Characteristics And Source Apportionment Of Water Soluble Components In PM_2.5 In Chongqing

As one of the key areas for air pollution prevention and control in China,its complex basin terrain and unique meteorological conditions play an important role in the formation of haze in Chengdu-Chongqing area.The sampling campaigns were conducted simultaneously in Yubei(YB)and Wanzhou(WZ)from April 2015 to January 2016.The spatial and temporal variations of water-soluble inorganic ions(WSIC)and water-soluble organic carbon(WSOC)in PM2.5 at urban and non-urban sites in Chongqing were analyzed.The UV-visible absorption characteristics of WSOC were investigated,and the key role of water-soluble components on the formation of heavy pollution in winter was illustrated.The main conclusions are as follows.The annual mean concentrations of PM2.5 in YB and WZ were 67.4±31.2 μg/m3 and 40.9±24.0 μg/m3,respectively,with the highest concentration in winter and the lowest in summer.The effect of rainfall on PM2.5 removal was obvious,however,when the rainfall amount was low,PM2.5 concentrations would increase rather than decrease due to the increase of relative humidity.Water-soluble ions were major chemical components of PM2.5,accounting for 38.9%-50.2%.SO42-,NO3-and NH4+were dominant components of water-soluble inorganic ions,accounting for an average of 90%of total water-soluble ions.Each water-soluble ion showed different seasonal variations,e.g.,the highest concentration of Na+was observed in summer,whereas the other inorganic ions were the highest in winter.(NH4)25O4,NH4NO3 and NH4CI were the primary forms of water-soluble inorganic ion species.The average annual values of NO3-/SO42-were 0.62 and 0.43 at YB and WZ,respectively,indicating that the stationary source of coal-fired emissions and the mobile source emissions of motor vehicles were both important for PM2.5 pollution in Chongqing,although the stationary source has a greater impact on the atmospheric environment.The annual mean values of[NH4+]/[NO3-]were 6.0 and 5.28,respectively,suggesting that agricultural emission was a major contributor to nitrogen.The average concentrations of WSOC in YB and WZ were 3.87±2.22 μg/m3 and 3.46±2.38 μg/m3 during the whole sampling period,respectively,accounting for 33.3%and 35.7%of OC.AAE in Chongqing varied from 4 to 13.The annual average light absorption were 6.5±4.5 Mm-1 and 4.7±4.9 Mm-1 at YB and WZ,respectively.Furthermore,the mass absorption efficiency were 1.4±0.6 m2/g and 1.0±0.4 m2/g at YB and WZ,respectively,indicating that WSOC had a strong light absorption capacity in Chongqing,which may have more complicated effects on the climate and tropospheric photochemical reactions in this region.Based on the analysis of optical parameters such as SUVA254,SUVA280 and A250/A365,results showed that WSOC at WZ had a lower aromaticity and molecular weight than that of YB.During the heavy pollution period,the mean values of PM2.5 mass concentration in YB and WZ were 132±39.7 and 92.8±10.6 μg/m3,respectively,and the corresponding SNA was 2.4-2.5 and 2.1-2.2 times higher than that during cleaning period.The high PM2.5 concentrations in winter at Yubei were primarily caused by the local emission,whereas both local emissions and long-distance transport were contributed to the PM2.5 pollution at WZ.