A very sensitive method of CO detection in the air, based on gas chromatography, has been developed in view of a study of the natural sources and atmospheric behavior of carbon monoxide. The CO variations found in forest air in France (44¿N) and in the Ivory Coast (5¿N) confirm the existence of a natural CO formation by vegetation. The similarity of the results suggests a homogeneous abundance in the planetary boundary layer over the continents. The tropospheric measurements are in good accordance with the data published in the literature except for the free atmosphere of the north hemisphere mid-latitudes where CO mixing ratios (70--110 ppb in oceanic air), inferior to those of Seiler (1974) (200--250 ppb), are obtained. From the CO budget based on various recent results it appears that the CO tropospheric residence time is short, of the order of 0.1 to 0.2 year. Consequently, the latitudinal CO distribution may be explained by the latitudinal repartition of natural sources rather than by the anthropogenic one (CO air concentrations, natural formation, residence time.)