Heterogeneous formation of HONO and its impacts on haze formation in the YRD region of China
Authors: Zheng Jun , Shi Xiaowen , Ma Yan 
Published: March 2020

&lt;p&gt;A suite of instruments were deployed to simultaneously measure nitrous acid (HONO), nitrogen oxides (NO&lt;sub&gt;x&lt;/sub&gt;= NO + NO&lt;sub&gt;2&lt;/sub&gt;), carbon monoxide (CO), ozone (O&lt;sub&gt;3&lt;/sub&gt;), 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 &amp;#177;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&lt;sub&gt;OH+NO&lt;/sub&gt;) alone, especially during periods with high loadings of particulate matters (PM&lt;sub&gt;2.5&lt;/sub&gt;). 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&amp;#215;10&lt;sup&gt;6&lt;/sup&gt;molecules cm&lt;sup&gt;-3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;), followed by ozonolysis of alkenes (3.94&amp;#215;10&lt;sup&gt;6&lt;/sup&gt;molecules cm&lt;sup&gt;-3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;), photolysis of O&lt;sub&gt;3&lt;/sub&gt;(2.46&amp;#215;10&lt;sup&gt;6&lt;/sup&gt;molecules cm&lt;sup&gt;-3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;) and photolysis of HCHO (1.60&amp;#215;10&lt;sup&gt;6&lt;/sup&gt;molecules cm&lt;sup&gt;-3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;), especially within the plumes originated from the industrial zone. The observed similarity between HONO/NO&lt;sub&gt;2&lt;/sub&gt;and HONO in diurnal profiles strongly suggests that HONO in the study area was likely originated from NO&lt;sub&gt;2&lt;/sub&gt;heterogeneous reactions. The averagenighttimeNO&lt;sub&gt;2&lt;/sub&gt;to HONO conversion ratewas determined to be ~0.9% hr&lt;sup&gt;-1&lt;/sup&gt;. Good correlation between nocturnal HONO/NO&lt;sub&gt;2&lt;/sub&gt;and the products of particle surface area density (S/V) and relative humidity (RH), S/V&amp;#215;RH,supports the heterogeneous NO&lt;sub&gt;2&lt;/sub&gt;/H&lt;sub&gt;2&lt;/sub&gt;O reaction mechanism. The other HONO source, designated as P&lt;sub&gt;unknonwn&lt;/sub&gt;, was about twice as much as P&lt;sub&gt;OH+NO &lt;/sub&gt;on average and displayed a diurnal profile with an evidently photo-enhanced feature, i.e., photosensitized reactions of NO&lt;sub&gt;2&lt;/sub&gt;may be an important daytime HONO source. Nevertheless, our results suggest that daytime HONO formation was mostly due to the light-induced conversion of NO&lt;sub&gt;2&lt;/sub&gt;on aerosol surfaces but heterogeneous NO&lt;sub&gt;2&lt;/sub&gt;reactions on ground surface dominated nocturnal HONO production. Concurred elevated HONO and PM&lt;sub&gt;2.5&lt;/sub&gt;levels 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 NO&lt;sub&gt;2&lt;/sub&gt;/HONO conversion by providing more heterogeneous reaction sites.&lt;/p&gt;