Remote Tropospheric Sulfur-Dioxide Cloud Transport of Reactive Sulfur Emissions.
Model simulations suggest that reactive sulfur gases emitted from the land or ocean surface can produce upper tropospheric sulfur dioxide at the levels of 80 (+OR -) 30 pptv measured over the remote oceans. Emissions of 20-40 Tg(S)/yr of dimethyl sulfide, hydrogen sulfide, carbon disulfide, or more reactive species appear sufficient to explain this SO(,2). Previously published estimates based on traditional K(,z) theory suggest 1/6 to 1/30 as much SO(,2) since they do not simulate the effects of rapid, but intermittent, convective transport. Transports due to penetrative cumulonimbus, organized into cloud clusters, are related to a simplified synoptic -scale overturning circulation characteristic of the tropical marine atmosphere. Low clouds and cloud-free regions occupy four-fifths of the area. A two-dimensional photochemical model is presented that has consistent descriptions of cloud effects on photolysis rates, transports, and water -phase chemistry. The transports used are based on diagnostic studies of cloud populations in the tropics and subtropics. Steady -state simulations of S chemistry with this model typically suggest that SO(,2) is constant or increases with height. Dimethyl sulfide emissions of (TURN)25 Tg(S)/yr from the oceans are sufficient to produce 60-120 pptv upper tropospheric SO(,2), but also require 60 pptv of boundary layer (CH(,3))(,2)S, ten times more than is observed. CS(,2) emissions could contribute one fourth to one half of the SO(,2), even though CS(,2) would be below 4 pptv in the middle troposphere. A simulation of continent-to-ocean transport and reaction of sulfur compounds suggests that 20 Tg(S)/yr of H(,2)S missions from the evergreen forests and wetlands of the tropics could also explain much oceanic SO(,2). The model results are quite sensitive to the correlation of sulfur emissions and convective activity. Further results indicate the importance of the possible transport, rainout, or reactions of peroxides; these appear to be as important as NO(,x) concentrations in determining tropical OH radical concentrations. Implications of this work for the description of O(,3), NO(,x), SO(,2) and other species in the tropics and mid-latitudes are briefly mentioned.
- Pub Date:
- Physics: Atmospheric Science