Observations of ice nucleation by ambient aerosol in the homogeneous freezing regime

We measured the freezing activation curves for ambient particles as a function of relative humidity with respect to water over the temperature range of −40° to −50°C using a continuous flow diffusion chamber, and compared the observations with those for ammonium sulfate particles and predictions from a parametric representation of homogeneous freezing of solution particles as a function of water activity and temperature. Since it has been proposed that the rate of homogeneous freezing depends on solution water activity, we made separate measurements of the hygroscopicity (κ) of the ambient aerosol. Observed κ ranged from 0.1 to 0.2, lower than that of ammonium sulfate (0.6) and representative of a continental aerosol. As predicted for this difference in κ, there was no significant difference between the homogeneous freezing conditions of size‐selected ammonium sulfate and the apparent homogeneous freezing conditions of same‐sized ambient aerosol. Further, the parameterization predicted freezing fraction‐relative humidity relationships for non size‐selected ambient aerosol that differed by only 0.5 to 1.5% relative humidity from observed relations at the tested temperatures, well within experimental uncertainty. Our findings confirm that the tested ambient aerosols, with hygroscopicities typical of continental regions, freeze homogeneously as expected based on present understanding for single component solution drops in the laboratory. Results also confirm that freezing is more sensitive to particle size than to composition, for particles containing at least a few percent by volume of hygroscopic species.