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

Abstract

Total suspended particulate (TSP) and particulate matter less than 2.5 μm in aerodynamic diameter (PM2.5) sam ples 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 PM2.5 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 PM2.5 the dominant ions were SO4 2−, NH4 +, and NO3 −. Most of the ions were more concentrated in the PM2.5 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 PM2.5, 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 PM2.5, 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 PM2.5, 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.