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Ice formation by black carbon particles was studied using a continuous flow thermal diffusion chamber. Submicron particles were treated to uptake H 2 SO 4 in amounts estimated to range from zero to several percent by weight. The particles were processed at constant temperatures (−40 to −60°C), and humidity from ice saturation to values exceeding water saturation. Conditions required for ice nucleation were determined for different acid exposures. Untreated soot particles showed activity as deposition/sorption ice nuclei. Particles with approximate monolayer equivalent coverage by H 2 SO 4 nucleated ice at humidities for which it was inferred that the acid was dilute enough to freeze homogeneously on soot surfaces. Particles with multi‐layer equivalent H 2 SO 4 coverage froze as solution droplets most readily. Heterogeneous freezing of the latter particles was observed to occur in preference to the homogeneous freezing of an externally mixed population of H 2 SO 4 droplets at lower temperatures.
DeMott et al. (Sun,) studied this question.