Improvements to the accuracy of zenith-sky measurements of total ozone by visible spectrometers II: use of daily air-mass factors

Ozone at Faraday in Antarctica is measured by a ground-based visible spectrometer. Total ozone is deduced from the slant column measurements by means of calculated air-mass factors (AMFs), which have now been calculated for each month of the observations with a mixture of climatological and measured parameters as input. The intercepts of Langley plots using these AMFs had less seasonal variability than hitherto, but the ratio of total ozone to that measured by the Dobson at Faraday still showed a significant dependence on season and on presence of the ozone hole. This latter dependence was reduced by blending ozone-hole AMFs on appropriate days, resulting in ratios which vary by less than ±9% over the three years of observations. When using these daily AMFs, we estimate the potential for errors in seasonal trends in total ozone at less than 6%/month in the worst-case outside ozone-hole break-up, with a more usual value between 2 and 3%/month. There remain clear artifacts due to the ozone hole and to summer–winter differences, which cannot be resolved without new ozone-sonde flights.