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Science Background

[Keeping An Eye on Ozone]

Ozone molecules are composed of three oxygen atoms; that is O3, is triatomic in the same state. The formation of ozone from oxygen is an endothermic reaction in which the energy is furnished in the form of an electrical discharge, heat, or ultraviolet light. Substances formed by endothermic reactions tend to be less stable than those formed by exothermic reactions, so ozone is more active chemically than oxygen and decomposes readily into oxygen by exothermic reaction. The rate of the decomposition of ozone is influenced by the presence of nitric oxide, NO, which acts as a catalyst in the decomposition.

Ozone is pale blue as a gas and deep blue as a liquid, with a sharp, irritating odor. Ozone is a powerful oxidizing agent and forms oxides with many elements when oxygen will not react. The molecular structure of ozone is shown in the graphic.

The electronic structure of ozone can be described by the following resonance forms.

[resonance structures]

The presence of ozone in gas mixtures can readily be detected by passing the gas through a solution of potassium iodide, KI containing some starch.

[chemical reactions]

The O3 oxidizes I- to I2, elemental iodine, which imparts a blue color to the starch. Although O2 reacts with almost all other elements, it usually does so only at elevated temperatures. Ozone is used as a bleaching agent for oils, waxes, fabrics, and starch. It is sometimes used instead of chlorine to purify water and is also used in air purifiers.

Ozone is prepared commercially by an electrical ozonizer, passing dry oxygen between two electrically charged plates, that is more complicated and more efficient then laboratory apparatus. An alternative method of commercial production is by electrolysis of very cold, concentrated sulfuric acid. The sharp odor associated with sparking electrical equipment, or lightening, is due in part to ozone.

Smog and Ground-Level Ozone

Smog is a term used to describe the results of chemical reactions in the atmosphere caused by the presence of nitrogen oxides (NOx) and volatile organic compounds (VOCs). These compounds react with oxygen in the air in the presence of heat and strong sunlight to produce ground-level ozone, the primary ingredient of smog. Unlike ozone in the upper atmosphere which is naturally occurring and beneficial due to its protective qualities, ozone at the earth's surface is a man-made air pollutant which can have harmful effects on both humans and the environment. Unlike other air pollutants, ground-level ozone is not emitted directly into the air, but is produced by the combination of pollutants (VOCs and NOx) from many sources. These ozone-producing pollutants come from local sources such as cars, trucks, industrial boilers, power plants, paints, solvents, and other commercial and consumer products. About half of the manmade VOC and NOx emissions in most areas come from automobiles and trucks! In addition to the pollutants emitted from local sources, ozone and its precursors can be transported from upwind sources up to several hundred miles away.

Ozone at ground-level poses health problems by irritating the nose, throat and lungs and causing chest pain, coughing and nausea. Long term exposure may result in permanent damage to the lungs. Ozone aggravates respiratory conditions such as allergies, asthma and emphysema and can have pronounced effects even on healthy individuals who work or play outdoors during hot, sunny summer months when levels of smog are usually at their highest. In addition to the effects on people, ozone is believed to harm forests and agricultural crops and may accelerate the deterioration of rubber tires, paints and dyes in fabrics. A great deal of the deal of the deterioration of automobile tires is due to contact with ozone in the air.

Other Effects of Ground-level Ozone;

  • Ground-level ozone interferes with the ability of plants to produce and store food, so that growth, reproduction and overall plant health are compromised.
  • By weakening sensitive vegetation, ozone makes plants more susceptible to disease, pests, and environmental stresses.
  • Ground-level ozone has been shown to reduce agricultural yields for many economically important crops (e.g., soybeans, kidney beans, wheat, cotton).
  • The effects of ground-level ozone on long-lived species such as trees are believed to add up over many years so that whole forests or ecosystems can be affected. For example, ozone can adversely impact ecological functions such as water movement, mineral nutrient cycling, and habitats for various animal and plant species.
  • Ground-level ozone can kill or damage leaves so that they fall off the plants too soon or become spotted or brown. These effects can significantly decrease the natural beauty of an area, such as in national parks and recreation areas.
  • One of the key components of ozone, nitrogen oxides, contributes to fish kills and algae blooms in sensitive waterways.

Other Ozone Resources Online:

EPA'S Updated Air Quality Standards for Smog (OZONE) and Particulate Matter

New EPA Standards - as of July 1997

AirNow - EPA Ozone Mapping Project

Skycast - Greater Kansas City Area

Philadelphia - Ozone Action Partnership Ground-Level Ozone Forecast

New England - EPA Region 1

Indiana - information and data concerning ozone levels across the state.

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