It depends on the measurement set up. Is the CO2 adsorbed from the gas phase? Are other gases adsorbed too?
If it is pure CO2 gas measure the pressure decreasure due to sorption and find the adsorbed mass from the law of prefect Gases. Also check for the BET method. It essentially measures sorption in that way, but requires sophisticated equipment and a special procedure regarding temperature. Also make sure to desorbed previously adsorbed gases in vacuum and apply heat.
If the sorption of other gases is likely you could use gas chromatography to analyze the changes of composition in the gases phase instead.