Pollution characteristics and potential sources of nitrous acid (HONO) in early autumn 2018 of Beijing.

Nitrous Acid (HONO) is an important precursor of hydroxyl radical (OH) and has significant impacts on the formation of Ozone (O) and Secondary Organic Aerosol (SOA). The atmospheric concentrations of HONO were measured during early autumn in downtown, Beijing (China). This study investigated HONO pollution characteristics and potential sources during day and night. The maximum hourly HONO levels reached 5.16 ppb, with 1.23 ppb on average. HONO concentration exhibited typical diurnal variation characteristics, with maximum at nighttime and minimum at daytime. The potential sources mainly included vehicle emission, heterogeneous reaction of NO on aerosol surfaces (Photo-enhanced at the daytime) and photolysis of particulate nitrate (NO) in Beijing. Vehicle emission was an important HONO source, particular at the morning rush period and lower HONO concentration. The simulated results highlighted that the main contribution of HONO was NO heterogeneous reaction on aerosol surfaces. The photolysis of particulate NO was also an important daytime HONO source, particularly in the pollution period. The main loss routine was the photolysis of HONO and dry deposition at day and night, respectively.