Chemical Reactions in the Atmosphere
We have seen, in earlier discussions, that substances in the lithosphere tend to become more reduced over time. In the lithosphere, for instance, biomass (CH2O) is slowly transformed through a sequence of steps to substances with no oxygen atoms, and then on to compounds with successively larger carbon to hydrogen ratios. The final product of this process is a form of pure carbon.
Chemical reactions in the atmosphere have the opposite effect on substances, causing an atom to become more oxidized over time in the atmosphere. Atoms that enter the atmosphere as gases in a reduced state are oxidized, in a stepwise fashion, to form ionic substances that are washed out of the atmosphere in rainfall. One example of this transformation would be where the sulfur atom in hydrogen sulfide (H2S, oxidation number of -2) is washed out as a sulfate molecule (SO42-, oxidation number of +6). Understanding these transformations is one of the primary objectives for this section of the course.
Atmospheric chemistry at ground level is very different from that which occurs in the thermosphere, and there are a number of reasons why this is so. The density of the atmosphere decreases at one progress from ground level to the edge of outer space. This means there are many more molecules per unit volume of gas to react with each other at ground level. Another factor is the fact that the temperature of the gases goes from near room temperature to very cold to very hot to very cold as one goes from ground level to the thermosphere. This affects the average velocity of the molecule, their kinetic energy, and the probability that a collision with another molecule will lead to new products. And finally, the composition of the gases themselves change dramatically from large, stable molecules at ground level to mostly small ions and highly charged atoms at the top to the thermosphere.
Let's begin our discussion by looking at the Figure of stratification of the atmosphere that we used in the last lecture. As you can see, the atmosphere consists of four distinct regions. The area closest to the earth's surface, the troposphere, extends up about 10-16 km from the earth's surface. The stratosphere is next, and reaches up to about 50 km. The mesosphere lies between 50 to 85 km from the earth's surface, and the thermosphere goes from 85 km to 500 km away from the earth's surface.
Figure 31.2 Stratification of the earth's atmosphere showing changes in temperature and pressure with altitude.
The composition of the troposphere consists of mostly nitrogen and oxygen gases. There are smaller amounts of water vapor, argon, carbon dioxide, nitrogen oxides, sulfur oxides, methane and additional trace gases. This region of the atmosphere is where all life processes occur, and this region of the atmosphere is the one most affected by anthropogenic pollution. The reactions that take place in the troposphere may be acid base reaction or photochemical reactions, and substances in the troposphere usually have a shorter lifetime than in other atmospheric regions. The ultimate fate of chemical reactions in the troposphere is to be washed out through precipitation events.
The stratosphere is not nearly as dense as the troposphere and molecules in the stratosphere are therefore exposed to much more intense radiation from the sun. This causes the stable form of molecules to be smaller is size and have a higher kinetic energy. Stratospheric ozone forms under these conditions, absorbing much of the ultraviolet light coming in from the sun. This absorption increases the average molecular velocity. The composition of the stratosphere is mainly nitrogen, oxygen, nitrogen oxides and ozone at this point of the atmosphere.
The mesosphere contains mostly ions of the same molecules that make up the stratosphere. Being closer to the sun, these molecules are exposed to even more intense radiation that has the ability to simply ionize small molecules into positive ions and electrons.
The thermosphere consists of a mixture of ions and highly charged atoms that are formed by the even more intense solar radiation that occurs at the outer edge of the atmosphere. The reason for these changes in atmospheric composition is the different amount of solar radiation present at each level of the atmosphere. Molecules act as very effective filters of light. Each layer of the atmosphere absorbs some sunlight, shielding the gases below from the radiation that it removes.
The reasons for these changes are based in Figure 31.2, which shows the variation of atmospheric pressure vs. altitude and temperature vs. altitude. The temperature of the atmosphere at earth's surface is determined by radiation of energy from the land back into the air, and by the density of the gases in the air. Regions with higher ground temperatures also have higher air temperatures. As you move away from the earth's surface, convective heating has a smaller effect and the air cools. Air temperature starts near 0° Celsius at ground level, and drops to about -60° 18 km from the earth's surface. The point where temperature begins to increase defines the break between the troposphere and the stratosphere. Temperature then increases to value of about 20° C at a distance of 50 km. This is the break between the stratosphere and the mesosphere, and going high results in temperature drops with increases in altitude, reaching a low temperature of -100° C eighty-five km from the earth's surface. This defines the break between the mesosphere and the thermosphere, where temperature again increases with increasing altitude. The earth's solar radiation budget is an issue of major importance, and underlies the concerns surrounding global warming. Figure 32.3 illustrates the current accepted values for the solar budget.
Figure 32.2 Earth's radiation budget expressed on the basis of portions of the 1,340 watts/m2 composing the solar flux.
On average, the earth receives 1,340 watts/m2 energy at the top of the atmosphere, and many different things happen to this radiation before it is ultimately returned back into space. The average temperature of the earth is determined by the ratio of energy received from the sun and the amount returned back into space by convection and reflection. The amount of light reflected from clouds has a definite effect on the temperature of the earth's surface, and some scientists argue that a short-term solution to global warming would be to generate more clouds. The composition of the atmosphere governs the rate at which infrared radiation is emitted back to space. By adding molecules to the atmosphere that absorb infrared radiation, we have effectively placed a "blanket" over the atmosphere--resulting in warmer temperatures at ground level.
Chemical reactions in the atmosphere can occur as gas phase collisions between molecules, on the surfaces of solid particles (particulate matter) or in aqueous solution (in water droplets). The reactions that take place in water droplets are predominately acid-base reactions (the same processes we studied in chapters 3 and 4). Reactions on particle surfaces are of minor importance, in most cases, because of the short residence time particles spend in the atmosphere. Gas phase reactions dominate the chemical changes that occur to substances in the atmosphere.
The hydroxyl radical (HO· ) is by far the most important single species in atmospheric chemistry. It has been called the "Ajax of the atmosphere." There are several reactions that form the hydroxyl radical, but the primary process is one where an O-H bond of the water molecule is broken to form a hydrogen atom (H· ) and a hydroxyl radical (HO· ). The hydrogen atom can then react with another water molecule to form hydrogen and a second hydroxyl radical, or with an oxygen molecule (O2) to form a second hydroxyl radical and an oxygen atom. The new oxygen atom can then react with another water molecule to form two new hydroxyl radicals. The result of these processes is a constant concentration of about a 10 million hydroxyl radicals per cubic centimeter of air at ground level. These reactions are summarized in Figure 32.3.
Figure 32.3 Reactions involved in the formation of the hydroxyl radical
These processes result in the steady state concentrations of atmospheric hydroxyl radicals shown in Table 32.1.
Table 32.1. Average background concentrations of the hydroxyl radical in the troposphere.
|Daytime summer concentrations||5-10 x 106 molecules/cm3|
|Daytime winter concentrations||1-5 x 106 molecules/cm3|
|Nighttime concentrations||< 2 x 105 molecules/cm3|
Figure 32.4 Atmospheric reactions involving the hydroxyl radical.
Original page created by Dr.Richard Foust, Professor of Chemistry and Environmental Science, Northern Arizona University