GOMOS data characterisation and error estimation

The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument uses stellar occultation tech- nique for monitoring ozone, other trace gases and aerosols in the stratosphere and mesosphere. The self-calibrating mea- surement principle of GOMOS together with a relatively simple data retrieval where only minimal use of a priori data is required provides excellent possibilities for long-term monitoring of atmospheric composition. GOMOS uses about 180 of the brightest stars as its light source. Depending on the individual spectral characteris- tics of the stars, the signal-to-noise ratio of GOMOS varies from star to star, resulting also in varying accuracy of re- trieved profiles. We present here an overview of the GOMOS data characterisation and error estimation, including model- ing errors, for O3, NO2, NO3 and aerosol profiles. The re- trieval error (precision) of night-time measurements in the stratosphere is typically 0.5-4% for ozone, about 10-20% for NO2, 20-40% for NO3 and 2-50% for aerosols. Meso- spheric O3, up to 100 km, can be measured with 2-10% precision. The main sources of the modeling error are in- completely corrected scintillation, inaccurate aerosol mod- eling, uncertainties in cross sections of trace gases and in atmospheric temperature. The sampling resolution of GO- MOS varies depending on the measurement geometry. In the data inversion a Tikhonov-type regularization with pre- defined target resolution requirement is applied leading to 2- 3 km vertical resolution for ozone and 4 km resolution for other trace gases and aerosols.