In order to understand transport of ozone in the stratosphere, we need to understand some key concepts. First, stratospheric air is very thin and becomes even thinner with increasing altitude. Second, stratospheric air is very stable, and vertical motion in the stratosphere is quite slow. This is because of the temperature structure there: temperatures rise with altitude. Third, the very long lived gases in our atmosphere become uniformly mixed by transport processes. Finally, the stratosphere can be divided into four distinct regions: (1) the tropics, which stretch from about 20°N to 20°S; (2) the middle latitudes or "surf zone"; (3) the polar vortex; and (4) the lowermost stratosphere. The structure of the stratosphere is primarily discussed in Chapter 2 as part of a discussion on the structure of the entire atmosphere. Here in this section, we will re-emphasize some of the key points of Chapter 2 with respect to mixing and transport processes in the stratosphere. We will discuss the density of air in the stratosphere, and how the temperature structure affects the stability (or buoyancy) of air. We will then discuss the well-mixed nature of our atmosphere. Finally, we will describe the basic regions of the stratosphere (a key background description for all of the subsequent sections of this Chapter).
The stratosphere is not a good place to be. First, the ozone in the stratosphere, which protects us from biologically destructive solar ultraviolet light, exists at such high levels that the air itself is toxic. Second, even this toxic air is much too thin for normal breathing. Finally, temperatures in the stratosphere are lethally frigid.
2.1.1 Air Density Change With Altitude -- The density of the atmosphere decreases with altitude. This density decrease is notable on high mountain tops (such as Mt. Everest) where the lower air density makes breathing more difficult. In fact, commercial airliners must be pressurized to provide enough air for normal breathing by passengers. The density of air at an altitude of 16 km (50,000 feet) is only about 13% of the density of air at the surface. The red curve in the top panel of Figure 6.01 shows a typical vertical profile of density from the ground to 60 km. The density of air near the top of the stratosphere is nearly zero.
2.1.2 Air Temperature Change With Altitude -- The temperature of the atmosphere at first decreases with altitude and then increases. Temperatures decrease with altitude in the troposphere, the region between the surface and about 11 km. This is the region where we live and where weather occurs. Temperatures are first steady and then increase in the stratosphere, the region of the atmosphere from 11 km to about 47 km. The black curve in the top panel of Figure 6.01 shows a typical vertical profile of temperature from the ground to 60 km. The temperature is given in degrees Kelvin, abbreviated K, where Kelvin temperature = Celsius temperature (°C) + 273. Thus, 273 K corresponds to 0°C (or 32°F).
Temperatures drop to just under 220 degrees Kelvin (-53°C or -63°F) at the top of the troposphere. Temperatures begin to rise in the stratosphere, though temperatures remain bitterly cold by most surface standards.
2.1.3 The Tropopause -- The troposphere is separated from the stratosphere by the tropopause. Shown in Figure 6.01 as the horizontal black line at 11 km, the tropopause is an important feature of the atmosphere, as it marks the region where the temperature structure changes. Below the tropopause, temperatures decrease with altitude, while above the tropopause, temperatures increase up to about 47 km, which marks the top of the stratosphere. The troposphere and stratosphere are thus defined by their vertical temperature structures.
It is commonly recognized that warm air rises, and cold air sinks. This is because warm air is less dense than cold air. A simple test of this is to put a layer of cold water (perhaps with a dye) on top of a layer of warm water. The layers will overturn and mix. In the troposphere (where we live and see our weather), it is almost always the case that colder air overlays warmer air. The warm air is heated at the surface and it rises up. Under the right conditions, water vapor condenses out of the air and we see large clouds that appear to boil upwards. This is a process known as convection. Thunderstorms form by such convection.
2.2.1 Static Stability -- Because temperature increases with altitude in the stratosphere, warmer air overlays colder air. As a result of this temperature structure, convection never happens in the stratosphere. If we could displace an air parcel to a higher altitude in the stratosphere, it would be colder than its surroundings. Cold air is more dense than warm air, and the parcel would sink back to its original location, though it would overshoot slightly because of its momentum. After overshooting, it would drop to a location where it would be warmer than its surroundings. Warm air is less dense than cold air, and the parcel would rise back to its original location, though it would once again overshoot slightly. This process would continue in a series of vertical oscillations about some equilibrium altitude where the parcel density and the surrounding air (ambient) density were the same.
Such up and down oscillations of air (like a bob on the end of a rubber band) are indeed observed in the atmosphere. In the stratosphere, this oscillation has a period of about 40 seconds. In the troposphere, the same sort of displacement has a period of about 70 seconds. The faster oscillation in the stratosphere occurs because of the fact that air in the stratosphere gets warmer with altitude. This means that the air has greater static stability or greater buoyancy in the stratosphere. The greater stability in the stratosphere is the reason why vertical motions of air are not easily accomplished there. We speak of the stratosphere as being "stably stratified".
2.2.2 Potential Temperature and Static Stability -- The stability is calculated from the vertical change of a quantity known as potential temperature. We discussed potential temperature in Chapter 2, Sections 3.3.2 and 5.2.1. Recall that potential temperature is defined as the temperature an air parcel would have if compressed adiabatically (i.e., without any heat being added or taken away, such as would happen if water vapor condensed out of the parcel) from its existing pressure to a reference pressure of 1000 millibars. The way potential temperature changes with height determines the static stability of the air. If potential temperature rises with height, the air is said to be stably stratified. If it falls with height, air is said to be unstably stratified. If it does not change with height, the air is said to be neutrally stratified.
2.2.3 Potential Temperature Profile in the Stratosphere -- In the stratosphere, potential temperature always rises with height. That is, the stratosphere is always stably stratified. Figure 6.01 Bottom Panel shows a typical potential temperature vertical profile.
In Figure 6.01, potential temperature is given in degrees Kelvin (K). We can see in the figure how potential temperature becomes quite large at higher altitudes in the stratosphere, reaching almost 400 K (127°C or 261°F) at 16 km altitude and 500 K (227°C or 441°F) at 20 km altitude. (Recall that this is the temperature the air at that level would have if compressed adiabatically to the 1000 mb).
2.2.4 Isentropic Surfaces and the Motion of Stratospheric Air -- If we choose a particular potential temperature, all of the air with this particular potential temperature will form a surface called an isentropic surface. In fact, potential temperature divided by 25 is about equal to the altitude in kilometers (i.e., 400 K = 400/25 = ~16 km and 500 K =500/25= 20 km). Because potential temperature becomes so large at higher altitudes in the stratosphere, it is difficult to move air upward or downward. Stratospheric air tends to remain on an isentropic surface for many days. Vertical motions are consequently very small.
2.2.5 Potential Temperature as a Vertical Coordinate -- Potential temperature is widely used as a vertical coordinate because air in the stratosphere tends to move along surfaces of constant potential temperature. The potential temperature of an air parcel is only changed by addition or removal of heat. This is known as a diabatic process, the opposite of an adiabatic process. Thus, the potential temperature of an air parcel remains about constant even if its temperature and pressure are changing.
Air is primarily composed molecular nitrogen and molecular oxygen, with an assortment of minor or trace gases, such as argon, carbon dioxide, water vapor, and ozone, as well as many others, making up the rest. A parcel of air contains about 78% nitrogen molecules (N2, molecular weight of 28), 21% oxygen molecules (O2, molecular weight of 32 kg/kmol), and the remaining 1% are the trace gases. From this basic composition, the apparent molecular weight of air is about 28.964 kg/kmol. Both molecular nitrogen and oxygen decrease with altitude at exactly the same rate as overall air density. This means that the composition of air is approximately the same in both the troposphere and the stratosphere. The relative amounts of nitrogen and oxygen (78% and 21%) persist up to about 120 km, where atmospheric pressure is a tiny fraction of that of surface pressure.
2.3.1 Turbulent Diffusion and the Homosphere -- While the molecular oxygen is heavier than molecular nitrogen, the two gases do not stratify in our atmosphere according to their weights. The gases don't stratify because parcels of air are thoroughly mixed into a uniform soup by wind currents, convection, and large-scale circulation patterns. These stirring processes are such that there is very little variation in the atmosphere for gases like nitrogen and oxygen. Such mixing is known as turbulent diffusion and it is very important from the surface up to about 120 km. This region is known as the homosphere, the region of the atmosphere where gases are uniformly mixed.
2.3.2 The Heterosphere -- Above 120 km, gases begin to stratify according to molecular weight. Air is so thin at this altitude that individual molecules are able to accelerate to high speeds before bumping into another molecule. The lighter gases accelerate more than the heavier gases, and as a result, the atmosphere begins to stratify according to their molecular weight. This region above 120 km is called the heterosphere, the region of the atmosphere where gases stratify according to their molecular weight.
2.3.3 Trace Gas Variability -- Many of the trace gases have variable concentrations. These include variations in both place and time. Trace gases variability can be due to a number of different reasons. Among the reasons for trace gas variability are phase change (e.g., water vapor changes to liquid water), chemical reaction (e.g., nitric acid is formed in the polar stratosphere when certain reactions occur, as shown in Chapter 11), creation by human activity (e.g., chlorofluorocarbons are developed), or photochemistry (e.g. ozone is created and destroyed by ultraviolet light from the sun). Each of these can cause trace gases to display large variations in their atmospheric concentrations.
We already know that the atmosphere is partitioned into distinct regions based on the temperature structure in the region. Temperatures fall with altitude in the troposphere. Temperatures rise with altitude in the stratosphere (see Figure 6.01). It is at the tropopause where the transition from decreasing temperature with altitude to increasing temperature with altitude occurs. The tropopause separates the troposphere from the stratosphere (see Chapter 2).
2.4.1 Position of the Tropopause -- The position of the tropopause varies with latitude. In the tropics, the tropopause is located at an altitude of about 16 km or 50,000 feet. This corresponds to the 380 K isentropic surface. In the polar regions, the tropopause is as low as 8 km or 30,000 feet. Figure 6.02 shows a color image of zonally-averaged January temperatures from the South Pole to the North Pole and between the surface and 48 km (158,000 feet), averaged from 1979-1998. The tropopause is superimposed on Figure 6.02 as the thick black line. The tropopause is highest in the tropics, and lowest at polar latitudes.
2.4.2 Regions of the Stratosphere -- The stratosphere itself can be divided into four distinct regions: (a) the tropics, which stretch from about 20°N to 20°S; (b) the middle latitudes or "surf zone," (c) the polar vortex; and (d) the lowermost stratosphere.
(a) The tropics -- The tropics is a region of the stratosphere that stretches from about 20°N to 20°S. It is here that ozone has its photochemical source region, since it is here that there is enough of the necessary highly energetic ultraviolet radiation from the Sun to create ozone. As we shall see in section 3, ozone is transported out of this region and poleward by a broad circulation pattern.
(b) The surf zone -- The middle latitudes of the stratosphere is known as the "surf zone." Much like surf on a beach is characterized by turbulent overturning and mixing of water, the stratospheric surf zone is analogously characterized by a turbulent looking mixture of air masses, each of which contain differing amounts of ozone. Because of the equator-to-pole circulation pattern, tropical air contains less ozone than polar air. As a result of weather systems in the middle latitudes, tropical (low ozone) and polar (high ozone) air are mixed together. This gives the surf zone its turbulently mixed appearance. (For a preview, look ahead at Figure 6.18, which shows the complicated structure of the surf zone.)
(c) The polar vortex -- In winter, stratospheric winds typically blow from west to east (referred to as the westerlies by meteorologists). As discussed in Chapter 2, a band of strong winds referred to as a jet stream sets up along the zone of greatest temperature change. In the stratosphere, this occurs in winter along the polar night terminator, the line that divides sunlight from the long polar night. (This occurs north of the Arctic Circle and south of the Antarctic Circle.) The jet stream that sets up here is called the polar night jet. It should not be confused with the polar jet stream, which together with the subtropical jet stream are features of the upper troposphere. The region poleward of the northern polar night jet is known as the Arctic polar vortex, while the region poleward of the southern polar night jet is known as the Antarctic polar vortex, which is a region of air isolated from the rest of the stratosphere where the long polar night allows extremely cold temperatures to develop. The degree of isolation, however, is quite different between the Arctic and Antarctic (see Chapter 11). Special conditions inside the more isolated Antarctic polar vortex allow human-produced chlorofluorocarbon (CFC) compounds to destroy ozone each spring, creating the "ozone hole" phenomenon (see Chapters 1, 5, 11).
Figure 6.02 shows the situation for January, the northern hemisphere winter and southern hemisphere summer. It reverses itself six months later. The wind patterns are indicated by the white lines on Figure 6.02, with jet streams indicated by the bold J's on the figure. Solid white lines indicate westerly winds, while dashed white lines indicate easterly winds. The (northern) polar night jet is located in the middle to upper stratosphere, with its core of fastest wind speeds around 32 kilometers.
We see in Figure 6.02 that in the summer hemisphere, the polar vortex has vanished. Stratospheric winds weaken and actually reverse direction, becoming easterly. This seasonal dynamical variability is discussed in Chapter 2, but it is related to the long period of sunlight over the summer pole (during the polar day) and the presence of ozone, which absorbs some of the solar energy and warms the region. This results in a reverse temperature gradient, and hence the winds reverse direction and become easterly.
(d) The lowermost stratosphere -- A special region of the stratosphere is known as the lowermost stratosphere. This part of the stratosphere contains a mixture of both tropospheric and stratospheric air. Air in the troposphere has a different chemical composition (or fingerprint) than air in the stratosphere. In the lowermost stratosphere region, we find a mixture of the two. The lowermost stratosphere is delineated on the bottom by the tropopause and at the top by the 380 K potential temperature surface (shown in Figure 6.02 as the dashed line). In the tropics, the lowermost stratosphere is separated on the bottom at the core of the subtropical jet stream.