Ozone is a chemical compound containing three oxygen atoms, that is one more than the gas we breathe. Though ever prevalent and important to earth’s atmosphere, it was not discovered until 1839 by Christian Friedrich Schonbein.
Ozone can actually be harmful to humans when present in high concentrations in the troposphere, the part of the atmosphere where humans live. The air we breathe contains about 78% nitrogen, 21% oxygen, and 1% other gases, like carbon dioxide. The toxic gas that is ozone is usually less that one part per million.
Ozone In the Atmosphere
The exact amount was first measured in 1918 by Robert Strutt by using a hydrogen lamp. The data in his studies showed an uneven distribution of ozone within the atmosphere, meaning it was lower at the surface of the earth. To further investigate this, Fabry and Buisson turned to spectrographic techniques. They argued their data suggested that ozone was predominantly found in the stratosphere, about 31 miles above Earth’s surface. This was not fully determined until the 1930s when the vertical distribution of ozone in the atmosphere was measured.
Today, the ozone column is measured by an ozone spectrophotometer that was first built by meteorology professor GMB Dobson in the 1920s. To date, about 120 of these are in production. The spectrophotometer measures ozone concentration by comparing the intensities of two types of ultraviolet light.
The sun’s radiation extends to all parts of the electromagnetic spectrum, including about 7% ultraviolet light, 41% in the visible range, and 52% in the infrared spectrum. Ultraviolet light may be further categorized into aging UV-A, cancerous UV-B, and ozone-blocked UV-C.
The photodissociation, or free radical annihilation, of an oxygen atom, turns life-giving oxygen gas into ultraviolet blocking ozone. This occurs in the stratosphere, as it requires wavelengths of radiation shorter than 240nm. Though ozone protects the stratosphere from the sun’s radiation, it too can breakdown under high energy. When it does, it simply goes through the photodissociation process again and reforms. Less high energy radiation penetrates the lower stratosphere, meaning higher concentrations of ozone may be found. Scientists are able to track the mixing of those ozone patches to model air patterns.
Though in low concentration, the ozone in the atmosphere absorbs a great deal of radiation, including the ultraviolet Hartley band (200 – 300 nm), the ultraviolet Huggins band (300 – 350 nm), and the visible Chappuis band (440 – 740 nm).
Generally speaking, ozone concentration holds stead at 300 Dobson Units or 200 milli-atmosphere-centimeters. This amounts to a 3 millimeter thick layer of ozone, spread across an atmospheric column in the highest density at the troposphere. This is what the ozone layer refers to.
Chloro-fluoro-carbons or CFCs and other ozone-damaging chemicals have been exponentially introduced into the atmosphere over the last half-century. Many times this can be harmless due to slow reaction times or regeneration of ozone. This is not the case for the atmosphere above Antarctica, where a large hole unveils every spring due to its unusual winter properties. While the Montreal Protocol, an international treaty, was drawn to control the use of these chemicals, much of the damage has already been done.
Antarctica contains this ozone hole due to its unique weather patterns. Ozone breaks down so quickly in what is called Stratosphere Clouds of Mother of Peart, or PSCs. In them, the chlorine or bromine from ozone-damaging chemicals are transformed into free-radicals, an especially reactive from that destroys ozone. These PSCs can only form in very cold conditions, like those seen during Antarctic winters. During this time, rapid ozone loss is seen, such that a hole appears in the springtime.
On an important side note, this same process does not occur in the Arctic. The northern hemisphere exhibits different weather patterns than its southern counterpart. It sends up warmer air closer to winter, resulting in the Artic being on average ten degrees warmer than the Antarctic in the winter.
The Recovery of the Ozone Layer in Antarctica
The big concern is the progress of the Antarctic ozone hole’s recovery. The media have been reporting some improvement, though there seems to be a lot of sensational speculation to that end. They seem to arise from surface measurements, rather than stratospheric ones. At surface monitoring stations, the concentration of ozone-damaging chemicals has dropped. Now, those chemicals are 6% lower at the surface than the peak in 1994. While this may be worth celebrating, the bigger issue is the stratosphere. It is there where the toxic PSC’s form that burns through the ozone layer over Antarctica.
Despite the progress being made at the surface, the ozone-damaging chemicals remain ever-present in the stratosphere. Their concentrations have seen no relevant decline over the last half-century.
Still, there is some room for encouragement. The depletion of ozone in the upper portion of the stratosphere does appear to be slowing down based upon satellite measurements. Again though, this small success is tempered by the fact that the overall ozone depletion in the stratosphere is not slowing down.
In 2002, the relatively small size of the ozone hole over Antarctica was not attributed to a decrease in ozone-damaging chemicals. Moreover, to say that the ozone hole is in recovery will require more than a decade of evidence. For this to happen the current decline in ozone-damaging chemicals must continue worldwide without interruption. More to the point, other events, like greenhouse gas emitting volcanic eruption or Tunguska-like event cannot occur. Assuming all of that is maintained, then by the middle of the 2000s the ozone hole over Antarctica may disappear. Though exciting, the brief decrease in the size of 2002 was just an outlier brought about by extreme weather. This is further supported by the extremely large ozone hole the Antarctic saw just a year later in 2003.
How the Ozone Hole is Impacted by Global Warming
Though the ozone hole and global warming are separate climate-related issues, they do have several similarities. First of these are the ozone-damaging chemicals, like CFCs. These chemicals, of course, increase the size of the ozone hole by destroying the ozone in the Antarctica stratosphere. They, too, are greenhouse gases and help drive up the average global temperature each year. The increased use and resulting concentration of greenhouse gases, such as carbon dioxide have likely resulted in an advanced global warming. The most auspicious connection between the two is how global warming affects the ozone hole. As the global temperature rises, the stratosphere cools, as energy conservation must be maintained in nature. As a result of this heat exchange, more dangerous PSCs may form to further tare away at the ozone layer.
What would happen if these ozone-damaging clouds found their way outside of the Antarctic winter? Well, they have. In the United Kingdom, these stratospheric clouds appear just around sunset and sunrise in the later, less dry months of winter. In particular, a stunning view came to be during the evening of February 16, 1996, in the United Kingdom. At an altitude between six to 19 miles high, these clouds are formed in a specific region of the stratosphere. To generate them, the atmospheric temperature must fall below -80°C. In them, scientists speculate, is ice crystal having a liquid coating of extremely corrosive nitric acid. These clouds can be beautiful to look at due to their luminosity. Well after sunset they are able to reflect back the sun’s light, leading to an intense pastel sky. This occurs due to various light diffraction, like rainbows and interference effects, like a film of oil floating on water. They may be seen in nearby Scotland during winter, as well.