The Accuracy of Marine Shadow-Band Sun Photometer Measurements of Aerosol Optical Thickness and Angstrom Exponent

An analytical uncertainty propagation model is used in conjunction with laboratory and field data to quantify the uncertainty in measurements of the direct-normal irradiance, aerosol optical thickness, and Angstrom exponent made with a ship-mounted fast-rotating shadow-band radiometer (FRSR). Total uncertainties in FRSR measurements of aerosol optical thickness are found to be 0.02–0.03 at the 95% confidence level (two standard deviations). The "lever-arm" effect, a salient characteristic of the Langely technique in which uncertainties in aerosol optical thickness measurements are reduced as the solar zenith angle increases, is essentially offset by orientation uncertainty. Lack of a lever-arm effect precludes Langley calibration of FRSRs while at sea; they must be calibrated on land. Uncertainties in FRSR measurements of the two-wavelength Angstrom exponent are shown to depend strongly on the aerosol optical thickness, with the maximum uncertainty of 0.6 associated with clean, maritime air masses.