Characteristics of raindrop acidification by co-washout of SO2(G)-H2O2(G)-HNO3(G)

In order to investigate the acid rain formation under the coexistence of SO 2(g), H2O2(g), and HNO3(g) in the air, a mathematical model has been built and some numerical simulations have been carried out with use of the model. The simulation reveals that SO2(g) absorbed into a raindrop is released an d then re-absorbed as the fall distance increases. The desorption and reabs orption processes of SO2(g) are caused by: (1) the fact that the equilibriu m concentration of H2O2(aq) and HNO3(aq) in raindrops are much higher than that of SO2(aq), and (2) the fact that the oxidation reaction rate of HSO3- with H2O2(aq) increases with H+ concentration in raindrops. The degree of acidification of the rainwater has been estimated by introducing a raindrop size distribution. The acidification is mainly caused by the adsorption of SO2(g) in the usual case where the atmospheric concentration of SO2(g) is much higher than that of HNO3(g). With the increase in the atmospheric conc entration of HNO3(g), the concentration of H+ generated from SO2(g) decreas es and the contribution of HNO3(g) to the generation of H+ becomes dominant .