Heterogeneous formation of HONO and its impacts on haze formation in the YRD region of China

A suite of instruments were deployed to simultaneously measure nitrous acid (HONO), nitrogen oxides (NO_x= NO + NO₂), carbon monoxide (CO), ozone (O₃), volatile organic compounds (VOCs, including formaldehyde (HCHO)) and meteorological parameters near a typical industrial zone in Nanjing of the Yangtze River Delta region, China. High levels of HONO were detected using a wet chemistry-based method. HONO ranged from 0.03-7.04 ppbv with an average of 1.32 ±0.92 ppbv. Elevated daytime HONO was frequently observed with a minimum of several hundreds of pptv on average, which cannot be explained by the homogeneous OH + NO reaction (P_OH+NO) alone, especially during periods with high loadings of particulate matters (PM_2.5). The HONO chemistry and its impact on atmospheric oxidation capacity in the study area were further investigated using a MCM-box model. The results show that the average hydroxyl radical (OH) production rate was dominated by the photolysis of HONO (7.13×10⁶molecules cm^-3 s^-1), followed by ozonolysis of alkenes (3.94×10⁶molecules cm^-3 s^-1), photolysis of O₃(2.46×10⁶molecules cm^-3 s^-1) and photolysis of HCHO (1.60×10⁶molecules cm^-3 s^-1), especially within the plumes originated from the industrial zone. The observed similarity between HONO/NO₂and HONO in diurnal profiles strongly suggests that HONO in the study area was likely originated from NO₂heterogeneous reactions. The averagenighttimeNO₂to HONO conversion ratewas determined to be ~0.9% hr^-1. Good correlation between nocturnal HONO/NO₂and the products 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 P_unknonwn, was about twice as much as P_OH+NO on average and displayed a diurnal profile with an evidently photo-enhanced feature, i.e., photosensitized reactions of NO₂may be an important daytime HONO source. Nevertheless, our results suggest that daytime HONO formation was mostly due to the light-induced conversion of NO₂on aerosol surfaces but heterogeneous NO₂reactions on ground surface dominated nocturnal HONO production. Concurred elevated HONO and PM_2.5levels strongly indicate that high HONO may increase the atmospheric oxidation capacity and further promote the formation of secondary aerosols, which may in turn synergistically boost NO₂/HONO conversion by providing more heterogeneous reaction sites.