Abstract
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.
Highlights
Nitrous acid (HONO) plays an important role in tropospheric photochemistry because its fast photolysis contributes to the formation of the hydroxyl (OH) radical, which is an essential atmospheric oxidant that initiates the oxidation of volatile organic compounds (VOC) to form organic peroxy radicals (RO2) and hydroperoxyl radicals (HO2)
The buildup of HONO frequently proceeded the accumulations of PM2.5, e.g., on the 7 December and from 21 to 22 December 2015, indicating that HONO may promote the formation of secondary aerosol by contributing to OH production, which will be further analyzed in detail
In a study conducted in New York City in winter 2004, it was found that 48 % of the net HOx production was from HONO photolysis, 36 % was from the ozonolysis of alkenes, only 6 % was from HCHO photolysis and 1 % was from O3 photolysis (Ren et al, 2006)
Summary
Nitrous acid (HONO) plays an important role in tropospheric photochemistry because its fast photolysis contributes to the formation of the hydroxyl (OH) radical, which is an essential atmospheric oxidant that initiates the oxidation of volatile organic compounds (VOC) to form organic peroxy radicals (RO2) and hydroperoxyl radicals (HO2). It has been suggested that HONO may be emitted directly by incomplete combustion processes, such as vehicle exhaust (Kirchstetter et al, 1996; Kurtenbach et al, 2001; Liang et al, 2017; Nakashima and Kajii, 2017; Trinh et al, 2017; Xu et al, 2015) and biomass burning (Müller et al, 2016; Neuman et al, 2016; Nie et al, 2015; Rondon and Sanhueza, 1989) Such strong but sporadic point sources could not account for the widely observed daytime HONO in the polluted areas (Elshorbany et al, 2012; Wang et al, 2017). The mechanisms of possible daytime HONO formation and their consequent impacts on air pollutants formation were explored
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