Although there have been a number of publications focused on heterogeneous of NO2 on mineral particles, most of these studies were focused on β-Al2O3 and performed in the dark. Less was known about the reaction process of NO2 on α-Al2O3, especially the effect of sunlight factor. The heterogeneous reaction between NO2 and α-Al2O3 was investigated by using diffuse reflectance infrared Fourier transform spectrometry. The effects of NO2 and O2 concentrations as well as simulated sunlight were examined, and the reaction mechanism including the consumption of surface OH groups, oxidation process of nitrites, and the formation of water was also discussed in detail. It was observed that the formation rates of nitrates and nitrites were sensitive to NO2 concentrations and O2 concentrations. Nitrite was identified to be an intermediate production and disappeared very soon as [NO2] was up to 4.035 × 1015 molecules/cm3. Light played an important role in the changes of the electronic configuration of mineral dust, such as electronic donating ability, surface OH groups orientation, as well as the conversion efficiency between proton acid and nonproton acid, all of which could significantly enhance the heterogeneous reaction process. The reaction order for NO2 and O2 was determined to be 0.960 ± 0.111 and 0.620 ± 0.028, respectively. The uptake coefficient of NO2, which dominated the first step of the heterogeneous reaction, was calculated by the infrared absorbance with the use of ion chromatography and determined to be 9.9 × 10–10 in the dark and varied from 2.54 to 3.33 × 10–9 under simulated sunlight from 0.45 to 1.35 mW/cm2. It was also found that γNO2 was independent of [NO2] and sunlight increased the uptake coefficient by three times, indicating that the heterogeneous reaction between NO2 and α-Al2O3 was enhanced under sunlight.

The characteristic variations of exhaust particles were investigated on a light-duty diesel engine. The commercial diesel fuel (D100) was used as a baseline fuel, and three different oxygenated fuels with the same oxygen content, including a 50% soybean-based biodiesel blend (B50), a 8% dimethyl carbonate blend (DMC8), and a 13% dimethoxy methane blend (DMM13), were tested in this study. From particle size distribution measurement, the oxygenated fuel blends reduce accumulation mode particles (diameter > 50 nm) but increase nucleation mode particles (diameter < 50 nm) in varying degrees compared to the base fuel. The particle size distribution for B50 shows the lowest accumulation mode particle concentration accompanying with the highest nucleation mode particle concentration. From OC/EC (organic carbon/elemental carbon) analysis, three fuel blends reduce the TC (total carbon) emission at a comparable level. With different OC/EC ratios, B50 and DMC8, respectively, produce the most OC and EC materials. On the basis of thermogravimetric analysis and Raman spectroscopy results, particles from B50 exhibit the highest oxidation reactivity and the most amorphous nanostructure. Particle surface functional groups were also measured using Fourier transform infrared spectroscopy. Particles from oxygenated fuel blends have more oxygen-related surface groups than those from the base fuel, and the DMM13-derirved particles show the highest amount comparing with other tested oxygenated fuels.

A classic H2SO4-H2O binary homogeneous nucleation model coupled to an aerosol dynamics model, suitable for studying the formation and transformation of volatile nanoparticles (VNPs) during diesel engine exhaust dilution, has been developed. Using the H2SO4-H2O binary homogeneous nucleation model, the nucleation ratio and molecular cluster size were calculated. The effect of aerosol dynamic processes on VNP number size distributions was studied. The effects of fuel sulfur content (FSC) and sampling conditions in the laboratory on VNP number size distributions were also calculated. Our simulations demonstrated that nucleation increased the cluster number concentration and that FSC, temperature and humidity significantly affected the nucleation ratio and molecular cluster size. Coagulation promoted the evolution of cluster-particle size distributions from monodisperse to polydisperse. Soot present in the exhaust can suppress the formation of VNPs. FSC and sampling conditions, like the primary dilution temperature, the primary dilution relative humidity, residence time and the primary dilution ratio have significant effects on VNP number size distributions.

Measurements of exhaust particle number concentration and size distribution from a dimethyl ether (DME) engine at different engine loads and speeds were carried out by using a two-stage dilution system and an SMPS. The results of the DME engine were compared with those of the original diesel engine. The fuel composition had significant effects on the exhaust particle size distribution, the total exhaust particle number and mass concentrations. Compared with those of the DME engine, the particle mass emissions of the diesel engine increased 5.7–17.7 times. At high engine speed (n=2200 r/min), compared with those of the DME engine, the total particle number emissions of the diesel engine increased 0.75–2.2 times, while the total particle number emissions of the diesel engine decreased by about 50%–80% for middle and high loads at middle engine speed (n=1400 r/min). Compared with those of the DME engine, the total exhaust particle number concentrations in the accumulation mode of the diesel engine increased 4.2–62.6 times and the exhaust particle geometric number mean diameters in the accumulation mode increased by about 10–30 nm. This correlated with higher oxygen level and lack of C-C bonds in DME. A lot of nucleation mode particles were emitted from the DME engine, this correlated with the processes of nucleation and condensation of the volatile and semi-volatile compounds in the exhaust gas.

Multiple-injection has been widely investigated to simultaneously reduce diesel NOx and soot emissions. While the comprehensive understanding of the effects of multiple injection on exhaust PM physical characteristics is still lacking. Three injection modes, single main, pilot-main and main-post injections, were compared in this work. The main objective is to better understand the influence of pilot and post injections on particle size and nanostructure characteristics in diesel exhaust. Experimental tests have shown that, for the pilot-main injection case, less premixed combustion and more diffusion combustion occur compared with the single main injection, which promotes formation of soot nuclei and results in a significant increase of number and mass of particles with the diameter above 100 nm. On the contrary, soot oxidation later in the combustion is improved due to the enhancement of gas mean temperature (GMT) and air/fuel mixing for the main-post injection case, which favors soot oxidation and leads to the decrease of particle number and diameter. A comparison of particle nanostructures for the pilot-main injection case and main-post injection case has been conducted, which indicates that both low in-cylinder temperature and relative short carbonization time lead to the particles of less carbonization level (short fringe length and large fringe tortuosity) for the pilot-main injection case. Particles exhibit highly ordered graphitic structure for the main-post injection case due to the increase of combustion duration and the enhancement of in-cylinder temperature during the later stage of combustion derived by post combustion, which presents potential negative effect on diesel particle filters (DPF) regeneration efficiency.