On the basis of the measurements at two monitoring sites located close to each other (220 km) in plain regions in Hungary, the representativeness of low-elevation continental CO2 measurements is estimated. It is shown that under such conditions only the measurements carried out in the early afternoon hours can be considered as regionally representative for the CO2 content of the planetary boundary layer (PBL). Filtering the data in this way, it is calculated that the characteristic CO2 mixing ratio in the PBL may be about 2.5 ppm higher over this part of Europe than at the Mauna Loa Observatory (National Oceanic and Atmospheric Administration), Hawaii. ¿ 1999 American Geophysical Union