Atmospheric CO₂ retrieval from ground based FTIR spectrometer over Shadnagar, India

Abstract. The column-averaged volume mixing ratio (vmr) of carbon dioxide (XCO₂) has been retrieved over Shadnagar (Latitude = 17.03° N, Longitude = 78.21° E), India at Near Infrared (NIR) spectra using ground based high resolution (∆ν = 0.01 cm⁻¹) Fourier Transform InfraRed (FTIR; model IFS125M) spectrometer. It is first of its kind in India for measuring columnar and vertical mixing ratios of atmospheric trace gases and other greenhouse gases (GHGs). In the present study, retrieval of XCO₂ could be performed for a week at near Sun's nadir view (solar zenith angle-SZA, ±75.0°) using line-by-line radiative transfer algorithm (LBLRTA). Vertical profiles of CO₂ have been retrieved at NIR and middle IR (MIR) spectra using a version of the Fast Atmospheric Signature Code 3 (FASCODE3) model which is for the retrieval of atmospheric trace gas profiles. Error residuals between measured and fitted atmospheric transmission lie with in ±1.0 % for CO₂ (6180–6380 cm⁻¹) and O₂ (7800–7940 cm⁻¹) respectively. During analysis period mean (standard deviation, 1σ) XCO₂ was observed to be 385.24 ppm (4.22 ppm) with 1.0 % of daily variation. Minimum and maximum averaged molecular column densities of CO₂ (O₂) are 6.15 × 10²¹ (3.3 × 10²⁴) molecules/cm² and 8.06 × 10²¹ (4.72 × 10²⁴) molecules/cm² respectively. Obtained an average high signal to noise ratio (SNR) of 833 and 625 for NIR and MIR spectra, respectively.