Evaluating Ocean Sources of Ice Nucleating Particles

Aerosols produced from sea spray emissions have been relatively little discussed and often dismissed as sources of atmospheric ice nucleating particles (INP), presumably paling in comparison to desert and other terrestrial sources that are transported long distances and to high altitudes. Nevertheless, the vastness of oceans and cloud systems over them make it inevitable that ocean emissions are the primary source of ice nucleating particles to cold clouds in some regions. Focused measurements over oceans have been limited in the last 30 years, as have been laboratory studies, such that both the nature and abundance of oceanic INP sources remain unclear. Thus, the extent to which INP emissions may influence cloud properties is also unclear. Poor prediction of cloud radiative properties by climate models, especially over Southern Hemisphere ocean regions, the deep supercooling of cloud systems noted via satellite and select ship-based remote sensing over these same areas, and the low ice crystal concentrations noted in the few aircraft-based measurement studies in mixed-phase clouds over Southern Oceans raise the question of the possible role of weak INP emissions in affecting cloud properties. This paper describes new measurements of the temperature spectrum of ice nucleating particle number concentrations and investigations of the nature of sea spray-produced INP in recent specialized laboratory studies at the NSF Center for Aerosol Impacts on Climate and the Environment at the University of California, San Diego and in field campaigns at sites in the Northern Hemisphere. These measurements will be compared to previous data collected from over global ocean regions.

Online and offline methods are being applied to measure the number concentrations of INP from realistically-generated laboratory sea spray particles (by wave generation or plunging water bubble production) and aerosols over marine and coastal regions. Measurements suggest that immersion freezing is the nearly exclusive mechanism by which such particles activate as ice nuclei, motivating online processing in this mode using a continuous flow diffusion chamber (CFDC) and comparison to offline processing of the immersion freezing temperature spectra of collected aerosols dispersed into small liquid volumes. The typical number concentrations of INP accompanying typical marine boundary layer aerosol concentrations are lower (e.g., only 1 in 10 million particles are typically active as INP at -15°C) than are measured over land regions, are consistent with previous measurements made over oceans, and show general agreement between laboratory and atmospheric measurements. Nevertheless, sometimes strong increases in INP concentrations are found in accord with biological processes occurring in surface seawater.

In the laboratory, we have thus far mimicked ocean situations dominated by growth (blooms) of bacteria and phytoplankton, finding instances where INP increase during bacterial blooms, increase during certain phytoplankton blooms, or increase following a phytoplankton bloom. Higher INP number concentrations were also noted during ship transects over biologically-productive ocean regions compared to oligotrophic regions, as indicated by satellite chlorophyll-a and particulate organic carbon measurements during sampling periods. Variation of the size of emitted sea spray in the laboratory and selective filtering of bulk and surface microlayer seawater for immersion freezing measurements indicate that INP in released in sea spray tend to be small units in the sub-200 nm range except in special circumstances. Heat sensitivity tests indicate that these INP are likely organic in composition, but proteinaceous (organisms) contributions are unclear. Scanning electron microscopy and mass spectrometric measurements of INP separate following activation in the CFDC instrument suggest that these INP components are usually distributed with sea salt particles.

The variation of INP emissions over the global oceans and their influences on clouds remain as a major topic for continued exploration. The exact nature of marine ice nucleating units of particles remains to be fully elucidated, and this will be required before informed quantitative predictions of these ice nuclei can be developed for modeling impacts on mixed-phase clouds and climate.