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As diesel PM regulation gets more stringent, the current gravimetric method which has been used for legal determination of PM mass for vehicle emissions will have difficulty accurately quantifying PM mass emissions. Progress in regulating diesel PM emissions by non-gravimetric means has been made in the Europe. The method, so called particle measurement program (PMP) protocol specifies counting solid particles larger than 23 nm. This method has already been adopted for Euro V and VI to regulate light-duty and heavy-duty vehicle emissions and it is also expected to be adopted to regulate emissions for aviation section. However, exclusion of sub-23 nm particles poses some potential issues. In this thesis, the PMP method was investigated with a focus on finding the nature of sub-23 nm particles which are excluded in the current PMP protocol.A PMP-compliant system, an AVL advanced particle counter (APC) and an alternative volatile particle removal system, a catalytic stripper (CS) were evaluated and compared for measuring solid particle number emissions in chapters 2 and 3. The evaluations and comparisons were conducted using sulfuric acid and hydrocarbon particles as model volatile particles in laboratory tests, and diluted exhaust from a diesel particle filter (DPF)-equipped heavy-duty diesel vehicle operated on a heavy-duty chassis dynamometer under steady speed conditions at two different engine loads. Thisstudy also compared particulate matter (PM) mass and particle number (PN) emissions from a heavy-duty diesel vehicle operating over the urban dynamometer driving schedule (UDDS) and actual on-road flow-of-traffic driving conditions, including two uphill (phases 1 and 2) and two downhill (phases 3 and 4) segments. The UDDS and on-road flow-of-traffic tests represent a broader engine operating conditions than those currently certified emissions testing cycles.For the laboratory test, both the APC and CS removed more than 99% of the volatile particles in terms of PN when using aerosols composed of pure sulfuric acid or hydrocarbons. When using laboratory test aerosols consisting of mixtures of sulfuric acidand hydrocarbons more than 99% of the particles were removed by the APC but the surviving particles were no longer entirely volatile, with 12-14% being solid.For the chassis dynamometer test, PN emissions of particles between 3 and 10 nm downstream of the APC were 2 and 7 times higher than the PN emissions of particles above 10 nm at the 74 and 26% engine load, respectively. At the 26% engine load, PNlevel of the 3 to 10 nm particles downstream of the APC were significantly higher than that in the dilution tunnel, demonstrating that the APC was making 3 to 10 nm particles. The PN emissions of 3 to 10 nm particles downstream of the APC were related to theheating temperature of the APC evaporation tube, suggesting these particles are artifacts formed by renucleation of semivolatiles. Considerably fewer particles between 3 to 10 nm were seen downstream of the CS for both engine loads due mainly to removal of semivolatile material by the catalytic substrates, although some of this difference could be attributed to diffusion and thermophoretic losses.Chapter 4 provides an evaluation of the nature of sub 23 nm particles downstream of the European PMP methodology with prescribed cycles and on-road flow-of-traffic driving conditions. Particle number concentrations and size distributions were measured using two PMP measurement systems in parallel. For this analysis, the focus is on the real-time results from multiple instruments. The results revealed that a significant fraction of particles downstream of both PMP systems for all tested cycles werebelow 11 nm. The fraction of sub 11 nm particles observed downstream of the PMP system decreased when the overall dilution ratio of one PMP system was increased from 300 to 1500, suggesting those sub 11 nm particles were formed through re-nucleation of semivolatile precursors. When the evaporation tube temperature was increased from 300 to 500°C, no difference in particle number concentrations was observed, suggesting incomplete evaporation of semivolatile particles did not contribute to those sub 11 nm particles. Particle emissions were about one order of magnitude higher during flow-of-traffic driving along a highway with a steep grade than during the prescribed driving cycles. During the same flow-of-traffic condition, a sudden jump of PMP operationally defined solid particle concentration was observed, while the accumulation mode particle concentrations in the constant volume sampling (CVS) tunnel measured by engine exhaust particle sizer (EEPS) only showed a slight increase. This discrepancy was attributed to the extensive growth of the re-nucleated particles downstream of the PMP systems.In chapter 5, the PMP system was evaluated over a standard laboratory testing cycle and uphill and downhill on-road, flow-of-traffic driving conditions. The PM mass emissions for the UDDS and on-road tests were more than 6 times lower than the U.S. 2007 heavy-duty PM mass standard. The PM mass emissions for the UDDS fell between those for the uphill and downhill driving on-road driving conditions. The PN emissions of particles larger than 23 nm for the UDDS and downhill on-road driving conditions were 3 times lower than the Euro VI heavy-duty PN limit for transient cycles, while the PN emissions from the uphill on-road driving conditions were 4 to 5 times higher than the Euro VI PN limit. The PN emissions of particles larger than 23 nm for the UDDS (with an average engine load of 38%) were comparable in magnitude to those for the phase 3 downhill segment (with an aveage engine load of 40%) of the on-road test, and were 25% lower than those for the phase 4 downhill segment (with an avearage engine load of 18%) of the on-road test. The variability of the PN emissions of particles larger than 23 nmranged from 10 to 40% for the UDDS and on-road tests, comparable to that found in the European PMP study.

Cholesterol is a component of cell membrane and plays a vital role in maintaining metabolism and normal functions in human body. Liver X receptor (LXR) is a nuclear receptor expressed in abundance in the liver. It influences the process of cholesterol metabolism through regulating the synthesis, transformation, and transportation of cholesterol and bile acid at the level of hepatocytes, and therefore, it plays an important role in maintaining cholesterol homeostasis in human body. In addition, LXR can inhibit the intestinal absorption of dietary cholesterol, reduce exogenous cholesterol level and total cholesterol level in human body, and prevent hypercholesterolemia and formation of gallstones. This article summarizes the mechanism of action of LXR in regulating cholesterol metabolism at both liver and intestinal levels.