High-level HONO exacerbates double high pollution of O and PM in China.

Double high pollution (DHP) of ozone (O) and fine particulate matter (PM) has frequently been observed in China in recent years. Numerous studies have speculated that DHP might be related to nitrous acid (HONO), but the chemical mechanism involved remains unclear. Field observation results of DHP in Shanghai indicate that the high concentration of HONO produced by nitrogen dioxide (NO) heterogeneous reactions under conditions of high temperature and high humidity promotes an increase in PM and O concentrations. The box model combined with field observations to reconstruct pollution events indicates that HONO photolysis generates abundant hydroxyl (OH) radicals that rapidly oxidize volatile organic compounds (VOCs), which in turn accelerates the RO (OH, hydroperoxyl (HO), and organic peroxy (RO) radicals) cycle and causes the accumulation of O. This elevated O along with high concentrations of HONO, produces particulate nitrate (pNO) by encouraging the NO + OH reaction. This process strengthens the chemical coupling between O and PM, which can exacerbate the DHP of O and PM. Sensitivity analysis of pNO/O-NO-VOCs suggests that under nitrogen oxides (NO = NO + NO) reduction conditions, simultaneous control of pNO and O can be expected to be successfully achieved through emission reduction of alkanes and oxygenated VOCs (OVOCs). Therefore, the present research will facilitate the design of appropriate PM and O control strategies for high HONO concentration conditions, and thus alleviate the current stresses of air pollution.