Abstract
Abstract. Since May 2009, high-resolution Fourier Transform Infrared (FTIR) solar absorption spectra have been recorded at Addis Ababa (9.01° N latitude, 38.76° E longitude, 2443 m altitude above sea level), 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 of the target gas shows that the major contribution to the retrieved information comes from the measurement. The degrees of freedom for signals is found to be 2.1 on average for the retrieval of O3 from the observed FTIR spectra. The ozone Volume Mixing Ratio (VMR) profiles and column amounts retrieved from FTIR spectra are compared with the coincident satellite observations of Microwave Limb Sounding (MLS), Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Tropospheric Emission Spectrometer (TES), Ozone Monitoring Instrument (OMI), Atmospheric Infrared Sounding (AIRS) and Global Ozone Monitoring Experiment (GOME-2) instruments. The mean relative differences in ozone profiles of FTIR from MLS and MIPAS are generally lower than 15% within the altitude range of 27 to 36 km, whereas difference from TES is lower than 1%. Comparisons of measurements of column amounts from the satellite and the ground-based FTIR show very good agreement as exhibited by relative differences within +0.2% to +2.8% for FTIR versus MLS and GOME-2; and −0.9 to −9.0% for FTIR versus OMI, TES and AIRS. The corresponding standard deviations are within 2.0 to 2.8% for FTIR versus MLS and GOME-2 comparisons whereas that of FTIR versus OMI, TES and AIRS are within 3.5 to 7.3%. 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 an approximate 3-yr period.
Highlights
The study of atmospherHic tyradcreogalosegs yhasaangdreat role to play in terms of global climateEchaanrgthe anSdyatsmtoesmpheric chemistry
Ground-based FTIR spectrometry is a very useful technique to derive total column amounts and vertical profiles of many important trace gases in the atmosphere. This measurement site is located in a tropical region which is poorly constrained by ground-based instruments
A good agreement is determined from the intercomparison of O3 profiles and column amounts from FTIR with that from the satellites instruments
Summary
Addis Ababa is located at 9.01◦ N latitude, 38.76◦ E longitude, 2443 m altitude above sea level, which is in the equatorial region. It is relatively dry due to its topography, making it robust for monitoring trace gas species in the tropics since interference of tropospheric water vapour absorption lines is of minor relevance. The typical tropopause height for tropical regions is between 16 to 18 km, and the corresponding temperature is about 200 K. The tropical tropopause region is the transition layer between the dynamical control of the vertical mass flux by tropospheric convection and by the stratospheric Brewer-Dobson circulation (Holton, 2004; Jacobson, 2005). The detailed climatology of the region is described by Mengistu Tsidu (2012) and references therein. This FTIR site is ideal to observe seasonal variation as ITCZ migrates north-south with season across the site
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