Why has one a beneficial and the other a detrimental role in the atmosphere?
This essay is going to explain the difference between ground-level and stratospheric ozone; why has one a beneficial and the other a detrimental role in the atmosphere.
The troposphere is the lowest layer of the earth’s atmosphere (Figure 1). It starts at the earth’s surface extending to a height between 7 and 20km above sea level. The troposphere is the thickest part of the atmosphere in which around 75-80% of atmospheric mass is located. As altitude increases through the troposphere, temperature, air pressure and density of the air decrease. The troposphere is heated from the earth’s surface; sunlight warms the ground or ocean and in turn, heats the air around which rises into the troposphere. On average the temperature drops around 6.5 degrees Celsius per km as altitude increases. At the top of the stratosphere, temperatures can reach a low of -55 degrees Celsius. As shown in figure 1 there is a layer between the troposphere and the stratosphere this is called the Tropopause, a boundary between the two layers. The altitude of the Tropopause varies with latitude, season, weather and time of day. At mid level latitudes the typical height of the Tropopause is around 11km whilst near the poles it can drop to a low of 7km. The lowest part of the troposphere is known as the atmospheric boundary layer which is affected by the earth’s surface, an example of this can be the wind being affected by hitting a mountain. In the polar regions just below the Tropopause is a jet stream which is defined as ‘a belt of powerful upper-level winds that sits atop the polar front’ (Britannica 2010)
(Figure 1 – ACCU Weather 2009)http://global-warming.accuweather.com/stratosphere_diagram_sm-thumb.jpg
Ozone (O3) ‘is a form of oxygen containing 3 molecules, usually found in the stratosphere, and responsible for filtering the sun’s ultraviolet radiation’ (Null, J 2003). Figure 2 shows the levels on ozone present in the earth’s atmosphere.
(Figure 2 – NASA 1999)
File:Atmospheric ozone.svg
According to NASA ‘10% of ozone (O3) in the atmosphere is found in the troposphere.’ Ozone in the troposphere is unwanted. Ozone can be found in low concentrations at ground-level. Ground level ozone and stratospheric ozone are two key sources of natural ozone. Ground level ozone are hydrocarbons which are released by soil and plants, whereas stratospheric ozone which rarely migrates downwards to the earth’s surface. These two natural sources do not give enough ozone to be thought of as a threat to life and the environment.
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With growing populations, increased transport and high industry has increased the ozone levels in the lower atmosphere. Ozone cannot be traced by coming directly from one source as it is formed by contact between the suns UV rays and hydrocarbons and nitrogen oxides which can be traced back to transport, fossil fuels, power plants and other industries. It can be referred to as ozone pollution or Smog. In urban areas it’s known as photochemical smog
(Figure 3 – Environmental Science 2010)
http://apesnature.homestead.com/files/fg22_04b.jpg
Ozone is a key element traced in smog and as a powerful greenhouse gas.
Greenhouse gases are a part of the atmosphere; they are both natural and created from human activity. These gases emit radiation within range thermal infrared radiation which is emitted from the earth’s surface, the atmosphere and clouds. This causes the greenhouse effect. The greenhouse effect is when sunlight reaches the earth’s surface and is reflected back into the atmosphere, but it is prevented from escaping by the layer of greenhouse gasses, this in turn heats the atmosphere causing a rise in the earth’s surface temperature. The rise in temperature causes ice caps to melt, raising sea levels. The four major gases that contribute to the greenhouse effect and their percentage contribution are: ‘Water Vapour, 36-70%, Carbon Dioxide, 9-26%, Methane, 4-9% and Ozone, 3-7%’. (Kiehl J, 2003)
Ozone levels are found at its peak during the summer months and are usually highest in built up areas. Concentrations are at its highest during the afternoon as the UV rays have had time to react. This causes ozone pollution.
Ozone pollution has many negative effects on human life, vegetation and the environment. It can cause health problems such as chest infections, breathing problems, headaches, eye irritation and aggravate pre existing conditions such as asthma, heart disease, bronchitis and emphysema. Ozone pollution according to SEQL damages leaves and crops, which reduce crop production and quality, The United Nations Environment programme estimated a ‘loss of $8.56 billion worth in crops in the European Union in 2000′(UNEP 2007) Buildings and decorating materials can be weakened or damaged and paint can fade faster than anticipated.
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The stratosphere is the second layer of the atmosphere from the earth’s surface (Figure 1). The stratosphere rises to around 50km above sea level. The lowest part of the stratosphere varies depending on the latitude and the season. It differs at around 8 – 16km near the equator, around 10km at the mid latitudes and 8km near the poles. The air within the stratosphere is dry and contains small amounts of water vapour. The air within the stratosphere is around a thousand times thinner than that at sea level. Due the lack of vertical convection within the stratosphere, compounds can stay for long periods of time which breaks down the ozone layer.
About 90% of the ozone in the earth’s atmosphere is located in the stratosphere, which is between 15 and 50km above the earth’s surface. The ozone in the stratosphere forms a layer called the ‘Ozone Layer’. This is where oxygen (O2) and Ozone (O3) molecules absorb UV light from the sun protecting the earth’s surface from UV radiation. Both oxygen and ozone combined absorb between 95 – 99.9% of the suns UV radiation. Without them UV radiation such as UV-C and UV-B can cause skin cancer and damage eye and plant tissue. The ‘Ozone Layer’ is being destroyed by chemicals released by humans. An example of this is Chlorofluorocarbons (CFC’s) and the ‘Green House Gases’ which break down the ozone in the stratosphere.
Figure 4 that the level that the ozone is having on the stratosphere and the troposphere. It shows that the levels of ozone have been reduced in the stratosphere and the levels of ozone have increased.
(Figure 4 – Institute for Chemistry)
seperated ozone trends
To conclude, this essay shows that ground level ozone has a detrimental effect of the earth’s atmosphere as it causes health problems or aggravates pre-existing problems, damage to natural vegetation, crops and buildings and the environment. It is not ozone itself that damages the earth’s atmosphere but the interactions between the suns UV rays and hydrocarbons and nitrogen oxides within the troposphere. Whereas stratospheric ozone has a beneficial role to play in the atmosphere. Stratospheric ozone is beneficial to the earth’s atmosphere as it absorbs UV radiation which prevents health problems such as skin cancer.
Bibliography
Guy, P, John J, Geoffrey, S. (1999). Tropospheric Ozone. In Birks, W Atmospheric Chemistry and Global Change. 4th ed. Oxford: Oxford University Press 465-486
Guy, P, John J, Geoffrey, S. (1999). Middle Atmospheric Ozone. In Birks, W Atmospheric Chemistry and Global Change. 4th ed. Oxford: Oxford University Press. 487-513
NAS. (2001). Ozone Layer. Available: http://www.nas.nasa.gov/About/Education/Ozone/ozonelayer.html. Last accessed 02/03/2010
RSPB. (2001) The Greenhouse Effect. Available: www.rspb.org.uk/ourwork/teaching/resources/science/greenhouse_effect.asp Last accessed 3/03/2010
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