Total Ozone Data Retrieval from the Phaethon DOAS System

Spectrally resolved measurements of the direct solar irradiance and scattered sky radiance at different elevation angles in the UV and visible regions are performed regularly at the Aristotle University of Thessaloniki by Phaethon, a miniature CCD-equipped spectrograph system. The direct irradiance spectra of Phaethon are analyzed using the Differential Optical Absorption Spectroscopy (DOAS) method to derive the total ozone column (TOC). The absorption features of the measured spectra are compared with those of a reference spectrum to derive the differential slant column density of ozone, which is subsequently converted to vertical column, i.e. to TOC. The error in the retrieved TOC was found to be less than 10 DU for 90 % of the data and below 5 DU for 75 % of the data. The relative difference between TOC data derived from Phaethon and a collocated Brewer spectrophotometer for several months was 0.96 +/- 3.2 %. A recent modification of the system increased the signal to noise ratio, leading to much smaller retrieval errors and improved the agreement with the Brewer to 0.76 +/- 1.5 %.