Identifying links between sea spray ice nucleating particles and oceanic biological activity

The impact of naturally-derived particles on cloud processes is one of the largest sources of uncertainty in climate model predictions. Previous studies regarding atmospheric ice nucleation in marine environments have suggested that higher levels of ice nucleating particles (INP) are associated with regions of oceanic upwelling (i.e. regions rich in biological activity). The goal of this study was to determine the abundance of INP in the marine environment, the attributes and compositional properties of sea spray INP and the dependence of INP emissions on ocean biological cycling.

We first observed links between biological activity, sea spray aerosol and ice nucleation in a series of mesocosm experiments, conducted as part of the Center for Aerosol Impacts on the Climate and the Environment (CAICE), by integrating measurements of number concentrations of INP active at temperatures spanning -32 to -10 °C and aerosol size distributions and chemical composition with phytoplankton and bacteria counts in bulk mesocosm seawater. Mesocosm manipulations included a range of nutrient additions to fresh seawater, offering an array of phytoplankton bloom scenarios. Aerosol was generated via plunging water or wave breaking, creating aerosol number concentrations and size distributions similar to typical marine boundary layer conditions. Number concentrations of INP were characterized using the Colorado State University (CSU) continuous flow diffusion chamber (CFDC) and the CSU ice spectrometer (immersion freezing device); single particle chemical composition was determined using an aerosol time of flight mass spectrometer (ATOFMS).

Sea spray INP were further characterized by single particle chemical and morphological analyses. Ice crystal residuals were collected downstream of the CFDC for subsequent analysis with scanning emission and scanning transmission x-ray microscopy and Raman microspectroscopy. A pumped counter-flow virtual impactor (PCVI) was also used to selectively send ice crystals from the CFDC output flow to an ATOFMS, providing in-situ single ice crystal residual chemical composition.

To complement the mesocosm experiments, we explored the contribution of sea spray INP to the total INP population in a remote coastal environment at the University of California Davis Bodega Bay Marine Laboratory (BML). In a short-term field deployment at BML, number concentrations of INP were monitored via the CFDC in conjunction with single particle chemical composition and aerosol size distribution measurements.

Preliminary results indicate number concentrations of natural sea spray INP in the marine boundary layer range from 1 to 10 L^-1 at -30 °C, with a typical decrease of one order of magnitude per 5 °C of temperature increase. Highest levels of INP in mesocosms were observed after peak fluorescence of the blooms, likely associated with increases in bacteria counts and biological byproducts, such as monolayer-forming aliphatics. INP compositions at BML had varied sources, although a common type appears to be organic coated sea salt. These results were similar in the laboratory and field locations.