The effect of nitrous acid (HONO) on ozone formation during pollution episodes in southeastern China: Results from model improvement and mechanism insights.

Two ozone (O) processes, summer episode dominated by local production and autumn episode dominated by regional transport, were chosen to investigate the role of HONO in different pollution processes. Meteorological conditions, diurnal variation of O, potential source contribution factor (PSCF) analysis, concentration weighted trajectory (CWT) models, and the distribution of the eight-hour maximum values of O on mainland China all prove that summer O was mainly locally generated while autumn O episode was more susceptible to regional transport. The gaps between observations and simulation results with the default HONO chemistry in Master Chemical Mechanism (MCM) of Observation Based Model (OBM) were higher in summer episode (0.58 ppb) than autumn episode (0.37 ppb). Although we implemented nine additional sources in the model to revise the HONO chemistry, the simulated values were still lower than the observed values. HONO promoted O production by accelerating the reaction of HO + NO and RO + NO, and promoted loss of O by the reaction of OH + NO and RO + NO. The net production rate of O with HONO constraint increased by 28.50 % in summer and 22.43 % in autumn, which also indicated that HONO played more important role in the O production in summer. The difference of NOx of daily RIR between the cases with and without HONO constraint was higher in summer O episode (0.15 %/%) than that in autumn O episode (0.09 %/%), the same as to VOCs with -0.20 %/% in summer O episode and - 0.14 %/% in autumn O episode, which indicated that the presence or absence of the HONO constraint has a greater impact on the RIR value in the case of dominant local generation. In brief, the O sensitivity would be more favorable for VOCs without HONO constrained in the model, which would inevitably mislead policy makers to develop efficient policies to control O pollution.