New method for deriving total ozone from Brewer zenith sky observations

[1] The Brewer spectrophotometer derives total column ozone from UV measurements using two operational modes: observing either direct sunlight (DS) or zenith sky (ZS) scattered sunlight. While DS measurements are more accurate, ZS measurements are necessary for generating long-term ozone time series unbiased by meteorological conditions and for the validation of satellite algorithms for cloudy scenes. In the ZS mode, column ozone (X) is obtained using the slant path (μ) and a weighted sum of the logarithms of the ZS intensities (F). Prior to this, however, a set of nine empirical coefficients, specific to each instrument, must be derived that relate μ and X to F. This requires a large set of near-simultaneous ZS and DS measurements that span the operational range of X and μ, which is difficult to obtain. In this work, a modified algorithm is presented in which radiative transfer model simulations of ZS observations are used to derive generic coefficients that describe the sky response to different observing conditions. Instrument-specific coefficients are obtained through a simple scaling and offset of these generic values, derived through ZS-DS comparisons. These two parameters may be accurately determined in a relatively short period of time and using a much more limited data set, such as during regular instrument calibrations. Since the new algorithm provides nine coefficients, no changes to the existing Brewer software are required. It is demonstrated that the new algorithm is as useful as the standard Brewer ZS algorithm with nine empirically estimated parameters derived using a vastly more extensive data set.