Pollution Characteristics of NH4+,NO3-,SO42- in PM2.5and Their Precursor Gases During 2015 in an Urban Area of Beijing

Abstract

Simultaneous measurements of precursor gases NH3,NO,NO2,SO2 and the main water-soluble ions in PM2.5 such as sulphate (SO42-),nitrate (NO3-) and ammonium (NH4+) (collectively called SNA)were carried out in the urban area of Beijing during 2015-01 to 2015-12,which obtained 325 groups of samples. PTFE membrane filters were used to collect particulate NH4+, NO3- and SO42-, followed by the online instruments to collect precursor gases. The pollution characteristics of the precursor gases and SNA were analyzed and their correlation was studied. The mean concentrations of NH3, NO, NO2, SO2, NH4+,NO3- and SO42- were 21.5, 17.7, 54.3, 14.2, 8.1, 13.5 and 12.7 μg·m-3 respectively during the period of monitoring, and SNA accounted for 43.4% of PM2.5. The concentrations of SO2,NOx and SNA declined compared to 2014. The concentrations of NO,NO2 and SO2 were highest in winter and lowest in summer. The concentration of NH3 was higher in summer and lower in autumn; The concentration and the percentage of NH4+ were stable during the four seasons,both the concentrations and the percentage of NO3- were lowest in summer. The concentrations of SO42- was highest in winter and the percentage was lowest in summer. The ratio of ([NO3-]+2[SO42-]) and NH4+ was 0.97 during the whole year, showing that anions mainly existed in the form of NO3- and SO42-. In summer, the ratio of[NO3-]+2[SO42-] and[NH4+] was slightly higher than 1.0, which was the reason why NO3- was bound to Ca2+, Mg2+ and Na+besides NH4+. With the increase of pollution, the mass concentration of precursor gases and SNA increased dramatically, among which NOx increased most rapidly, and SO2 decreased from severe pollution to serious pollution. The contribution rate of NH4+ was maintained at a relatively stable level. SO42- had a higher contribution when the pollution level was lower, whereas the concentration of NO3- was higher than others and contributed most to PM2.5 in heavy pollution. Heterogeneous transformation on the surface of particulate matter played a more important role in the formation of SO42- and NO3-. The correlations between NO3-, NO2 and NO,NH4+ and NH3,SO42- and SO2 were significant at the confidence level of 0.01. SO42- had negative correlation with SO2, and NO3- had positive correlation with NO2. Compared with NH3, the NH4+ concentration was more obviously affected by acid gases NO2, SO2.