| Secondary organic aerosols(SOA) affect air quality, global climate and human health. The formation of SOA is a frontier of atmospheric chemistry. Due to the quite limited knowledge about the formation of SOA, chemical transport models always underestimate the concentration of SOA in regional and global scale, which puzzles the atmospheric scientists. Recent years China faces serious air quality problems of fine particles(PM2.5) in its megacities. The frequent haze events significantly endanger human health and industrial production. The fine particles leading to haze are mainly secondary formed particles including SOA and so on. In recent years the vehicle number of China has been shapely increasing. Vehicle exhaust is an important source of PM2.5 and its contribution to PM2.5 is still uncertain. The primary emissions of vehicles are widely studies in China, but there is still no information on secondary particles formed from vehicle exhaust, thus making it difficult to evaluate the contribution of vehicle exhaust to PM2.5. Moreover, the differences of fuel quality, vehicle types, emission standards and driving conditions between China and other countries as well as the complex atmospheric conditions of China will make the formation of SOA complex and the results of other countries unsuitable to China. In this thesis, the big indoor smog chamber of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences(GIG-CAS) was evaluated in detail. A series of experiments were conducted in this chamber to investigate the SOA formation from gasoline vehicle exhaust. The number distribution, composition, and concentration were online characterized by a high-resolution time-of-flight aerosol mass spectrometer(HR-TOF-AMS) and a scanning mobility particle sizer(SMPS). The formation, yield and production factors of SOA as well as the effects of sulfur dioxide(SO2) and ammonia(NH3) were evaluated. The main results were listed below:1. The mixing time of reactants in the reactor was 120 s. Temperature can be set in the range from-10 ?C to 40 ?C at accuracy of ?1 ?C. The relative humidity was adjustable from 5% to 80%. The volume of the reactor was also adjustable. The purified dry air includes <500 ppt non-methane hydrocarbons(NMHCs), <50 ppt NOx, <1 ppb O3, SO2 and carbonyl compounds, and no detectable particles. Four arc xenon lamps and banks of black lamps were used as light source with light density and distribution similar with solar irradiation. The NO2 photolysis rate of xenon lamps and black lamps were 0.17-0.18 min-1 and 0.487±0.009 min-1, respectively. The average wall loss rates of NO, NO2 and O3 were measured to be 1.41±0.16×10-6 s-1, 2.31±0.39×10-6 s-1 and 2.19±0.39×10-6 s-1, respectively; and the particle number wall loss rate to be 0.17 h-1, lower than those of other famous smog chambers. Auxiliary mechanisms of GIG-CAS smog chamber were determined by simulating clean air, low NOx-air, CO-NOx-air and CO-air irradiation experiments. Applying these auxiliary mechanisms to the propene-NOx irradiation system, concentrations of propene, NO, O3, HCHO and CH3 CHO except for NO2 were well fit by the Master Chemical Mechanism version 3.2(MCM 3.2). The prediction biases of Δ([O3]-[NO]) are calculated to be varied from-2.2% to 23.7%. Results of α-pinene dark ozonolysis experiments revealed SOA yields comparable to those form other chamber studies. Characterization experiments demonstrate that GIG-CAS smog chamber can be used to provide valuable data for gas-phase chemistry and SOA formation.2. After 5hr photo-oxidation, SOA formed from gasoline vehicle exhaust was 12–259 times of primary OA(POA). Substantial formed ammonium and nitrate indicate that the contribution of gasoline vehicle exhaust to PM2.5 in ambient air can be mainly attributed to secondary aerosols such as SOA, ammonium and nitrate. Effective SOA yield data in this study were well fit by a one-product gas-particle partitioning model and quite lower than those of a previous study investigating SOA formation form Euro 2–Euro 4 passenger vehicles. Traditional single-ring aromatic precursors and naphthalene could explain 51%–90% of the formed SOA. Unspeciated species such as branched and cyclic alkanes might be the possible precursors for the unexplained SOA. A high-resolution time-of-flight aerosol mass spectrometer was used to characterize the chemical composition of SOA. The relationship between f43(ratio of m/z 43, mostly C2H3O+, to the total signal in mass spectrum) and f44(mostly CO2+) of the gasoline vehicle exhaust SOA is similar to the ambient semi-volatile oxygenated organic aerosol(SV-OOA). We plot the O:C and H:C molar ratios of SOA in a Van Krevelen diagram. The slopes of ΔH:C/ΔO:C ranged from-0.59 to-0.36, suggesting that the oxidation chemistry in these experiments was a combination of carboxylic acid and alcohol/peroxide formation.3. The production factors(PF) of SOA were 0.001–0.044 g kg-1, within the range of previous studies. Though primary PM2.5 emission factors of LDGVs would decline with upgrade of emission standards, when SOA was included, a Euro I gasoline car might be even inferior to a Euro IV one in the ultimate contribution to ambient PM2.5. The formation of SOA is thus needed to be considered when controlling emissions of vehicle exhaust. With the SOA/POA ratios we estimated that exhausts-derived SOA alone could reach about 1-3 times of primarily emitted PM2.5 for on-road vehicles in China. Moreover, if both primary and secondary contributions to PM2.5 are considered, gasoline vehicles may be comparable to or even dominate over diesel vehicles, particularly in megacities like Beijing.4. The presence of high concentration of SO2 increased the SOA production factor of gasoline vehicle exhausts(GVE) by 60–200% largely through acid-catalyzed SOA formation. The presence of high concentration of SO2 also enhanced the new particle formation, increasing the number concentrations of particles by 5.4–48. In addition, the presence of high concentration of SO2 increased the SOA formation rate and leaded to a relative lower oxidation degree probably due to the higher mass of organic loadings and heterogeneous acid-catalyzed reactions. The presence of GVE enhances the conversion of SO2 to sulfate predominantly through reactions with stabilized Criegee intermediates and heterogeneous processes. This synergy between SO2 and GVE in forming secondary aerosols partly explains frequent heavy haze events in North China Plain where SO2 from massive coal burning meets exhausts from a surging number of cars. This synergy indicated that better air quality in the Pearl River Delta region than North China Plain might be associated with its faster decrease and relatively lower concentrations of SO2. The synergy also implies that reducing SO2 emission would not only lower atmospheric sulfate, but indirectly decrease SOA formation; and both would aggravate the warming due to lowered cooling aerosols.5. Removing NH3 from GVE would greatly suppress the formation and growth of particles. Adding NH3 into the reactor after 3 h photo-oxidation of GVE, the particle number concentration and mass concentrations jumped explosively to much higher levels, indicating that the numbers and mass of particles might be enhanced when aged vehicle exhausts are transported to rural areas and mixed with NH3-rich plumes. We also found that the presence of NH3 had no significant influence on SOA formation from GVE. Very similar O:C and hydrogen to carbon H:C ratios resolved by aerosol mass spectrometer with and without NH3 indicated that the presence of NH3 also had no impact on the average carbon oxidation state of SOA... |
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