Retrieval and validation of O₃ measurements from ground-based FTIR spectrometer at equatorial station: Addis Ababa, Ethiopia

Abstract

Abstract. Since May 2009 high-resolution Fourier transform infrared (FTIR) solar absorption spectra are recorded at Addis Ababa (9.01° N latitude, 38.76° E longitude, 2443 m altitude a.s.l.), Ethiopia. The vertical profiles and total column amounts of ozone (O3) are deduced from the spectra by using the retrieval code PROFFIT (V9.5) and regularly determined instrumental line shape (ILS). A detailed error analysis of the O3 retrieval is performed. Averaging kernels analysis of the target gas shows that the major contribution to the retrieved information always comes from the measurement. We obtained 2.1 degrees of freedom on average for signals in the retrieval of O3 from the observed FTIR spectra. We have compared the FTIR retrieval of ozone Volume Mixing Ratio (VMR) profiles and column amounts with the coincident satellite observations of Microwave Limb Sounding (MLS), Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and Tropospheric Emission Spectrometer (TES), Ozone Monitoring Instrument (OMI), Atmospheric Infrared Sounding (AIRS) and Global Ozone Monitoring Experiment (GOME-2) instrument. The mean relative differences are generally found below +15% in the altitude range of 27 to 36 km for comparison of VMR profiles made between MLS and MIPAS, whereas comparison with TES has shown below 9.4% relative difference. Furthermore, the mean relative difference is positive above 31 km, suggesting positive bias in the FTIR measurement of O3 VMR with respect to MLS, MIPAS and TES. The overall comparisons of column amounts of satellite measurements with the ground-based FTIR instruments show better agreement exhibiting mean relative differences of ground-based FTIR with respect to MLS and GOME-2 within +0.4% to +4.0% and corresponding standard deviations of 2.2 to 4.3% whereas, in the case of OMI, TES, AIRS, the mean relative differences are from −0.38 to −6.8%. Thus, the retrieved O3 VMR and column amounts from a tropical site, Addis Ababa, is found to exhibit very good agreement with all coincident satellite observations over the 2-yr period.