High-level HONO exacerbates double high pollution of O₃ and PM_2.5 in China

Double high pollution (DHP) of ozone (O₃) and fine particulate matter (PM_2.5) 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 (NO₂) heterogeneous reactions under conditions of high temperature and high humidity promotes an increase in PM_2.5 and O₃ 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_x (OH, hydroperoxyl (HO₂), and organic peroxy (RO₂) radicals) cycle and causes the accumulation of O₃. This elevated O₃ along with high concentrations of HONO, produces particulate nitrate (pNO₃) by encouraging the NO₂ + OH reaction. This process strengthens the chemical coupling between O₃ and PM_2.5, which can exacerbate the DHP of O₃ and PM_2.5. Sensitivity analysis of pNO₃/O₃-NO_x-VOCs suggests that under nitrogen oxides (NO_x = NO + NO₂) reduction conditions, simultaneous control of pNO₃ and O₃ 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_2.5 and O₃ control strategies for high HONO concentration conditions, and thus alleviate the current stresses of air pollution.