[HONO Pollution Characteristics and Nighttime Sources During Autumn in Guangzhou, China]

Nitrous acid (HONO) plays an important role in atmospheric photochemistry processes because its photolysis provides an efficient source of hydroxyl (OH) radicals in the troposphere. However, few studies exist on HONO in nocturnal chemistry processes. Using the observation data of HONO and related parameters for a super site at Guangzhou Jinan University in October 2015, the pollution processes and sources of HONO during nighttime were analyzed in this study. The results showed that the average concentration of HONO was 4.32 μg ·m^-3 during the nighttime, which was 2.6 times its concentration of 1.67 μg ·m^-3 in the daytime. The conversion rate of HONO, C_HONO, during the nighttime was 0.0068 h^-1, and the average contribution of vehicle emissions to HONO was 15.1%, with a peak of 37.8% at 20:00. The average net HONO generation rate from the reaction of NO and OH radicals during the night was 0.44 μg ·(m³ ·h)^-1. Correlation analysis suggested that particles could not be the major HONO source, but that relative humidity (RH; 33%-78%) was a key factor. Relevant calculation indicated that the heterogeneous reaction of deposited NO₂ on humid ground surfaces could be the main source of HONO during nighttime.