Dust Cools Climate Due to Effects on High-Level Cloud Ice Particles
Atmospheric particles, or aerosols, influence climate by blocking incoming solar radiation and by influencing
clouds. One of the least understood effects of aerosols is their influence on very cold clouds, which exist toward polar regions and high in the atmosphere. High-atmosphere clouds trap long-wave radiation and warm climate. Dust from natural sources, such as deserts, and from human activities, including disruption of soils and some industrial activities, appears to have an important effect on cold clouds. Researchers, led by a U.S. Department of Energy scientist at Pacific Northwest National Laboratory, used the Community Atmospheric Model version 5 (CAM5) to study the effect of dust on upper tropospheric cirrus clouds through their tendency to enhance ice particle formation as vapor or droplets that freeze on dust (heterogeneous ice nucleation). These ice particles typically fall out or precipitate. Although scarce, heterogeneous ice nuclei could impact ice crystal number concentration, compared to standard droplet (homogeneous) freezing, by initiating ice nucleation earlier, depleting available water vapor, and hindering the occurrence of homogeneous freezing. Using two model formulations that consider homogeneous and heterogeneous nucleation and the competition between them, the team found heterogeneous nucleation on dust aerosols reduces the occurrence frequency of homogeneous
nucleation and thus the ice crystal number concentration in the northern hemisphere. These results highlight the importance of quantifying the number concentrations and properties of heterogeneous ice nuclei (mainly dust) in the upper troposphere.
Liu, X., X. Shi, K. Zhang, E. J. Jensen, A. Gettelman, D. Barahona, A. Nenes, and P. Lawson. 2012. “Sensitivity Studies of Dust Ice Nuclei Effect on Cirrus Clouds with the Community Atmosphere Model CAM5,”
Atmospheric Chemistry and Physics 12(24), 12061–79. DOI: 10.5194/acp-12-12061-2012.