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Far IR optical properties have been measured for smoke from diesel fires. Concentrations of both gaseous and particulate combustion products have been measured and chemical species contributing to the optical effects identified. To obtain these results, a variety of sampling instruments were lofted into large plumes on a mobile and open structure. The smoke plumes of diesel fires have been found to consist largely of carbonaceous material (in fibrous form) and heavy liquid hydrocarbons infused with the expected gaseous products of the combustion process. Strong attenuation at a wavelength of 10.6 microm is found to be due largely to the carbonaceous aerosol. The absorption coefficient is typically ~500 km(-1) at 10 m from the source with a variable but often comparable total scattering coefficient. The extinction coefficient, normalized to the mass density of the aerosol in a unit volume of space, is found to be 1.2 m(2)-g(-1) with an estimated variance of 20%. luctuational spectra of the attenuation follow a form similar to turbulence spectra.

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A technique is presented for measuring atmospheric effects on image metrics by comparing infrared images simultaneously collected from positions close to and far away from a target. An image acquired close to the target provides a measure of the radiance inherent to the target and its background, while an image acquired far from the target provides a measure of the radiance after propagation through the atmosphere.Therefore, changes in radiance can be separated into those due to a change in the inherent radiance and those due to the propagation of the inherent radiance through the atmosphere. A "complexity metric" is used to quantify the effects of environmental and atmospheric changes on target-to-background contrast. Examples of the effects of cloud cover, wind speed, dust clouds and optical turbulence on the complexity metric are presented.