Title: How does tropospheric OH variability influence the transport of SO2 and COS from the surface to the stratosphere? A conceptual study
Abstract: The stratospheric sulfate aerosol layer is a key element in the climate system as it affects both the chemistry of the stratosphere and the absorption and scattering of radiation in the stratosphere. In the absence of major volcanic eruptions, the stratospheric aerosol layer is maintained by transport of sulfur containing species, predominantly carbonyl sulfide (COS) and sulfur dioxide (SO2), from the troposphere, through the tropical tropopause layer (TTL), to the stratosphere. The lifetimes of these species, both in the troposphere and in the TTL, are largely determined by hydroxyl (OH) concentration. The product of the OH oxidation, SO3, can combine with water vapor to form sulfuric acid (H2SO4). The acid can precipitate in water droplets forming a sulfate aerosol which is largely removed by rainout or deposition on ice. The part of tropospheric sulfate aerosol which is not removed and predominantly the non-soluble fraction of these sulfur containing species determine the flux of sulfur entering the lower stratosphere and contributing to the stratospheric aerosol abundances. Therefore changes in the OH concentration have a strong impact on the stratospheric sulfur burden. The use of Lagrangrian trajectories to study transport processes from the troposphere to the stratosphere through the TTL is well established. In this study a Lagrangrian (trajectory based) chemistry transport model (ATLAS) is used to investigate the sensitivity of sulfur containing species entering the stratosphere to tropospheric OH concentrations. Recent observations of very low ozone concentrations in the tropical western Pacific implied very low OH concentrations in that region. Using 4 scenarios of OH concentrations derived from the GEOS-Chem chemistry transport model, as initial concentrations, trajectories were initialized in the tropical lower stratosphere (440K) and then followed backward. Only trajectories that extend back to the boundary layer were used for further study. A simplified chemical box model including the breakdown of COS, the conversion of SO2 into highly soluble SO3, and the photolysis of COS and SO2 was run along these trajectories. Furthermore, heterogeneous processes for SO2, its uptake into droplets and on ice and its phase conversion into sulfate are implemented within the box model. To estimate the extent to which sulfate is transported into the stratosphere in the form of aerosol, microphysical processes (such as nucleation, condensation, and sedimentation) need to be considered. The fraction of the initial COS and SO2 abundances that reaches the stratosphere has been determined. The implications of reduced tropospheric OH concentrations on sulfur transport into the stratosphere will be shown. The results suggest that reduced OH over the Pacific warm pool (the main entry region for stratospheric air masses), enhances the fraction of SO2 reaching the stratosphere and suggest that direct emissions of SO2 into the troposphere from anthropogenic activity in South East Asia or from small scale volcanic activity in that region can have a larger effect on the stratospheric sulfur budget than would be derived from standard OH fields.
Publication Year: 2013
Publication Date: 2013-02-01
Language: en
Type: article
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