Simulating nitrate formation mechanisms during PM2.5 events in Taiwan and their implications for the controlling direction

The long-term downward trend of NOX concentrations does not reflect the reduction of nitrate (NO3-) in Taiwan. Instead, the proportion of NO3- in PM2.5 increased in recent years. To probe the increasing importance of NO3- in PM2.5, this study applied the WRF/CMAQ modeling system to implement a simulation from 16 March 2017 to 30 April 2017, in which 5 p.m.2.5 events with daily average concentrations & GE;35 mu g m(-3) and their corresponding correlation coefficients (R) of simulated and observed PM2.5 above 0.6 were selected for analysis. During the daytime, the reaction of NO2 and OH contributed more than 90% of the total HNO3. After sunset, the high concentrations of NO3 and N2O5 peaked, followed soon by the simultaneous rise of NO3-, aerosol water content, and HNO3 concentrations around midnight, which indicated that the heterogeneous reaction was the main formation mechanism of NO3-, accounting for approximately 30%-90% of total HNO3. At nighttime, the daytime-formed gaseous phase NO3- condensed, and low wind and low boundary layer height favored accumulation; therefore, PM2.5 peaked around the midnight period to the early morning. The sensitivity test showed that doubling and halving the NOX and NH3 & nbsp;emissions could directly lead to the highest production and reduction of NO3-, respectively, followed by doubling and halving NMHC emissions, which caused the highest and lowest O-3 & nbsp;concentrations. The correlation analysis of the simulation results showed that the daytime maximum O-3 & nbsp;and HNO3 & nbsp;were highly correlated. The relationships between daytime maximum O-3, nighttime maximum NO3, N2O5, and HNO(3 & nbsp;)in pairs were also moderately to highly correlated. This study implies that in addition to direct reduction of NOX or NH3 & nbsp;emissions, controlling O-3 & nbsp;is possibly another useful strategy to reduce NO3-. Because NOX emission reduction could conflict with controlling O-3, this study suggests to re-examine the determination of NOX-limited and VOCS-limited regions in order to develop strategies for reducing NOX emission and O-3 simultaneously.