查看更多>>摘要：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 μg 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 sig-nificant 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(R2＞0.95).Among these variables,CO,NH3,and N02 were found to be the main factors affecting the NO3-concentrations.Compared with the period without vehicle re-striction,the contributions of NO3- and NH4+ to PM2.5 mass decreased by 5.3％and 3.4％in traffic re-striction 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.
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