Heterogeneous ice nucleation on particles composed of humic-like substances impacted by O3

Heterogeneous ice nucleation plays important roles in cirrus and mixed-phase cloud formation, but the efficiency of organic particles to act as ice nuclei (IN) is still not well understood. Furthermore, the effect of particle oxidation by O-3 on corresponding IN efficiencies has not yet been sufficiently assessed. We present heterogeneous ice nucleation on kaolinite, Suwannee River standard fulvic acid (SRFA), and leonardite standard humic acid particles as a function of particle temperature (T-p), relative humidity with respect to ice (RHice), nucleation mode, and O-3 exposure. Ice nucleation and water uptake were studied for T-p >203 K and RHice up to water saturation using a novel ice nucleation apparatus. This study shows that SRFA, leonardite, and corresponding O-3-exposed particles can nucleate ice via different modes at relevant atmospheric conditions. These particles nucleated ice via deposition mode at T-p <= 231 K, and for higher T-p water was taken up or ice was nucleated via deposition or immersion mode. Oxidation of leonardite and SRFA particles by O-3 led to a decrease in deposition nucleation efficiency and to water uptake at lower temperatures for the former and to an increase in the lowest temperature at which deposition nucleation was observed for the latter. Activated IN fractions and heterogeneous ice nucleation rate coefficients (J(het)) were derived, and corresponding contact angles (theta) were calculated. A parameterization of theta as a function of RHice is presented which allows derivation of J(het) for various deposition IN and corresponding ice crystal production rates for application in cloud-resolving models.