Abstract
Abstract 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.
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