Atmospheric Reactions of Sulfur and Nitrogen
Reading Assignment: Read the material on the sulfur and nitrogen cycles linked below, and the information provided in the lecture notes. You should be reading chapter 11 in Manahan.
Homework: HW-12, Due Friday, April 18
Global Sulfur cycle
Atmospheric reactions of sulfur
Global nitrogen cycle
Tropospheric chemistry of nitrogen
Stratospheric chemistry of nitrogen
The modern global sulfur cycle differs quite dramatically from the "pre-industrial" sulfur cycle from the large portion of anthropogenic sulfur added to the atmosphere each year. 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.
Figure 37.1 The modern global sulfur cycle. The units in this figure are expressed as teragrams of sulfur.
Figure 37.2 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.
The atmospheric reactions of SO2 are very complex, and proceed through three different pathways to the sulfate ion (SO42-). Sulfur dioxide can react with the hydroxyl radical to form an HSO3 radical, which can react with another hydroxyl radical to form water and SO3 or H2SO4 . Sulfur dioxide also dissolves in water droplets where it can react with oxygen gas to form SO42-. The third pathway to sulfate is when sulfur dioxide reacts with hydrogen peroxide to sulfuric acid as shown in Figure 37.3.
Figure 37.4 Reactions of sulfur dioxide to form sulfuric acid.
The ultimate fate of all sulfur in the atmosphere is to be oxidized to the sulfate ion, usually as sulfuric acid (H2SO4). The most common base present in the atmosphere is ammonia (NH3) which reacts with sulfuric acid to form ammonium bisulfate (NH4HSO4) and ammonium sulfate ((NH4)2SO4). Sulfuric acid, ammonium bisulfate and ammonium sulfate are all hydroscopic substances, readily dissolving in water. They wash out of the atmosphere during precipitation events.
Atmospheric reactions of nitrogen are much more complex than the atmospheric reactions of sulfur. Ammonia (NH3) is the most reduced form of nitrogen, and is released in small quantities from anaerobic degradation of organic matter containing nitrogen. Just like hydrogen sulfide, ammonia reacts with the hydroxyl radical to form oxidized nitrogen species. Nitrogen oxides are released to the atmosphere from both natural and anthropogenic sources. The two most common nitrogen gases released to the atmosphere from biological processes are nitrous oxide (N2O) and nitrogen dioxide (NO2). Combustion processes release mostly nitroghe oxide (NO) and nitrogen dioxide. The exact composition of nitrogen oxides emitted from combustion processes varies with temperature of the combustion process, and the nitrogen oxides from combustion are often referred to as NOx to indicate the uncertainty in chemical composition.
Like sulfur, the modern global nitrogen cycle is very different than the "Pre-industrial" nitrogen cycle. The difference, again, is the large amount of nitrogen added to the atmosphere through combustion processes. The excess atmospheric nitrogen oxides contribute to acid rain in the same way that excess sulfur oxides do. Table 37.1 lists the important atmospheric nitrogen oxides and their oxidation states.
Table 37.1 Important Atmospheric nitrogen oxides
Figure 37.5 The modern global nitrogen cycle. The units in this figure are expressed as teragrams of nitrogen.
The chemistry of atmospheric nitrogen in the troposphere is different that the chemistry in the stratosphere. Nitrogen chemistry at both levels is driven by the photochemical dissociation of nitrogen dioxide (NO2), but the products formed depend on other substances with which the photochemically excited NO2 molecules can react. At ground level the air is more dense than in the troposphere, so the concentration of O2 is much greater. Also at ground level are volatile organic carbon substances (from automobile traffic, solvents and industrial processes) that react with nitrogen oxides to form peroxyacylnitrates, a product of photochemical smog. The many different substances that can form from nitrogen oxides in the troposphere are shown in Figure 37.6.
Figure 37.6 Forms of nitrogen oxides in the troposphere important in the formation of photochemical smog.
Stratospheric nitrogen chemistry includes ozone as a major player. The intense ultraviolet radiation 50 km from the earth's surface causes diatomic oxygen to dissociate into oxygen atoms. These single oxygen atoms react with oxygen molecules to form ozone. This sequences of reactions is known as the Chapman cycle, shown below.
Nitrogen oxides then react with ozone as shown in the two reaction schemes shown below:
The overall effect of these reactions in the undistrubed stratosphere is a steady-state concentration of each of the gases involved. We are most concerned about the ozone concentration because of its ability to absorb ultraviolet radiation, protecting the earth's surface in the process. The natural concentration of stratospheric ozone is 6-10 ppm, or about 350 Daltons.
The sequence of reactions that take nitrous oxide, nitrogen oxide and nitrogen dioxide to nitric acid are shown in Figure 37.5-7. Nitorgen enters the atmosphere in several different forms, but it leaves the atmosphere by being washed with precipitation as nitric acid.
Figure 37.7 Equilibrium between oxygen and nitrogen gases responsible for formation of nitrogne oxide in combustion processes.
Figure 37.8 Photochemical conversion of nitrous oxide to nitrogen oxide.
Figure 37.9. Photochemical conversion of nitrous oxide and nitrogen oxide to nitrogen dioxide.
Figure 37.10. The atmospheric reactions that convert nitrogen dioxide into nitric acid.
Environmental Chemistry -- ENV 440