The anthropogenic sulfur is added in the form of sulfur dioxide, SO2, and is produced when fossil fuels containing sulfur are burned. Figure 37.1 shows the main components of the modern sulfur cycle. The modern global sulfur cycle. The units in this figure are expressed as teragrams of sulfur. . Another example of the modern global sulfur cycle demonstrating the uncertainty in the sulfur fluxes.
Sulfur is released to the atmosphere as either hydrogen sulfide or sulfur dioxide.
Both forms are toxic gases that are primary air pollutants. Hydrogen sulfide is oxidized to sulfur dioxide in a three step process as shown in Figure 37.2. Note that the hydroxyl radical is responsible for initiating the transformation from hydrogen sulfide to sulfur dioxide. Figure 37.3 Reaction of hydrogen sulfide with hydroxyl radical and diatomic oxygen to form sulfur dioxide. Sulfur dioxide can react with the hydroxyl radical to form an HSO 3 radical, which can react with another hydroxyl radical to form water and SO 3 or H 2 SO 4 .