Satellite sensing of aerosol absorption

A method is developed for remote sensing of aerosol absorption from satellite images of the earth's surface. The method is based on the measurement of the change in the upward radiances between a clear and a hazy day over a varying surface reflectance. For a zero change a balance between brightening due to scattering and darkening due to absorption and scattering is reached. This balance is utilized via a radiative transfer model to derive the aerosol single-scattering albedo. A sensitivity study is performed and the method is tested against laboratory measurements. It is suggested that for the case of haze introduced on top of an existing background aerosol (e.g., forest fire smoke, dust storms, etc.) and with a fair estimate of the scattering phase function, the error in the remotely sensed single-scattering albedo (ω0, ratio between scattering and scattering plus absorption) is in the range of 0.03–0.05. The method is applied to Landsat Multi-Spectral Sensor imagery of forest fire smoke detected over the Washington D.C., area, resulting in ω0 = 0.90. The variability of the remotely sensed ω0 across the image was Δω0 = ±0.01. No substantial dependence on wavelength between 550–750 nm was found. Application of the method to aircraft radiances measured during regional industrial air pollution resulted in ω0 = 0.94 ± 0.04. The main advantage of the method is that it provides a simple measure of ω0 of the undisturbed aerosol. The main errors in the method arise from variations in the surface reflectance between the clear and the hazy days, uncertainty in the scattering phase function, and variation of the aerosol and gaseous absorption between these two days. If the satellite calibration varies with time, the measurements of ω0 can be substantially affected.