Budget of nitrous acid (HONO) at an urban site in the fall season of Guangzhou, China

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 P-emis 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 P-Unknown (0.65 +/- 0.46 ppbv h(-1)). Similar to previous studies, P-Unknown 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.