Seasonal variations and evidence for the effectiveness of pollution controls on water-soluble inorganic species in total suspended particulates and fine particulate matter from Xi'an, China

Total suspended particulate (TSP) and particulate matter less than 2.5 μm in aerodynamic diameter (PM_2.5) samples were collected over Xi'an for a 1-yr period to characterize the seasonal variations of water-soluble inorganic ions and to evaluate the effectiveness of the pollution policies and controls during the past 10 yr. Mass concentrations of five cations (sodium [Na⁺], potassium [K⁺], ammonium [NH₄⁺], calcium [Ca²⁺], and magnesium [Mg²⁺]) and four anions (fluoride [F^-], chloride [Cl^-], nitrate [NO₃^-], and sulfate [SO₄^2-]) were determined by ion chromatography. The yearly arithmetic-mean mass concentrations of the total measured water-soluble ions in TSP and PM_2.5 were 83.9 ± 58.4 and 45 ± 34.3 μg · m^-3. The most abundant ions in TSP were SO₄^2-, NO₃^-, Ca²⁺, and NH₄⁺; whereas in PM_2.5 the dominant ions were SO₄^2-, NH₄⁺, and NO₃^-. Most of the ions were more concentrated in the PM_2.5 than in TSP, but two exceptions were Ca²⁺ and Mg²⁺. Comparisons of the molar ratios of Mg²⁺/Ca²⁺ in TSP indicated that fugitive dust was the main source for these two ions, and the influence of soil dust from outside of the city was most evident during dust storms. The mass concentrations of SO₄^2-, NO₃^-, NH₄⁺, and K⁺ in TSP were highest in winter and lowest in spring, but Ca²⁺ was much higher in spring than other seasons because of suspended mineral dust. In PM_2.5, NO₃^- and K⁺ also showed winter maxima, but SO₄ ^2- and NH₄⁺ were highest in summer. Calculations of ion equivalents showed that TSP samples were more alkaline than PM_2.5, the latter being weakly acidic in winter and autumn. High sulfur and nitrogen oxidation ratios occurred in summer and autumn, and there was evidence for the formation of ammonium bisulfate in TSP, ammonium sulfate in PM_2.5, and ammonium nitrate in both fractions. Comparisons with the results of prior studies indicate that pollution controls in Xi'an have reduced the levels of air pollution over the past 10 yr. The SO₄^2- concentration during the heating season in 2006 was only about one-eighth of that in 1996, and NH₄⁺ decreased to one-ninth of that in 1996. Seasonal variations in the NO₃ ^-/SO₄^2- ratio are different than the patterns observed 10 yr ago, suggesting that emission sources have changed, with those from motor vehicles becoming increasingly important.