Retrieval of CO₂, CH₄, CO and N₂O using ground- based FTIR data and validation against satellite observations over the Shadnagar, India

Abstract. An improved column averaged concentration (X) of greenhouse gases (GHGs) namely CO₂, CH₄, CO and N₂O are retrieved using ground-based Fourier Transform Infrared (FTIR; model IFS125M) Spectrometer data collected at Atmospheric Science Lab (ASL) of National Remote Sensing Centre (NRSC), Shadnagar, India during 2016 period in clear sky days. Indium Antimonide (InSb) detector and Calcium Fluoride (CaF₂) beam splitter in combination with the spectral range between 1800 cm⁻¹ to 11000 cm⁻¹ (5.50 μm to 0.90 μm) with 0.01 cm⁻¹ spectral resolution (∆ν) are set for the present study. Atmospheric transmittance for each gas was computed using PcModWin6 (MODTRAN v6) and compared against measured spectrum. In this study, spectra are analyzed using non-linear least squares spectral fitting algorithm (GFIT) developed by the California Institute of Technology, U. S. A. The Total Carbon Column Observing Network (TCCON) identified standard spectral windows were selected for retrieving the GHGs over the study site. With the present retrieval scheme, precision of the FTIR achieved are 0.17 % to 0.52 % for CO₂, 0.30 % to 0.77 % for CH₄, 6.33 % to 8.92 % for CO and 0.41 % to 0.75 % for N₂O, respectively. Observed little to clear diurnal and seasonal variations in XCO₂, XCH₄, XCO and XN₂O respectively. In this work, we compared FTIR retrieved GHGs against Orbiting Carbon Observatory-2 (OCO-2) retrieved XCO₂ and Measurements of Pollution in the Troposphere (MOPITT) retrieved XCO. With the present retrievals, comparative study yields a mean relative bias between ground-based FTIR retrieved XCO₂ and XCO are −1.50 % and 0.60 % against OCO-2 (XCO₂) and MOPITT (XCO) respectively. Pearson correlation coefficient (r) between FTIR retrieval and satellite retrievals are 0.80 (XCO₂, N=14 co-located) and 0.85 (XCO, N=18), respectively.