Abstract:

Karamay is one of the most important cities in the economic belt on the northern slope of Tianshan Mountains,Xinjiang,China,and is also an important petrochemical base in China.Located in the special leeward slope topography,it displays the climate characteristics of drought,less rain and frequent gale in spring and autumn.In order to fully understand the chemical composition and sources of atmospheric precipitation in Karamay,the precipitation samples collected at the acid rain observatory in Karamay in 2016 were used to analyze the concentration characteristics of major ions (F^-、Cl^-、NO₃^-、SO₄^2-、Na⁺、NH₄⁺、K⁺、Mg²⁺、Ca²⁺),heavy metals (Al,Mn,Fe,Ni,Cu,Zn,Cd,Pb,Hg),total carbon and total nitrogen,and the sources of water soluble ions and heavy metals were analyzed by correlation analysis.The results show that the pH value of precipitation is between 5.1 and 6.88,and the weighted average is 6.25.There are 94% of samples with pH value being larger than 5.6.The order of weighted concentration of each ion in precipitation is Ca²⁺> NH₄⁺> SO₄^2-> NO₃^-> Cl^-> Mg²⁺> Na⁺>K⁺>F^-,which shows the characteristics of inland precipitation. Ca²⁺ is the main cation with an annual average of 182.09 μeq·L^-1 and SO₄^2- is the main anion with an annual average of 87.28 μeq·L^-1,indicating that sulfate is the main acid-causing substance in Karamay precipitation.The total ion concentration in precipitation is the highest in autumn,followed by spring and summer,and the lowest in winter,and the concentration changes of SO₄^2-, NO₃^-, NH₄⁺, Ca²⁺, Na⁺ are obvious.The average FA value was 0.005,indicating that 99.5% of precipitation acidity was neutralized by alkaline components,and the neutralization ability of Ca²⁺ was the strongest,followed by NH₄⁺.The average concentration of heavy metal element Zn in precipitation is the largest,followed by Fe,and the smallest is Pb.Compared with domestic and foreign cities,the average concentrations of heavy metals in precipitation in Karamay are not high,and the pollution of toxic heavy metals in precipitation is lighter. The concentrations of DTC, DOC and DIC varied widely,ranging from 1.62-9.97 mg·L^-1,1.62-7.19 mg·L^-1 and 0-3.75 mg·L^-1,with average concentrations of 4.37 mg·L^-1,3.60 mg·L^-1 and 0.78 mg·L^-1,respectively.The concentration range of DTN varied from 0.64-8.01 mg·L^-1 with average concentration of 2.69 mg·L^-1,which were negatively correlated with precipitation,while the wet deposition flux of DOC and DTN were positively correlated with precipitation.Volatile organic compounds (VOCs) produced by fossil fuel combustion and industrial exhaust are the main sources of DOC.The concentration of DIC in precipitation is generally higher than the theoretical saturated solubility of atmospheric CO²,indicating that calcium carbonate in fine particulate matter is likely to be an important reason for the higher concentration of DIC than the theoretical value,and human emissions are an important factor for the higher concentration of DTN.Based on correlation analysis, SO₄^2- and NO₃^- mainly come from coal combustion and fossil fuel combustion,Na⁺,Mg²⁺ and Ca²⁺ mainly come from wind sand and dust,and ammonium compounds mainly exist in the form of ammonium nitrate in the atmosphere.The correlation among heavy metal elements is quite different.The main sources of heavy metal pollution are metal smelting,coal burning and human activities.The chemical composition of precipitation in Karamay is mainly affected by human activities,chemical production,coal combustion and dust activities.

Key words: Karamay City, precipitation, chemical composition, source analysis