A GLOBAL 3-DIMENSIONAL MODEL OF THE STRATOSPHERIC SULFURIC-ACID LAYER

We present the results of a theoretical study of the chemistry and circulation which maintains the lower stratospheric sulfuric acid (Junge) layer in nonvolcanically perturbed periods. We use a global three-dimensional chemical-dynamical model which includes production of SO2 from OCS, oxidation of SO2 to gaseous H2SO4, condensation-evaporation equilibrium of gaseous and particulate H2SO4, condensation growth of particulates as they enter the tropopause-upper troposphere region, and particulate rainout in the lower troposphere. We have compared our results with the NIMBUS 7 SAM II and AEM-2 SAGE stratospheric aerosol extinction data for periods when the stratosphere was not perturbed by recent volcanic eruptions. The model simulates the general behavior of stratospheric aerosol extinction including the existence of a polar tropopause enhancement in this extinction. Agreement is good in the tropics but there is a tendency for the model in high latitudes to significantly overpredict aerosol extinction above 15 km due perhaps to an overly vigorous predicted circulation or to inadequate knowledge of particle sizes. We identify two major sources for stratospheric H2SO4: one is upwardly transported and photodissociated OCS and the other is upwardly transported SO2. The importance of upwardly transported SO2 is a new and significant result whose validity is dependent on the realism of the vertical transport and chemical loss of SO2 above 9.3 km in the model. We have studied the roles of chemical sources, circulation, and sinks in the global sulfur compound budgets and we find certain similarities in the behavior of H2SO4 and 03 in the stratosphere; each is chemically produced predominantly at lower latitudes in the stratosphere with poleward transport maximizing in the winter and spring months.