Ice formation by black carbon particles

Ice formation by black carbon particles was studied using a continuous flow thermal diffusion chamber. Submicron particles were treated to uptake H2SO 4 in amounts estimated to range from zero to several percent by weight. The particles were processed at constant temperatures (-40 to -60 degrees C), and humidity from ice saturation to values exceeding water saturation. Cond itions required for ice nucleation were determined for different acid expos ures. Untreated soot particles showed activity as deposition/sorption ice n uclei. Particles with approximate monolayer equivalent coverage by H2SO4 nu cleated ice at humidities for which it was inferred that the acid was dilut e enough to freeze homogeneously on soot surfaces. Particles with multi-lay er equivalent H2SO4 coverage froze as solution droplets most readily. Heter ogeneous freezing of the latter particles was observed to occur in preferen ce to the homogeneous freezing of an externally mixed population of H2SO4 d roplets at lower temperatures.