What is the ozone layer?
The ozone layer is 90% of the stratosphere, the second layer of the Earth’s atmosphere. About 90% of the ozone in Earth’s atmosphere is contained in the stratosphere. The ozone layer absorbs most of the sun’s ultraviolet radiation (UV), hence protecting the Earth from its harmful UV rays.
“Ozone holes”
“Ozone holes” are popular names for areas of damage to the ozone layer. This is inaccurate as ozone layer damage is more like a really thin patch than a hole. The ozone layer is thinnest near the poles. During mid 1980s, an “ozone hole” appeared over Antarctica. Scientists determined that the ‘hole’ was caused by humans due to man-made chemicals such as chlorofluorocarbons (CFCs) which interact with ozone and deplete the ozone layer.
How is ozone formed in the atmosphere?
Ozone (O3) is gas made up of three oxygen atoms, naturally produced in the stratosphere by a two-step reactive process.
In the first step, ultraviolet radiation (sunlight) breaks apart an oxygen molecule (O2) to form two separate oxygen atoms (O).
In the second step, each of these highly reactive oxygen atoms (O) combine with an oxygen molecule (O2) to produce an ozone molecule (O3).
In the overall process, three oxygen molecules (O2) plus sunlight react to form two ozone molecules (O3).
Ozone molecules are very effective at absorbing UV rays. Hence, the thin layer of ozone in the Earth’s atmosphere serves as a shield, protecting the Earth from harmful UV rays given out by the sun.
Ozone as a secondary pollutant
Ozone layer depletion results in the decrease in ozone in the stratosphere and an increase in ozone present in the lower atmosphere. The latter resultant is both a pollutant and a greenhouse gas and it contributes to global warming and climate change.
Hydrocarbons and stratospheric ozone are 2 major sources which contribute to the ground-level ozone by migrating to the Earth’s surface. Both plants and soil release the former source. The increased ozone is due to the increasing population, more automobiles, and more factories and industries. Tropospheric ozone is the product of the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides. Hydrocarbons and nitrogen oxides are emitted by automobiles, gasoline vapors, fossil fuel power plants, refineries, and certain other industries. Ozone in the troposphere is destroyed by naturally occurring chemical reactions and by reactions involving human-produced chemicals. Also, tropospheric ozone can be destroyed when ozone reacts with soils and plants on the surface of Earth.
There are many other substances that lead to ozone layer depletion such as hydrochlorofluorocarbons (HCFCs) and volatile organic compounds (VOCs). These ozone depleting substances are found in vehicular emissions, by-products of industrial processes, aerosols, and refrigerants and is broken down upon being exposed to the ultra violet rays in the stratosphere. This results in the release of free chlorine atoms which reacts with the ozone gas, and hence, leading to the depletion of the ozone layer.
How CFCs destroy the ozone layer
Man-made CFCs are the main cause of stratospheric ozone depletion. CFCs have a lifetime of about 20 to 100 years, and consequently one free chlorine atom (Cl) from a CFC molecule can do a lot of damage, destroying ozone molecules for a long time.
- When UV rays hit a CFC molecule, it causes one chlorine atom (Cl) to break away.
- The free chlorine atom (Cl) then hits an ozone molecule (O3) and takes one oxygen atom (O) away.
- A chlorine monoxide molecule (ClO) is formed. The ozone molecule (O3) is destroyed, becoming an oxygen molecule (O2).
- When a free oxygen atom (O) hits the chlorine monoxide molecule (ClO), the two oxygen atoms form an oxygen molecule (O2).
- The chlorine atom is released and now free to destroy more ozone.
References
http://www.nationalgeographic.org/encyclopedia/ozone-layer/
http://climate.ncsu.edu/edu/k12/.ozonelayer
https://www.esrl.noaa.gov/csd/assessments/ozone/2010/twentyquestions/Q2.pdf
http://www.theozonehole.com/ozonecreation.htm
http://www.7continents5oceans.com/ozone-layer-depletion-causes-effects-and-solutions