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

Abstract

[1] 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 O3 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 (Tp), relative humidity with respect to ice (RHice), nucleation mode, and O3 exposure. Ice nucleation and water uptake were studied for Tp >203 K and RHice up to water saturation using a novel ice nucleation apparatus. This study shows that SRFA, leonardite, and corresponding O3-exposed particles can nucleate ice via different modes at relevant atmospheric conditions. These particles nucleated ice via deposition mode at Tp ≤ 231 K, and for higher Tp water was taken up or ice was nucleated via deposition or immersion mode. Oxidation of leonardite and SRFA particles by O3 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 (Jhet) were derived, and corresponding contact angles (θ) were calculated. A parameterization of θ as a function of RHice is presented which allows derivation of Jhet for various deposition IN and corresponding ice crystal production rates for application in cloud-resolving models.