As a consequence of the long lifetime of ozone molecules below the profile peak region, ozone possesses an important characteristic as a tracer. That is, gases such as ozone with long lifetimes are used to trace the motion of air parcels, showing how air from one region follows a path into another region. This characteristic of being a tracer should not be confused with the fact that gases such as ozone are also referred to as trace gases. A trace gas is one that exists in small quantities in the atmosphere. A trace gas can be used as a tracer of air parcel motions.
The long lifetime of ozone gives it the property of constant mixing ratio within a specified parcel of air. This property is then used to follow, or trace, the air parcel as it moves along, possessing the equivalent of an identifying name tag.
As an analogy, drop a rubber ducky in your bathtub. Then stir the water in the tub. The rubber ducky floats around the tub, following the currents in the water. By tracking the position of the rubber ducky as a function of time, you can determine the motion of the water in the tub. In the same way, ozone at this level acts as a tracer of air motion. The figures discussed in Section 6.2 illustrate how ozone behaves as a tracer. They contain satellite measurements of ozone concentration. The units are in parts per million by volume, or ppmv. The measurements are taken at different altitudes in the atmosphere.
As you look at the figures, you will note that altitudes are expressed in units of millibars, which you probably know as the familiar unit of air pressure. Meteorologists and other atmospheric scientists frequently use air pressure instead of actual height measurements (e.g., miles, kilometers) as a vertical coordinate. Why they do this is explained in Section 6.1.
Why are vertical coordinates expressed in terms of air pressure? The reason that pressure is used as a vertical coordinate in place of ordinary elevation or altitude surfaces is that many atmospheric motions and temperature properties occur along constant pressure surfaces, as opposed to constant altitude surfaces. The topography of mountain ranges frequently interrupts atmospheric motions, so it is easier to follow constant pressure surfaces. It also greatly simplifies the complicated equations that govern these atmospheric motions, so researchers prefer to use pressure as a vertical coordinate.
Surface pressure is typically referenced to 1000 mb, because this is approximately standard sea level pressure, which has a value of 1013.25 millibars (or 29.92" of mercury). This is a measure of the average weight of the air in a column above a spot at sea level. Another unit of pressure is the Pascal (Pa), which has MKS units of Newton per meter squared, N/m2. The Pascal is a small unit, with 1 mb equal to 100 Pa or 1 hectoPascal (hPa). Thus, 1000 mb equals 1000 hPa. The surface pressure reported anywhere in the world is typically referenced to the 1000 mb surface, even for places that are well above sea level. For instance, Denver, Colorado, is nearly a mile (1.6 km) above sea level. Typical uncorrected air pressure readings there are around 850 mb, but a correction is made to account for this. Air pressure falls exponentially with height, dropping to 500 mb at roughly the 5 km level, to 50 mb by about the 21 km level, to 15 mb near the 29 km level, and to 0.5 mb near the 53 km level.
The degree to which ozone acts as a tracer with height is explored through the following figures. Each is a map of ozone as measured by the Solar Backscatter Ultraviolet (SBUV) satellite instrument for different altitudes. The ozone is expressed in ppmv (parts per million by volume), the unit of mixing ratio. Recall that mixing ratio in ppmv relates the fractional concentration of ozone as the number of ozone molecules per million molecules of air.
6.2.1 Ozone at the 50 Millibar Level -- In Figure 3.06 we have a map of SBUV ozone at the 50 millibar level. This is just below the peak in ozone level (see Figure 3.04).
We see from the figure that ozone mixing ratio is low in the tropics and higher in the mid- latitudes of both hemispheres, especially the southern. A ring of high ozone in the southern mid-latitudes, often called the "collar" region, surrounds a region of very low ozone centered on the south pole. This is discussed further in Section 7.
6.2.2 Ozone at the 15 Millibar Level -- Looking a bit higher up, Figure 3.07 shows the SBUV ozone measured at 15 mb (about 29 km, near the peak of the ozone profile expressed as a mixing ratio). A very different picture appears.
Here we see very high ozone in the tropics and very low ozone in the polar regions. A filament of low ozone can also be seen extending away from the south polar region in the southern hemisphere between 30°-60° S and 210°-290° E. The air within this filament traces back to the polar vortex region (the isolated polar air mass of low ozone described in Chapter 10).
6.2.3 Ozone at the 0.5 Millibar Level -- If we now look at a map of ozone at an even higher altitude on exactly the same day, we see another different picture. Figure 3.08 shows a map of ozone, again from SBUV, this time at 0.5 mb (about 53 km).
At this altitude few of the UV photons have been blocked out. Ozone molecules are, as a result, created and destroyed very rapidly. The lifetime of ozone is very short, so ozone is described as being photochemically controlled. We'll look into the photochemical properties of ozone in Chapter 5. For now, we just need to know that at altitudes where ozone is photochemically controlled, the ozone distribution will show no dynamical features. Compare Figures 3.06 and Figure 3.07 with Figure 3.08. Notice that neither the filament of low ozone emerging from the southern polar vortex at 15 mb (29 km) nor the collar region of high ozone around the polar region at 50 mb (21 km) appear at the 0.5 mb (53 km) level. We will see shortly if and how these differences affect the amount of ozone in the total column.
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