Ice nucleation by particles containing long-chain fatty acids of relevance to freezing by sea spray aerosols

Heterogeneous ice nucleation in the atmosphere regulates cloud properties, such as phase (ice versus liquid) and lifetime. Aerosol particles of marine origin are relevant ice nucleating particle sources when marine aerosol layers are lifted over mountainous terrain and in higher latitude ocean boundary layers, distant from terrestrial aerosol sources. Among many particle compositions associated with ice nucleation by sea spray aerosols are highly saturated fatty acids. Previous studies have not demonstrated their ability to freeze dilute water droplets. This study investigates ice nucleation by monolayers at the surface of supercooled droplets and as crystalline particles at temperatures exceeding the threshold for homogeneous freezing. Results show the poor efficiency of long chain fatty acid (C16, C18) monolayers in templating freezing of pure water droplets and seawater subphase to temperatures of at least -30 degrees C, consistent with theory. This contrasts with freezing of fatty alcohols (C22 used here) at nearly 20 degrees C warmer. Evaporation of mL-sized droplets to promote structural compression of a C19 acid monolayer did not favor warmer ice formation of drops. Heterogeneous ice nucleation occurred for nL-sized droplets condensed on 5 to 100 mm crystalline particles of fatty acid (C12 to C20) at a range of temperatures below -28 degrees C. These experiments suggest that fatty acids nucleate ice at warmer than -36 degrees C only when the crystalline phase is present. Rough estimates of ice active site densities are consistent with those of marine aerosols, but require knowledge of the proportion of surface area comprised of fatty acids for application.