PM2.5 acidity and secondary nitrate formation during typical pollution episodes of four seasons in Beijing
Based on the online monitoring data from urban sites in Beijing during 2022,the ISORROPIA-Ⅱ model was used to analyze the acidity of PM2.5 during four typical pollution episodes,namely haze pollution in winter,dust pollution in spring,high ozone pollution in summer,and PM2.5 and O3 combined pollution in autumn.The evolution of aerosol pH with the rapid formation of nitrate was obtained.The results show that the aerosol pH in Beijing was moderately acidic,in ranges of 3.59～5.07,3.70～7.76,2.44～6.15,and 2.80～4.69 for the four episodes,respectively.The aerosol pH exhibited normal distribution for the four pollution episodes with median values of 4.60,4.59,3.91 and 4.09,respectively.During winter haze pollution,aerosol water content and the aerosol pH were the highest.During spring dust pollution,the aerosol pH presented bimodal distribution,affected by both anthropogenic and natural sources.The acidity of PM2.5 in summer and autumn episodes was the strongest and the second strongest respectively,which might be related to the oxidation of acid gas enhanced by strong atmospheric oxidation conditions.During high ozone pollution in summer,with high temperature and low pH,HNO3 tended to be distributed in the gas phase,and the nitrate proportion in PM2.5 was the lowest(22％).For PM2.5 and O3 combined pollution in autumn,pH was low in the daytime and high in the nighttime,conducive to the nitrate accumulation at night after the gaseous HNO3 formation in the daytime.The nitrate proportion was comparable to that during pollution episodes in winter and spring,reaching 27％～28％.Gaseous NH3 was abundant,and the neutralization(homogenization)reaction between HNO3 and NH3 was the main formation mechanism of NO3-.Abundant of NH4+,(NH4)2SO4,NH4NO3 and NH4C1 in aerosol could also be fully coupled.This research shows that high aerosol water content and aerosol pH were responsible for the rapid growth of nitrate during pollution.Further reduction of the precursors NH3 and NOx would be an effective means to control the fine particles in Beijing.
万方数据
