Budget of nitrous acid (HONO) and its impacts on atmospheric oxidation capacity at an urban site in the fall season of Guangzhou, China

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 P_emis was 0.12 ± 0.01 ppbv h⁻¹, and the homogeneous reaction P_OH + NO was 0.79 ± 0.61 ppbv h⁻¹, larger than the unknown sources P_Unknown (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, P_Unknown, 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.