During June 1980, the Hawaii Mesoscale Energy and Climate Project (HAMEC) field program was conducted in the vicinity of the island of Hawaii. The objective of the program was to use the NOAA P3 aircraft to measure meteorological variables upwind and downwind of the island to provide data to evaluate mesoscale models of airflow and cloud physics. One specific objective was to obtain flask samples upwind of the island to confirm that the CO2 values observed at the Mauna Loa Observatory (MLO) are representative of the free air at comparable altitudes. On 2 days, carbon dioxide flask samples were exposed aboard the aircraft at the altitude of the observatory and immediately above the trade inversion. Flask pairs in reasonable agreement were obtained on both occasions. During the same period the sampling conditions at MLO were free of obvious local contamination. The average difference between the aircraft measurements at the altitude of the observatory and the continuous CO2 record from the observatory over the same period of time was 0.8 mole fraction in ppm. Differences in the individual measurements are discussed with respect to prevailing meteorological conditions.