Restructuring of carbonaceous particles upon exposure to organic and water vapours

Abstract

In this study, the change in mobility diameter (dm) was investigated for a range carbonaceous particle types (diesel exhaust, petrol exhaust, cigarette smoke, candle smoke, particles generated in a heptane/toluene flame, and wood smoke particles) exposed to different organic (heptane, ethanol, and dimethyl sulfoxide/water (1:1vol%) mixture) and water vapours. Particles were first size-selected and then bubbled through an impinger (bubbler) containing either an organic solvent or water, where particles trapped inside rising bubbles were exposed to saturated vapours of the solvent in the impinger. The size distribution of particles was simultaneously measured upstream and downstream from the impinger. A size-dependent reduction in dm was observed when bubbling fractal-like particles (diesel exhaust, particles generated in a heptane/toluene flame, and candle smoke particles) through heptane, ethanol and a dimethyl sulfoxide/water mixture. In addition, the size distributions of these particles after bubbling through an impinger were broader and thisincrease of the geometric standard deviation (σ) of the size distributions was also found to be size-dependent. Size-dependent reduction in dm and an increase of σ indicate that particles undergo restructuring to a more compact form, which was confirmed by TEM analysis. However, bubbling of these particles through water did not result in a size-dependent reduction in dm, nor in an increase of σ. Liquid particles or particles assumed to be spherical (cigarette smoke, petrol exhaust, and wood smoke particles) did not result in any substantial change in dm, or σ, when bubbled through organic solvents or water. Compaction of fractal-like particles was attributed to the ability of condensing vapours to efficiently wet the particles. Our results also show that the presence of an organic layer on the surface of fractal-like particles, or the surface tension of the condensed liquid do not influence the extent of compaction.