Because the gaseous carbon dioxide is in equilibrium with the carbon dioxide derivatives in the solution, the concentrations of carbon dioxide derivatives, including bicarbonate, should also increase. The fall in pH is also not surprising, since the formation of a bicarbonate molecule is concomitant with the release of a proton (see Fig. 1).
The equilibrium between carbon dioxide gas and its derivatives in the solution results in an increase in the concentrations of these derivatives, including bicarbonate. This phenomenon is also accompanied by a decrease in pH due to the simultaneous formation of a bicarbonate molecule and the release of a hydrogen ion.