Based on?
CO2 is a well-mixed gas in the atmosphere. There is variability right at the surface of the ocean, for example, where there is air-sea gas exchange, but once you're 500 meters over it there's not much difference. Now, they do CO2 measurements on Mauna Loa because it samples the global atmospheric circulation; the fact that they see the basic seasonal variability cycle every year indicates that CO2 must mix quickly. There might a bit of hemispheric variability, of course.