Research On Pollution Characteristics And Source Apportionment Of PM_2.5 In Urban Nanchang

In this study,monitoring data of PM_2.5.5 and meteorological data were collected in Nanchang city from in 2013 at nine sites.PM_2.5 and pollution source samples were collected in September of 2013 to analyze the concentrations of inorganic elements,polycyclic aromatic hydrocarbons?PAHs?,water-soluble ions,elemental carbon?EC?and organic carbon?OC?in the PM_2.5 samples.Meanwhile,the list of atmospheric PM_2.5 emission sources in local areas of Nanchang was established,and a platform for simulating the regional air quality in Nanchang city was set up based on the localized emission simulation and multi-scale air quality model?CMAQ?.Then,the pollution characteristics and sources of PM_2.5 in Nanchang city were studied by using the multivariate statistical analysis,backward trajectory,PSCF?potential source contribution factor?,CWT?concentration weight trajectory analysis?,CMB?chemical mass balance method?and CMAQ.The main results are as follows.?1?The annual average PM2.5 concentration in Nanchang in 2013 was 69.1?g/m³,which was 1.97 times of the secondary national standard limit?35?g/m³?.The diurnal variation curve showed a bimodal distribution with the peaks at 9:00¹1:00and 20:00²2:00.The monthly average concentration value of PM_2.5.5 at each station showed a"V"shaped trend,i.e.,higher on both sides while lower in the middle.The seasonal variation of PM_2.5 was significant with the sequence of winter>spring>autumn>summer.The spatial characteristics of PM_2.5 showed a significant decreasing trend from southeast to northwest.PM_2.5 concentration was positively correlated with air pressure,while negatively correlated with relative humidity,temperature and mean wind velocity.?2?The potential sources and transport characteristics of PM2.5 in the main air flow trajectory in four seasons and during the heavy pollution events in Nanchang in2013 were studied using the methods of trajectory clustering,PSCF and CWT.The airflow trajectories were mainly from south and southeast in summer,with the contribution of 65.6%.However,the inland airflow trajectories were mainly from northeast and north in winter,spring and autumn,with the contributions of 62.0%,59.6%and 54.7%,respectively,which greatly affected the PM2.5 level in Nanchang.The higher PM_2.5 values analyzed by PSCF and CWT methods were in Nanchang,Zhejiang province and northern Fujian province,while southern Henan province and central Jiangsu province were also the potential source areas of PM_2.5 in Nanchang.The results of the two PM_2.5 heavy pollution processes showed that there was a PM_2.5pollutant air transport channel in the north and northeast of Nanchang,and the PM2.5mainly came from the surrounding areas of Nanchang during the two heavy pollution events.?3?In September of 2013,the average concentrations of inorganic elements in PM_2.5 show an order of S>Ca>Si>Na>A1>K>Fe>Mg>Ba>Zn>Ni>Pb>Mn>Ti>Br>Cr>Cu>As>V>Cd>Co>Hg.The results of enrichment factors showed that Mn,Ti,Al and V were mainly influenced by natural sources;Fe,Cr,Co,Mg,K,Ba,Ca,Cuand As were influenced by both natural sources and anthropogenic sources;Zn,Pb,Hg,Ni and Cd were significantly affected by anthropogenic emissions.The combined results of cluster analysis and factor analysis revealed the pollution sources of these metals in PM_2.5:Mg,K,Al,Ca and Ti mainly came from natural soil and building material dust;As and Hg were mainly from coal combustion;Ba,Ni and Mn were mainly from industrial emission of metal smelting;V,Cu,Fe,Cd,Pb,Cr and Co mainly came from traffic sources;Zn mainly came from metal smelting and coal burning.The average concentrations of water-soluble ions in PM_2.5.5 showed the order of SO₄^2->NO₃^->NH₄⁺>Ca²⁺>K⁺>Na⁺>Cl^->Mg²⁺>F^-,in which the sum contribution of SO₄^2-,NH₄⁺and NO₃^-was 79.4%⁹5.2%.The PAHs of 5 to 6 ring were the main components?accounting for 68.4%?of total PAHs,showing significant characteristics of vehicle exhaust emission.The threat of PAHs was calculated by BaP equivalent concentrations,indicating that PAHs was potentially harmful to human health in Nanchang city.The results of the correlation analysis and the ratio of OC/EC showed that the sources of OC and EC in PM2.5 were the same.OC and EC were mainly affected by coal combustion and vehicle exhaust emissions.The average ratios of primary organic carbon?POC?to OC in PM_2.5 was 81.0%,indicating that the pollution of secondary organic carbon in PM_2.5.5 in Nanchang city.?4?The simulating system for Nanchang regional air quality was established using the CMAQ model.A preliminary validation study was conducted for the period of September 2013,by comparing the simulated results from the model system with the oberseved data from regional air quality monitoring network.The results showed that the established CMAQ model could be employed to simulate the situation and the trend of meteorological factors?temperature,relative humidity,wind speed and wind direction?and pollutant(SO₂,NO_x,PM_2.5,PM₁₀0 and O₃).The simulation error was within 50%at most monitoring sites,indicating that the system had good reliability and could basically meet the needs for Nanchang air quality simulation and pollution control scenario analysis.?5?The"source switch method"was applied to identify and analyze the pollution transmission and contributions of various sources from other cities in Jiangxi province to the PM_2.5 in Nanchang city.The results showed that the largest contributor of PM_2.5 was the local emissions with the contribution rate of 67.2%,while about 20.7%of PM_2.5 was transported from the regions outside the Jiangxi provice.Besides Nanchangcity within Jiangxi provice,Jiujiang city contributed the highest rate of 7.8%,followed by Shangrao city with the contribution rate of 2.8%,the total contribution rate of other cities was less than 1%.The results were consistent with those of PM_2.5 backward trajectory,PSCF and CWT analysis.?6?In order to quantitatively analyse the origin of PM_2.5 in Nanchang,CMB method was introduced and the results indicated that dust and motor vehicle exhaust dust had significant contribution to PM_2.5 in autumn with the contribution rates of24.5%and 22.5%,respectively.The contribution of coal dust,construction cement dust and metallurgical dust to PM_2.5 were only 5.2%,3.4%and 1.4%,respectively.The contribution rate of other source was 17.3%.CMAQ and CMB methods were used to analyze the external contribution rate of PM_2.5 in autumn in Nanchang,and the results indicated that the local sources and the external sources contributed 67.2%and 32.8%,respectively.In the local sources,the main contributors were the dust and traffic pollution with the contribution rates of 23.2%and 19.8%,respectively.