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 bioacrosols resulting from bubble bursting at the ocean surface. The performance of a home-built droplet freezing assay (DFA) device for quantifying 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 INPs at moderate concentrations, ranging from 6.0 x 10(1) to 1.1 x 10(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.

Automated summary

Ice nucleating particles (INPs) in the atmosphere are crucial for ice crystal formation in mixed-phase clouds. The sources and composition of INPs vary, but they can be found even in the absence of phytoplankton blooms. A home-built droplet freezing assay (DFA) device has been validated for quantifying the ice nucleating abilities of water samples, showing that higher-latitude coastal waters have a greater potential for cloud formation and precipitation.