P4.6 SOME AEROSOL EFFECTS ON CIRRUS ICE FORMATION

This study reports on laboratory measurements to investigate homogeneous and heterogeneous ice nucleation involving various aerosol particle types in the environmental regime of cirrus clouds. The motivation for such studies is to determine the role of different natural and manmade aerosols in affecting cirrus microphysical properties and thereby potentially altering the radiative characteristics of cirrus. Current estimates of aerosol effects on radiative forcing of the climate system (Intergovernmental Panel on Climate Change 2001) do not yet consider the effects of aerosol changes on cirrus. These aerosol effects are certainly quite different than they are in warm, low altitude clouds. Numerical model studies have made clear that cirrus microphysics may be much less sensitive to aerosol particle increases than are warm clouds, but that this conclusion depends strongly on assumptions of the predominant nucleation mechanism for cirrus formation (DeMott et al., 1997; Jensen et al. 2001, Lin et al. 2002). The predominant ice formation mechanism could be homogeneous, involving soluble particles that deliquesce and dilute as relative humidity rises during lifting, or heterogeneous involving insoluble aerosol components. Which mechanism dominates depends both on the types of aerosols reaching cirrus altitudes and on the forcing dynamics of the clouds. Recent studies, including some summarized in this paper, have placed constraints on the effect of soluble aerosol composition on homogeneous freezing nucleation rates. These results make it difficult to explain the full range of observed conditions of cirrus formation (DeMott 2002), a point that will be reinforced in this paper. There has been limited information published on the composition of natural ice nuclei (IN) in the upper troposphere. The primary information has come from studies of cirrus ice crystal residuals (Heintzenberg et al., 1996; Petzold et al., 1998) and from collections of IN from ambient air activated in a diffusion chamber (Chen et al. 1998). These studies clearly indicated the presence of refractory aerosols at cirrus levels that might act as heterogeneous nuclei for cirrus formation. However, Chen et al. did not reproduce cirrus conditions at temperatures below -40°C to validate the role of IN there. Some studies have correlated the presence of carbonaceous material from aircraft with ice crystal concentrations in cirrus clouds (Strom and Ohlson, 1998). However, insoluble aerosols may also enter cirrus crystals by scavenging processes. ____________________________________________ *Corresponding author address: Paul J. DeMott, Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523-1371. pdemott@lamar.colostate. edu One approach to resolving the role of different aerosols in forming cirrus is to investigate in the laboratory the nucleation behavior, at the appropriate conditions, of particle types detected in the upper troposphere. An important aspect is to do such studies with aerosols of the most relevant sizes for the atmosphere. This paper describes the generation, physical and chemical characterization, and measurement of ice nucleation characteristics of such aerosols. The key measurements of ice nucleation are made using the continuous flow diffusion chamber technique (Rogers et al. 2001). To minimize the need for an exhaustive “survey” of all types of atmospheric aerosols in the laboratory, related new studies are being done to measure the activation conditions and compositions of atmospheric IN. Some of these new studies are reported elsewhere in this volume (DeMott et al. 2002).