Soot effects on clouds

Abstract

Soot particles act in the atmosphere by a variety of pathways. Being black they absorb radiation and heat the ambient air, which is called the direct effect. When the absorption process occurs close to or within clouds, the cloud dynamics can be changed, leading to changes in cloud properties and lifetime. This is called the semi-direct effect. Finally, soot particles and coated soot particles can act as cloud condensation and ice nuclei in the atmosphere, which also can alter cloud properties and lifetimes. A striking example are ship tracks. In the case of ice clouds still another process can occur: According to our general understanding ice clouds naturally form at rather high supersaturations. Soot particles and coated soot particles may serve as ice nuclei at lower supersaturations, thus leading to cirrus clouds where natural cirrus would not form. The striking example here are contrails that often can be observed in otherwise clear air. Hence, soot particles interfere with the flow of radiative energy in the atmosphere directly and by affecting cloud properties, lifetimes, and cloud coverage. In my presentation I shall concentrate on the indirect effects of soot on clouds. I shall give an overview on the current knowledge and shall address the open questions for further discussion on the workshop.