Growth of nitrate contribution to aerosol pollution during wintertime in Xi'an, northwest China: Formation mechanism and effects of NH3

With the strengthened controls on SO2 emissions and extensive increases in motor vehicles' exhaust, aerosol pollution shifts from sulfate-rich to nitrate-rich in recent years in Xi'an, China. To further gain insights into the factors on nitrate formation and efficiently mitigate air pollution, highly time-resolved observations of water-soluble inorganic ions (WSIIs) in PM2.5 were measured in a suburban area of Xi'an, China during wintertime. Hourly concentration of total WSIIs is 39.8 mu g m(-3) on average, accounting for 50.3% of PM2.5 mass. In contrast to a slight decrease in the mass fraction of SO42-, NO3- shows a significant increase of the PM2.5 contribution with the aggravation of aerosol pollution. This suggests the importance of NO3- formation to haze evolution. Furthermore, homogeneous reactions govern the formation of NO3-, while alkali metals such as calcium and sodium play an additional role in retaining NO3- in PM2.5 during clean periods. However, the heterogeneous hydrolysis reaction contributed more to NO3- formation during the pollution periods under high relative humidity. Our investigation reveals that temperature, relative humidity, oxidant, and ammonia emissions facilitate rapid NO3- formation. Using the random forest (RF) model, NO3- concentrations were successfully simulated with measured variables for the training and testing datasets (R-2 > 0.95). Among these variables, CO, NH3, and NO2 were found to be the main factors affecting the NO3- concentrations. Compared with the period without vehicle restriction, the contributions of NO3- and NH4+ to PM2.5 mass decreased by 5.3% and 3.4% in traffic restriction periods, respectively. The vehicle restriction leads to the decreases of precursor gases of NO2, SO2, and NH3 by 12.8%, 5.9%, and 27.6%, respectively. The results demonstrate collaborative emission reduction of NOx and NH3 by vehicle restrictions, and using new energy vehicles (or electric vehicles) can effectively alleviate particulate matter pollution in northwest China.