As an important component of fine particulate matter (PM2.5), secondary organic aerosol (SOA) has an important impact on air quality, climate change and human health. The formation mechanism and influencing factors of SOA have become the frontier areas in current atmospheric environmental chemistry. Due to the lack of comprehensive understanding of SOA formation mechanisms and influencing factors, the SOA concentration in field observations is always underestimated by air quality models based on laboratory parameters. In recent years, vehicle ownership in China has increased rapidly, and vehicle-related pollutants have exacerbated the severity of compound atmospheric pollution, and caused frequent regional haze events in China. Meanwhile, with the implementation of tailpipe exhaust emission control measures, the proportionate share of vehicular evaporation emissions to this pollution has grown, so that they have become non-negligible contributors. Therefore, the study of the contribution of VOCs emitted by gasoline evaporation to SOA formation and the influence of various factors have important environmental significance for understanding the causes of haze events.
In this thesis, the 30 m 3 indoor smog chamber at the Research Center for Eco
Environment Sciences, Chinese Academy of Sciences (RCEES CAS) was evaluated in detail, then a series of experiments were conducted in this chamber to investigate SOA formation from gasoline vapors . L iquid gasoline s collected from different gas stations
located in Beijing and Hebei province etc. were used as a proxy for gasoline evaporation emissions. A series of instruments such as a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), scanning mobility particle sizer (SMPS), thermal desorption coupled with gas chromatograph/mass spectrometer (TD-GCMS), proton-transfer-reaction time of flight mass spectrometry (PTR-TOF) and gas analyzers were employed to perform real-time online monitoring of the particulate and gas phase species. Combined with in-depth analysis of the relevant data, the effects of aromatic hydrocarbon content, sulfur dioxide (SO2), ammonia (NH3) and precursor concentration on the formation of SOA were systematically studied.
Firstly, the RCEES-CAS smog chamber system was comprehensively characterized. A fully equipped 30 m3 indoor smog chamber system has been built. It can be used to simulate the different light intensities observed in different seasons and different periods, and the highest NO2 photolysis rate can reach 0.55 min-1, which could be comparable to the irradiation intensity at noon in Beijing. Temperature and relative humidity in the reactor can be flexibly controlled. In addition, the wall loss rates of both gas-phase (NOx, O3, SO2, VOCs) and particulate-phase species are lower than those of other smog chambers. A calculation method for the wall loss of semi-volatile organic compounds/low-volatility organic compounds (S/LVOCs) was established, which provided methodological support for accurately correcting the concentration and yield of particulate matter. Results of α-pinene/O3 dark experiments and toluene/NOx photo-oxidation experiments revealed SOA yields could be comparable to those derived from other chamber studies. Characterization experiments demonstrated that the RCEES-CAS smog chamber could be used to investigate the formation mechanism of SOA.
Secondly, the effect of aromatic hydrocarbon content on gasoline vapor SOA formation was systematically investigated. This study found that with the increase of aromatic content (from 23 % to 50 %), the SOA yield was significantly enhanced by a factor of 4.0−6.7. This phenomenon might be related to the synergistic effects between precursors in the mixtures. With the increase of aromatic content, higher amounts of LVOCs and SVOCs formed, which is conducive to SOA formation. Our study suggested that it is urgent to reduce the aromatic hydrocarbon content in gasoline by optimizing the refining process and improving the quality of gasoline, which will help to relieve the current situation of secondary fine particulate pollution in China.
Thirdly, the effects of SO2 and NH3 on gasoline vapor SOA formation were systematically investigated. Studies have shown that coexisting of SO2 and NH3 under environmental conditions have a significant effect on the formation of gasoline vapor SOA, with increases of 1.6−3.1 times and 1.4−1.7 times, respectively. The presence of both SO2 and NH3 could promote the initial nucleation rate and new particle formation (NPF). In the presence of SO2, acid-catalyzed heterogeneous reactions between sulfuric acid and carbonyl that take place in the particle phase and the formation of low-volatility sulfur-containing organics might be the main reasons for the increase in SOA formation. On the one hand, when NH3 is present, it can react with small-molecule organic carboxylic acids. On the other hand, NH4+ ions can react with carbonyl compounds (such as glyoxal) to form nitrogen-containing organics and oligomers, thereby promoting SOA formation. Under the compound pollution conditions of SO2 (coal-fired flue gas) and NH3 (agricultural non-point source and traffic emissions), the contribution of vehicle evaporative emissions to SOA formation was estimated to be 0.49 ± 0.04 Tg yr 1 , which could not be ignored when comparing the SOA production
(2.62 Tg yr 1 ) from anthropogenic precursors. Therefore, in order to relieve the secondary PM2.5 pollution, synergistic emission reduction of vehicular evaporation emissions, coal-fired flue gas and agricultural non-point source pollution should be realized.
Finally, the effect of precursor concentration on SOA yield was also investigated. This study found that the SOA yield at low concentrations is significantly higher (3−4 times) than that predicted from the yield curve obtained at high concentrations. At high concentrations, the precursor will compete with the gas-phase intermediates for the consumption of oxidants such as OH radicals, which is not conducive to the conversion of intermediates to LVOCs, nor to the increase of SOA yield. At low concentrations, gas-phase intermediates would be converted into advanced oxidation products more readily. In addition, LVOCs were the main products, which were prone to nucleate, promoting the formation of SOA, and then increasing the SOA yield. Therefore, the SOA yield obtained at high concentrations might underestimate the SOA yield under real atmospheric conditions. This might be one of the reasons that current model predictions generally underestimate the SOA concentration observed in field observations.