Retrieval of Optical Depth for Heavy Smoke Aerosol Plumes: Uncertainties and Sensitivities to the Optical Properties

Abstract

This paper is concerned with uncertainties in the Advanced Very High Resolution Radiometer (AVHRR)-based retrieval of optical depth for heavy smoke aerosol plumes generated from forest fires that occurred in Canada due to a lack of knowledge on their optical properties (single-scattering albedo and asymmetry parameter). Typical values of the optical properties for smoke aerosols derived from such field experiments as Smoke, Clouds, and Radiation-Brazil (SCAR-B); Transport and Atmospheric Chemistry near the Equator-Atlantic (TRACE-A); Biomass Burning Airborne and Spaceborne Experiment in the Amazonas (BASE-A); and Boreal Ecosystem?Atmosphere Study (BOREAS) were first assumed for retrieving smoke optical depths. It is found that the maximum top-of-atmosphere (TOA) reflectance values calculated by models with these aerosol parameters are less than observations whose values are considerably higher. A successful retrieval would require an aerosol model that either has a substantially smaller asymmetry parameter (g < 0.4 versus g > 0.5), or higher single-scattering albedo (? ? 0.9 versus ? < 0.9), or both (e.g., g = 0.39 and ? = 0.91 versus g = 0.57 and ? = 0.87) than the existing models. Several potential causes were examined including small smoke particle size, low black carbon content, humidity effect, calibration errors, inaccurate surface albedo, mixture of cloud and aerosol layers, etc. A more sound smoke aerosol model is proposed that has a lower content of black carbon (mass ratio = 0.015) and smaller size (mean radius = 0.02 ?m for dry smoke particles), together with consideration of the effect of relative humidity. Ground-based observations of smoke suggest that for τ < 2.5 there is an increasing trend in ? and a decreasing trend in g with increases in τ, which is consistent with the results of satellite retrievals. Using these relationships as constraints, more plausible values of τ can be obtained for heavy smoke aerosol. The possibility of smoke?cloud mixtures is also considered, which can also lead to high TOA reflectances. However, without measurements, the hypothesis can be neither accepted nor negated. The study demonstrates that without independent assessments of the optical properties, large uncertainties would be incurred in the retrieved values of optical depth for heavy smoke aerosol plumes.