Retrieval and Study of Columnar CO2 and CH4 using ground-based FTIR Spectrometer

The environmental impact on the atmosphere due to the anthropogenic processes by the human activity has been consistently increasing since recent decades. Due to which, global environmental problems such as climate change, depletion of stratospheric ozone and increase in regional air pollution are witnessed. Currently, these are major issues even for international agenda and agreement like Kyoto Protocol and Climate Conference (total number of COP’s 26 so far across globe) which came to a conclusion to regulate the emissions of climate destructors such as Chlorofluorocarbons (CFCs) and other greenhouse gases namely CO2 and CH4. Hence, monitoring of these gases globally with a distributed dense observational network of CO2 and CH4 more accurately since global carbon cycle plays a major role in earth’s climate system. Ground-based measurements are representative of only small locations around observation locations and is difficult to extrapolate this information over regional areas. Due to various constraints, it is not possible to have many observations in any given area. Satellite remote sensing on the other hand is able to give information over regional areas and with relatively low cost. Satellite retrievals add synoptic and geospatial extent to ground based measurements. Due to the advantage of high spatial coverage, remote sensing of atmospheric greenhouse gases (GHGs) is needed to play a vital role to understand the climate change in which CO2 and CH4 are of major concern. ISRO started National Carbon Project under the CAP-IGBP which focused on robust observational network across the country. Ground-based Fourier Transform infrared (FTIR) spectrometers are used around the globe for the measurement of atmospheric trace gases by solar absorption spectroscopy. Precise measurements of global columnar concentrations of GHGs are essential to investigate the major regions of sources and sinks on the Earth. Columnar measurements are especially important in the tropics, as convection is consistently strong and as a result flux signal are only weakly seen in surface measurements. Satellite based measurements complement ground-based observations by producing frequent global measurements of atmospheric constituents. Though the surface fluxes on CO2 and CH4 are strengthened in the recent decade, precise ground -based columnar GHGs measurements were found to be a gap area. Hence, first of its kind FTIR (Model: IFS 125M) in the country is established at National Remote Sensing Centre (NRSC), Shadnagar to generate long-term records and to study the columnar GHGs variability along with their vertical transport induced changes on surface fluxes. Thus, this thesis covers the retrieval of dry columnar CO2 and CH4 concentrations from the ground based FTIR data. Observations from ground based FTIR instrument have been collected at NRSC, Shadnagar, Telangana since 2016. We get direct solar radiation spectra from FTIR at a high spectral resolution of 0.01 cm during clear sky days. With the combination of InSb detector and CaF2 beam splitter, the observations were made in the 0.905. 50μm during 2016 and 2019. The solar spectra data are collected at 5 min interval during pre-monsoon season. The hourly mean and 5 min XCO2 is mostly ranging from 405 ppmv to 412 ppmv with maximum deviation of 7 ppmv during the study period. The diurnal variation of XCO2, XCH4 are clearly observed with diurnal amplitude of ~2 ppmv and ~10 ppbv respectively. With the present retrieval scheme, precision of the FTIR achieved are 0.17 % to 0.52 % for CO2 and 0.30 % to 0.77 % for CH4 while meeting the global standards. A correlation coefficient of ‘r’ of 0.80 is observed between FTIR retrieved XCO2 against Orbiting Carbon Observatory-2 (OCO-2) satellite retrieved XCO2. The potential reasons for obtaining ‘r’ of 0.80 are satellite footprint which is chosen optimally due to lack of co-located points in finer grid size and global retrieval algorithm is different from regional retrieval algorithm. Further this thesis also has attempted to retrieve the columnar CH4 from the hyperspectral imaging sensor, the Airborne Visible/Infrared Imaging Spectrometer-Next Generation (AVIRIS-NG) covering spectral range from visible (380 nm) to shortwave infrared (2510 nm) with a spectral resolution of 5±0.5 nm. Line-by-line radiative transfer algorithm (LBLRTA) is applied on the AVIRIS-NG airborne data collected during December 2015-January 2016. Using LBLRTA the columnar density of methane was estimated from AVIRIS-NG transmittance data. For Shadnagar site, comparative analysis is done with the ground based FTIR observations and retrieved XCH4 concentration during the same period. Thus, this study demonstrated the potentiality of the AVIRIS-NG hyper spectral imaging sensor data application in the atmospheric species retrievals. With this, the present thesis covered the importance of columnar GHGs observations under primary climate forcers namely atmospheric CO2 and CH4.