Knowledge of vertical CO₂ distribution is important for development of CO₂ transport models and calibration/validation of satellite-borne measurements. In this study, vertical profiles of CO₂ concentration within 0-1000 m were measured using a tethered balloon at a suburban site in Xiamen, which is undergoing fast urbanization. The characteristics of CO₂ vertical distribution were investigated under both stable and convective boundary-layer conditions. The correlation of ground level CO₂ concentrations and those at high altitudes decreased with altitude and show significant correlation in the first 300 m with R=0.78 at 100 m, R=0.52 at 200 m, R=0.40 at 300 m (P<0.01). The correlation keeps almost constant for 300-800 m, and there is no obvious correlation at 800 m, indicating that the impact of ground level CO₂ was restricted within the 300 m above the ground. When comparing the vertical profiles obtained at different times during a 24 h period, it was found that CO₂ concentration exhibited more obvious diurnal pattern at surface level than at high altitude because of the variation of sources and sinks of CO₂ at ground level. Most profiles demonstrated declining trends of CO₂ concentration with increasing altitude. The vertical profiles of CO₂ were fitted to obtain an empirical equation for estimating CO₂ vertical concentration in the lower atmosphere (0-1000 m): y=-75.04+1.17×10⁹e^-x/28.01, R²=0.59 (P<0.05). However, for some cases opposite patterns were observed that the CO₂ concentration profiles showed a turning point at a certain altitude or little variation with altitude under certain meteorological conditions. The atmospheric boundary layer depth and atmospheric stability are two major factors controlling the vertical structure of CO₂ profile. The results would improve our understanding of the spatial and temporal variation of CO₂ in urban environment, which would facilitate using 3-D transport model to study the impacts of CO₂ on urban environment.