The Effect of Nitrous Acid (Hono) on Ozone Formation During Pollution Episodes in Southeastern China: Results from Model Improvement and Mechanism Insights

Abstract

Two ozone (O3) processes, summer episode dominated by local production with IOA=0.94 and autumn episode dominated by regional transport with IOA=0.60, were chosen to investigate the role of HONO in different pollution processes. 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 are still lower than the observed values. The maximum differences of OH with or without HONO constraints appeared at 9:00 (0.72×107 molecules×cm-3) and 12:00 (0.43×107 molecules×cm-3) for summer O3 episode and autumn O3 episode, respectively, which indicated that OH concentration would be underestimated if no measurement data available. HONO promoted production of O3 by HO2+NO and RO2+NO, and promoted loss of O3 by the reaction of OH+NO2, followed by RO2+NO2. The net production rate of O3 with HONO constraint increased 28.50% in summer and 22.43% in autumn, which also indicated that HONO played more important role in the O3 production in summer. The difference of NOx of daily RIR between with and without HONO constraint was higher in summer O3 episode (0.15 %/%) than that in autumn O3 episode (0.09 %/%), the same as to VOCs with -0.20 %/% in summer O3 episode and -0.14 %/% in autumn O3 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. Therefore, the O3 sensitivity would be more favorable for VOCs without HONO constrained in the model, which could unavoidably mislead policy makers to develop efficient policies to control O3 pollution especially in the O3 episode dominant by local production.

Keywords: HONO, O3 pollution, OBM, model improvement, O3 sensitivity