Absorption of NH₃ and SO₂ during activation of atmospheric cloud condensation nuclei

A numerical model is presented that considers the micro-physics and chemistry of cloud condensation nuclei (CCN) as the nuclei are transported vertically from the base of a cloud. The CCN are initially composed of mixtures of (NH₄)₂SO₄, H₂SO₄ and H₂O and are in equilibrium with gaseous SO₂ and NH₃ concentrations. The model incorporates liquid phase oxidation of S(IV) to S(VI) during adiabatic lifting of the CCN. Simultaneous absorption of SO₂ and NH₃ between the cloud droplets and gaseous dispersion medium is also considered. The model also evaluates whether the droplets are in chemical equilibrium with respect to gaseous SO₂ and NH₃ concentrations. Results from the model indicate that oxidation of S(IV) increases cloud droplet acidity during activation of the CCN. Large cloud droplets also exhibit gas phase mass transfer limitations with respect to SO₂ and NH₃. pH values of the resulting cloud droplet size distribution range over 3 pH units within the cloud at typical atmospheric conditions.