Seasonal differences in carbon composition,water-soluble organic matter molecular characteristics and sources of PM2.5 in Xi'an
By using parallel factor analysis,backward trajectory models,UV spectroscopy,and three-dimensional fluorescence spectroscopy to analyze the carbon composition,molecular characteristics,and sources of Water-Soluble Organic Matter(WSOM)in PM2.5 in Xi'an.The results show that the seasonal variations of PM2.5,OC and EC in Xi'an follow a descending order of winter,autumn,spring,and summer,with no significant difference between the southern and northern suburbs.The concentrations of WSOM are 3.50-17.29 μg/m3,with the highest concentration in winter.WSOM in all seasons both contained UV humus-like and visible humus-like substances.In autumn,winter,and summer,the fluorescence intensity of fulvic acid-like accounts for the largest proportion.The seasonal variation of E2/E3,E3/E4,and AAE values follow the descending order of winter,spring,summer,and autumn.The values of SUVA254 and MAE365 are highest in winter and lowest in summer.The E2/E3,E3/E4 and AAE values in the southern suburbs are higher than those in the northern suburbs,while UV253/UV203,SUVA254,and MAE365 values were lower.The molecular weight and humification degree of WSOM are smaller,the degree of molecular benzene ring substitution is the largest,and the capacity and wavelength dependence of light absorption is stronger in winter;the molecular weight and humification degree of WSOM are larger,and the wavelength dependence of light absorption was weaker in autumn;the degree of aromatization and light absorption capacity in summer and the degree of molecular benzene ring substitution of WSOM in spring were the weakest.The concentrations of carbon composition,UV254,α350,and fluorescence intensity were significantly positively correlated with each other(p＜0.01).The values of FI,BIX,and HIX are 1.51-2.15,0.88-1.46,and 1.18-3.19,respectively.The autochthonous source of WSOM is the highest in winter,while terrestrial sources are larger in summer.The humification degree of WSOM in autumn and summer is higher.Polluted air mass in Xi'an mainly comes from local transmission.There are seasonal differences in carbon composition,molecular characteristics,and sources of WSOM in Xi'an,but there is no significant difference between the northern and southern suburbs.
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