Soot aerosols are well known to be atmospheric constituents, but the hydrophobic nature of fresh soot likely prohibits them from encouraging cloud development. Soot aged through contact with oxygenated organic compounds may become hydrophilic enough to promote water uptake. In this study, the tandem differential mobility analyzer (TDMA) and differential mobility analyzer?aerosol particle mass analyzer (DMA?APM) were employed to measure the changes in morphology and hygroscopicity of soot aerosol particles upon coating with succinic and glutaric acids. The effective densities, fractal dimensions and dynamic shape factors of fresh and coated soot aerosol particles have been determined. Significant size-dependent increases of soot particle mobility diameter, mass, and effective density (?eff) were observed upon coating of aggregates with succinic acid. These properties were restored back to their initial states once the acid was removed by heating, suggesting no restructuring of the soot core had occurred. Coating of soot with glutaric acid, on the other hand, leads to a strong size shrinking with a diameter growth factor ~0.60, even after the acid has been removed by heating suggesting the strong restructuring of the soot agglomerate. The additional 90% RH cycle can evidently enhance the restructuring process. The extinction and scattering properties at 532 nm of soot particles internally mixed with dicarboxylic acids were investigated experimentally using a cavity ring-down spectrometer and an integrating nephelometer, respectively, and the absorption is derived as the difference between extinction and scattering. It was found that the organic coatings significantly affect the optical and microphysical properties of the soot aggregates. The size-dependent amplification factors of light scattering were as much as 3.8 and 1.7 with glutaric and succinic acids coatings, respectively. Additional measurements with soot particles that are first coated with glutaric acid and then heated to remove the coating show that both scattering and absorption are enhanced by irreversible restructuring of soot aggregates to more compact globules. These results reveal the microphysical state of soot aerosol with incomplete restructuring in the atmosphere, and advance the treatment of atmospheric aged soot aerosol in the Mie theory shell-and-core model.

Practical combustion devices such as gas turbines and diesel engines operate at high pressures to increase their efficiency. Pressure significantly increases the overall soot yield. Morphology of these ultra-fine particles determines their airborne lifetime and their interaction with the human respiratory system. Therefore, investigating soot morphology at high pressure is of practical relevance. In this work, a novel experimental setup has been designed and built to study the soot morphology at elevated pressures. The experimental setup consists of a pressure vessel, which can provide optical access from 10° to 165° for multi-angle light scattering, and a counterflow burner which produces laminar flames at elevated pressures. In the first part of the study, N2-diluted ethylene/air and ethane air counterflow flames are stabilized from 2 to 5 atm. Two-angle light scattering and extinction technique have been used to study the effects of pressure on soot parameters. Path averaged soot volume fraction is found to be very sensitive to pressure and increased significantly from 2 to 5 atm. Primary particle size and aggregate size also increased with pressure. Multi-angle light scattering is also performed and flames are investigated from 3 to 5 atm. Scattering to absorption ratio is calculated from multi-angle light scattering and extinction data. Scattering to absorption ratio increased with pressure whereas the number of primary particles in an aggregate decreased with increasing pressure. In the next part of the study, Thermophoretic Sampling of soot is performed, in counterflow flames from 3 to 10 atm, followed by transmission electron microscopy. Mean primary particle size increased with pressure and these trends are consistent withour light scattering measurements. Fractal properties of soot aggregates are found to be insensitive to pressure. 2D diffused light line of sight attenuation (LOSA) and Laser Induced Incandescence (LII) are used to measure local soot volume fraction from 2 to 10 atm. Local soot volume fraction increased with pressure and soot concentration profiles showed good agreements when measured by both techniques. Experimental data obtained in this work is very helpful for the modelers for validating their codes and predicting the soot formation in pressurized flames.

An experimental study was performed to study soot formation and evolution in atmospheric, laminar, coflow, diffusion flames of Jet-A1, Synthetic Paraffinic Kerosene and their surrogates. Light extinction, rapid thermocouple insertion and thermophoretic sampling followed by transmission electron microscopy and atomic forced microscopy were used to obtain soot volume fraction profiles, temperature profiles and soot morphologies, respectively. Different soot evolution processes were observed on the flame centerline and on a streamline with a significantly different temperature history. Formation and agglomeration of the first soot particles are different on the two streamlines. Transparent liquid-like particles are produced in large volumes in the early regions of the flame centerline where T < 1500 K; these particles are undetectable by the extinction method with the wavelength of 632.8 nm. Most of the currently used computational soot models do not predict the liquid-like nature of nascent soot particles which has major effects on the modeling.

An experimental study was performed to study soot formation and evolution in atmospheric, laminar, coflow, diffusion flames of Jet-A1, Synthetic Paraffinic Kerosene and their surrogates. Light extinction, rapid thermocouple insertion and thermophoretic sampling followed by transmission electron microscopy and atomic forced microscopy were used to obtain soot volume fraction profiles, temperature profiles and soot morphologies, respectively. Different soot evolution processes were observed on the flame centerline and on a streamline with a significantly different temperature history. Formation and agglomeration of the first soot particles are different on the two streamlines. Transparent liquid-like particles are produced in large volumes in the early regions of the flame centerline where T < 1500 K; these particles are undetectable by the extinction method with the wavelength of 632.8 nm. Most of the currently used computational soot models do not predict the liquid-like nature of nascent soot particles which has major effects on the modeling.