Now that we have a better understanding of the language of ozone measurements, let's take a closer look at the distribution of ozone. We have seen that a typical ozone profile has low values at the surface and the troposphere, increasing values with altitude into the low and middle stratosphere, and decreasing values in the upper stratosphere and mesosphere. The exact altitude of the peak varies with latitude and season. Figure 3.04 shows an average of a number of ozone profiles taken at tropical, mid-, and high (polar) latitudes in the northern hemisphere.
The profiles in this figure are based on SAGE II measurements from September 1994. The altitude of the peak of the ozone distribution is clearly higher in the tropical profiles than the other two profiles. The somewhat narrower peaks of both the tropical and the midlatitude profiles lead to smaller total column ozone amounts in these latitudes. There are exceptions to this behavior: ozone profiles frequently show sharp positive and negative variations in ozone from the normal ozone profile shape (Reid and Vaughan, 1991). These variations show up as spikes in an ozone profile curve. Known as laminae, they usually are associated with certain dynamic phenomena, such as those described in Chapter 6. High latitude profiles (outside the ozone hole) have peaks that are typically lower in altitude than both the midlatitude and tropical peaks. These peaks (in number density) are also typically larger in magnitude than their tropical and midlatitude counterparts, as also demonstrated in Figure 3.04.
The reason that we observe the smallest maximum number density (peak profiles) of ozone in the tropics and the highest maximum number density (peak profiles) in the high latitudes is because of a hemispheric circulation pattern between the Equator and the poles. It is known as the Brewer-Dobson circulation (see Section 7 and also Chapter 6 for more explanation). Rising motion in the tropics first elevates the height of the peak profile and then poleward transport carries ozone away from the tropics, lowering the total column amount. This is why the maximum number density is smallest in the tropics. At high latitudes, sinking motion lowers the height of the peak profile and causes ozone to accumulate, increasing the total column amount. This is why the maximum number density is greatest in high latitudes.
The most noteworthy, aberrant ozone profiles to date are those associated with the Antarctic ozone hole. Figure 3.05 shows both an average high latitude ozone profile taken outside the Antarctic ozone hole and an average profile taken inside the hole by the same SAGE II instrument in September 1994.
Note the large inward notch in the ozone hole profile. Note that it is occurring exactly at the point where ozone values reach their maximum in the more typical high latitude profile. What is clear from comparing these two ozone profiles is that something unusual and worrying is happening inside the ozone hole. Whatever is happening (see Chapter 10), the effect is to destroy ozone most severely at the very altitudes where it is normally most plentiful. The cause of this depleted notch in the profile is a combination of two key factors. The first is related to the chemical reactions between ozone and the chlorine introduced by manmade chlorofluorcarbons (CFCs). The second is related to the unique meteorology of the southern hemisphere winter. The details of both and exactly how they facilitate the loss of ozone are explored thoroughly in Chapter 10.