The influence of marine biogenic particles on ice phase initiation

Aerosol particles vary in composition with many being biogenic and of terrestrial or marine origin. Efficient ice forming biogenic particles are typically thought to be of terrestrial origin; however, recent data demonstrate that marine biogenic particles can act as ice nuclei (IN) in both immersion and deposition modes, with and without association of NaCl. These results are significant given that ocean derived particles including phytoplankton, microorganisms, transparent exopolymers, and colloidal gels become aerosolized from the sea surface microlayer through wave breaking and bubble bursting. Such particles typically include sea salt, but in situ observations of air masses associated with phytoplankton blooms have identified organic compounds as significant mass contributors to aerosol loading. Here we present results from experiments with Thalassiosira pseudonana, Emiliania huxleyi, and Nanochloris atomus, phytoplankton with distinctly different cell walls: silica, calcite, and cellulose fibrils, respectively, as efficient IN in immersion and deposition modes at typical tropospheric conditions. In a separate set of experiments, submicron size particles with and without organics are generated through bubble bursting in a custom built seawater tank. Subsequently collected, these particles are observed using a coupled cooling stage/optical microscope, for their ice nucleation potential as a function of particle temperature (T), water activity (aw), relative humidity with respect to ice (RHice), droplet volume, and particle surface area. In the immersion mode, fragmented and intact cells of T. pseudonana and N. atomus enhance ice nucleation in aqueous NaCl solution droplets by ~10-30 K and 10-20 K above the homogeneous freezing limit, and for a range of aw of 1.0-0.8, while E. huxleyi do not enhance freezing temperatures. In the deposition mode, all three species nucleate ice for RHice as low as ~120%, however, for each, different nucleation modes occur at warmer temperatures. T. pseudonana and N. atomus take up water as low as ~85% relative humidity and subsequently nucleate via immersion freezing. E. huxleyi can nucleate ice via deposition freezing for T as high as 245 K. Direct measurements of cell surface area are used to derive ice nucleation rate coefficients and contact angles, α, following classical nucleation theory, a time-dependent description of ice nucleation. A time-independent deterministic description is used to derive ice active surface site densities. Values of α range from 60° to 100° and depend on T and RHice in the immersion mode; however, for deposition freezing, α can be reproduced as a function of RHice between 16° to 30°. These results underscore the importance of ocean derived biogenic particles for the formation and evolution of ice and mixed phase clouds in the atmosphere.