What is the Antarctic ozone hole?
In the previous 10 chapters of this online textbook on stratospheric ozone, we prepared the groundwork to discuss Antarctic ozone losses and to answer this question. Chapter 1 introduced stratospheric ozone and had a short, preliminary discussion of the ozone hole. Chapter 2 provided us with a broad overview of the atmosphere -- its composition, its temperature structure, and its general wind circulation patterns. Chapter 3 introduced us to a few ways of measuring ozone and what the different records show. Chapters 4, 5, and 6 dealt with the basic radiative physics, photochemical processes, and dynamical motions of the stratosphere, respectively. Chapter 7 discussed various ozone measurement techniques by actual instruments in use today. Chapters 8 and 9 detailed variability and trends in stratospheric ozone. Chapter 10 examined stratospheric pollution by different sorts of gases in the stratosphere, including nitrogen oxides.
All of these chapters have together provided the scientific background and theoretical framework for understanding how and why stratospheric ozone is being depleted. This depletion is caused by manmade chemical compounds, the chlorofluorocarbons or CFC's, developed in this century. Depletion of stratospheric ozone is symbolized most dramatically in the sudden, seasonal loss of ozone over Antarctica. Ozone amounts fall by half in a few weeks. This large ozone loss is called the "Antarctic ozone hole." Arguably the most important ecological problem facing the world today, the Antarctic ozone hole is the topic of this chapter.
This chapter investigates the data that support our knowledge of the Antarctic ozone hole. It also examines the special atmospheric factors that contribute to the development of the ozone hole in the context of current scientific theories for its formation. At the end of the chapter, we will also look at the trend in the recent data for the Arctic region to see if any evidence exists for similar, dramatic, seasonal losses of ozone there.
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The Antarctic ozone hole is a region of extreme ozone loss that has been appearing annually since the 1970s. Ozone amounts over Antarctica drop dramatically in the course of a few weeks. The "hole" begins to develop each August and culminates by early October, corresponding to the arrival of spring in the southern hemisphere, and it subsequently disappears by early December. Total ozone amounts in this period fall by up to 50% inside the hole. Because we have ozone records stretching back to the late 1950s for Antarctica, we can compare ozone amounts for the same period from then and now. Once again, we find that ozone amounts during this period fall by about half compared to amounts measured during the same time of year as recently as the early 1970s. Observations taken in the Antarctic region from aircraft, the ground, and satellites have demonstrated that the ozone hole results from the increased amounts of chlorine and bromine in the stratosphere, combined with the peculiar meteorology of the southern hemisphere winter.
The discovery of the Antarctic ozone hole in the mid-1980's was a big surprise to atmospheric scientists who had until then thought they understood the physical and photochemical processes which controlled stratospheric ozone. Research as late as 1983 had produced reasonable computer model simulations of ozone with respect to observed measurements. The 1985 announcement of large losses springtime total ozone over Antartica between 1975-1984, stunned the atmospheric science community. Ozone amounts had decreased between the measurements taken back in the late 1950s and early 1960s from over 300 Dobson Units to 200 Dobson Units.
Why and how this was happening became an area of intense research. Early on, it was recognized that the dramatic drop in ozone was probably the result of some combination of the extremely cold temperatures over Antarctica inside the polar vortex region (see Chapter 6) and an increasing level of chlorine.
Measurements by the TOMS instrument aboard the Nimbus-7 satellite showed that the depletion of ozone during the southern hemisphere spring occurred over the entire Antarctic continent, centered on the South Pole. Because of the visual appearance of this Antarctic low ozone region, the phenomena was quickly dubbed the "Antarctic ozone hole."
Various theories were proposed to account for the existence of the "hole." These included the dynamical theory, the nitrogen oxide theory, and the heterogeneous chemistry theory. The dynamical theory proposed that the atmospheric circulation over Antarctica had changed in such a way that air from the troposphere, where there is little ozone, was being carried into the polar lower stratosphere, and hence the observed reductions. The nitrogen oxide theory proposed that increased levels of NOx produced by the photochemical effects of a sunspot peak period in 1979 were responsible for destroying more ozone. The heterogeneous chemistry theory proposed that reactions were occurring on the surfaces of tiny cloud particles that form in the extremely cold conditions of the Antarctic winter stratosphere. The "surfaces" provided by the cloud particles, known as polar stratospheric clouds (PSC's), were altering the polar stratospheric chemistry. (See Section 5.1 of this chapter for more detailed information on PSCs.) The compounds formed by the reactions on these PSCs allowed nonreactive compounds containing chlorine to become reactive compounds. These reactive chlorine compounds catalytically destroyed ozone at an extremely rapid rate, the particulars of which are discussed at length in Chapter 5. In addition to chlorine compounds, bromine compounds also partcipate in catalytic destruction of ozone.
Measurements over Antarctica have shown that the heterogeneous chemistry theory is correct. Antarctic ozone loss was caused by the heterogeneous reactions of chlorine compounds on the surfaces of polar stratospheric clouds. Once the chlorine is freed by these heterogeneous reactions, the weak levels of sunlight initiate and maintain the catalytic ozone loss photochemistry. The chlorine compounds are principally of manmade origin: the chlorofluorocarbons (CFCs) we discussed in Chapter 1. These were the safe, inert compounds developed in the late 1920s for refrigeration and aerosol propellants. CFCs gained enormous usage worldwide since their creation. Measurement of extremely high levels of chlorine monoxide over Antarctica was the "smoking gun" that provided clear evidence that the CFCs were the culprit behind these ozone losses.
The answer to the question of why do temperatures get so cold inside the stratosphere above Antartica, as opposed to elsewhere, and allow PSCs to form involves the unique circulation pattern there. First, you need sunshine (actually UV light) to heat the stratosphere. Since the Antarctic stratosphere is dark during polar night, there isn't any heating of the polar stratosphere by the Sun. The Antarctic stratosphere cools off by emitting IR radiation to space, just like an electric stove element that cools from red hot after you turn off the stove. Second, the weather systems in the stratosphere warm the polar regions. During the southern winter these stratospheric weather systems are very weak, and there is nothing to heat the Antarctic stratosphere. Hence, because of the IR cooling and weak weather systems, the polar stratosphere gets very, very cold.
A second aspect of these very cold temperatures is something we've discussed in previous chapters, the polar night jet stream (see Chapter 6). The polar night jet is a ribbon of fast moving winds that develops near the edge of the polar night and the cold temperatures. Wind speeds reach 100 mph or greater at 70,000 feet. This jet results from the same two processes that give us the very cold polar region, weak weather systems and infrared cooling. We also tend to think of this jet as a "vortex" of air that swirls west to east around the South Pole. The jet also acts as a barrier to transport of air between the south polar region and the southern midlatitudes.
The reactions required for ozone loss ultimately also involve sunlight, so it is necessary for there to be sunlight, as well as extremely cold temperatures with polar stratospheric clouds (PSCs) Such conditions exist for a few weeks of the year in September and early October, at the start of the southern spring. By December, conditions are too warm for PSCs and also the strong polar vortex breaks down as temperature differences become less. The warming temperatures in spring, and the breakup of the polar vortex shuts down the rapid ozone-destroying reactions that occur via heterogeneous chemistry.
This chapter explores the topics associated with polar stratospheric ozone loss in more detail. We begin with observations of ozone and the early theories for the existence of the Antarctic ozone hole (Section 2). Observations of Antarctic ozone and the discovery of the seasonal ozone hole phenomenon are our first topic of discussion (Section 2.1). We next turn to the theories behind the existence of this Antarctic ozone hole (Section 2.2). These include the dynamical theory (2.2.1), the nitrogen oxide theory (2.2.2), and the heterogeneous chemistry theory (2.2.3).
The next section (3) explains aspects of the Antarctic polar vortex, the setting in which the ozone hole occurs. Aspects include wind circulation around the vortex (3.1); temperatures inside the vortex (3.2); potential vorticity and how it is used to identify the polar vortex (3.3); radiative heating and cooling processes within the vortex (3.4); and transport of air parcels inside the vortex (3.5).
We then turn to the structure and dynamics of the Antarctic ozone hole (Section 4). This includes a discussion of horizontal (4.1) and vertical (4.2) structure of the ozone field; the annual or seasonal cycle in ozone over Antarctica (4.3); the breakup of the ozone hole (4.4); and a summary (4.5).
Our next topic revisits ozone hole theory (Section 5), focusing on aspects of the heteorogeneous chemistry theory from before. A key ingredient required for the ozone loss reactions to occur are polar stratospheric clouds (5.1). Next we review the key heterogeneous reactions (5.2) and the actual catalytic loss reactions for ozone (5.3). The evolution of the ozone hole through a season is our next topic (5.4).
This brings us to the topic of Arctic ozone and the search for any possible hole there (Section 6).We review the Arctic record from 1988 to 1995 (6.1) and 1996 to 1998 (6.2). We explore differences between the Arctic and Antarctic stratosphere to find the reasons for the differences in ozone amount between the two locations (6.3). The question here is: why is there no ozone hole over the Arctic? We finish the section by looking at future prospects for Arctic ozone (6.4) and for the Antarctic ozone hole phenomenon (6.5) in the context of expected change in atmospheric chlorine content and temperature changes produced by the atmospheric greenhouse effect, both of which are related to human activity.
We then summarize the necessary conditions required for and the steps involved in the ozone hole and provide review questions for your use in the classroom.