The Truth Behind Ozone
This article presents the applications of ozone monitoring.
What is ozone? How does it impact air quality? Why is it important to monitor ozone concentrations? What are the industrial applications of ozone?
Ozone (O3) is a triatomic molecule consisting of three oxygen atoms. It is a colorless gas with a pungent smell.
Ozone at ground level – in the air we breathe – is an air pollutant and is not to be confused with the ozone layer in the upper atmosphere. Ozone in the upper atmosphere filters potentially damaging ultraviolet light from reaching the Earth's surface.
In the troposphere, near the Earth's surface, human activities lead to ozone concentrations several times higher than the natural background level.
...And in the afternoon, because the formation of ozone needs sunlight:
The highest concentrations of ozone are not always found in city centers where the pollutants that form ozone are emitted. The reason is that where there is an abundance of nitrogen oxide from traffic, ozone formation is suppressed. As a result, its concentration is sometimes low in busy urban centers. Ozone may also be transported by the wind over distances of 400-500 km a day. Hence, ozone pollution may eventually be found in suburban and rural areas far away from the sources of the pollutants. So, ozone levels in suburban areas can actually be higher than in urban centers. When ozone mixes with other air pollutants
like particulate matter (PM) and nitrogen oxides (NOx), it can form harmful smog.
Ozone impact on air quality
Ozone is the main product of complex photochemical processes in the lower atmosphere involving oxides of nitrogen and volatile organic compounds as precursors. Ozone is a strong photochemical oxidant. The main sectors that emit ozone precursors are road transport, power and heat generation plants, household (heating), industry, and petrol storage and distribution. The dominant issues determining short-term air quality are nitrogen dioxide, particulate matter and ozone. All three pollutants are strongly related to the use of fossil fuels. Ground-level
ozone (O3), unlike other pollutants mentioned, is not emitted directly into the atmosphere, but is a secondary pollutant produced by reaction
between nitrogen dioxide (NO2), hydrocarbons and sunlight. Sunlight provides the energy to initiate ozone formation; consequently, high levels of ozone are generally observed during hot, still sunny, summertime weather. During the day, ozone concentrations will usually build up and peak in the afternoon. At night-time, ozone concentrations decline.
Ozone concentrations peak in summer-time:
Importance to monitor ozone concentrations for health and crop
Two pollutants, fine particulate matter and ground-level ozone, are now generally recognized as the most significant in terms of health impacts. Long-term and peak exposure can lead to a variety of health effects, ranging from minor effects on the respiratory system to premature mortality.
The main health effects of short-term (a few hours) exposure to ozone include irritation of throat and eyes, coughing, wheezing. Ozone causes breathing difficulties because it attacks the lining of our lungs. This means that the lung function may be affected and the lining may lose some of its ability to serve as a protective barrier against microbes, harmful chemicals and allergens. It can also trigger asthma attacks. These effects may occur when the ozone concentration reaches a level higher than 180 µg/m3
(EU information threshold). Ozone exposure has
been linked to a number of health effects and is thought to be the cause of the premature deaths of thousands of people in Europe each year. Children, because they breathe more air per kilogram of body weight and have smaller lungs than adult, and people with asthma or other respiratory illnesses are particularly high-risk groups. Also, recent research studies suggest that long-term exposure to ozone may be associated with lung cancer. Ozone can aggravate asthma and may also worsen other respiratory illnesses such as pneumonia and bronchitis. Asthma is now the most common respiratory disease among western European children, afflicting 7% of children aged between 4 and 10.
Ozone also has serious ecological and economic effects. It can reduce crop yields and damage vegetation that are not marketable anymore. High ozone concentrations can affect not only plant growth and aspect, but soil fertility. Plants that are exposed to high ozone concentrations metabolise less carbon dioxide, so less carbon is available in the soil, and fewer soil microbes grow and thrive. In order to take into account such effects, an indicator called AOT, Accumulated exposure Over a Threshold, has been introduced. This indicator is calculated as the cumulative exceedance of a given pollutant concentration, over a selected period. The hourly concentration of 40 ppb (part per billion) was proposed as the threshold for vegetation. The critical AOT 40 level of 3, 000 ppb.h responds to decrease in agricultural
May/June 2010
IET
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