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Ozone plays complex role in atmosphere


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POSTED: Sunday, March 15, 2009

Ozone is a devious chemical. High in the sky, it protects us from UV light, while low in the atmosphere, it is a pollutant and a health hazard.

In the stratosphere, oxygen molecules absorb ultraviolet photons, break apart and recombine with molecular oxygen to form ozone, thus preventing the UV photon from reaching the surface.

On the other hand, in the lower atmosphere the visible photons of sunlight that penetrate through the stratosphere interact with nitrogen oxides to form ozone.

The formation of ozone via this reaction is especially efficient in regions that receive a high incidence of sunlight, notably those in the southwestern U.S.

Normally oxygen and nitrogen do not combine chemically, and coexist peacefully in the atmosphere, but the high-temperature, high-pressure environment inside internal combustion engines provides the energy to force their combination.

Because nitrogen forms five unique compounds with oxygen, the nitrogen oxides that form are variable. The nitrogen oxide composition of the atmosphere is usually referred to simply as NOx, the “;x”; referring to an unspecified ratio of nitrogen to oxygen in the mixture of compounds.

The dual role of ozone underscores the importance of oxygen and the critical roles it plays in Earth's environment. Although ozone is an unstable form of oxygen, that instability is the reason for its harmful effects in the lower atmosphere.

Oxygen's affinity for electrons is the primary reason for its various effects. When ozone decomposes into molecular oxygen and a single oxygen atom, it is even more “;hungry”; for electrons and will steal them from any surrounding atoms, causing oxidation of the substance that contained those atoms.

Hydrogen fuel is touted as a savior for carbon emissions from internal combustion engines, but in the literature for and discussions about hydrogen engines, the subject of nitrogen oxide production is usually not mentioned.

Because of its high burning temperature, hydrogen combustion in a conventional engine would produce high levels of NOx. Replacing today's gasoline engines with internal combustion hydrogen solves the carbon and hydrocarbon emission problem but still leaves a high level of emission of potentially hazardous NOx gases.

Although the focus of transportation-related emissions is on carbon, it is only one branch of a complex problem that centers on the undeniable fact that we are facing an energy crisis, one that must be alleviated by finding less hazardous sources.

Burning fossil fuels to produce electricity that is then used to produce hydrogen is a solution only if the hydrogen can be used other than in internal combustion engines.

The process is slightly more efficient and thus adds less carbon per vehicle-mile but is not enough to have much of an impact on either the carbon or NOx problem.

There is no energy conversion that is without environmental effects whether they are chemical, physical or aesthetic. It is unreasonable to think that we will change our entire lifestyle paradigm within a few generations, so we must find solutions that produce the least deleterious effects.

Carbon dioxide emissions are a serious problem, but a Princeton University report concluded, “;Hydrogen combustion could be an effective way to utilize hydrogen energy, however, many developments need to be made before it can be implemented to ensure that it does not cause more environmental problems than it solves.”;

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Richard Brill is a professor of science at Honolulu Community College. E-mail questions and comments to .(JavaScript must be enabled to view this email address).