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.