Since stratospheric ozone is the principal factor in the screening of ultraviolet radiation, it is critical to understand the processes that control the levels of ozone and the distribution of ozone in the atmosphere (Chapter 3). Figure 1.04 schematically shows the life cycle of an ozone molecule.
In the first step, an ozone molecule's life begins when intense ultraviolet radiation (less than 240 nm) breaks apart an oxygen molecule (O2) into two oxygen atoms. These atoms react with other oxygen molecules to form 2 ozone molecules (see Chapter 5). As a chemical equation, this process is represented by
O2 + hc/ --> O + O ( 240 nm)
where hc/ represents the ultraviolet ray or photon, with h representing Planck's constant, c representing the speed of light, and denoting the wavelength of the photon. The rate at which ozone is formed is slow, since there isn't a lot of solar energy at wavelengths less than 240 nm. If we destroyed all of the ozone that now exists at around 20 km, this production process would take about 1 year to replace the destroyed ozone (see Chapter 5 for a discussion of the photochemistry of the stratosphere).
Step 2 in Figure 1.04 shows that the ozone molecule spends most of its life absorbing UV radiation. This absorption process occurs when the UV ray breaks the ozone (O3) molecule into an oxygen molecule (O2) and an oxygen atom (O), followed by the recombination of the oxygen atom with another oxygen molecule to reform ozone. In this process UV radiation is converted to heat energy. Again, we can represent this chemically as
O3 + hc/ --> O2 + O
O + O2 + M --> O3 + M
The M in the 2nd step is another molecule (typically N2 or O2, the two most abundant molecules in the atmosphere). It carries away the extra energy of this 3 body reaction. As we saw in Figures 1.01 and 1.02, this process of absorption is an extremely efficient process since ultraviolet radiation is effectively screened out before it reaches Earth's surface.
In step 3 of Figure 1.04, we see that the ozone molecule's life ends when it reacts with one of a variety of chemicals in the stratosphere such as chlorine, nitrogen, bromine or hydrogen. These loss reactions generally occur in a catalytic process. A catalyst is a substance that facilitates a chemical reaction, but which itself remains unchanged or is reformed by the end of the reaction, so that it can take part in a similar reaction again. In this catalytic process, the ozone molecule is lost while the catalyst (chlorine, nitrogen, bromine or hydrogen) is reformed to potentially destroy another ozone molecule. An example of a typical loss reaction is shown below where ClO (chlorine monoxide) reacts with an oxygen atom to form Cl (free chlorine) and O2 (molecular oxygen). The Cl atom then reacts with an ozone molecule to reform ClO and another O2.
O3 + h --> O + O2
ClO + O --> O2 + Cl
Cl + O3 --> O2 + ClO
Net: O3 + O3 --> 3 O2
The net effect of this reaction is the formation of 2 oxygen molecules from an oxygen atom and an ozone molecule, while the ClO molecule is unaffected. At 40 km, this Cl-ClO catalytic chain can destroy nearly 1000 ozone molecules before the Cl or ClO is converted to a benign chlorine form such as HCl (hydrochloric acid) or ClONO2 (chlorine nitrate). HCl and ClONO2 typically last for a few days, and are photolyzed by UV radiation. This again frees the chlorine to destroy more ozone. Eventually, the Cl atom is carried out of the stratosphere. Over its lifetime in the stratosphere, an individual Cl atom can destroy about 100,000 ozone molecules.
The amount of ozone in the stratosphere results from a balance between the solar production and the loss by a number of these catalytic reactions. If we could increase the Sun's ultraviolet output at wavelengths below 240 nm, ozone levels would rise. The loss of ozone is a natural process resulting from normal levels of gases such as methane, nitrous oxide, methyl bromide, and methyl chloride. If we increase these natural levels of chlorine, nitrogen, bromine or hydrogen in the stratosphere, or if we add new compounds to the stratosphere, the loss of ozone will increase, and the level of ozone will decrease, until a new equilibrium between production and loss is achieved.