Notable effects of crustal matters on HONO formation by the redox reaction of NO2 with SO2 in an inland city of China

Abstract

Nitrous acid (HONO) is a vital precursor of the hydroxyl radicals, which plays a significant role in atmospheric chemistry. HONO can be produced by the redox reaction of NO2 with SO2. However, this reaction has often been neglected in previous studies on the HONO sources. To study this reaction on HONO production, continuous observation was carried out for one year in an inland city of China, and a sandy haze process was selected, divided into clean days (CD) and sand haze days (SHD). The mean HONO concentrations increased from 0.8 ± 0.4 ppb (CD) to 1.4 ± 1.0 ppb (SHD) during the nighttime. Since the rate of HONO sinks higher than that of sources, other elevation sources may contribute to the nighttime HONO concentration. In addition, the daytime HONO concentrations during the SHD periods also increased by 0.2 ppb, primarily due to the elevated photolysis of nitrate (0.20 ppb/h) and unknown sources (0.20 ppb/h) compared to the CD period. The positive correlations between HONO with NO2, SO2, particle pH, and SO42– indicate that HONO might be formed through the NO2 and SO2 redox reaction, and the calculated HONO production rates increased by two orders of magnitude under high pH conditions during the SHD period. In particular, on a heavily polluted SHD, this reaction explained 5% and 22% of HONO formation during the daytime and nighttime, respectively. In short, the NO2 and SO2 redox reaction may be more critical in high pH regions/periods and thus cannot be ignored when analyzing the HONO sources.