The Ozone Layer

Importance Ozone, O 3, shields the earth’s surface from biologically harmful UV-B radiation, which damages the genetic information in the DNA of all living cells. In humans, UV-B radiation causes sunburn, can eventually lead to skin cancer, and may suppress the normal functioning of the immune system. Only a trace of the gas is sufficient.

Location 90% of the world’s O 3 is high above the surface in the stratosphere, where its concentration is 4 parts per million. Here nearly all of the sun’s UV-B radiation is absorbed by the ozone molecules, effectively filtering out most of the harmful rays so they do not reach the lower troposphere and earth surface.

Ultraviolet (UV) Radiation The sun emits light energy mostly in a broad band of wavelengths from 400 to 700 nm, which is the visible spectrum our eyes detect. A small portion of sunlight energy is carried by photons of shorter wavelengths (<400 nm), called Ultraviolet light. UV-A radiation ranges from 400 to 320 nm, UV-B ranges from 320 to 280 nm, and UV-C is <280 nm. Photons of UV light have more energy than those of visible light, thus causing more damage to cells. UV-A is not filtered out as it passes through the atmosphere to the surface, but DNA is not sensitive to UV-A. Harmful UV-C is totally absorbed by O 2 molecules, which make up 21% of the air in the atmosphere.

Ozone Stability Ozone is kept at a steady state concentration in the stratosphere by equal rates of reactions for its formation and destruction. But now air pollutants, mainly CFCs, or chlorofluorocarbons (produced for roughly 60 years), have accelerated ozone destruction, effectively depleting stratospheric ozone concentrations.

Ozone Formation Ozone formation involves gaseous oxygen (O 2 ) and ultraviolet radiation from the sun. First: O 2 + UV-C → O + O Then: O + O 2 → O 3

Natural Ozone Destruction Ozone destruction occurs naturally a few different ways: O 3 + UV-B → O 2 + O O 3 + O → 2O 2 O 3 + NO → NO 2 + O 2

Destruction of Ozone by Pollutants Pollutant X + O 3 → XO + O 2 Then X is regenerated back into the atmosphere by the following reaction, perpetuating the cycle until X is removed by a side reaction. XO + O → X + O 2 Cl (from CFCl 3 ) and NO (from car exhaust) are some examples of pollutants that are causing ozone depletion.

Antarctic Ozone Hole In September and October, during the first few weeks of spring following the six month darkness of the Antarctic night, an enormous gap in the stratospheric ozone layer appears above the Antarctic continent and the Southern Ocean, including at least one city – Ushuaia, Argentina (in Patagonia). Scientists blame this effect on CFCs

NASA’s Goddard Space Flight Center used a mapping spectrometer to visualize the hole in the ozone layer.

Why Over Antarctica? During the sunless winter, nitric acid vapor (HNO 3 ), which helps to lessen the amount of ozone destroying Cl in the air when there is sunlight, is frozen in polar stratospheric clouds and catalyzes the formation of chlorine gas. ClONO 2 + HCl → Cl 2 (gas) + HNO 3 (ice) Because the Antarctic air is colder than anywhere on earth, the clouds form at much higher altitudes. Strong winds form a vortex, isolating the clouds from air of lower altitudes until spring.

Spring Sunrise The UV-A radiation breaks the Cl 2 into individual Cl atoms that, because there is little or no HNO 3 vapor to react with, begin the cycle of ozone destroying reactions until virtually all of the ozone inside the vortex has been used up (the hole seen in the 3-D image). Gradually the vortex looses its integrity and in mid-November the hole breaks up and mixes with the air of the rest of the world, further diluting the world’s ozone concentration as the Antarctic ozone void is filled.

Overall Effects In closing, studies have shown that the stratospheric ozone layer thinned by 10% from the early 1950’s to the early 1990’s. For every 1% decrease in ozone, there is a corresponding 2% increase in the amount of ultraviolet radiation that reaches the surface of the earth and a 4% increase in skin cancer. So it is evident that this is one of the more pertinent issues that we will face in the near future.