Abstract

Abstract. A ground-based, high-spectral-resolution Fourier transform infrared (FTIR) spectrometer has been operational in Addis Ababa, Ethiopia (9.01∘ N latitude, 38.76∘ E longitude; 2443 m altitude above sea level), since May 2009 to obtain information on column abundances and profiles of various constituents in the atmosphere. Vertical profile and column abundances of methane and nitrous oxide are derived from solar absorption measurements taken by FTIR for a period that covers May 2009 to March 2013 using the retrieval code PROFFIT (V9.5). A detailed error analysis of CH4 and N2O retrieval are performed. Averaging kernels of the target gases shows that the major contribution to the retrieved information comes from the measurement. Thus, average degrees of freedom for signals are found to be 2.1 and 3.4, from the retrieval of CH4 and N2O for the total observed FTIR spectra. Methane and nitrous oxide volume mixing ratio (VMR) profiles and column amounts retrieved from FTIR spectra are compared with data from the reduced spectral resolution Institute of Meteorology and Climate Research/Instituto de Astrofísica de Andalucía (IMK/IAA) MIPAS (Version V5R_CH4_224 and V5R_N2O_224), the Microwave Limb Sounder (MLS) (MLS v3.3 of N2O and CH4 derived from MLS v3.3 products of CO, N2O, and H2O), and the Atmospheric Infrared Sounder (AIRS) sensors on board satellites. The averaged mean relative difference between FTIR methane and the three correlative instruments MIPAS, MLS, and AIRS are 4.2 %, 5.8 %, and 5.3 % in the altitude ranges of 20 to 27 km, respectively. However, the biases below 20 km are negative, which indicates the profile of CH4 from FTIR is less than the profiles derived from correlative instruments by −4.9 %, −1.8 %, and −2.8 %. The averaged positive bias between FTIR nitrous oxide and correlative instrument, MIPAS, in the altitude range of 20 to 27 km is 7.8 %, and a negative bias of −4 % at altitudes below 20 km. An averaged positive bias of 9.3 % in the altitude range of 17 to 27 km is obtained for FTIR N2O with MLS. In all the comparisons of CH4 from FTIR with data from MIPAS, MLS, and AIRS, sensors on board satellites indicate a negative bias below 20 km and a positive bias above 20 km. The mean error between partial-column amounts of methane from MIPAS and the ground-based FTIR is −5.5 %, with a standard deviation of 5 % that shows very good agreement as exhibited by relative differences between vertical profiles. Thus, the retrieved CH4 and N2O VMR and column amounts from Addis Ababa, tropical site, is found to exhibit very good agreement with all coincident satellite observations. Therefore, the bias obtained from the comparison is comparable to the precision of FTIR measurement, which allows the use of data in further scientific studies as it represents a unique environment of tropical Africa, a region poorly investigated in the past.

Highlights

  • Methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs) are tropospheric species, which are the mainPublished by Copernicus Publications on behalf of the European Geosciences Union.T

  • The mean relative difference in Fourier transform infrared (FTIR) and Microwave Limb Sounder (MLS) N2O value increases as altitude increases; its value is less than 18.6 % at altitudes below 27 km, and its bias below 22 km is less than 8 %, which can be explained in terms of the systematic error of FTIR N2O

  • The mean partial column of CH4 and N2O within the sensitivity ranges of the instrument, which is from the surface to around 27 km, is determined as 2.85 × 1019 molecules cm−2 ± 5.3 % and 5.16 × 1018 molecules cm−2 ± 6.95 %, respectively

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Summary

Introduction

Methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs) are tropospheric species, which are the main. Addis Ababa FTIR measurements of atmospheric trace gases and their importance to understand various lower and middle atmospheric processes have been reported in a number of previous studies (Takele Kenea et al, 2013; Mengistu Tsidu et al, 2015; Schneider et al, 2015, 2016; Barthlott et al, 2017). H2O volume mixing ratio (VMR) profiles and integrated column amounts from ground-based FTIR measurements of the Addis Ababa site were compared with the coincident satellite observations of the Tropospheric Emission Spectrometer (TES), Atmospheric Infrared Sounding (AIRS), and Modular Earth Submodel System (MESSy) model, and the result confirmed reasonably good agreement (Samuel, 2014). Intercomparisons of vertical profiles and column amounts retrieved from solar spectra observed by the Fourier transform spectrometer at the Addis Ababa site with data from MIPAS, MLS, and AIRS sensors on board satellites were made to assess the quality of the data derived from FTIR.

Measurement site
The FTIR spectrometer and retrieval
Spectroscopic data and a priori profiles
Vertical resolution and sensitivity assessment
Error estimation
Satellite measurements
Comparison methodology
Comparison of FTIR CH4
Comparison of FTIR N2O
Comparisons of partial columns
Findings
Summary and conclusions
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