OZONE MEASUREMENTS BY ZENITH-SKY SPECTROMETERS - AN EVALUATION OF ERRORS IN AIR-MASS FACTORS CALCULATED BY RADIATIVE-TRANSFER MODELS

Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV -visible spectrometers are presented and discussed. Causes and size of errors in AMFs of ozone in the visible are evaluated. Errors can be c aused by approximations in the calculation (intensity-weight approxima tion, ignoring the finite field of view of the instrument); by approxi mation in the scheme of the calculation (single scattering, ignoring r efraction); or by variable geophysical parameters (vertical profile of constituents). These relative errors in AMF cause identical relative errors in vertical columns of ozone deduced from measurements by zenit h-sky spectrometers. The mean of the relative errors of ozone AMFs due to using one set of AMFs for all seasons and locations is +/- 2.4% wh en averaged over the commonly used range of solar zenith angles.