Recovery of spectral optical depth and zero-airmass solar spectral irradiance under conditions of temporally varying optical depth.

The effect of temporal variations in atmospheric optical depth on the determination of the zero-airmass voltage via Langley plot fitting of solar radiometer data is analyzed. Short-term variations in optical depth can be detected fairly accurately with only an approximate estimate of the zero-airmass voltage. Simulations are presented to assess the effect of specific time variations in optical depth on zero-airmass voltage determinations. It is shown that the fractional error in the zero-airmass voltage determination varies in an approximately one-to-one relation to the absolute change in optical depth. The error in the zone-airmass voltage determination can be 10-100 times greater than the standard deviation of this voltage obtained from the least-squares straight-line fit of the Langley plot data.-Authors