Baseline total atmospheric ozone values have been derived, using the Chappuis band, from historical data for 11 Smithsonian sites, including both Northern and Southern Hemispheres. The main baselines consist of 1194 and 970 days respectively for Mt. Montezuma, Chile and Table Mountain, California, covering a period of time from about 1923 to 1955. These are the two baselines reported on here. An earlier reported decrease in the ozone beginning in 1940 (Angione et al.) is erroneous and resulted from the use of the wavelengths published in the Smithsonian Annals, one of which was incorrect.

A modern filter wheel radiometer was used to further develop and evaluate the Chappuis-band method. Two intercomparisons at Mauna Loa Observatory show the Chappuis-band measurements to be systematically lower than simultaneous Dobson values by about 14 percent. Because this difference could result from errors in the ozone absorption coefficients, it is important to firmly establish the correct values of these coefficients in the Chappuis band.

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