Molecular dynamics simulations predict an accelerated dissociation of H2CO3 at the air-water interface

The dissociation and decomposition reactions of carbonic acid (H2CO3) in bulk water have been thoroughly studied, but little is known about its reactivity at the air-water interface. Herein, we investigate the dissociation reaction of H2CO3 at the air-water interface using ab initio molecular dynamics and metadynamics. Our results indicate that H2CO3 (pK(a) = 3.45) dissociates faster at the water surface than in bulk water, in contrast to recent experiments and simulations which have shown that HNO3 (pK(a) = -1.3) has a lower propensity to dissociate at the water surface than in bulk water. We find that the water surface allows for a more structured solvation environment around H2CO3 than in bulk water, which contributes to a decrease in the dissociation energy barrier via a stabilization of the transition state relative to the undissociated acid. Given its decreased kinetic stability at the air-water interface, H2CO3 may play an important role in the acidification of atmospheric aerosols and water droplets.