Determination And Characterization Of Water-Soluble Species Of PM_2.5 In Beijing

A simple and sensitive ion chromatography (IC) method was established for the simultaneous determination of seven water-soluble inorganic anions and fifteen water-soluble organic acids in atmospheric aerosols in this study, which expanded the scale of identifying water-soluble dicarboxylic acids from C2-C6 to C2-C12 and obviously shortened the run time and improved the discovery rate of individual sample analysis. Ambient PM2.5 samples were collected for over two years at a downtown site (Chegongzhuang), a residential site (Tsinghua Campus) and a rural site (Changping) in Beijing by using low volume and high volume PM2.5 samplers. Based on these, the chemical compositions, temporal and spatial variations, size distributions and phase partitioning of water-soluble ionic species of PM2.5 in Beijing were characterized and the source apportionments were performed in this study. There were several important findings in this dissertation. Firstly, sulfate, nitrate and ammonium were found to be the dominant ionic species of PM2.5 in Beijing, which accounted for about 21% of PM2.5 mass in total. The other inorganic ions accounted for about 7% of PM2.5 mass. Fifteen water-soluble organic acids possessed about 1.7% of PM2.5 mass, among which oxalate was the most abundant species. Secondly, the mass of water-soluble species of PM2.5 in Beijing were trimodally distributed in the size range of 0.1-10(m: a droplet mode with peak at 0.7(m and two coarse modes with the lower peak at 2(m and the upper peak at 5(m. Most of their mass was distributed in the fine modes except those of Sodium, Magnesium and Calcium, which was about half distributed in the fine modes and half distributed in the coarse modes. Thirdly, ion balances were almost achieved only in the particles with the size of 3.3(m in aerodynamic diameter. For the particles less than 3.3(m, they showed cation deficit and seemed to be acidic, while for the particles larger than 3.3(m, they were some anion deficit and seemed to be slightly alkaline. Fourthly, denuder experiment showed that about 12% of formate, 14% of acetate and 8% of nitrite distributed in gaseous phase, while about 31% of acetate and 26% of nitrate distributed in particle phase. Fifthly, sulfate seemed to be high correlation with relative humidity and poor correlation with gaseous sulfur dioxide and ozone, which might indicate in-cloud or aqueous reaction to be the principal formation pathway of particulate sulfate. Nitrate and oxalate were highly correlated with both relative humidity and their gaseous precursors and ozone, which suggested aqueous reaction and gas-phase reaction to be two formation pathways of particulate nitrate and oxalate. Sixthly, the results of principal component analysis suggested that secondary particles explained 43% of the total variance of the water-soluble inorganic species and 39% of the water-soluble organic species of PM2.5 in Beijing. Soil dusts, industries or vehicles and sea-salt particles explained about 20%, 11% and 7% of the total variance of the water-soluble inorganic species, respectively. Biogenic sources explained about 51% of the total variance of the water-soluble organic species. Lastly, the results of air quality modeling suggested that the regional sources contributed about 57% of sulfate, 58% of nitrate and 69% of ammonium concentrations of Beijing urban areas. Local industries and heating boilers contributed about 29% for sulfate, 13% for nitrate and 17% for ammonium. Vehicles contributed about 4% for sulfate, 27% for nitrate and 10% for ammonium.