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 microm in aerodynamic diameter (PM2.5) 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 PM2.5 were 83.9 +/- 58.4 and 45 +/- 34.3 microg x 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.

Chemical Characteristics of Fine Particles (PM1) from Xi'an, China

“…The water soluble Ca 2+ concentrations in PM 1 were much lower than those reported for TSP or PM 2.5 in a prior study at Xi'an (Shen et al 2008). This ion has been used as an indicator of Asian dust, which typically exhibits its highest concentrations in spring (Choi et al 2001;Cao et al 2005b;Shen et al 2007).…”

“…In general, sulfate, nitrate, and ammonium dominated the WS-i species studied, combining to account for 77.1%, 85.2%, 81.9%, and 83.0% of the total measured mass from spring to winter, respectively. Previous studies have suggested that gas-to-particle conversion is important, relative to the formation of these three ions, that is, SO 2 to SO 2-4 , NO 2 to NO -3 and NH 3 to NH + 4 (Lin 2002;Wang et al 2005;Shen et al 2008); and therefore, much of the PM 1 in our study apparently results from heterogeneous processes.…”

“…Prior aerosol studies in Xi'an have focused mainly on the chemical composition of TSP, PM 10 , and PM 2.5 Cao et al 2005a;Shen et al 2008Shen et al , 2009. To the best of our knowledge, no data for PM 1 over Xi'an has yet been reported.…”

“…One-fourth of each filter sample was used to determine the mass concentrations of selected WS-i ions, following procedures described in detail in Shen et al (2008). The concentrations of four anions (SO 2-4 , NO -3 , Cl -, and F -) and five cations (Na + , NH + 4 , K + , Mg 2+ , and Ca 2+ ) were determined in aqueous extracts of the filters prepared in three steps using ultrapure (18 M ) water.…”

“…Air quality in Xi'an is a public health concern due to the persistently high loadings of PM (Cao et al 2005a;Shen et al 2008). Some actions taken by the local government, beginning around 1998, have led to improvements in air quality, but PM pollution in Xi'an continues to be a serious problem.…”