Effect of liquid-phase oxidation of sulphur dioxide on acidification of growing cloud droplet

Cloud droplets grow by condensation of atmospheric water vapor into themselves, during which they absorb also atmospheric pollutants as SO2(g) and H2O2(g). In order to estimate the contribution of liquid-phase oxidation during growth to cloud droplet acidity, the oxidation of HSO3- to SO42- by H2O2(aq) is investigated numerically, using a mathematical model based on physicochemical considerations. The results of the numerical simulations show that: (1) the time variation of [H+] behaves as if it attained the equilibrium state, although after a while it breaks the state to increase (i.e., there is a plateau in the curve of [H+] vs time); (2) the contribution of the oxidation to the cloud droplet acidity increases with laplse of time; (3) the larger the initial radius of CCN is, the smaller the contribution of the oxidation becomes; and (4) the quasi-equilibrium [H+] is reached much faster than the equilibrium droplet radius.