Greenhouse gasses (GHGs) play an important role in controlling local air pollution as well as climate change. In the present study, we retrieved column-averaged dry-air (X) mole fractions of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) using a ground-based EM27/SUN Fourier Transform Infrared Spectrometer (FTIR). The EM27/SUN spectrometers are widely in use in the COllaborative Carbon Column Observing Network (COCCON). The PROFFAST software provided by COCCON has been used to analyze the measured atmospheric solar absorption spectra. In this paper, the diurnal variation and the time series of daily averaged XCO2, XCH4, and XCO covering the period December 2020 to May 2021 are analyzed. The maximum values of XCO2, XCH4, and XCO are observed to be 420.57 ppm, 1.93 ppm, and 170.40 ppb respectively. Less diurnal (XCO2 ~0.44 ppm; XCH4=0.004 ppm, and XCO=4.84 ppb) but clear seasonal changes are observed during the study period. The XCH4 and XCO from the Sentinel-5Precursor (S5P)/TROPOspheric Monitoring Instrument (TROPOMI) are compared against the EM27/SUN retrievals. The correlation coefficient ‘r’ between the EM27/SUN retrieved XCH4 and XCO against S5P/TROPOMI are 0.75 and 0.94 respectively.

The seasonal diurnal variability of atmospheric CO2 and driving factors are studied using its continuous isotopic fractionation (δ13C and δ18O) over Shadnagar, a sub-urban location of India with high precision in-situ data from November 2018 to October 2019. The annual averaged atmospheric CO2 concentrations and 13C-CO2 and 18O-CO2 are 415.03±9.77 ppm, -11.18±1.73 ‰ and 9.1±13.35 ‰. Seasonal amplitudes of atmospheric CO2 was observed in summer monsoon (17.30 ± 9.29 ppm) and the minimum was noticed in winter (7.19 ± 0.11 ppm) indicating strong seasonality at the study site. To characterize the atmospheric CO2 sources/sinks, an improved model of Miller and Tans was implemented by plotting ΔCO2 against Δ(CO2 × δ13C) respectively during day and night. An averaged seasonal δ13C source/sink signature (δs) is -32.84 ‰ in the day time and -26.09‰ in night time representing the source of atmospheric CO2 is related to combustion and dominance of C3 ecosystem respiration respectively. The seasonal relationship between δ18O and δ13C is strongly correlated during pre-monsoon ( ‘r’ = 0.93 to 0.95) than post monsoon (‘r’ = 0.07 to 0.13), which might be due to high vapour pressure deficit. A Lagrangian back-trajectory model confirms the influence of the Indian Summer Monsoon on the variability of atmospheric CO2 concentration during the summer monsoon season.