| The important chemical components of Atmospheric Fine Particulate Matter (PM2.5), such as the Carbonaceous Aerosol (Organic Carbon/Elemental Carbon, OC/EC) and Water Soluble Ion (WSIN), have significant impacts on the climate change, radiation force, visibility, precipitation acidity, air quality and human health in Lanzhou city. Altough the concentration of Polycyclic Aromatic Hydrocarbons (PAHs) is normally very low in PM2.5, its strong toxicity, such as "carcinogenic, teratogenic and mutagenic", attracts wide attention from scientific and social communities. In this study, PM2.5 samples were collected in Lanzhou city during the winter of 2012 and the summer of 2013, by the VFC-PM2.5 large flow sampler from Thermo company of USA, and then all samples are run on the DRI Model 2001A thermal/optical Carbon Analyzer, the Ion Chromatography and the GC (HP6890)-MS (Micromass VG Platform II) respectively, to get the concentrations of OC/EC, WSIN and PAHs. Combined with meteorological parameters, the OC/EC, WSIN and PAHs in Lanzhou PM2.5 were used to analysize the pollution levels, seasonal variation, possible sources as well as the main factors which cause pollutants to change daily. The results indicated that:1) The concentration of carbonaceous aerosol in winter was significantly higher than that in summer. The correlation coefficients between OC and EC was 0.91 in winter and 0.76 in summer, revealing that sources of OC and EC in winter were much similar. The relative contain of Secondary Organic Carbon (SOC) in OC of PM2.5 were ca.22.46±11.93% and 31.29±18.51% repectively for winter and summer, which manifested that OC are mainly from primary sources, and the contributions of secondary transformation were much higher in summer than those in winter. Analysis on the temperature-dependant eight fraction of carbonaceous aerosol showed that the main sources of OC and EC should be coal emissions and vehicle emissions in winter, and mainly vehicle emissions in summer, as well as biomass burning have a higher contribution for carbonaceous aerosol in PM2.5 in winter than that in summer.2) The average value of total concentration of water soluble zwitterions was obviously higher in winter than that in summer, as well as the mass concentration of each WSIN monomers; Among them, some secondary ions, such as SO42-, NH4+and NO3-, were the main ions in WISN in Lanzhou during both winter and summer, also that the secondary transformation ratio were happened higher in summer than that in winter. Analysis of ions balance between total cations and total anions as well as the main existed forms of WISN showed that airborne PM2.5 in Lanzhou was just a little acid in winter and alkaline in summer, water-soluble ions mainly existed in the forms of NH4NO3, (NH4)2SO4, H4HSO4 and NH4Cl, with a few of KNO3, NaNO3, K2SO4, Na2SO4, KCl and NaCl in winter. Crucial pollution sources devoted to WISN could divided into stable sources (for example coal emissions), and flow sources (such as motor vehicles) in winter. But the sources of summer pollution are quite complex, particularly with motor vehicle emissions.3) Total concentration of PAHs in winter was high, which show positive relation with pollution level. BaP level among that was significantly higher than international standards slightly higher than the national standard, which indicated that PM2.5 of Lanzhou made a threat to human health. Total concentration of PAHs was lowed which made the pollution levels lesser in summer. The proportion of middle molecular weight of PM2.5 was highest in Lanzhou. The ratio of large molecular weight of PM2.5 in summer was higher than that in winter, but the small molecular weight being the opposite. The sources of PAHs in winter were given priority to coal emission and then motor vehicle exhaust, particularly diesel, nevertheless, the absolute weight of all sorts of pollution sources emission were very light that in summer, in comparison, civil coal, wood combustion and gasoline vehicles for the contribution of PAHs is bigger.4) The daily changing trend of carbonaceous aerosol, WSIN and PAHs were consistent, and snowfall in winter was the main meteorological factors of the concentration declined, while continuous rainfall, dust by wind and the status of pollution sources in summer may cause the concentration fluctuation. The carbonaceous aerosol were negatively related with relative humidity, and that in winter was more obvious than summer; WSIN were positively correlated with the wind in winter, but negatively related with wind speed in summer; PAHs were negatively correlated with the wind speed in winter, however with the low correlation in summer, almost no correlation.These results of OC/EC, WSIN and PAHs in Lanzhou PM2.5 not only provide theory and data supports for improving air quality further, but also improve the pertinence, scientific nature and rationality for Lanzhou city prevention and control work. Therefore, these research conclusions have important significance for making the air pollution control plan and management strategy in Lanzhou. |
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