Oxidation and Atmospheric Transformation of Vehicle Exhaust Particle

Master of Science, Graduate Program in Mechanical Engineering
University of California, Riverside, May 2014
Dr. Heejung Jung, Chairperson

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°C 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. Result from the current study shows significantly lower activation energy with NO2 oxidation as compared to O2 at temperatures below 500°C. At higher temperature (800 to 900°C), the  soot  oxidation  rates  with  NO2  is  still  comparable  to  that  with  air.  However,  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. Higher effective densities  and  stronger  evaporation  of  semi-volatile  species  is  observed  from  the  dilution  with ejector dilutor as opposed to the raw exhaust injection into a full bag inside the environmental chamber.