Chemistry of sulfur in the atmosphere

Industrially produced sulfur enters the atmosphere in reactive forms, primarily SO2 and SO3, that should not be treated as inert pollutants for purposes of modeling. An effort was made to describe the equilibrium and kinetic aspects of sulfur chemistry in a form suitable for inclusion in the atmospheric transport model (ATM). At the usual low concentrations of SO2 in the atmosphere (typically 10-8 atm), it reacts with water only if condensed water already is present, producing the dissolved species H+, SO2(aq), HSO3-, and SO32-. In contrast, SO3 reacts vigorously and completely with either gaseous or liquid water to produce acid droplets containing the additional species HSO4- and SO42-. These droplets can be partially or completely neutralized by atmospheric NH3, the end product being solid particles of (NH4)2SO4 or, if the relative humidity exceeds approximately 80 percent, droplets of ammonium sulfate