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Biological residues on soil dust are ice nucleation active

2011/09/16

Interest in the interaction of biological ice nucleators with the atmosphere has focused on micro-organisms.  Arguments against their importance in the global rainfall budget are, in part, based on the small numbers of potentially ice nucleation active  micro-organisms in the atmosphere relative to abundance of mineral dust particles that are several orders of magnitude more numerous.  A new paper in Atmosphere Chemistry and Physics (Conen et al, Biological residues define the ice nucleation properties of soil dust.  http://www.atmos-chem-phys.net/11/9643/2011/acp-11-9643-2011.html ) demonstrates that the condensed organic matter on soil dust particles is ice nucleation active, rendering the soil particles much more ice nucleation active than if they consisted solely of the mineral fraction. 

Organic matter on soil consists of the condensed remains of plant tissues, microbial cells, invertebrates and other soil-borne organisms.  The ice nucleation activity of bacteria such as Pseudomonas syringae necessitates a certain configuration of the ice nucleation protein – this configuration can be maintained even if cells are dead. This principle is the basis for the activity of the Snomax product, for example, for which the cells of P. syringae are killed in such a way as to preserve their ice nucleation activity.  Furthermore, other micro-organisms and certain plant tissues contain active ice nucleators.  Via degradation of organic matter in the soil, a large spectrum of biological ice nucleating materials could condense and accumulate on the mineral constituent of soil particles in such a way that their ice nucleation activity is maintained.

Parameterization of ice nuclei in cloud models is often dominated by the rates of ice nucleation activity of desert dust, which is abundant in the atmosphere but inherently devoid of organic matter.  However, rich organic soils such as those in Central Asia erode thereby contributing to the atmospheric load of soil dust.  The Dust Bowl environmental disaster of the 1930’s in the western US is a testament of the extremities of erosion of agricultural lands and its contribution to atmospheric soil dust.  The work of Conen et al suggests that organic soil dust might have important regional effects on ice nucleation processes. Its potential impact is worth examining via a refinement of the parameterization of cloud models. 

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