Chemical Composition And Atmosphere Pollutant Source Of Wet Deposition In Chang’an District,Xi’an City

Wet deposition refers to the natural process that suspended particles in the atmosphere scoured and sedimented in the form of atmospheric precipitation.On the one hand,wet sedimentation process played an important role in cleaning the air,on the other hand,a large number of toxic pollutants in the air washed out and fell to the earth’s surface subsidence by rained or snowed,which damaged ecosystem seriously,even threaten human health.For the research of water chemistry composition of wet subsidence,pollutant sources and cross-regional transmission are not only domestic and international research focus and hotspot in the field of atmospheric environment,but also has important guiding significance for the control and governance of air pollution.This study through the wet sedimentation in Xi’an from 2016 to 2017 by hour physical index,water chemical composition(F^-,Cl^-and NO₂^-,NO₃^-,SO₄^2-,Na⁺,NH₄⁺,K⁺,Mg²⁺,Ca²⁺)in the measurement and the atmospheric pollutants（PM2.5 and PM10,O₃,CO,SO₂,NO₂）,combined with PMF and HYSPLIT models,analyzed seasonal change characteristics of pH and wet subsidence water chemical characteristics in the process of the wet subsidence,clarified the purification effect of wet sedimentation process to air,revealed the structure and the structure and contribution of local air pollutants and cross-regional pollutant sources in different time.The conclusion is as follows:（1）In terms of rainfall,the overall trend of ionic and atmospheric pollutants（except O₃）was similar in the process of wet settlement in different months,that was the subsidence process decreases gradually,but there were also differences.The change of O₃ was opposite.The pH value of precipitation were more complex,the pH range of precipitation was 5.2-6.3,the mean value was 5.8,which was slightly higher than the acid rain threshold（5.6）,which indicated that the precipitation was neutral.Cl^-,NO₂^-,F^-,SO₄^2-and NO₃^-were highest in the first and third hours of the wet deposition in January and November,and the anion concentrations were higher in January and November.The highest levels of Ca²⁺,NH₄⁺,K⁺and Mg²⁺were in the first hour in January,and Na⁺was in the third hour.In April,the highest levels of Ca²⁺,K⁺and Mg²⁺appeared in the sixth hour,with NH₄⁺and Na⁺appeared in the first and third hours respectively.In July and September each of the captions was highest in the first hour,and in November the third hour.PM2.5,PM10,SO₂ and NO₂ peaked at the first hour in January,July,September and November,and the lowest was in the seventh hour.In April,PM10 and PM2.5 levels were in the fifth and third hours respectively,suggested that there may be different sources.（2）In terms of seasonal changes,the pH value of precipitation in spring and autumn was slightly neutral,and acid rain might occur in summer and winter.SO₄^2-and NO₃^-were the main anions,showed a low level in spring and summer and high in autumn and winter.F^-,NO₂^-and Cl^-were highest in winter and lowest in spring.Ca²⁺and NH₄⁺are the main cations,Ca²⁺,Mg²⁺and Na⁺were highest in autumn,NH₄⁺and K⁺were highest in winter,indicated that pollutant discharge had seasonal characteristics.The wet subsidence type in spring was carbonate type,and in winter and summer it was nitric acid-sulfuric acid mixed type,in autumn it was carbonic acid-nitric acid-sulfuric acid mixture type.PM2.5 and PM10 showed a low level in spring and summer and high in autumn and winter,and PM2.5 was lower than PM10.SO₂ was highest in winter and NO₂ concentration was highest in autumn.O₃ was mainly distributed in spring and summer.（3）As far as pollution sources were concerned,there were obvious seasonal changes in the sources of local pollutants and the input of trans-regional pollutants.The main emission source were traffic industrial source（27.08%）,and the earth’s crust source（29.16%）in April,the proportion of traffic industry source reached up to 40%in July,transportion of salt also achieved maximum（11.2%）,in addition to transportation industry source and crust source on the high side in September,biomass combustion also reached the highest（12.1%）.Coal-fired heating increased in November and January significantly,reached to 17.16%and 25.21%.The input path of short-range pollutants in January（64%）,September（66%）and November（73%）are from northwest and might be related to westerly transport.In April and July,the input of pollutants mainly came from the northwest direction,followed by southwest direction.（4）Research shows that,the air pollution in spring and summer was relatively light,which may be related to the meteorological conditions of windy and rainy weather.Air pollution was severer in autumn and winter.In particular,local coal-fired heating increased significantly in November and January,which reached 17.16%and 25.21%,while the input from the northwest direction of short-range and cross-regional pollutants accounted for 64%and 73%respectively in January and November.Suggested that paying attention to industrial pollution emissions in spring and summer,advocating the use of clean energy and strengthening the control of spring dust and constructional dust,reducing the contribution to air pollution source from traffic industry source and crust source effectively.In autumn and winter,in addition to control of traffic and industrial source,more attention should focus on controling burning coal pollution,such as reducing the burning of coal heating in the surrounding area,coal cleaning emissions and increasing the use of clean energy and other measures.Control the air pollutant source pointed in right crop for right time,can be just able to improve the management of air quality more effectively.