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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.

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The effect of antifreeze protein type III (one type of fish antifreeze protein) on ice crystallization was examined quantitatively based on a "micro-sized ice nucleation" technique. It was found for the first time that antifreeze proteins can inhibit the ice nucleation process by adsorbing onto both the surfaces of ice nuclei and dust particles. This leads to an increase of the ice nucleation barrier and the desolvation kink kinetics barrier, respectively. Based on the latest nucleation model, the increases in the ice nucleation barrier and the kink kinetics barrier were measured. This enables us to quantitatively examine the antifreeze mechanism of antifreeze proteins for the first time.Antifreeze proteins are found in the blood and tissues of organisms that live in freezing environments (1). In these organisms, ranging from fish to bacteria, the effect of freezing is retarded or the damage incurred upon freezing and thawing is reduced (2-4). Applications of the antifreeze effect of these antifreeze proteins (AFPs), 1 which is the capacity to inhibit ice crystallization, have been sought for maintaining the texture in frozen food, improving storage of blood, tissues, and organs, cryosurgery, and protecting crops from freezing (4). Freezing is a process of ice crystallization from supercooled water. In this process, water should undergo the stage of ice nucleation, followed by the growth of ice (5). Actually, whether or not freezing takes place is determined to a large extent by ice nucleation. In other words, there would be no ice growth if ice nucleation did not occur. The freezing inhibition brought about by antifreeze proteins is actually to impede the nucleation and the growth of ice by reducing the associated kinetics.Previous studies of the AFPs were mainly focused on the modification of the crystal morphology of ice and the inhibition of ice crystal growth in terms of the adsorption of antifreeze protein molecules on specific surfaces of ice (6 -8). It is believed (9 -11) that antifreeze proteins lower the freezing point of water merely by adsorbing their residues onto the ice crystal surfaces and thereby inhibiting their growth. Although some reports show the modification of the ice morphology caused by AFPs (7), no study has thus far been carried out to show how AFPs inhibit ice crystallization, in particular ice nucleation. We notice that the neglect of the initial and key stage of ice crystallization, i.e. nucleation, is likely due to the fact that a well defined kinetics measurement is a difficult and challenging task.In this paper, we will present the first study on the effect of AFPs on the nucleation of ice in supercooled water using a newly developed technique, the so-called ''double oil layer micro-sized ice crystallization technique'' (12). This technique allows us to minimize the influence of the wall of the container on the nucleation of micro-sized ice, therefore obtaining reliable and reproducible data on ice nucleation. Because the water is confined in a micro-sized droplet, it is a model...