PNNL

The Science

Ice crystal formation in clouds is a major uncertainty in current climate models and increasing scientific understanding of cloud ice formation can help make future climate predictions more accurate. Sea spray transports carbon-based, or organic, compounds and larger particles associated with marine biological processes from the sea surface to altitudes where they can jumpstart the formation of tiny ice crystals in clouds. For biologically derived, or biogenic, particles, emissions and high concentrations of ice-nucleating particles (INPs) tend to occur in association with specific events like storm-induced mixing. Researchers used these observations and model simulations to identify the missing links between ocean biology, sea spray emission, and cloud ice formation.

The Impact

Clouds over the oceans are a very important part of Earth’s climate system. Marine clouds that contain ice crystals could be significantly affected by a warming climate, but researchers are uncertain about the exact type and extent of these changes. Representing the complex interplay between oceanic and atmospheric processes in climate models is crucial to developing a better understanding of marine clouds, which are influenced by particles emitted from the ocean.

Summary

Sea spray is a major source of INPs over the oceans. There are some indications that biogenic particles (e.g., marine bacteria) may play a role contributing to exceptionally high INP concentrations. Previous studies focused on understanding INPs associated with organic material in smaller sea spray particles. In contrast, the larger biogenic particles are emitted only episodically, making them more difficult to study. Their global atmospheric relevance has not been previously quantified using model simulations. One reason for this omission is the very complex interplay between marine and atmospheric processes (e.g., the enrichment of marine particulate matter in aerosol particles during sea-air transfer with dramatically varying estimates between different laboratory and field experiments). To tackle this challenge, researchers developed a numerical framework to describe these INP emissions using bacteria and polymers as proxies for marine biogenic particles. By exploring different physical scenarios, they estimated the range of plausible values for the atmospheric contributions of marine biogenic INPs. A comparison with observations shows that marine biogenic particles likely make only minor contributions to the atmosphere unless three conditions are met: the particles are (1) highly concentrated in ocean water, (2) strongly enriched during sea-air transfer, and (3) highly efficient as INPs. These findings help to clarify priorities for future research aiming to understand the global relevance of this unique class of INPs.

PNNL Contact

Susannah Burrows, Pacific Northwest National Laboratory, susannah.burrows@pnnl.gov

Funding

This research was supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research program through the Early Career Research Program and used data from the Atmospheric Radiation Measurement User Facility. A portion of the research for this study was performed using resources available through Research Computing at Pacific Northwest National Laboratory. Simulation data were obtained from the Energy Exascale Earth System Model project, sponsored by the DOE, Office of Science, Biological and Environmental Research program. Part of this research was also funded by the National Science Foundation through its Center for Aerosol Impacts on Chemistry of the Environment.