Temporal And Spatial Characteristics Of Secondary Components Of Pm_2.5 In Nanjing

Secondary components are important constitute of PM_2.5,mainly including SO₄^2-,NO₃^-,NH4+ and SOA.513 PM_2.5 samples were collected in six sampling sites in Nanjing during four seasons from October 2014 to August 2015.Based on the chemical compositions,temporal and spatial characteristics of secondary components of PM_2.5 in Nanjing were analyzed.（1）Averagely,the secondary components accounted for about 57.7%of the PM_2.5 mass concentration.Concentrations of SO₄^2-,NO₃^-,NH4+ and SOA were 11.9,12.5,7.7,and 8.7μg/m3,with proportions of 17.2%,16.9%,10.5%,and 13.1%,respectively.The value of[NO₃^-]/[SO₄^2-]has been increasing rapidly in recent years and reached about 1.0 in 2015,suggesting that vehicle pollution has been aggravated in Nanjing and Nanjing has transformed from the traditional coal pollution to coal and vehicle joint pollution.The proportions of NO₃^-,NH4+ and SOA in Nanjing are high in autumn and winter,lower in spring and summer（gap in 4.5%,3.1%,7.0%respectively）,but SO₄^2-goes the opposite trend（gap in 2.5%）.Proportion of secondary components is the highest（73.5%）in the outer suburb of the city,followed by the suburb regions（59.0%）,and lowest in city center and industrial region（57.3%,57.4%）,reflecting that PM_2.5 in outer suburb are mainly come from secondary transformation in the process of regional transport,primary pollution has important effect to the city center and industrial region,and suburb regions significantly influenced by the dual pollution of industrial coal and urban moter vehicle.Comparison with similar study of Nanjing,the concentration of SO₄^2-droped sharply showing that Nanjing coal pollution reduce obviously,meanwhile he concentration and propotion of NO₃^-increase substantially showing that Nanjing vehicle pollution rise obviously,and secondary pollution level in Nanjing enhanced relatively.（2）SIA propotion increasing with the rise of pollution level and SOA goes the opposite trend,suggesting that more inorganic aerosols secondary transformation and less organic aerosols secondary transformation in heavy PM_2.5 pollution.Proportions of SO₄^2-up and down while proportions of NO₃^-keep steadily increasing with PM_2.5 concentrations rise（increase from 14.6%in light pollution to 19.8%in heavy pollution）,suggesting vehicle pollution may play a more important role in heavy haze pollution episodes,and the conversion rate of various secondary components may change in different pollution level.Proportions of SA were highest in winter（65.86%）under mild pollution level and highest in autumn（63.47%,65.80%）under moderate and severe pollution lever,SOA accounted for about 2 times higher in autumn and winter than spring and summer,the formation of SO₄^2-in summer play an important role in moderate pollution and the formation of NO₃^-play an important role in severe pollution.The secondary components also have obvious difference in different seasons under different pollution levels,with pollution level rise,secondary component in PM_2.5 have fallen sharply in spring and rised in summer and autumn(especially SO₄^2-).During haze in winter,SA proportion decli1e reflecting that the contribution of primary pollution enhanced relatively.Precursor control measures can reduce emissions and primary pollutants effectively,lead to secondary aerosols accounted increase.（3）SO₄^2-,NO₃^-and NH4+have some degree of correlations showing that these secondary inorganic components may have same formation process,NO₃^-and NH4+have good correlation speculated that NH4NO3 exist,SO₄^2-+NO₃^-and NH4+ have good correlation speculated that NH4HSO4 and NH4NO3 coexist in the system.There is strongest correlation between SO₄^2-and SO2,SO₄^2-and O3 in summer,suggesting that summer photochemical oxidation reaction has important influence to SO₄^2-formation.There is strongest correlation between NO₃^-and NO2 in winter and between NO₃^-and O3 in summer,suggesting that NO2 has more conversion rate in winter and summer photochemical oxidation reaction has important influence to NO₃^-formation.SOA and O3 did not show good correlation,suggesting that the atmospheric photochemical oxidation used O3 as oxidizer is not the important way of SOA formation.This study deepened the understanding of the sources of PM_2.5 in Nanjing,and can provide scientific basis for the haze pollution control.