Sea Spray Aerosols Are a Unique Source of Ice Nucleating Particles
Laboratory studies and field campaign measurements are in agreement on the role of waves in producing particles that can lead to ice crystal formation in the atmosphere.
In clouds that are not cold enough for homogeneous freezing of condensed water to occur, the initiation of ice crystals requires the presence of relatively rare particles called ice nucleating particles (INPs). Fewer than one in a million aerosol particles in the atmosphere are INPs. Understanding the characteristics and sources of INPs is important for understanding and modeling cloud processes in cold regions, including cloud formation and lifetime, precipitation, and cloud radiative forcing. The source and concentration of INPs in remote oceanic regions, far from typical sources of INP such as soil and dust, is not well understood.
Measurements of INP concentrations from sea spray aerosol particles produced from breaking waves in a laboratory wave channel were found to be consistent with measurements from a diverse set of oceanic field campaigns including the Atmospheric Radiation Measurement (ARM)-funded “MAGIC” campaign. The results confirm that sea spray aerosols can be an important source of INPs over remote ocean regions and that number concentrations of INP and active site densities of the aerosol particles (likelihood that an aerosol will be an INP) are lower over remote regions than over continental regions. These findings have important implications for cloud radiative forcing and precipitation within marine clouds in remote regions that are unaffected by continental INP sources.
Global climate models consistently underestimate the reflected shortwave solar radiation in regions dominated by oceans, especially in the Southern Hemisphere. This underestimation has been attributed to the models producing too few and too short-lived clouds, in contrast with the persistent cloud layers observed by satellites in this region. This discrepancy may be due to errors in the number concentration of INPs, and therefore of ice and mixed-phase clouds, over remote ocean regions. Many studies have examined terrestrial sources of INPs, including mineral and arable soil dust, biomass burning, anthropogenic pollution particles, and biological organisms released from disturbed soil. The relative role of the ocean as a source of INPs, through sea spray aerosols produced as bubbles burst when waves break, is less well understood. Measurements of INPs over remote ocean regions cannot always determine whether the particles were produced from terrestrial or oceanic sources. In this study, laboratory experiments were conducted to isolate and characterize sea spray aerosols produced from wave breaking. Sea spray aerosol particles were produced in the laboratory using two different wave breaking methods. INP number concentrations were obtained from the laboratory experiments using an on-line and two off-line sampling techniques. INP number concentrations measured in the laboratory were shown to be within the range of INP concentrations measured in several field studies over the ocean, including the ARM-funded MAGIC campaign. These results confirm that the INPs measured in these remote locations are likely produced from sea spray, and not from transport of continental aerosols. The study also shows that as a function of surface area, sea spray aerosols have lower active site densities than continental aerosols such as mineral dust, indicating that sea spray aerosols are less likely to form INP.
Colorado State University
Funding support was provided by the National Science Foundation, National Aeronautics and Space Administration; Department of Energy, Office of Science, Office of Biological and Environmental Research (ASR and ARM programs); Korean Polar Research Program; and Natural Sciences and Engineering Research Council of Canada.
DeMott, P. J., T. C. Hill, C. S. McCluskey, K. A. Prather, D. B. Collins, R. C. Sullivan, M. J. Ruppel, R. H. Mason, V. E. Irish, T. Lee, C. Y. Hwang, T. S. Rhee, J. R. Snider, G. R. McMeeking, S. Dhaniyala, E. R. Lewis, J. J. Wentzell, J. Abbatt, C. Lee, C. M. Sultana, A. P. Ault, J. L. Axson, M. D. Martinez, I. Venero, G. Santos-Figueroa, M. D. Stokes, G. B. Deane, O. L. Mayol-Bracero, V. H. Grassian, T. H. Bertram, A. K. Bertram, B. F. Moffett, and G. D. Franc. 2016. “Sea Spray Aerosol as a Unique Source of Ice Nucleating Particles,” Proceedings of the National Academy of Sciences (USA) 113(21), 5797-803. DOI: 10.1073/pnas.1514034112.