FOCUSED SUN OBSERVATIONS USING A BREWER OZONE SPECTROPHOTOMETER

Accurate measurements of total ozone at high latitudes during winter have become increasingly important for studies of polar ozone depletion. The direct Sun measurements are the most direct and physically well defined type of measurement of total ozone with Dobson and Brewer spectrophotometers along with the focused Sun measurements. The direct Sun and focused Sun method are influenced by many sources of error at low solar elevations. A large error arises from the addition of radiation scattered from air along the path of the solar beam. A method to correct for this error using the Brewer instrument is described. The method gives reasonably accurate values down to solar elevations of 5-degrees compared to 10-degrees-20-degrees without corrections. This is especially important for high-latitude stations, where the low solar elevations are a limiting factor for accurate measurements of the total ozone particularly during the winter.