[Pollution and Deposition Characteristics of Precipitation and Its Source Apportionment in Xi'an City]

Chemical and deposition characteristics as well as potential sources of precipitation in Xi'an from 2000 to 2017 were analyzed based on the acid deposition monitoring network in East Asia (EANET). The pH of precipitation showed increasing trends, whereas the conductivity of precipitation showed decreasing trends between 2000 and 2017 in Xi'an. The decreasing order of ion concentration in precipitation was as follows:SO₄^2- > Ca²⁺ > NH₄⁺ > NO₃^- > Na⁺ > Cl^- > Mg²⁺ > K⁺. The percentage of SO₄^2- in the total ion concentration decreased from 38.6% in 2000 to 27.9% in 2017. The ratio of SO₄^2- to NO₃^- in recent years indicated that the type of pollutant in Xi'an changed from the coal-burning type to the mixed type. The deposition of SO₄^2- in precipitation had a declining trend, which was consistent with the results of the Mann-Kendall test. Nitrogen and sulphur depositions in precipitation were 16.89 kg·(hm²·a)^-1 and 33.52 kg·(hm²·a)^-1, respectively, and ammonium nitrogen (NH₄⁺-N) was the main deposition component of atmospheric active nitrogen in the precipitation of Xi'an. Vehicle emissions were the major contributors to SO₄^2- (15.43%) and NO₃^- (72.99%). NH₄⁺ originated mainly from agriculture sources, and the percentage reached up to 75.47%.