Variations in the surface boundary layer carbon dioxide concentration (<CO2>) during the 1971 growing season have been analyzed in terms of three periodicities. The diurnal periodicity has an amplitude maximum in the vegetation canopy, of up to 100 vpm, and this diminishes with increasing height above the surface. This variation is driven by the daytime intake of carbon dioxide by the vegetation in photosynthesis and the nighttime release through plant and soil respiration. Spectral analysis of the <CO2> shows the presence of irregular variations, with periods between 3 and 5 days, that appear to be related to synoptic scale weather patterns. Highest <CO2> is found in air that has most recently come from the Pacific Ocean and <CO2> decreases with longer trajectories over land causing a difference of up to 7 vpm in the daily minimum. A 28 vpm decrease in mean daily <CO2> from June to September is considered part of the annual <CO2>variation.