|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||||
|
|
|
|
|
|
|
|
|
||
Sulfur Cycle
The sulfur cycle (Figure 10) can be
divided into a cycle over water and a cycle over land.
In comparison with the nitrogen cycle, we notice immediately that the
sulfur cycle
has no connection to the stratosphere, since there are no long-lived species of sulfur.
Sulfur, of course, is a problem in the troposphere as an air pollutant.
We will come back to this in a future unit.
Figure 11 shows a simulation of sulfate () concentrations in the lower atmosphere (at a level of 900 hectopascals, which is about a kilometer above the earth's surface) over the earth. The numbers plotted are dimensionless ratios of total concentration of anthropogenic plus natural sulfate divided by the concentration based on natural emissions. Highly populated and industrial areas of the eastern part of the U.S. and eastern Europe have highest concentrations. Concentrations over Asia are increasing, particularly in the industrial area of southeast China. Of course, as the Chinese industrialization rapidly increases, this problem is likely to increase.
A recent finding, and one that has put environmentalists in a perplexing dilemma, is that there also is a secondary effect of sulfur dioxide in the earth's atmosphere. Although it is not a greenhouse gas, it does contribute to the radiation balance of the earth. In the presence of clouds, atmospheric becomes dissolved in the water droplets and forms weak sulfuric acid, . Such clouds observed from space appear brighter than natural clouds, suggesting that these clouds are reflecting more solar radiation than natural clouds. This process is called cloud brightening and reduces the amount of solar energy entering the earth/atmosphere/ocean system, thereby contributing to a cooling of the planet . The net result of burning fossil fuels containing sulfur (mainly coal) is that the emitted leads to global warming and the leads to global cooling. Environmentalists have fought for years to have emissions reduced, but one result of these efforts seems to be that global warming will be exacerbated. The relative amounts of warming and cooling will be discussed in Unit 1-12.
Another possible linkage of sulfur to global change is the role of dimethylsulfide (DMS) in formation of clouds over ocean areas. DMS is produced naturally in ocean areas by biological activity. Studies have shown that DMS can promote the production of cloud condensation nuclei, which are favored particles for cloud droplet growth. Therefore, an abundance of marine plant life can produce sufficient amounts of DMS to enhance local cloud formation and possibly increased precipitation. This constitutes a direct link between the biosphere and local meteorology. It also opens the possibility that there might be a link between changes in the stratospheric ozone and local meteorology in that, for instance, increased ultraviolet levels over the ocean could suppress ocean biology, which in turn would reduce the emission of DMS, which could reduce cloudiness and precipitation in ocean areas. We don't know enough about the magnitude of this effect to evaluate its importance in relation to other global change processes.
Several other questions relating to the sulfur cycle and its component fluxes and reservoirs indicate that much research remains to be done on this trace gas.
