Total ozone column from direct and diffuse spectral solar irradiance in the southwest of the Iberian Peninsula

Daily total ozone column (TOC) measurements from the Brewer spectroradiometer located at El Arenosillo (Spain) and the Spectrometer for Atmospheric TRAcers Monitoring (SPATRAM) located at Evora (Portugal) are analyzed for the period 2006-2008. The main goal of this study is to compare TOC estimates retrieved with the passive differential optical absorption spectroscopy (DOAS) methodology from zenith-sky observations by the SPATRAM spectrometer with highly accurate TOC estimates retrieved from direct-sun recordings by a well-calibrated Brewer instrument. On average, SPATRAM TOC estimates are (1.69 +/- 0.18)% smaller than collocated Brewer TOC estimates. Significant relative differences between SPATRAM and Brewer total ozone data are detected in some periods caused by the influence of high aerosol load episodes over the SPATRAM data. Satellite data recorded by the ozone monitoring instrument (OMI)-DOAS algorithm aboard the NASA EOS-Aura satellite have been used to quantify the natural spatial variability of TOC over the region of interest as a potential source of uncertainty in the SPATRAM-Brewer comparison. Our results show that the differences observed between both systems is not associated with the natural spatial variability of TOC. In addition, the OMI -DOAS total ozone data have been compared to the ground-based spectroradiometer data showing an excellent agreement for the Brewer spectroradiometer (R-2 similar to 0.95) and a fairly good agreement for the SPATRAM instrument (R-2 similar to 0.89).