CO2 conversion reactions in catalytic microenvironments produce variable amounts of bicarbonate (HCO− 3 ), carbonic acid (H2CO3), and water ions. A description of the pH at such a scale requires atomistic modeling of all competitive reactions. We first use a recently developed reactive polarizable and universal non-bond based force-field - RexPoN - to model the recombination of H3O+ and OH− ions in small water pools. We observe significantly longer ion recombination times in small water droplets of size 1000 and 3000 molecules. Unexpectedly, the process of recombination in surface vs. bulk of these droplets is more diffusion controlled than that depending on the local geometry. We then use DFT-D3 based metadynamics simulations to model bicarbonate/carbonic acid equilibria and assess energetics of all the possible reactions. The simulations predict the formation of both cis-cis and cis-trans carbonic acid from bicarbonate. An interconversion between cis-cis and cis-trans is also observed via a bridged intermediate. A reactive force-field (ReaxFF) is also parametrized to model bigger droplets (∼ 1000 atoms) – otherwise, inaccessible from DFT due to size limits. These reactions are further to be incorporated into the numerical simulations to predict concentrations of various molecular/ionic forms of bicarbonate/carbonic acid in the microenvironments.