[Pollution Characteristics of NH 4 + ,NO 3 - ,SO 4 2- in PM 2.5 and Their Precursor Gases During 2015 in an Urban Area of Beijing].

Simultaneous measurements of precursor gases NH ₃ ,NO,NO ₂ ,SO ₂ and the main water-soluble ions in PM _2.5 such as sulphate (SO ₄ ^2- ),nitrate (NO ₃ ^- ) and ammonium (NH ₄ ⁺ ) (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 NH ₄ ⁺ , NO ₃ ^- and SO ₄ ^2- , 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 NH ₃ , NO, NO ₂ , SO ₂ , NH ₄ ⁺ ,NO ₃ ^- and SO ₄ ^2- 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 PM _2.5 . The concentrations of SO ₂ ,NO _x and SNA declined compared to 2014. The concentrations of NO,NO ₂ and SO ₂ were highest in winter and lowest in summer. The concentration of NH ₃ was higher in summer and lower in autumn; The concentration and the percentage of NH ₄ ⁺ were stable during the four seasons,both the concentrations and the percentage of NO ₃ ^- were lowest in summer. The concentrations of SO ₄ ^2- was highest in winter and the percentage was lowest in summer. The ratio of ([NO ₃ ^- ]+2[SO ₄ ^2- ]) and NH ₄ ⁺ was 0.97 during the whole year, showing that anions mainly existed in the form of NO ₃ ^- and SO ₄ ^2- . In summer, the ratio of[NO ₃ ^- ]+2[SO ₄ ^2- ] and[NH ₄ ⁺ ] was slightly higher than 1.0, which was the reason why NO ₃ ^- was bound to Ca ²⁺ , Mg ²⁺ and Na ⁺ besides NH ₄ ⁺ . With the increase of pollution, the mass concentration of precursor gases and SNA increased dramatically, among which NO _x increased most rapidly, and SO ₂ decreased from severe pollution to serious pollution. The contribution rate of NH ₄ ⁺ was maintained at a relatively stable level. SO ₄ ^2- had a higher contribution when the pollution level was lower, whereas the concentration of NO ₃ ^- was higher than others and contributed most to PM _2.5 in heavy pollution. Heterogeneous transformation on the surface of particulate matter played a more important role in the formation of SO ₄ ^2- and NO ₃ ^- . The correlations between NO ₃ ^- , NO ₂ and NO,NH ₄ ⁺ and NH ₃ ,SO ₄ ^2- and SO ₂ were significant at the confidence level of 0.01. SO ₄ ^2- had negative correlation with SO ₂ , and NO ₃ ^- had positive correlation with NO ₂ . Compared with NH ₃ , the NH ₄ ⁺ concentration was more obviously affected by acid gases NO ₂ , SO ₂ .