Winter variability over periods of 1 to 5 days in surface CO2 concentration at Barrow, Alaska was studied by examining the relation between CO2 concentration and air mass. The largest positive anomalies of CO2 concentration occurred with relatively deep surface-based Arctic air masses. The long residence time in the Arctic of these air masses is qualitatively compatible with both a natural CO2 source, such as the Arctic Ocean, and transport of anthropogenic CO2 from mid-latitudes in a manner similar to that proposed for the Arctic haze. Trajectories suggest that the anthropogenic CO2 source region of eastern Asia does not contribute significantly to the positive anomalies. The largest negative CO2 anomalies were associated with the influx of air from the North Pacific or North Atlantic regions above a shallow surface-based Arctic layer. The moisture sounding data suggest mixing or diffusion of this air aloft to the surface through the inversion layer.