The traditional method used to derive the vertical profile of ozone is the Umkehr method (Gotz et al., 1934), which has been adapted for use with the Brewer instrument (Mateer et al., 1985; McElroy and Kerr, 1995). Measurements are made of radiation scattered from the zenith sky at twilight during sunrise or sunset for solar zenith angles between 90° and about 75°. Zenith sky radiation as a function of wavelength depends on total ozone, the solar zenith angle, and the vertical distribution of ozone. It is therefore possible to determine the vertical profile of ozone from measurements made as the SZA passes between 90° and 75°. It takes between one and three hours to cover this SZA range depending on latitude and time of year. One condition for good quality Umkehr measurements of the vertical profile of ozone is that the sky remains clear during the observing period. The fully automated capabilities of the Brewer instrument allow measurements to be attempted every morning and every evening during twilight. Good quality measurements can be selected during the analysis of the data set.
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