If we want to understand how ozone is distributed vertically, we examine profile measurements of ozone concentration. Profile measurements can be made by balloon borne instruments known as ozonesondes, laser instruments called lidar, and profiling satellite instruments (see Chapter 7 for more instrument details). These measurements are usually reported in mixing ratio, number density, or partial pressure. Mixing ratio in ppmv relates the fractional concentration of ozone as the number of ozone molecules per million air molecules. Number density refers to the absolute concentration as the number of ozone molecules per cubic centimeter. Partial pressure refers to the fraction of the atmospheric pressure at a given altitude for which ozone is responsible.
Ozone profiles (ozone versus altitude) measured in each of these units appear somewhat different, as evidenced by Figure 3.02. This figure shows three ways of looking at the same ozone data. The profiles were measured by a satellite instrument known as the Stratospheric Aerosol and Gas Experiment II (SAGE II). This measurement was taken at 40° S on September 11, 1994 (i.e., late winter in the southern hemisphere). Notice that the partial pressure and number density profiles are very similar. This similarity is due to the fact that the partial pressure of ozone can be expressed as a function of the number density.
where Pozone is the number density, k is Boltzman's constant (1.38 x 1023 J/K) and T is temperature measured in degrees Kelvin. While all three profiles peak between 20 and 40 km and fall off rapidly above and below this peak, in the case of the mixing ratio profile, the peak is significantly higher in altitude than for the number density and partial pressure profiles. Also note that features which appear prominently in the number density and partial pressure profiles between 8 to 10 km vanish in the mixing ratio profile. The basis for these differences can be derived from material presented in Chapter 2.
Recall from Chapter 2 that Earth's atmosphere rapidly thins with height: the higher you go in altitude, the fewer air molecules there are. A cube 1 meter on each side at the ground will contain about 45 moles of air (1 mole = 6.022 x 1023 molecules). That same cube at 10 km altitude will contain just over 13 moles of air. Clearly, number density declines with altitude. In addition, as a variable trace constituent, the absolute amount of ozone at any given height varies significantly. Thus, the percentage (or concentration) of ozone of the air varies.
Mixing ratio, however, accounts for this fact that there are fewer molecules higher up, and reports the fractional composition of the air molecules everywhere. Here's another way to think about mixing ratio: the cube we used to compute number density was rigid; it always contained 1 cubic meter of air. To compute mixing ratio, we will use an elastic balloon that always holds 1/20th of a mole of air. At the ground, this balloon will fill a volume of about one liter (10-3 m3). At 10 km, however, the balloon will have to be quite a bit larger (about 4 times as large, in fact) to contain the same number of molecules. In each case, the ozone mixing ratio will be the fractional number of the total number of "air" molecules in our balloon that are ozone molecules. If we were to fill our balloon at the ground with a mixture of one part ozone to nine parts air (an ozone mixing ratio of 0.10), seal it, then slowly carry it upward to 10 km, the balloon would expand to a volume about four times as large as it occupied at the ground. Nevertheless, the balloon would still contain exactly the same ozone mixing ratio since we've neither let gas into nor out of the balloon. Ozone mixing ratio is therefore said to be conserved following air parcel motion. This conservative characteristic makes mixing ratio an excellent tool for diagnosing atmospheric motion. The type of question you are trying to answer helps to determine whether mixing ratio or number density is more appropriate. Can you now explain why the small bumps in the mixing ratio profile result in such large bumps in the number density profile near 10 km?
We still have not explained why ozone profiles have such high altitude peaks. It seems counter-intuitive given what we've already learned, namely, that the number density of air molecules in the atmosphere drops off rapidly with height (see Chapter 2). Why should ozone behave differently? The answer to this question relates to photochemistry. It will be explored more thoroughly in Chapter 5, but here's a brief explanation for now.
The ozone in any given location is a balance between three processes: in situ creation, in situ destruction, and transport into or out of the location. We ignore for the time being transport processes. Recall from Chapter 2 that creation of ozone occurs mainly in the stratosphere where there is enough of the necessary ultraviolet (UV) light from the sun. Thus, most ozone molecules are found in the stratosphere. Closer to Earth's surface, less ultraviolet (UV) light penetrates to break apart the regular oxygen molecules (see Chapter 5) that are necessary to create ozone. The shielding of the lower atmosphere from UV light is actually provided by the layer of ozone molecules above (see Chapter 4). This reflects the fact that ozone creation at a given spot depends on the amount of ozone already in a column above the spot. Thus, we may say that ozone formation in the stratosphere inhibits the formation of ozone lower down in the troposphere. (The troposphere is the lowest level of the atmosphere where dynamical instability creates most of the observed weather.) This keeps ozone concentrations small at low altitudes. Ozone concentrations do not rise indefinitely, but instead fall off above a certain height. Why? Recall that the atmosphere thins rapidly (indeed, exponentially) with height. The amount of available oxygen for ozone creation drops off quickly. Less ozone is able to form even though the UV light is available. The result of these two opposing effects: less UV light with decreasing height and less oxygen with increasing height produce the observed ozone profiles. Ozone concentrations are observed to peak between 20 and 40 kilometers with a steady decrease above and below the peak.