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

Carbonic acid, H 2 CO 3 , is of fundamental importance in nature both in living and non-living systems. A lot of research has been devoted to this molecule and it has been characterized in solution, in the solid state and in gas phase, however its existence under certain conditions and its properties remain controversial. Providing direct spectroscopic evidence for carbonic acid formation is a challenge. Furthermore, the methods for carbonic acid generation are indirect e.g. high-energy irradiation or protonation of carbonate and bicarbonate ions by strong acids. Here we provide clear evidence by in situ attenuated total reflection infrared spectroscopy combined with modulation excitation spectroscopy and phase sensitive detection that CO 2 adsorption on ice surfaces is accompanied by carbonic acid formation. Hence we demonstrate that carbonic acid can be formed from CO 2 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 (HCO 3 - ) and carbonate (CO 3 2- ). The direct formation of carbonic acid from adsorption of CO 2 on ice could play a role in the upper troposphere in cirrus clouds, where all the necessary ingredients to form carbonic acid, i.e. low temperature, CO 2 gas and ice, are present.