Study On The Spatio-Temporal Variational Characteristics And Sources Of Of Main Chemical Components In Precipitation In Xi'an

Atmospheric pollution is harmful to the ecological environment and human health,and precipitation is an effective way to remove pollutants from the atmosphere.In this study,66precipitation samples from urban and rural sites in Xi'an were collected in 2019,and the p H,conductivity,water-soluble ions(Na⁺?NH₄⁺?K⁺?Mg²⁺?Ca²⁺?F^-?Cl^-?NO₂^-?SO₄^2-?NO₃^-)and free state heavy metal?Zn,Al,Fe,Pb,Sr,Mn,Ni,Cd?were analyzed,focusing on the characteristics of black carbon pollution in precipitation.Based on the sources of these chemical components,the following research conclusions are drawn:?1?The p H values of precipitation in the urban and rural sites in Xi'an were 6.61±0.44?5.53–7.49?and 6.80±0.32?6.22–7.47?,which were higher than the critical value of acid rain?p H 5.6?,indicating that precipitation in Xi'an was not acid rain.The electrical conductivity of precipitation in urban and rural sites were?63.12±63.94,9.46–260.27?S/cm?and?53.85±54.49,5.66–225.60?S/cm?,respectively.Due to coal burning produces acid gas,less rainfall and the heating in winter,and the frequent,heavy rainfall in summer,the significantly higher electrical conductivity was found in winter than those in summer and autumn?p<0.05?.?2?The average concentrations of ions in precipitation in Xi'an urban area appeared the ?99.92±168.84?eq/L?>NO₃^–?59.45±120.50?eq/L?>NO₂^–?18.26±28.61?eq/L?>Cl^–?14.86±40.60?eq/L?>K⁺?13.06±28.19?eq/L?>Na⁺?10.81±26.53?eq/L?>F^–?5.81±14.74?eq/L?>Mg²⁺?5.22±8.49?eq/L??p<0.05?,whereas the rural ion concentration ordered:Ca²⁺?153.91±201.56?eq/L?>NH₄⁺?117.64±157.93?eq/L?>SO₄^2–?76.37±133.37?eq/L?>NO₃^–?43.32±88.19?eq/L?>Cl^–?14.75±77.83?eq/L?>Na⁺?11.65±28.50?eq/L?>K⁺?7.70±61.89?eq/L?>NO₂^–?5.91±16.41?eq/L?>Mg²⁺?4.08±7.43?eq/L?>F^–(2.90±8.58 of Xi'an,and the concentrations of ions were significantly higher in winter than those in other[SO₄^2–]/[NO₃^–]radio ranged from 0.72–1.87 in urban site,and 0.55–2.23 in rural site,respectively,which showed that the precipitation in Xi'an belongs to the type of acid caused by sulfur-nitrate mixture.?3?The free state heavy metal pollution in precipitation in Xi'an was lighter.The concentrations of heavy metals in urban were generally higher than those in rural areas,and Pb,Cd,Ni and Al were significantly different between urban and rural areas?p<0.05?.The concentrations of free state heavy metal elements in urban site precipitation ordered:Zn?3.00±2.26?g/L?>Al?2.47±1.97?g/L?>Fe?1.66±0.57?g/L?>Pb?1.43±0.35?g/L?>Sr?1.12±1.23?g/L?>Mn?0.71±1.16?g/L?>Ni?0.30±0.08?g/L?>Cd?0.22±0.03?g/L??p<0.05?;the free state heavy metals concentrations of rural site appeared the following rules:Zn?2.32±1.68?g/L?>Fe?1.40±0.52?g/L?>Al?1.39±0.92?g/L?>Pb?0.97±0.41?g/L?>Sr?0.87±1.04?g/L?>Mn?0.84±1.39?g/L?>Ni?0.19±0.09?g/L?=Cd?0.19±0.04?g/L??p<0.05?.The concentration of free state heavy metals in precipitation in Xi'an was higher in autumn and winter than in spring and summer?p<0.05?.The source free state of Zn,Al and Fe elements in precipitation may be the same,mainly from industrial releases such as metal smelting and electroplating.The source free state of Pb and Ni and Cd elements may be petroleum combustion and coal burning in power plants.The correlation between free state of Mn and Sr elements is significant?p<0.01?,and the monthly change trend is consistent,the source may be crustal dust.?4?The concentration of black carbon?BC?in the urban and rural sites in precipitation in Xi'an were 80.05±99.04?g/L and 71.89±89.06?g/L,respectively,which was higher than that in many other areas at home and abroad,indicating that the degree of BC pollution in Xi'an was heavy.Due to heating in winter,coal combustion increased,resulting in increased black carbon emissions.The BC concentration of precipitation in urban site order was winter?313.09±90.20?g/L?>autumn?76.85±103.00?g/L?>spring?61.94±26.17?g/L?>Summer?44.58±28.74?g/L??p<0.05?;rural site was winter?308.22±36.47?g/L?>summer?68.38±68.08?g/L?>autumn?47.97±24.46?g/L?>spring?33.39±22.09?g/L??p<0.05?.The?¹³C of precipitation black carbon in urban and rural sites were-19.82±0.23‰and-19.55±0.23‰,respectively,and its?¹³C value were higher than that in other months from August to September?p<0.05?.The?¹³C precipitation in urban and rural sites were significantly lower in winter than in other seasons?p<0.05?.Comparing different sources of?¹³C,it can be found that the burning of C₃ plant and C₄ plant stalks in spring and summer may be the reason for the increase of black carbon?¹³C value in precipitation in Xi'an.Coal combustion is the reason for the decrease of black carbon?¹³C value in winter.Automobile exhaust is also one of the important sources of black carbon.