How do Chlorofluorocarbons Affect Ozone Levels?

The production and destruction of ozone in the stratosphere are nearly in balance, thus we say ozone is in a steady state. The steady state means that for every ozone molecule that is destroyed, one is produced. This steady state is imbalanced by chlorine chemistry that increases the loss rate of ozone and leads to a lower steady state level of ozone. Most of the chlorine present in the stratosphere is the result of chlorofluorocarbons (CFC's) produced by humans. CFC's were used heavily as a coolant in refrigerators and air conditioners, as a propellant in aerosol cans, and other uses from the 1940's to the 1980's. The production of CFC's was banned in developed countries in 1995. CFC's were considered safe because CFC's are inert or non-reactive. While it is true that CFC's are inert in the troposphere, they are not inert in the stratosphere. In the presence of high energy UV light in the upper stratosphere, the carbon-chlorine bond is broken. This produces a free chlorine atom (Cl) that reacts with an ozone molecule (O3) to form chlorine monoxide (ClO). The chlorine monoxide then reacts with an oxygen atom (O) to form an oxygen molecule (O2) leaving the chlorine atom free once again to destroy another ozone molecule. A chlorine atom can remain in the stratosphere for many years and destroy many ozone molecules. One chlorine atom can destroy up to 100,000 ozone molecules. The steady state, or the balance between production and loss of ozone, has been disrupted by the presence of chlorine in the stratosphere. The reaction is shown below.

CF2Cl2* + UV => CF2Cl + Cl

Cl + O3 => ClO + O2

ClO + O => O2 + Cl

* There are other formula's for chlorofluorocarbon. The formula shown here is CFC-12.

The animation illustrates how one chlorine atom in the stratosphere can destroy up to 100,000 ozone molecules.

 

The most obvious effect of this chlorine chemistry is the "ozone hole," which opens over Antarctica in the Southern hemisphere's spring (which is fall in the Northern hemisphere). Check out NASA's data on the "ozone hole" and a top story from NASA about the Antarctic "ozone hole".

In the Northern hemisphere, different weather conditions in the stratosphere prevent the formation of an obvious ozone hole, but chlorine and other anthropogenic (man made) gases may be affecting ozone levels. Therefore, we need to monitor and study ozone at high-latitude sites such as Fairbanks. Check out articles about Arctic ozone depletion at NOAA and NASA.


How is Ultraviolet Light Classified?

Ultraviolet (UV) light is high energy light that is not visible to humans. Ultraviolet light is classified into 3 groups, UV-A, UV-B, and UV-C. UV-A is lower energy ultraviolet light with a wavelength between 320 and 400 nm. Only about 5% of the UV-A light is absorbed by ozone. UV-B light is a higher energy light with a wavelength between 290 and 320 nm. Ozone absorbs most of the UV-B light before it reaches the Earth's surface. It is a concern that if ozone levels become too low and more UV-B light was to reach the surface of the Earth, damage to living cells could lead to serious health issues, such as higher rates of skin cancer. UV-C light is very high energy light with a wavelength between 200 and 290 nm. UV-C light would be detrimental to plants and animal if it was able to reach the Earth's surface, but ozone blocks 100% of UV-C light. UV-C light also photolyzes O2 to produce O3.

 

This illustration shows how the different groups of UV light are absorbed by ozone in the stratosphere before reaching the surface of the Earth.

Why do Ozone Levels Change Seasonally?

There are short term and long term influences that effect column levels of the ozone layer. Short term fluctuations in ozone include temperature, high and low pressure systems, solar UV variations, and seasonal changes. Long term effects on ozone levels include solar maximum and minimums, and chlorofluorocarbons (CFC's). Seasonal changes in ozone levels follow long term patterns. Most ozone is produced over the tropics near the equator. During the winter, stratospheric winds carry ozone to middle and upper latitudes. Because the sun is low in the sky during the winter in higher latitudes, UV light is not as intense. Ozone is not destroyed by UV light as quickly during the winter months, so ozone builds up during the winter to its highest levels in the spring. In the summer, with the sun higher in the sky and UV light more intense, ozone is destroyed at a faster rate. Ozone is destroyed during the summer and reaches its lowest levels in the fall. Below is a figure that shows ozone levels in Fairbanks from 1996 to 2000. Notice how the ozone levels fluctuate with the seasons, but follows the same pattern each year. There are areas on the graph where data is missing. This is because, in Fairbanks, during the months of November through February, the sun is too low in the sky to take ozone observations. The ozone measurements were taken with a Dobson Spectrophotometer.

Ozone observations taken with a Dobson Spectrophotometer from 1996 to 2000 in Fairbanks, Alaska. Data is missing during the late fall and early winter because the sun is too low in the sky to take observations in Fairbanks. Ozone levels change with seasonal changes, but follow similar patterns every year.