The reaction of the greenhouse gas, CO2, with Ca(OH)2 was explored using single-particle Raman spectroscopy to track the chemical reaction. Single particles were levitated in an electrodynamic balance (EDB) to maintain the particle in a laser beam. The levitation voltage provided gravimetric data. The humidity in the EDB chamber was varied to determine the effects of humidity on the reaction. It is demonstrated that no appreciable reaction occurs at low relative humidities (RH<70%). This is evidenced by no disappearance of the Raman peak associated with the [OH]-vibrational bond, but at high humidities (RH>70%) the Raman spectrum of CaCO3 developed as the reaction proceeded. The results are consistent with results from packed-bed studies of the reaction and are in agreement with similar findings for the SO2/Ca (OH) 2 reaction used for desulfurization, that is, the reaction does not proceed until multiple monolayers of water are adsorbed on the particle surface.