Abstract
The Intergovernmental Panel for Climate Change (IPCC) reported that carbon dioxide (CO2)
from anthropogenic emissions is the major contributor to radiative forcing in the atmosphere
(IPCC, AR6). The recorded monthly average atmospheric CO2 level in the Mauna Loa
Observatory (elevation of about 3397 m) was 421 ppm during March 2023, the highest level
since the accurate measurements began 65 years ago. The accelerating CO2 mixing ratios were
attributed to changes in land use and land cover (LU/LC). After industrialization (around 1760),
CO2 emissions from fossil fuel burning started and became the most significant contributor to
the global carbon budget (Ballantyne et al., 2012). Since the pre-industrial period, the burning
fossil fuel and LU/LC changes have increased by 40 and 150 % for CO2 and CH4 mixing ratios,
respectively (Huang et al., 2016). Recent studies suggest that the CO2 emissions from cement
production exceeded global fossil fuel emissions in the past two decades (Andrew, 2018). CO2
mixing ratio levels in the atmosphere are controlled by the combined effect of sources and
sinks. Only about half the amount of CO2 released remains in the atmosphere, whereas two
major sinks, the terrestrial biosphere, and oceans, absorb the remaining part (Andres et al.,
1996). Hence, monitoring and maintaining long-term records of atmospheric CO2
measurements are essential to understanding the carbon cycle and predicting the future
behaviour of the controlling factors: photosynthesis, respiration, biomass, fossil fuel burning,
and air-sea exchange processes (Machida et al., 2002).
The present thesis work is aimed at understanding and assessing the CO2 variability over the
Indian region using three approaches: ground-based, satellite data, and model simulations, and
further integrating them to provide a comprehensive estimate of CO2 concentrations and their
variability. The impact of LU/LC and its changes on CO2 emissions are quantified using
ground-based observations and model simulations from 2013 to 2022. In addition, the vertical
profiles of atmospheric CO2 were studied to help understand temporal variations in the vertical
column, which was used to improve the satellite-based CO2 estimates. A study was conducted
to understand the neighbourhood effects on atmospheric CO2 using space-based CO2 data,
model simulations, and LU/LC. Further, the objective was to quantify and generate a national
CO2 emission dataset as a function of climatic zones, LU/LC, satellite-derived CO2
concentration, and MIROC4-ACTM simulated inverse fluxes.Column-averaged CO2 concentrations (X) were obtained using data from a ground-based
Fourier Transform Infrared (FTIR) Spectrometer that was gathered in clear sky days at the
NRSC in Shadnagar, India throughout the 2016 period. Calcium Fluoride (CaF2) beam splitter
and Indium Antimonide (InSb) detector are used to collect the solar absorption spectra, with
the range between 1800 cm-1
to 11000 cm-1
(5.50 μm to 0.90 μm) and the spectral resolution
(∆ν) of 0.01 cm-1
for this study. This study analyses spectra using a non-linear least squares
Gas Fitting Spectral Analysis (GFIT) developed by the Jet Propulsion Laboratory (JPL), and
licenced by California Institute of Technology (Caltech), U.S.A. With the present retrieval
scheme, the precision of the FTIR achieved is 0.04 % for XCO2. During the study period,
the diurnal amplitude of XCO2 was about 2 ppmv, and the present work compared FTIRretrieved XCO2 data against XCO2 observed by Orbiting Carbon Observatory-2 (OCO-2). A
comparative study yields a mean relative bias of -0.78 % and -0.63 % between FTIR data and
satellite data for XCO2. The Pearson correlation coefficient (R) between FTIR and satellite data
is 0.96 (XCO2, N=13 co-located). In the present work, we also retrieved XCO2 using a groundbased portable EM27/SUN FTIR. The EM27/SUN spectrometers are widely used in the
COllaborative Carbon Column Observing Network (COCCON). The advantages of portability
of this FTIR allow experiments in a campaign mode so that data gaps in India in terms of
geographical coverage can be minimized. The PROFFAST software provided by COCCON
has been used to analyse the measured atmospheric solar absorption spectra. This work studied
the diurnal variation and the time series of daily averaged XCO2, covering December 2020 to
May 2021. The maximum value of XCO2 was observed to be 420.57 ppm. Less diurnal
(XCO2~0.44 ppm), but clear seasonal changes are observed during the study period. The FTIRbased XCO2 retrievals help estimate the uncertainties in the satellite-based products and enable
the highly accurate products from the satellites by adjusting the biases between ground and
space-based retrievals. Thus, ground-based FTIR retrievals are decisive in validating satellite
products.
An airborne campaign was conducted to investigate the vertical distribution of the atmospheric
CO2 using a twin-turbo prope