Water nucleation properties of carbon black and diesel soot particles

In laboratory experiments we studied the hygroscopic properties of carbon black and diesel soot aerosols with and without chemical modification. The fraction of particles activated to droplets was determined by simultaneous measurement of condensation nuclei (CN) and cloud condensation nuclei (CCN) for a range of supersaturations (0.2–1.3%) and particle sizes (r = 0.02−0.2 μm). The chemical modification enhanced the soluble mass fraction of the particles produced. At fixed supersaturation the CCN/CN ratio was a function of particle size (large CCN/CN ratios at large sizes) and the particle soluble mass fraction. This ratio was also influenced by the method of aerosol generation. The nucleation ability of chemically modified carbonaceous particles increased with increasing soluble mass fraction and was comparable to that of (NH4)2SO4 when the soluble mass fraction exceeded about 10%. The hygroscopicity of particles generated by combustion of diesel fuel in a diffusion flame increased when a sulfur-containing compound was added to the fuel. The CCN characteristics of diesel soot appears to be comparable to that of wood smoke aerosol.