Interannual Evolutionary Characteristics Of PM_2.5 Chemical Composition In Nanjing

In recent years,the concentration and chemical composition of atmospheric fine particles(PM2.5)have changed significantly with the implementation of a series of action plans and measures on air pollution control in China.PM2.5 chemical composition is not only closely related to human health and environmental effects,but also important for studying the source of PM2.5 and formulating pollution control strategies.Therefore,studing the evolutionary characteristics of PM2.5 chemical composition is significantly for understanding the sources of PM2.5 and improving pollution control strategies.This study is based on the monitoring data of PM2.5 concentration and chemical composition in multiple locations and different seasons in Nanjing from 2011 to 2016.It focused on the following three aspects.(1)Concentration and chemical composition evolutionary characteristics of PM2.5 were analyzed at full-scale and different seasons in Nanjing from 2011 to 2016,based on concentration and reconstruction of PM2.5 speciation.(2)The evolutionary characteristics of water-soluble ions,carbon components and inorganic elements in PM2.5 were studied separately at full-scale and different seasons.(3)The sources of PM2.5 in Nanjing from 2011 to 2016 were analyzed using correlation analysis,enrichment factor,principal component analysis and other methods.(4)The efficency of pollution control measures was discussed in light of the changes in PM2.5 concentrations and chemical composition during the Asia Youth Association and Youth Olympic Games.The main conclusions of this study are as follows:(1)Interannual evolution of PM2.5 chemical composition in Nanjing:the study period(2011-2016)was divided into early period(2011-2012),medium period(2014-2015)and recent period(2016)during the annual scale discussion.The average annual concentrations of PM2.5 were 71.28 ?g/m3,67.76 ?g/m3 and 66.90 ?g/m3,respectively,showing a decreasing trend year by year.Water-soluble ions dominated by secondary sources(SNA)and organic carbon components(OC)were the two largest components of PM2.5 during 2011-2016.The average proportion of SNA and OC were 44.8%and 16.8%,respectively.The reconstruction of PM2.5 speciation showed that the relative proportions of sulfate,nitrate and soil dust(CM)in PM2.5 had a decreasing trend.The proportions of them in PM2.5 decreased by 7.6%,13.5%,and 1%,respectively,comparing to the early period,indicating that the control measures for coal and dust pollution in Nanjing have been taken effectively recently.(2)Interannual evolution of chemical species in PM2.5:the average concentrations of water-soluble ions decreased at different levels throughout the years.The concentrations of NO3-,SO42-and NH4+ decreased by 24.44%,27.11%,and 11.92%,respectively,the concentrations of Ca2+,Na+,and Mg2+all decreased more than 60%,and the concentrations of K+also decreased by nearly half,comparing to the early and recent period,.In addition,the ratio of[NO3-]/[SO42-]increased from 0.99 to 1.05,indicating that the contribution of motor vehicles has an increasing trend.The ratio of AE/CE increased from 0.92 to 1.04,indicating that PM2.5 in Nanjing evolved from weakly alkaline to slightly acidic.There was no significant change on carbon composition at the annual scale.The average concentrations of OC in the three periods were 11.57 ?g/m3,11.95 ?g/m3,12.36?g/m3,respectively,and the average concentrations of EC were 3.97 ?g/m3,3.64 ?g/m3,4.45 ?g/m3.The concentrations of OC and EC were relatively low during the period of 2014-2015,indicating that the control measures implemented during the Youth Olympic Games have lightened the carbon component pollution.The concentrations of inorganic elements such as K,Ca,and Mg decreased during the period of 2014-2015 due to control measures such as the prohibition of burning of straw and dust control during and after the Youth Olympic Games.(3)Changes in the source of chemical constituents of PM2.5 in Nanjing:there was a strong correlation among NO3-,SO42-and NH4+,suggesting that they were mainly derived from the same secondary conversion process.It showed that the contribution of coal combustion has declined while the contribution of motor vehicles has increased,based on the decline of SNA and the rising trend of[NO3-]/[SO42-].The higher concentration of K+b(with a mean value of 0.50 ?g/m3)in autumn indicated that the biomass combustion pollution was more serious in this season.Enrichment of elements indicated that the contribution of dust from soil to elements in PM2.5 declined,while the impact of other human activities increased significantly.In general,the main sources of PM2.5 in Nanjing are coal burning,industrial processes,motor vehicles,and construction dust.The relative contribution of engine power has increased significantly,but the source of building and soil dust has decreased.(4)The concentrations of SO42-,NH4+ and EC decreased by 16.9%,36.6%and 51.3%during the Asia Youth Association(August 16-August 24,2013),comparing to the same period of 2011-2012.The concentrations of SO2,NO2,and O3 decreased by 41.6%,43.8%and 30.5%,compared with the the eight days before the association.During the Youth Olympic Games(August 16-August 28,2014),the concentration of SOC decreased by 57.8%compared with the same period of 2015-2016,but the concentration of NO3-and SO42-increased compared with that during the Asia Youth Association.The temporary emission reduction and control measures have obviously reduced the concentration of the main components and gaseous pollutants of PM2.5 during the Asian Youth Council and the Youth Olympic Games.