We have seen that ozone can change on a variety of time scales. Up to this point, we have focused mainly on changes in the total column amount of ozone as a function of time. Column changes in ozone reflect changes in ozone occurring at different altitudes, since the column is the total ozone from the surface to the top of the atmosphere. So what we really want to know about are the changes in ozone at different altitudes or levels in the atmosphere as a function of time.
As with total ozone changes, changes in ozone at a given altitude are explained in terms of production, loss, and transport processes. Photochemical processes are the principle causes of production and loss of ozone. Changes in ozone at different altitudes are driven in differing degrees by both photochemistry and transport. If ozone molecules at some altitude are shielded from energetic ultraviolet radiation from the sun, their life expectancy and the way they vary will differ from ozone molecules higher up that are not similarly shielded. It is for this reason that we divide up our overview of ozone variability into lower stratosphere and upper stratosphere sections.
Because there is not much ozone above the stratosphere (i.e., in the mesosphere and thermosphere), most of the ozone in the upper stratosphere is exposed to strong ultraviolet radiation from the Sun. Ozone and chemical radicals (see Chapters 3, 5, 11, and 12) which affect ozone photochemistry are rapidly created and destroyed by ultraviolet light. Ozone lifetimes are short. In order to determine ozone concentrations due to photochemistry, we use the rates of photochemical processes that are measured in the laboratory. A standard assumption is that the system is in a steady state. This approximation works in places where the rates of ozone production and loss are fast compared to the rate of transport processes.
The steady-state approximation assumes that the rate at which ozone is being produced is equal to the rate at which it is being destroyed at a given location. Such an approximation allows us to determine theoretically what the concentration of ozone would be if only photochemical processes were operating. The calculated concentrations can be compared to measurements of ozone in the atmosphere to test the accuracy of the photochemical approximation.
In the upper stratosphere, we find that observed ozone concentrations are consistent with photochemical steady-state to within the uncertainties in our knowledge of the reaction rates used in the calculations. In the lower stratosphere of the midlatitudes, we find that observed ozone concentrations are significantly higher than that predicted by the photochemical steady-state calculation. This is because the time for photochemical loss is longer than the time for transport processes to bring air with high concentrations of ozone into the region. Thus, observed ozone levels remain elevated above those obtained from simple production/loss steady-state calculations.
The time rates of photochemical processes are discussed in the next section. Knowing these time constants and how rapidly air is moving tells us whether photochemical or transport processes dominate the ozone budget of a region in the atmosphere.
Photochemical replacement time (abbreviated PRT) is defined as the time it would take to generate the observed ozone concentration at a specific location at the existing production rate with no loss processes. It is a convenient measure that helps us to determine whether photochemistry or transport is the controlling factor in determining the ozone concentration at a particular location.
The PRT, t, in seconds is
where n(O3) is the density of ozone in molecules per cm3 and P(O3) is the production rate in molecules per cm3 per second.
In the lower stratosphere where PRT constants are long, the amount of ozone in a given location is strongly influenced by the transport of air into and out of that location. The large ozone concentrations in the lower stratosphere in winter are a result of net transport downward and poleward from the more active photochemical region in the middle and upper stratosphere in the tropics. This transport cycle is associated with the Brewer-Dobson circulation discussed in detail in Chapter 6.
Figure 8.02 shows the important features of the ozone distribution as a function of latitude and altitude for January.
The color contours in Figure 8.02 show the ozone concentration in units of 1018 molecules per cubic meter (which is the same units as 1012 molecules per cubic centimeter). Superimposed on the figure are contours of calculated PRTs for ozone. In the summer hemisphere, the replacement times are less than one day at altitudes above 40 km. In the winter hemisphere, the replacement times increase as one moves poleward, and reach values greater than one year. In the lower stratosphere, the PRTs range from months to years as the ozone higher up absorbs the ultraviolet radiation which drives the chemistry, and hence photochemical processes are slowed.