H2CO3 → CO2 + H2O decomposition in the presence of H2O, HCOOH, CH3COOH, H2SO4 and HO2 radical: instability of the gas-phase H2CO3 molecule in the troposphere and lower stratosphere

Abstract

To understand the stability of the gas-phase carbonic acid (H₂CO₃) molecule, especially in the Earth's troposphere and lower stratosphere, here we report high level quantum chemistry calculations investigating the energetics for the H₂CO₃ → CO₂ + H₂O decomposition reaction via its shortest route in the presence of one to three water (H₂O) molecules as well as in the presence of formic acid (FA), acetic acid (AA), sulfuric acid (SA) and hydroperoxide (HO₂) radical. The calculations have been performed at the MP2/aug-cc-pVDZ, MP2/aug-cc-pVTZ, MP2/6-311++G(3df,3pd) and CCSD(T)/aug-cc-pVTZ levels of theory. The comparison of the reaction rates including tunneling corrections according to the unsymmetrical Eckart potential barriers suggests that at 0 km altitude in the clean environment of the Earth's atmosphere, the gaseous H₂CO₃ molecule becomes an unstable species in the presence of the H₂O monomer, dimer, FA and AA. This follows as the FA- and AA-assisted H₂CO₃ → CO₂ + H₂O decomposition reactions are effectively near-barrierless processes and the reaction rates for the H₂O monomer-, dimer-, FA- and AA-assisted H₂CO₃ decomposition reactions are comparable within a factor of ∼15. Similarly, at 0 km altitude in a polluted environment and also in the 5 to 15 km altitude range, only the FA- or AA-assisted H₂CO₃ decomposition is the dominant reaction pathway, especially, among all the pathways that have been considered here. It is seen from the CCSD(T)/aug-cc-pVTZ level prediction results that at altitudes of 5, 10, and 15 km in the Earth's atmosphere, the reaction rates for the FA-assisted H₂CO₃ decomposition depending upon the average concentrations of FA are respectively ∼10², 10⁵ and 10⁶ times higher than the reaction rates associated with the water monomer-assisted H₂CO₃ decomposition. Moreover, it is thought that the catalytic efficiencies of FA, AA and SA upon the H₂CO₃ → CO₂ + H₂O decomposition reaction are similar to each other, but nevertheless, SA, because of its low concentration, does not play a significant role in the H₂CO₃ → CO₂ + H₂O decomposition reaction, especially in the 0 to 15 km altitude range of the Earth's atmosphere.