The UNAM-Droplet Freezing Assay: An Evaluation of the Ice Nucleating Capacity of the Sea-Surface Microlayer and Surface Mixed Layer in Tropical and Subpolar Waters

Ice nucleating particles (INPs) in the atmosphere are necessary to generate ice crystals in mixed-phase clouds, a crucial component for precipitation development. The sources and composition of INPs are varied: from mineral dust derived from continental erosion to bioaerosols resulting bubble bursting at the ocean surface. The performance of a home-built droplet freezing assay (DFA) device for quantificatying of the ice nucleating abilities of water samples via immersion freezing has been validated against both published results and analyses of samples from sea surface microlayer (SML) and bulk surface water (BSW) from the Gulf of Mexico (GoM) and Saanich Inlet, off Vancouver Island (VI), Canada. Even in the absence of phytoplankton blooms, all the samples contained ice nucleating particles at moderate concentrations, ranging from 6.0x10 1 to 1.1x10 5 L -1 water. The freezing temperatures (i.e., T 50 , the temperature at which 50% of the droplets freeze) of the samples decreased in order of VI SML > GoM BSW > GoM SML, indicating that the higher-latitude coastal waters have a greater potential to initiate cloud formation and precipitation.