Carbon-dioxide is one of the most abundant gasses in the atmosphere. Due to human induced anthropogenic activities, the amount of CO2 released into the atmosphere has been rising extensively during the last 150 years. Present investigation was carried out to assess the carbon dioxide concentration in ambient air of Tarai region of Uttarakhand. The online monitoring of CO2 concentration from ambient air was carried out with the help of a Non-dispersive Infra-red (NDIR) gas analyzer at one minute time interval. Simultaneously, air samples were collected from various sampling sites starting. The ambient air CO2 concentration at Pantnagar has shown a large fluctuation that varied in the range of 306 ppmv to 427 ppmv with an average concentration of 350.82 ppmv. Among different sites of sampling, the highest concentration of surface air CO2 was found at the vehicular sites followed by Industrial site. Higher CO2 concentration values were found near the ground and a decreasing pattern was observed with the increase in height. In general, CO2 concentrations were higher during winter and lowest during summer season.