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Oxidation and Atmospheric Transformation of Vehicle Exhaust Particles


Soot particles pose significant adverse health effects and influence on earth's temperature and climate. Hence, the elimination of soot emissions from diesel engines has attracted a lot of attention. Prior to this work, the investigation of soot oxidation is done mostly under air environment. However, since it is typical for NO2 to be used in promoting regeneration under low-temperature soot oxidation for modern DPF, this study focuses on deriving the kinetics of soot oxidation under the influence of NO2 being a major oxidant. An online aerosol technique of high temperature oxidation-tandem-differential mobility analysis is used with soot being oxidized in a laminar flow reactor at temperature ranging from 500 to 900℃ with NO2 concentration from 0 to 600 ppm. The exposure of soot particles to the non-uniform temperature and NO2 mixing ratio inside the furnace causing NO2 thermal decomposition is thoroughly accounted for the first time. The soot oxidation rates being calculated as a function of frequency factor Asoot and activation energy Esoot are found to be: Asoot = 2.4 x 10-14 nmK-0.5s-1cm3molecule-1 and Esoot = 47.1 kJ mol-1. NO2 oxidation shows significantly lower activation energy when compared to that for O2 at temperatures below 500℃. Even at higher temperature (800 to 900℃), the soot oxidation rates with NO2 is still comparable to that with air. This indicates that NO2 is a stronger oxidant than O2 since only parts per million levels of NO2 are sufficient to cause such significant soot oxidation.

After the emissions from diesel exhausts are released into the atmosphere, secondary organic aerosol (SOA) are formed from the photo-chemical oxidation of organic vapors and followed by gas-to-particle partitioning. This study aims to advance the understanding of diesel emission influences on SOA formation with a major focus on the photo-oxidation of aromatic hydrocarbons inside an environmental chamber that simulates atmospheric chemistry. The investigation through combining the real-time density measurement and other physical/chemical analysis (APM-SMPS, HR-ToF-AMS) demonstrates that mass-based measurement techniques are necessary for interpreting the physical processes (evaporation of SVOCs and condensation of secondary organic compounds) during SOA formation due to the external void space in the agglomerate particles. Analysis of O/C ratio has shown to increase rapidly from 0.05 to 0.25 within 12 hours inside the chamber, emphasizing that the multigenerational oxidation of volatile organic vapors from the exhaust to be an important source of SOA formation. The impact of different dilution methods on the diesel particles evolution inside the chamber is investigated for the first time. Lower effective densities and stronger evaporation of semi-volatile species is observed from the intense dilution by the ejector dilutor as opposed to the raw exhaust injection into a full bag inside the environmental chamber.

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