Observation of Carbonic Acid Formation from Interaction between Carbon Dioxide and Ice by Using In Situ Modulation Excitation IR Spectroscopy.

Carbonic acid, H2 CO3 , is of fundamental importance in nature both in living and non-living systems. Providing direct spectroscopic evidence for carbonic acid formation is however a challenge. Here we provide clear evidence by in situ attenuated total reflection IR spectroscopy combined with modulation excitation spectroscopy and phase-sensitive detection that CO2 adsorption on ice surfaces is accompanied by carbonic acid formation. We demonstrate that carbonic acid can be formed from CO2 on ice in the absence of high-energy irradiation and without protonation by strong acids. The formation of carbonic acid is favored at low temperature, whereas at high temperature it rapidly dissociates to form bicarbonate (HCO3 - ) and carbonate (CO3 2- ). The direct formation of carbonic acid from adsorption of CO2 on ice could play a role in the upper troposphere in cirrus clouds, where all the necessary ingredients to form carbonic acid, that is, low temperature, CO2 gas, and ice, are present.