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.

Seasonal and diurnal variability of atmospheric CO2 and its driving factors are studied using its continuous monitoring of concentration and isotopic ratios (δ13C-CO2 and δ18O-CO2) for the first time 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, δ13C-CO2 and δ18O-CO2 are 414.76±4.26 ppm, -11.19±1.63 ‰ and 9.02 ±12.78 ‰. Maximum seasonal diurnal variability 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, an improved model of Miller and Tans was implemented by plotting DCO2 against D(CO2 ×δ13C) respectively during day and night. However, a strong source/sink signature of δ13C was observed during nighttime of summer monsoon with a slope of -37.42 ± 0.73 ‰, obtained using a reduced major axis (RMA) regression.The source identified is attributed to combustion and dominance of C3 ecosystem respiration respectively. The seasonal relationship between δ18O-CO2 and δ13C-CO2 is strongly correlated during pre-monsoon ( ‘r’ = 0.93-0.95) than post-monsoon (‘r’ = 0.07-0.13), 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.