Smoke optical depths: Magnitude, variability, and wavelength dependence

The Ames airborne, autotracking sunphotometer has been operated aboard a Sandia Laboratories research aircraft to measure magnitudes, temporal/spatial variabilities, and wavelength dependence of optical depths in the near-ultraviolet to near-infrared spectrum of smoke from two forest fires and one jet fuel fire and of background air. The results were corrected for Rayleigh scattering and for estimated absorption by ozone and nitrogen dioxide. Characteristic differences in the aerosol optical depths of background atmospheres and of different types of smokes are the following: (1) the magnitude and wavelength dependence of "background" optical depths vary with the geographic location at which the measurements are performed; (2) the wavelength dependence of smoke optical depths depends on the fuels that feed the fires and on the residence time of the smoke cloud in the atmosphere. In general, the jet fuel smoke optical depths tended to be less wavelength dependent (near-ultraviolet to near-infrared) than background aerosol optical depths. Forest fire smoke optical depths showed a wide range of wavelength dependences, including incidents of wavelength-independent extinction.