Presence of atmospheric PAHs in urban and suburban region (Beijing, China) was studied in April and July 2011. Forty-four pairs of gas and particle (TSP) phase samples were collected every six day by high volume (Hi-Vol) air samplers at four sampling sites, and determined separately by GC/MS based on USEPA Method TO-13A. Average total concentration (gas + particles) of PAHs (T-PAHs) was 135.1±49.0 ng/m3 and 181.2±40.9 ng/m3 in April and July, respectively. Gas phase PAHs (G-PAHs) was the major fraction, comprising 63–92% of T-PAHs. Lighter (2-, 3-, 4-ring) and heavier (5-,6-ring) PAHs were found predominantly in gas and particle phase, respectively. 2- to 6- ring PAHs contributed 10%, 53%, 26%, 7% and 4% of T-PAHs, respectively. Five major PAHs, naphthalene (NAP), fluorene (FLU), PHE, fluoranthene (FLA), and pyrene (PYR) contributed 70 – 90% of T-PAHs. G-PAHs increased significantly while PAHs in particle phase (P-PAHs) decreased from April to July. Volatilization from soil and more emission from power generation increase might explain the increase of G-PAHs, and the washout of P-PAHs along with particles might explain the decrease of P-PAHs. Given particulate organic carbon (OC) and elemental carbon (EC) being well correlated, P-PAHs was moderately correlated with OC and EC, suggesting that there were other mechanisms contributing to P-PAHs different from those of OC/EC. Significant correlation between P-PAHs with SO2 and NO2 suggested coal combustion and automobile exhaust to be contamination contributors.
AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) with thick (>30 nm), high-kappa (TiO2/NiO), submicron-footprint (0.4 mu m) gate dielectric on SiC substrate are demonstrated, which are found to exhibit low gate leakage current (similar to 1 nA/mm of gate periphery), high I-MAX (1 A/mm), and high drain breakdown voltage (188 V). The derived current gain cutoff frequency is 30 GHz (from S-parameter measurements). The output power density is 6.6 W/mm, and the associated power-add ed-efficiency is 46% at 2.5 GHz frequency and 50 V drain bias. This high performance submicron-footprint MOSHEMT is highly promising for microwave power amplifier applications in communication and radar systems.
In this study, 121 daily PM2.5 (aerosol particle with aerodynamic diameter less than 2.5 μm) samples were collected from an urban site in Beijing in four months between April 2009 and January 2010 representing the four seasons. The samples were determined for various compositions, including elements, ions, and organic/elemental carbon. Various approaches, such as chemical mass balance, positive matrix factorization (PMF), trajectory clustering, and potential source contribution function (PSCF), were employed for characterizing aerosol speciation, identifying likely sources, and apportioning contributions from each likely source. Our results have shown distinctive seasonality for various aerosol speciations associated with PM2.5 in Beijing. Soil dust waxes in the spring and wanes in the summer. Regarding the secondary aerosol components, inorganic and organic species may behave in different manners. The former preferentially forms in the hot and humid summer via photochemical reactions, although their precursor gases, such as SO2 and NOx, are emitted much more in winter. The latter seems to favorably form in the cold and dry winter. Synoptic meteorological and climate conditions can overwhelm the emission pattern in the formation of secondary aerosols. The PMF model identified six main sources: soil dust, coal combustion, biomass burning, traffic and waste incineration emission, industrial pollution, and secondary inorganic aerosol. Each of these sources has an annual mean contribution of 16, 14, 13, 3, 28, and 26%, respectively, to PM2.5. However, the relative contributions of these identified sources significantly vary with changing seasons. The results of trajectory clustering and the PSCF method demonstrated that regional sources could be crucial contributors to PM pollution in Beijing. In conclusion, we have unraveled some complex aspects of the pollution sources and formation processes of PM2.5 in Beijing. To our knowledge, this is the first systematic study that comprehensively explores the chemical characterizations and source apportionments of PM2.5 aerosol speciation in Beijing by applying multiple approaches based on a completely seasonal perspective.
From July 2009–April 2011, 304 whole-air samples were collected at urban and suburban sites in four Chinese cities. The results indicated that recent chlorofluorocarbon (CFC) concentrations in all four cities are approaching Northern Hemispheric (NH) background levels, suggesting that the phase-out of CFCs in China is underway. However, hydrochlorofluorocarbon (HCFC) and 1,1,1,2-tetrafluoroethane (HFC-134a, CH2FCF3) concentrations have risen far above the NH background levels. Their concentration variability is evident, suggesting significant regional emissions. One-way analysis of variance (ANOVA) and independent-sample t-tests were applied to analyze the spatial distributions and emissions. Monochlorodifluoromethane (HCFC-22, CHClF2) levels at the urban sites were 30% higher than those observed at the suburban sites (P < 0.05), likely owing to larger population density in the urban areas. The largest 1-dichloro-1,1-fluoroethane (HCFC-142b, CH3CClF2) mean concentrations were detected in Beijing (131 and 52 pptv for urban and suburban sites, respectively), likely because of more widespread use of extruded polystyrene board. The variation in HFC-134a concentration levels in different cities was mainly related to the vehicle population. In addition, the different HCFC species exhibited significant positive correlations amongst themselves for each city (P < 0.05), suggesting that either they were emitted from similar or co-located sources or they shared similar emission patterns. These results are helpful to confirm the phase-out of ozone-depleting substances (ODSs) and to provide guidance for implementing effective phase-out strategies for ODSs and greenhouse gases.
From July 2009–April 2011, 304 whole-air samples were collected at urban and suburban sites in four Chinese cities. The results indicated that recent chlorofluorocarbon (CFC) concentrations in all four cities are approaching Northern Hemispheric (NH) background levels, suggesting that the phase-out of CFCs in China is underway. However, hydrochlorofluorocarbon (HCFC) and 1,1,1,2-tetrafluoroethane (HFC-134a, CH2FCF3) concentrations have risen far above the NH background levels. Their concentration variability is evident, suggesting significant regional emissions. One-way analysis of variance (ANOVA) and independent-sample t-tests were applied to analyze the spatial distributions and emissions. Monochlorodifluoromethane (HCFC-22, CHClF2) levels at the urban sites were 30% higher than those observed at the suburban sites (P < 0.05), likely owing to larger population density in the urban areas. The largest 1-dichloro-1,1-fluoroethane (HCFC-142b, CH3CClF2) mean concentrations were detected in Beijing (131 and 52 pptv for urban and suburban sites, respectively), likely because of more widespread use of extruded polystyrene board. The variation in HFC-134a concentration levels in different cities was mainly related to the vehicle population. In addition, the different HCFC species exhibited significant positive correlations amongst themselves for each city (P < 0.05), suggesting that either they were emitted from similar or co-located sources or they shared similar emission patterns. These results are helpful to confirm the phase-out of ozone-depleting substances (ODSs) and to provide guidance for implementing effective phase-out strategies for ODSs and greenhouse gases.