Pollution Characteristics Of Water-soluble Components Of PM_2.5 And Precipitation In Xi’an

Sampling sites of PM_2.5and precipitation were set in the south of Xi’an.The pollution characteristics of water-soluble components of PM_2.5andprecipitation were analysed. Then the relations between PM_2.5and precipitation werediscussedSO₄^2-, NO₃^-, NH₄⁺were major water-soluble components of PM_2.5and the relativecontents of remaining water-soluble components were lower, not higher than5%. Theseasonal variations of water-soluble ions of PM_2.5were obvious. The massconcentration of each ion in general highest in the winter which shown the mostserious air pollution was happend in the winter in Xi’an. The highest concentrationsof low molecular weight organic acids was oxalic acid. Concentration of organicacids was the highest in the winter and summer, followed by the spring and the lowestin the autumn.The highest concentration of SO₄^2-was in09:00-12:00time period in the springand autumn, in12:00-15:00time period in the summer and in09:00-12:00and21:00-24:00time period in the winter. NO-3had merged as a low concentration of theafternoon session and high concentrations of the night and early morning in fourseasons. The period fluctuations of concentration of NH+4was not obvious in thespring and autumn, but different in the summer which appeared slightly higher in12:00-15:00and15:00-18:00time period. The highest concentration of NH₄⁺was in09:00-12:00and21:00-24:00time period in the winter. The average massconcentration of Na⁺, Mg²⁺showed small component and hours at the average in thefour seasons. The mass concentration of Ca²⁺was evenly distributed of periodfluctuations, but significantly higher in the spring than in other seasons. The concentrations of SO₄^2-, NO₃^-, NH₄in haze weather were significantlyhigher than normal weather in Xi’an. SO₄^2-was more likely to be converted to SO₄^2-inhaze weather and concentrations of organic acids in haze weather also increased. PM_2.5in haze weather was more acid compared with normal weather. A large number ofsetting off fireworks would cause serious air pollution in a short period of time andK⁺、Mg²⁺would be used as source tracer for fireworks.pH of the precipitation ranged from4.49to7.26, with average of6.06, andfrequency of the acid rain was18.18%in Xi’an2012. There were significant seasonapollution characteristics of acid rain in Xi’ an. pH of the precipitation was high in thespring and summe and low in the autumn and winter. SO₄^2-, NO₃^-, NH₄⁺and Ca²⁺werethe major water-soluble inorganic ions of precipitation. The seasonal variations of ionsof precipitation was obvious in Xi’an. The content of SO₄^2-, NO₃^-, Na+in the winterwas significantly higher than in other seasons; the content of NH₄⁺was the highest inthe winter and summer; the content of Ca²⁺, Mg²⁺was the highest in the spring; thecontent of K⁺was the highest in the summer. The seasonal distribution of lowmolecular weight organic acids shown: winter> spring> summer> autumn.There were scavenging effects of the precipitation to PM_2.5,but not high. Theremoval efficiency of the precipitation to cation was greater than anion, which was oneof the reasons that the pH value decreased with increasing precipitation andprecipitation time. The equivalent ratio of SO₄^2-/NO₃^-in PM_2.5and precipitation wasdeclining trend which shown type of air pollution was coal-fired and mixed vehicleemission pollution, and vehicle emission pollution was increasing year by year in Xi’an.