Fast response (5-s resolution) CO2 measurements were recorded on two recent NASA-sponsored airborne atmospheric chemical survey missions: the summer/fall 1991 Pacific Exploratory Mission-West A (PEM-West A) and the winter 1992 Airborne Arctic Stratospheric Expedition II (AASE II). Both missions were conducted aboard the NASA Ames Research Center DC-8 aircraft and included sampling between the surface and 12-km altitudes over a wide range of latitudes and longitudes. In the following text, these data, along with simultaneous surface measurements from the NOAA flask sampling network, are examined to establish the vertical distribution and variability of CO2 as a function of location and season. Results indicate that middle to upper tropospheric (>5 km altitude) CO2 concentrations often differ considerably (>5 parts per million by volume in some cases) from values recorded at surface stations within the same geographic region. These differences are particularly notable at middle to high northern latitudes where midtropospheric concentration changes and seasonal cycles are generally delayed in time and highly damped in amplitude relative to surface observations. ¿ American Geophysical Union 1996