Investigations of Marine Ice Nucleating Particles (Invited Presentation)

Marine air can contrast strongly with continental air in terms of total particle concentrations, CCN and ice nucleating particle properties. To understand the implications of this aerosol state on clouds and precipitation involving the ice phase over oceans, and to understand how perturbations on this system imposed by dust and biomass burning aerosols may alter clouds over the oceans, it is necessary to understand the sources and concentrations of ice nucleating particles emitted from oceans. In this study, measurements are presented of the temperature spectra of ice nucleating particle concentrations in recent specialized laboratory sea spray studies and in field campaigns conducted in the Northern Hemisphere. Comparison is made to previous data collected over oceans.

Measurements of ice nuclei were made on laboratory-generated sea spray particles produced from breaking waves in a large wave channel (Prather et al. 2013), from bubble production induced by plunging waterfalls with varied microbiological conditions in the laboratory, and from ground-based (Puerto Rico), aircraft-based (St. Croix), and ship-based measurements (various regions of the Pacific Ocean) in the marine boundary layer. Ice nucleation measurements were made in real-time using the CSU continuous flow diffusion chamber operating in the condensation/immersion freezing regime and using offline immersion freezing measurements of particles collected on filters or other substrates. The two measurement methods were consistent in their overlapping temperature regimes.

In general, measured number concentrations of ice nucleating particles in laboratory studies were consistent with the average range of previous measurements over oceans. These concentrations of ice nuclei tend to be lower than are measured over land regions, at least for modestly supercooled cloud conditions. Certain but complex connections of ice nucleating particle production to ocean microbiological processes affecting the chemical composition of the sea surface microlayer were seen, but the composition of the ice nucleating units remains to be identified. Associations of ice nucleating particle concentrations with heterotrophic bacterial concentrations were noted in some experiments (Prather et al. 2013), while correlation with chlorophyll-a concentration in seawater was seen in laboratory simulations of phytoplankton blooms.

The immersion freezing spectra of particles measured in field studies were similar to those found in the laboratory studies, but varied by location. The majority of particles detected thus far as ice nuclei from sea spray and in marine air show episodic direct participation of biological ice nucleating organisms on the basis of sensitivity to high temperatures (95°C). However, assembled measurements are still sparse, the nuclei could be a product of biological processes, and ship-collected particles from the Bering Sea showed a high labile fraction associated with the presence of a high proportion, based on pyrosequencing of DNA extracted from the collected particles, of species from the Gammaproteobacteria, the same class that contains the ice nucleating bacteria.

These studies are ongoing. Data are intended to serve as the basis for parameterization development for ice initiation in numerical model simulations of mixed-phase clouds. New measurement plans, including ship-based aerosol collections over Southern Oceans, will also be described.

Prather, K. A., T. H. Bertram, V. H. Grassian, G. B. Deane, M. D. Stokes, P. J. DeMott, L. I. Aluwihare, B. Palenik, F. Azam, J. H. Seinfeld, R. C. Moffet, M. J. Molina, C. D. Cappa, F. M. Geiger, G. C. Roberts, L. M. Russell, A. P. Ault, J. Baltrusaitis, D. B. Collins, C. E. Corrigan, L. A. Cuadra-Rodriguez, C. J. Ebben, S. D. Forestieri, T. L. Guasco, S. P. Hersey, M. J. Kim, W. Lambert, R. L. Modini, W. Mui, B. E. Pedler, M. J. Ruppel, O. S. Ryder, N. Schoepp, R. C. Sullivan, and D. Zhao, 2013: Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol, Proc. Natnl. Acad. Sci., 110 (19) 7550-7555, www.pnas.org/cgi/doi/10.1073/pnas.1300262110.