Dead time effect on the Brewer measurements: correction and estimated uncertainties

Abstract

Abstract. Brewer spectrophotometers are widely used instruments which perform spectral measurements of the direct and the global solar UV irradiance. By processing these measurements a variety of secondary products can be derived such as the total columns of ozone, sulfur dioxide and nitrogen dioxide, and aerosol optical properties. Estimating and limiting the uncertainties of the final products is of critical importance. High quality data have a lot of applications and can provide accurate estimations of trends. The dead time is characteristic for each instrument and non-proper correction of the raw data for its effect may lead to important errors in the final products. It may change with time and the currently used methodology is not always sufficient to accurately determine the correct dead time. For specific cases, such as for low ozone slant columns and high intensities of the direct solar irradiance, the error in the retrieved TOC, due a 10 ns change in the dead time from its nominal value, is found to be up to 5 %. The error in the calculation of UV irradiance is about 3–4 % near the maximum operational limit of light intensities. While in the existing documentation it is indicated that the dead time effects are important when the error in the used value is greater than 2 ns, we found that for single monochromator Brewers a 2 ns error in the dead time may lead to uncertainties above the limit of 1 % in the calculation of TOC; thus the tolerance limit should be lowered. A new routine for the determination of the dead time from direct solar irradiance measurements has been created and tested and a validation of the operational algorithm has been performed. Additionally, new methods for the estimation and the validation of the dead time have been developed and are analytically described. Therefore, the present study in addition to highlighting the importance of the dead time for the processing of Brewer datasets, also provide useful information for their quality control and re-evaluation.