People often associate ozone as being a layer in the stratosphere, which absorbs dangerous UV rays that can cause cancer and that we are, by burning fossil fuels and releasing gases into the air with unknown consequences, creating a hole in this layer. This stratospheric layer is thought of as “good ozone”. But ozone can exist at ground level as well which is thought of as “bad ozone”. This ground level (or troposhperic) ozone has the exact same chemical make up as stratospheric ozone, which is hazardous to human health and so at ground level, can become very dangerous (Brimblecombe and Maynard, 2001). Another more common name for this form of pollution is smog, a word that brings harrowing memories of the 4000 deaths attributed to smog in the 1952 episode.

This essay will attempt to analyse the formation of troposhperic ozone pollution through time and space along with its variables and a number of case studies to illustrate the causes and effects of such events.

Firstly, what exactly is ozone? It is an odourless, colourless gas that, as has already been established, occurs in two layers of the atmosphere. The stratosphere (approximately 9 to 18 miles up) and the troposphere (ground level to 9 miles up) ( Unlike stratospheric ozone, tropospheric ozone is not naturally created. It has the same chemical structure (O3) but can sometimes undergo photochemical reactions between oxides of nitrogen (NOx) and Volatile Organic Compounds (VOCs) in the presence of sunlight. The reaction for ozone formation in the stratoshere is: –

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O2 + UV light O + O; O2 + O O3

As a result of this, almost no Ultra Violet rays from the sun pass through. (Briggs et al. 1997)

However, VOCs + Nox + Sunlight = Ozone in the troposphere ( Ozone is continually being created and destroyed by the troposphere by reactions involving oxygen molecules and ultra violet light. The rate of synthesis is greatly increased by the presence of high concentrations of NOx and VOCs (Seaton, 1994).

NOx is an all round term for a group of gases all containing Nitrogen and Oxygen but in different amounts. These gases are found to come from source such as car exhausts and commercial, industrial and residential sources that burn fuels. This alone can cause serious risks to human health. High levels may be fatal, while lower levels affect the delicate structure of lung tissue. In experimental animals this leads to a lung disease that resembles emphysema in humans. As with ozone, long-term exposure to nitrogen oxides makes animals more susceptible to respiratory infections. Nitrogen dioxide exposure lowers the resistance of animals to such diseases as pneumonia and influenza. Humans exposed to high concentrations suffer lung irritation and potentially lung damage. Increased respiratory disease has been associated with lower level exposures. (

VOCs are found in many household products. Sources such as cosmetic, cleaning, varnishing and degreasing products all contain VOCs. Some of these sources are safe for handling where as some of them are highly toxic ( VOCs also occur naturally. Trees emit small amounts of VOCs in to the atmosphere. Exposure to elevated levels of VOCs may cause irritation to the eyes, nose, and throat. Headaches, nausea, and nerve problems can also occur. Some people do not appear to have any kind of reaction to fairly “low” amounts of VOCs, while other people are fairly sensitive. Studies of animals have shown that breathing some types of VOCs over a long period of time can increase the risk of getting cancer.

Ground level ozone is thought to be more of an urban problem as there are more sources of NOx and VOCs in urban areas but this is not necessarily the case. It has been known for winds to carry ozone and the pollutants that form it hundreds of miles from its original source toxic ( This process is called transport. But even if there are NOx and VOCs present, tropospheric ozone needs the right weather conditions to form.

The largest and undoubtedly most famous smog event was the 1952 smog in London. The weather in London on December 4th 1952 was anti-cyclonic ( and a fog began to form. It had also been colder than average and people had been burning more fuel ( all the ingredients for a disastrous smog. The smog lasted for five days and is said to have killed anywhere between 4000 and 12,000 people. The graph below shows deaths that occurred per day against the smoke and sulphur dioxide rates. As it can be seen from the graph, even after the smog had cleared, people were continuing to die from diseases such as pneumonia and bronchitis. The great London smog led to the establishment of the 1956 clean air act. Although it didn’t pun an immediate end to smog events, it pushed down the levels of emissions and now the thick “pea-soup” smog events of the past no longer occur in London.

Another place in particular that suffers heavily from ground level ozone pollution is Ontario, Canada. Smog events in Ontario can blanket large areas, both urban and rural for several days and in the summer months; ozone concentrations often greatly exceed the maximum acceptable air quality in the most populated areas. (Fuentes JD and Dann TF). The pollutants that cause these episodes are transport from Ohio in the USA and this greatly contributes to the ozone levels and smog events. The majority of adverse health effects experienced by people living in Ontario are caused by exposure to ground level ozone pollution (

Fuentes and Dann conducted a study in Canada to research concentrations ground level ozone over many different sites in the summertime when these ozone events occur most frequently. They found that there was no particular pattern for the eastern sites in Canada. In other places however they received significant results. In Montreal there seemed to be a very downward trend, which was associated with a fluctuations in nitric oxide concentrations. Whereas in Toronto, there appeared to be a significant upward trend, again associated with fluctuations in concentrations of nitric oxide.

Lin et al studied the spatial variation of ground level ozone concentrations in three areas in Taiwan, one an air quality management district, another a neighbourhood region and the final a field of an elementary school surrounded by high buildings. The results gained on this study showed to be highly non-uniform and that there was a great deal of day to day variability in the spatial distribution of ground level ozone.

In Vermont, the Environmental Protection Agency offer a service called “Smog Alert,” which automatically notifies participants by e-mail when high concentrations of ground-level ozone have been predicted for their area. Smog Alerts are issued throughout the summer smog season, which lasts from May until September. This service is offered ac there are a large number of adults and children spending large amounts of time outdoors in the summer and if they suffer from respiratory diseases such as asthma, exposure to elevated ozone levels can cause serious breathing problems (

Exposure to tropospheric ozone can cause numerous other health risks such as temporary decrease in lung capacity, inflammation of the lung tissue, impairs the bodies immune system leaving people more susceptible to diseases such as bronchitis and pneumonia.

There is a very complex relationship in terms of the spatial and temporal dimensions of the formation of ground level ozone pollution. The right ingredients do not necessarily have to be present in the same place as they can be transported by the wind to cause problems in other areas as in the Ontario case. There is one major limit however and that is that smog can only form in times when sunlight is present because the reaction that occurs between Nox and VOCs is photochemical. This is why these events are more likely to occur in the summer months. In a study conducted by Shaw and Griffis, they attempted to understand the relationship between ground level ozone and its adverse effects upon humans and crops from a meteorological perspective. They found that the maximum daily temperature is the single most important variable and that it was this that accounted for the largest percentage of variance.

It is very difficult to completely eradicate this problem due to the transport factor. In London it was simpler as it was London’s pollution that was causing the problems. In Ontario, it is US pollution that is being transported to cause problems and so some kind of international agreement needs to be made. This has shown in the past not to be as easy as it sounds. The US at the moment sees their economic wealth and growth to be at the forefront of their policies with the environment taking a back seat. The chances it seems that they would change their policy for another country appears to be very slim.


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