Characteristics of PM2.5 and SNA components and meteorological factors impact on air pollution through 2013-2017 in Beijing, China

Beijing, one of the world's largest cities, has been tackling its severe air pollution problems in the past decades, and achieved pronounced reduction on PM2.5 concentration from 99.4 mu g/m(3) to 69.4 mu g/m(3) during 2013-2017. This study focused on the variation trends of SO42-, NO3(-) and NH4+ (SNA) of PM2.5 during this effective improvement period, through a five-year PM2.5 sampling project in urban area. Measurement showed that SNA presented a noticeable decline trend, from 16.6 +/- 19.8 mu g/m(3) to 9.7 +/- 13.3 mu g/m(3) for SO42-, from 16.5 +/- 16.2 mu g/m(3) to 13.1 +/- 16.9 mu g/m(3) for NO3-, from 10.2 +/- 10.1 mu g/m(3) to 9.1 +/- 9.9 mu g/m(3) for NH4+. However, their proportions presented different variations, a decline trend from 14.3 +/- 7.7% to 12.9 +/- 6.6% for SO42-, a raise trend from 15.2 +/- 7.2% to 16.0 +/- 7.7% for NO3- and from 9.6 +/- 3.7% to 11.7 +/- 4.4% for NH4+. The concentration and proportion of SNA significantly increased in heavy polluted days (HPD) compared to non-heavy polluted days (NHPD) through 2013-2017. The ratio of NO3- /SO42- was lower in HPD than NHPD in 2013-2015. However, it was generally larger in 2016 and 2017, indicating the importance of NO3- in the formation of PM2.5 pollution. SOR increased obviously through 2013-2017, which was mainly owed to the enhanced atmospheric oxidation and the regional transport related to regional annual emission variations. HPD mainly occurred in autumn and winter, which was generally related to the south and east wind. RH was a major cause for SNA pollution. Higher increment of SNA concentration as well as SOR and NOR was observed when the RH increased from 40-60% to 60-80%.