕V) and
relative humidity (RH), S/V⋅RH, supports the heterogeneous
NO2∕H2O reaction mechanism. The other HONO source, designated as
Punknonwn, was about twice as high as POH+NO on average and
displayed a diurnal profile with an evidently photo-enhanced feature, i.e.,
photosensitized reactions of NO2 may be an important daytime HONO
source. Nevertheless, our results suggest that daytime HONO formation was
mostly due to the light-induced conversion of NO2 on aerosol surfaces,
whereas heterogeneous NO2 reactions on the ground surface dominated nocturnal
HONO production. Our study indicated that an elevated PM2.5 level during haze events can promote the conversion of NO2 to HONO by providing more
heterogeneous reaction sites, thereby increasing the atmospheric oxidation
capacity, which may further promote the formation of secondary air
pollutants. Highlights: High levels of HONO, with an average of 1.32±0.92 ppbv, were
observed near one of the largest industrial zones in the YRD region of
China. HONO photolysis and alkene ozonolyses contributed the most to OH production
and, hence, the atmospheric oxidation capacity. High loading of PM2.5 provided additional reaction surfaces for HONO
formation. Heterogeneous formation mechanisms were the most important daytime HONO
sources and were further enhanced by sunlight.