Spectral Characteristics And Source Analysis Of DOM In Precipitation In The Southern Suburbs Of Xi'an

The test methods of 3DEEM and UV-Vis,the analytical methods EEMs-PARAFAC and correlation analysis were used to study the fluorescence components of DOM in the precipitation of the southern suburbs of Xi'an,reveal the changes of precipitation pH,conductivity,turbidity,Zeta potential,and spectral characteristics during precipitation,explore the source of the precipitation DOM and the correlation between the spectral parameters.They were used to identify spectral parameters characterize and achieve rapid detection of DOM content,deepen the understanding of the geochemical characteristics of the organic matter in the precipitation,and provide an analytical data basis for researching the migration and environmental impact of precipitation to atmospheric pollutants in the environment.Through EEMs-PARAFAC analysis,it was determined that the DOM of the precipitation samples in the southern suburbs of Xi'an mainly contained protein-like?tryptophan-low excitation zone?,protein-like?tryptophan-high excitation zone?,protein-like?tyrosine-low excitation region?,ultraviolet humus-like and marine humus.In addition,the autumn,winter and summer precipitation's DOM contains protein-like?tyrosine-high excitation region?,and the winter snowfall DOM also contains ultraviolet fulvic acid-like and visible fulvic acid-like.Among them,the fluorescent components with the highest proportion of fluorescence intensities in the DOM in autumn,winter,spring and summer were protein-like?tryptophan-low excitation zone?,ultraviolet humus-like,protein-like?tryptophan-low excitation zone?and protein-like?tyrosine-high excitation region?,respectively.The lowest fluorescent components were marine humus,visible fulvic acid-like,marine humus and marine humus,respectively.During the monitoring period,there was no acid precipitation?pH was from 5.80 to7.42?in the southern suburbs of Xi'an.The conductivity and Zeta potential of the four seasons'precipitation were 5.1?S/cm-85.6?S/cm and-22.77 mV--0.68 mV,respectively.In the late summer and early autumn,the precipitation's conductivity and turbidity were lower,the winter snow conductivity increased significantly.And the DOM in the precipitation had negative charge.The absolute values of the Zeta potential in the spring rainfall DOM showed a decreasing trend.There was a strong absorption band appeared at the wavelength of 190 nm-240 nm in the ultraviolet spectrum of most precipitation samples.The content of DOM and the degree of substitution of benzene ring in winter snowfall were relatively higher.The values of UV₂₅₄,?₃₀₀,?₃₅₀ and?₃₅₅ of DOM in spring and summer precipitation showed a fluctuating downward trend,and the winter molecular weight of DOM was lower than that of early spring.With the precipitation process,the absorbance at the maximum absorption wavelength of DOM in some precipitation decreased or decreased first and then rised.The BIX of the DOM in precipitation of the southern suburbs of Xi'an was 0.73-1.25,and its organic matter was mainly newly produced or had strong self-source characteristics.The FI and HIX of DOM in precipitation were 1.40-2.59 and 0.34-1.84,respectively.The humus contained in DOM was mainly mixed with terrestrial and biological sources.Some precipitation was mainly biological sources.The source of DOM in spring rainfall was changed from the mixture of terrestrial and biological sources to biological sources,gradually.The degree of humification and aromaticity in the winter snowfall DOM was relatively stronger.There was a strong correlation between the UV parameters(UV₂₅₄,?₃₀₀,?₃₅₀,?₃₅₅)and fluorescence intensity?fluorescence peak A,peak M,peaks A and M,and the sum of all fluorescence peaks?of DOM in precipitation.They could be used to indicate the content or the relative content of organic matter in precipitation.The ultraviolet humus-like,marine humus,protein-like?tryptophan-low excitation zone?and protein-like?tryptophan-high excitation zone?in the DOM of the southern suburb of Xi'an had similar sources.And the sources of the same humus or tryptophan-like protein were more similar.The values of UV₂₅₃/UV₂₀₃ and S_275-295,UV₂₅₃/UV₂₀₃ and FI had significant negative correlation and positive correlation at 0.01 level,respectively.And the values of?₂₅₀/?₃₆₅ and HIX,FI and HIX had positive correlation and negative correlation at 0.05 levels,respectively.