Abstract. High concentrations of nitrous acid (HONO) have been observed in the Pearl River Delta (PRD) region of China in recent years, contributing to an elevated atmospheric oxidation capacity due to the production of OH through HONO photolysis. We investigated the budget of HONO at an urban site in Guangzhou from 27 September to 9 November 2018 using data from a comprehensive atmospheric observation campaign. During this period, measured concentrations of HONO were 0.02 to 4.43 ppbv, with an average of 0.74 ± 0.70 ppbv. An emission ratio (HONO/NOx) of 0.9 ± 0.4 % was derived from 11 fresh plumes. The primary emission rate of HONO at night was calculated to be between 0.04 ± 0.02 and 0.30 ± 0.15 ppbv h−1 based on a high-resolution NOx emission inventory. Heterogeneous conversion of NO2 on the ground surface (0.27 ± 0.13 ppbv h−1), primary emissions from vehicle exhaust (between 0.04 ± 0.02 and 0.30 ± 0.15 ppbv h−1, with a middle value of 0.16 ± 0.07 ppbv h−1), and the homogeneous reaction of NO + OH (0.14 ± 0.30 ppbv h−1) were found to be the three largest sources of HONO at night. Heterogeneous NO2 conversion on aerosol surfaces (0.03 ± 0.02 ppbv h−1) and soil emission (0.019 ± 0.009 ppbv h−1) were two other minor sources. Correlation analysis shows that NH3 and the relative humidity (RH) may have participated in the heterogeneous transformation of NO2 to HONO at night. Dry deposition (0.41 ± 0.31 ppbv h−1) was the main removal process of HONO at night, followed by dilution (0.18 ± 0.16 ppbv h−1), while HONO loss at aerosol surfaces was much slower (0.008 ± 0.006 ppbv h−1). In the daytime, the average primary emission Pemis was 0.12 ± 0.02 ppbv h−1, and the homogeneous reaction POH+NO was 0.79 ± 0.61 ppbv h−1, larger than the unknown source PUnknown (0.65 ± 0.46 ppbv h−1). Similar to previous studies, PUnknown appeared to be related to the photo-enhanced conversion of NO2. Our results show that primary emissions and the reaction of NO + OH can significantly affect HONO at a site with intensive emissions during both the daytime and nighttime. Uncertainty in parameter values assumed in the calculation of HONO sources can have a strong impact on the relative importance of HONO sources at night, and could be reduced by improving knowledge of key parameters such as the NO2 uptake coefficient. The uncertainty in the estimated direct emission can be reduced by using emission data with higher resolution and quality. Our study highlights the importance of better constraining both conventional and novel HONO sources by reducing uncertainties in their key parameters for advancing our knowledge of this important source of atmospheric OH.

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Abstract. Nitrous acid (HONO) can produce hydroxyl radicals (OH) by photolysis and plays an important role in atmospheric photochemistry. Over the years, high concentrations of HONO have been observed in the Pearl River Delta region (PRD) of China, which may be one reason for the elevated atmospheric oxidation capacity. A comprehensive atmospheric observation campaign was conducted at an urban site in Guangzhou from 27 September to 9 November 2018. During the period, HONO was measured from 0.02 to 4.43 ppbv with an average of 0.74 ± 0.70 ppbv. The emission ratios (HONO/NOx) of 0.9 ± 0.4 % were derived from 11 fresh plumes. The primary emission rates of HONO at night were calculated to be between 0.04 ± 0.02 ppbv h−1 and 0.30 ± 0.15 ppbv h−1 based on a high-resolution emission inventory. The HONO formation rate by the homogeneous reaction of OH + NO at night was 0.26 ± 0.08 ppbv h−1, which can be seen as secondary results from primary emission. They were both much higher than the increase rate of HONO (0.02 ppbv h−1) during night. Soil emission rate of HONO at night was calculated to be 0.019 ± 0.001 ppbv h−1. Assuming dry deposition as the dominant removal process of HONO at night, and a deposition velocity of at least ~2.5 cm s−1 is required to balance the direct emissions and OH + NO reaction. Correlation analysis shows that NH3 and relative humidity (RH) may participate in the heterogeneous transformation from NO2 to HONO at night. In the daytime, the average primary emission Pemis was 0.12 ± 0.01 ppbv h−1, and the homogeneous reaction POH + NO was 0.79 ± 0.61 ppbv h−1, larger than the unknown sources PUnknown (0.65 ± 0.46 ppbv h−1). These results suggest primary emissions as a key factor affecting HONO at our site, both during daytime and nighttime. Similar to previous studies, the daytime unknown source of HONO, PUnknown, appeared to be related to the photo-enhanced conversion of NO2. The daytime average OH production rates by photolysis of HONO was 3.7 × 106 cm−3 s−1, lower than that from O1D + H2O at 4.9 × 106 cm−3 s−1. Simulations of OH and O3 with the Master Chemical Mechanism (MCM) box model suggested strong enhancement effect of HONO on OH and O3 by 59 % and 68.8 %, respectively, showing a remarkable contribution of HONO to the atmospheric oxidation in the fall season of Guangzhou.

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<p>Nitrous acid (HONO) can produce hydroxyl radicals (OH) by photolysis and plays an important role in atmospheric photochemistry. Over the years, high concentrations of HONO have been found in the Pearl River Delta region (PRD), which may be one of the reasons for the high atmospheric oxidation capacity. A comprehensive atmospheric observation was conducted at an urban site in Guangzhou from 27 September to 9 November 2018. During the period, HONO ranged from 0.02 to 4.43 ppbv with an average of 0.74±0.70 ppbv. The combustion emission ratio (HONO/NOx) of 0.9±0.4% was derived from 11 fresh plumes. The primary emission rate of HONO during night was calculated with the emission source inventory data to be between 0.04±0.02 and 0.30±0.15 ppbv/h. And the HONO produced by the homogeneous reaction of OH+NO at night was 0.26±0.08 ppbv/h, which can be seemed as secondary results from primary emission. They were both much higher than the increase rate of HONO (0.02 ppbv/h) during night. Soil emission rate of HONO at night was calculated to be 0.019±0.0003 ppbv/h. Deposition was the dominant removal process of HONO during night, and a deposition rate of at least 2.5 cm/s is required to balance the direct emissions and OH+NO reaction. Correlation analysis shows that NH<sub>3</sub> and relative humidity (RH) may participate in the heterogeneous transformation from NO<sub>2</sub> to HONO during night. In the daytime, the average primary emission P<sub>emis</sub> was 0.12±0.01 ppbv/h, and the homogeneous reaction P<sub>OH+NO</sub> was 0.79±0.61 ppbv/h, which was even larger than the unknown sources P<sub>Unknown</sub> (0.65±0.46 ppbv/h). The results showed that the direct and indirect contributions of primary emission to HONO are great at the site, both during daytime and nighttime. Similar to previous studies, P<sub>Unknown</sub> was suggested to be related to the photo-enhanced reaction of NO<sub>2</sub>. The mean OH production rates by photolysis of HONO and O<sub>3</sub> were 3.7×10<sup>6</sup> cm<sup>-3</sup>·s<sup>-1</sup> and 4.9×10<sup>6</sup> cm<sup>-3</sup>·s<sup>-1</sup>, respectively. We further studied the impact of HONO on the atmospheric oxidation by a Master Chemical Mechanism (MCM) box model. When constraining observed HONO in the model, OH and O<sub>3 </sub>increased 59% and 68.8% respectively, showing a remarkable contribution of HONO to the atmospheric oxidation of Guangzhou.</p><p> </p>