【摘要】 本文通过对2012年3月至2013年3月宝鸡市大气PM_2.5中各个水溶性无机离子组分的质量浓度进行研究,获得了水溶性离子的时间变化特征,并结合主成分分析方法讨论了不同离子的来源。结果显示,宝鸡市PM_2.5中水溶性离子主要由、和组成,分别占总水溶性离子质量浓度的40.47%、30.75%和15.07%;PM_2.5整体偏酸性。/比值随API指数的升高而增大,当空气质量较好时PM_2.5中硫酸盐居多,而随着空气污染发生硝酸盐逐渐增多并占优势。主成分分析结果表明PM_2.5中水溶性离子的主要来源有二次气溶胶、生物质燃烧和土壤尘。更多还原

【Abstract】 Background,aim,and scope Fine particulate matter(PM_2.5) causes air quality problems in urban areas,especially visibility reduction and health problems,such as asthma and even mortality.Water-soluble inorganic ions are major components of the atmospheric aerosols,especially PM_2.5.They can compose up to 60%—70%of the total mass of suspended particulate matter.Therefore,observations on the chemical composition of watersoluble fine aerosols would be valuable for understanding their physical/chemical characteristics,sources,and behavior and formation mechanism.Baoji（33°35’—35°06’N,106°18’—108°03’E） is an inland city situated in the mid-west part of China.Due to the rapid increase of motor vehicles and the growth in energy consumption over the past few decades,Baoji is facing serious air quality problems,mainly due to the high aerosol loadings in the region.However there has been no much attention paid for the problem in these mid-scale cities.In the current study,we collected the water-soluble ions in PM_2.5 samples at urban site of Baoji and to investigate the temporal variations and possible sources for these species.This study can provide useful information for establishing control strategies of aerosol pollution.Materials and methods Water-soluble inorganic ions in PM_2.5 were collected in Baoji from March 2012 to March 2013.The sampling site was located in Baoji Environmental Monitoring Station that is surrounded by a big residential area.Aerosols were collected by mini-volume samplers equipped with pre-baked quartz fiber filters.A total of 10 water-soluble ions(Na⁺,NH₄⁺,K⁺,Mg²⁺,Ca²⁺,F^-,Cl^-,NO₂^-,NO₃^-,and SO₄^2-) were analyzed by Ion Chromatograph in the aqueous extracts of the air filters.Results The 24-hr average mass concentrations of total water-soluble ions in PM_2.5 varied from 10.54 μg·m^-3 to 207.00 μg·m^-3,with an overall average of 51.62 μg·m^-3.Monthly average concentrations of total water-soluble ions were highest during February(92.33 μg·m^-3) and lowest during April(22.1 μg·m^-3).In anion,the concentrations of the most abundant ionic species followed the order of SO₄^2->NO₃^->Cl^->F^-,while in cation were NH₄⁺>Ca²⁺>Na⁺>K⁺>Mg²⁺.Overall,SO₄^2-,NO₃^- and NH₄⁺ were dominant ionic species in PM_2.5,the annual average concentrations were20.89 μg·m^-3,15.88 μg·m^-3 and 7.78 μg·m^-3,respectively.A strong correlation（R²>0.95） between cation and anion equivalents for all samples indicates that the five cations and five anions were the major ions extracted from the filters.The slope（anion/cation） of the linear regression was close to 1.25 in summer and winter,1.08 in spring and autumn.The annual volume-weighted mean values of NF for Ca²⁺,NH₄⁺ and Mg²⁺ were 0.23,0.50 and 0.04,respectively,indicating that Ca²⁺ and NH₄⁺ were the major neutralizers in PM_2.5 in Baoji.The seasonal variation of NO₃^-/SO₄^2- was shown winter（0.85）> autumn（0.83）>spring（0.66）>summer（0.36）.Discussion The seasonal variations of secondary components,SO₄^2-,NO₃^- and NH₄⁺,were similar,i.e.high concentrations in winter and low concentrations in spring and autumn.The seasonal variation trend may be ascribed to（1） the increase of emission sources（i.e.,residential and commercial coal combustion）;（2） meteorological conditions during winter were characterized by stagnation with a low inversion layer,which intensify secondary components levels through accumulation of air pollutants.Ion balance calculations are useful for studying the acid base balance of aerosol particles.This result implies that the aerosol particles from summer and winter are more acidic.The buffering of acidity in spring is likely due to the high dust loadings.The neutralization factor（NF） of Ca²⁺,NH₄⁺ and Mg²⁺ were calculated using their equivalent concentrations that can be used to describe the interaction between cations and anions.The NF for NH₄⁺ was higher in four seasons and NF for Ca²⁺ was higher in spring and autumn,suggesting that Ca²⁺ played a dominant role in the spring and autumn acid neutralization while NH₄^- had a stronger buffering ability in the whole year.It showed clearly in correlation coefficients between major ions that SO₄^2- mainly existed as（NH₄）₂SO₄ and NH₄HSO₄,while NO₃^- as NH₄NO₃.The mass ratio of NO₃^-/SO₄^2- was not suitable for indicator of mobile vs.stationary sources of sulfur and nitrogen in the atmosphere in Baoji.NO₃^-/SO₄^2- ratio was relatively high with increasing API,it means sulfate decreasing and nitrate increasing while poor air quality.A preliminary source identification study of the ws-ions was carried out by principal component analysis.The two factor PCA model for the ws-ions in the PM_2.5 aerosol samples accounted for ⁸0.0%of the total variance in the concentration data.Factor 1 account for ⁴9.9%of the total variance,and it was strongly loaded with NH₄⁺,NO₃^-,SO₄^2- and K⁺,suggesting likely origins from secondary aerosol and biomass burning.Factor 2 is dominated by F^-,Cl^-,Mg²⁺ and Ca²⁺,and it accounted for ³0.1%of the total variance,suggesting their concentrations are affected by primary soil dust.Conclusions The annual average concentration of total water-soluble ions in PM_2.5was 51.62 μg·m^-3 in Baoji from March 2012 to March 2013.Water-soluble ions were mainly composed of SO₄^2-,NO₃^- and NH₄⁺,accounting for 40.47%,30.75%and 15.07%in total water-soluble ions,respectively.PM_2.5 was generally acidic.NO₃^-/SO₄^2- ratio was relatively high with increasing API,it means sulfate decreasing and nitrate increasing while poor air quality.The PCA results indicated that secondary aerosol,biomass burning and soil dust were mainly sources of water-soluble ion in PM_2.5.Recommendations and perspectives The search provide a significant scientific basis for understanding the pollution characteristics of water-soluble ion in PM_2.5 at Baoji.To alleviate the PM_2.5 pollution,reducing coal burning,biomass burning and controlling soil dust should be performed.更多还原