| In order to understand the physicochemical characteristics of heavy metals and single particles in atmospheric particulate matter in Nyingchi City,this study set two sampling points at the key points of the transport channel of South Asian warm and humid air flow in the great bend of Yarlung Zangbo River in Nyingchi City,and collected atmospheric single particulate matter samples and atmospheric particulate matter PM10and PM2.5samples.An inductively coupled plasma mass spectrometer（ICP-MS）was used to detect and analyze 15 kinds of metals in atmospheric particulate matter PM10and PM2.5samples in Nyingchi City,including Mg,Al and Ca,3 light metals,Fe,Ti,Zn,Mn,Ba,V,Cu,Ni,Pb,Co,Cd and Cr.The physical and chemical composition of atmospheric single particulate matter samples in Nyingchi City was analyzed by transmission electron microscopy（TEM-EDS）with energy dispersitive spectrometer.The main research results are as follows:（1）The daily mean concentrations of PM10and PM2.5in Nyingchi urban area were 15.28μg/m3and 7.45μg/m3,respectively,while those in suburban area were 12.86μg/m3and 5.80μg/m3.All are lower than the ambient air quality standard（GB3095-2012）PM10and PM2.5levelⅠstandards of50μg/m3,35μg/m3concentration limits.The sampling point of Nyingchi City is located near Bayi Bridge.Under the background of more traffic sources and the highest concentration of particulate matter,the PM10/PM2.5ratio ranges from 0.20 to 0.84,with an average value of 0.53.On December9,the ratio is 0.84,indicating that the PM10/PM2.5ratio in Nyingchi City is relatively high in winter and is mainly formed secondary.It is mainly fine particulate matter and the pollution degree is relatively light.On December 2,the PM10/PM2.5ratio was 0.2,indicating that the primary particle emission of coarse particles was mainly caused by the high wind speed in December and the large contribution of dust to coarse particles.Combined with the time series distribution of air quality parameters in Nyingchi for 6 years,the six conventional pollutants were all lower than the annual concentration limit of NAAQS Level I,reflecting the overall excellent level of air quality in Nyingchi.（2）The average sum of Mg,Al and Ca light metals in PM10samples from Bayi Town is 977.23ng/m3,accounting for 6.4%of the total particles and 73.53%of the total metal elements,which are the main metal elements in the particles.The concentration of heavy metals in atmospheric PM10in Nyingchi City from high to low is Fe>Ti>Zn>Mn>Ba>V>Cu>Ni>Pb>Co>Cd>Cr.The concentration of heavy metal elements is 301.62 ng/m3Fe,17.53 ng/m3Ti and 9.63 ng/m3Zn.These three elements account for 93%of the total heavy metals,so they are the main components of heavy metals in particulate matter,and the sum of the remaining heavy metal elements is less than2%of the total elements.Cr is below the detection limit.The aggregate average concentration of Mg,Al and Ca light metals in PM2.5is 440.17 ng/m3,accounting for 2.9%of the total particulate matter and 64.46%of the total metal element.The remaining 13 heavy metals account for25.5%～18.6%of the total metal element in PM10and PM2.5.The distribution of heavy metals in PM2.5is similar to the sequence of PM10:Fe>Zn>Ti>Ba>V>Mn>Cu>Ni>Pb>Cd>Co>Cr.Fe concentration of 83.78 ng/m3,Ti concentration of 5.03 ng/m3and Zn concentration of 6.09ng/m3 are the main components of heavy metals in the particulate matter,accounting for 89%of the total heavy metals,and the sum of the remaining heavy metals is less than 2%of the total elements.The lowest values of Co and Cd were 0.01 ng/m3,and Cr was lower than the detection limit.In addition,the element content of PM10and PM2.5showed relatively regular change characteristics,indicating that they had similar sources and compositions.（3）Cr was not detected in the concentration of heavy metals in particulate matter,so it did not participate in the risk assessment.Starting from the background values of major elements such as Mn,Zn,Cu,Pb,Ni and Co in Xizang soil.The evaluation results of potential ecological risk of heavy metals showed that the potential ecological risk index RI value of PM10and PM2.5was less than 150,and the overall arrangement was Cd>Pb>Ni>Co>Zn>Cu>Mn.The ecological harm degree of heavy metals Mn,Zn,Cu,Pb,Ni and Co was slight.The potential risk coefficient of heavy metal Cd in PM10was between 40 and 80,which was a moderate ecological hazard.The potential risk coefficient of heavy metal Cd in PM2.5is less than 40,indicating a slight ecological risk.Under the background that Cd occupies a large proportion in the potential risk index of all heavy metals,the research results provide a certain basis for the preliminary quantitative assessment of the potential ecological risk of the atmosphere in Nyingchi City and contribute to further development.（4）Compared with the contribution ratio diagram and time series of PMF source identifical factors of heavy metals in PM10atmospheric particles,the analytical results showed that there were four main factors,including biomass combustion（37.3%）,crust/soil dust（27.3%）,vehicle emission（13.6%）and mineral dust（21.6%）.Factor 1 is the crustal source/soil source,and the tracer elements are Ca,Fe and Al.These three are alkaline earth metal elements in the crust,accounting for about4%of the crust abundance,accounting for the fifth place;The second factor is transportation source,typical elements Pb,Cd,Ni,Co,Ni is fossil fuel;The third factor is the mixed source of coal combustion and biomass combustion,and the signature elements are Mn,Cu and Cd.The fourth factor is the source of mineral exploitation,and the characteristic elements are V,Zn and Cu.The PMF of heavy metals in PM2.5atmospheric particles can be divided into four main factors:biomass combustion（26.7%）main tracer elements Co,Cr,Mn,Fe;The main tracer elements of crust/soil dust（18.5%）were Ca,Al and Fe;The iconic elements of vehicle emission（27.2%）were Pb,Zn,Cu and Ni;In residential fires and fireworks activities（27.6%）,the iconic elements were Mg,Ti and Ba.According to the source,the main pollution sources and contribution degree were determined,and the sources of heavy metals under different particle sizes were analyzed.（5）The phase characteristics and chemical composition analysis of atmospheric single particulate matter samples from Nyingchi City showed that the atmospheric single particulate matter was mainly composed of mineral particles,fly ash,organic matter,S-rich particles,soot particles and metal particles.Usually,each type of particulate matter does not contain only one component,or is mixed by one component or multiple components.In this study,single particulate matter contains sulfate（potassium-calcium and sodium mixed）,mineral（iron-rich particles）,fly ash,organic particulate matter（OM）,organic-sulfate（aluminum-potassium）and other salts.The single particulate matter samples in urban areas are more rich in compound salts and fly ash than those in suburban areas.The shape characteristics are mainly divided into 10 types:long flat,square,nearly round,irregular,circular,elliptical,mitsubishi enclosed,circular enclosed organic matter,chain enclosed and transparent organic matter enclosed.The air mass backward trajectory was used to simulate the different altitudes of Nyingchi City in 72 hours on December 3,5,7,9,11,13.The meteorological data further showed that the air flow with the height of 50 meters,500 meters and1000 meters was dominated by the air flow from northeast India and part of the air flow from northwest Nepal.The transmission process of these air flows through southeast Tibet and India may have an impact on the underlying particles.The transboundary transport of pollutants from some parts of South Asia across the Himalayas has a certain impact on the concentration of particulate matter and heavy metal in the lower atmosphere of Nyingchi Prefecture and even southeast Tibet.It is helpful to judge the origin mechanism and complex reaction process of single particle by statistics on the type and component characteristics of single particle.（6）From the perspective of meteorological factors in the past six years,except for the mean temperature and the mean temperature,the mean wind speed was 4.19±0.52 m/s,and the mean temperature was 10.32±0.42℃.It can be seen that the peak value of ozone is about 84μg/m3in2018,and the peak value is 76μg/m3in the second half of 2020,with an average value of81.26±2.98μg/m3.Precipitation showed a regular cosine function,which reached 550±86.97 mm in2018.Relative humidity is 81.76±4.09%.The relative humidity and precipitation showed that during the rain-heat period in Nyingchi Prefecture,the ozone content was higher in 2018 under humid environment,high altitude and sufficient light,and the photochemical reaction may also lead to the secondary formation of particulate matter.The results of the annual and seasonal concentration changes of the six conventional pollutants PM10,PM2.5,SO2,NO2,CO,O3（8 h）in Nyingchi City from the industrial layout and surrounding sources that may cause pollution in Nyingchi City during 2017-2022 show that:The time series analysis of atmospheric pollutants such as SO2,NO2,CO,O3（8 h）in Nyingchi City from 2018 to 2022 showed that the annual average of the six pollutants during the observation period was lower than the first-order concentration limit of NAAQS,and fluctuated around the concentration limit.Correlation analysis of conventional pollutants found that the correlation between PM10and PM2.5was 0.256 and showed a significant correlation（p<0.05）.The correlation between PM2.5and SO2was-0.248,indicating that there was a significant negative correlation between PM2.5and SO2（p<0.05）.The correlation value between PM2.5and O3（8 h）is 0.476,indicating that there is a significant positive correlation between PM2.5and O3（8h）（p<0.01）.In addition,there is no significant correlation between PM2.5and NO2and CO（p<0.05）.Correlation analysis of conventional pollutants monitored online in Nyingchi City showed that except for NO2,the other five types of pollutants showed good correlation,indicating that they had consistent sources.Combined with the environmental air quality parameters of Nyingchi City and social development needs,the current situation is discussed,and then the trend concentration distribution of atmospheric environmental quality is analyzed. |
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