Ozone as a Pollutant

Reaction 8.6 produces atomic oxygen in an excited ( D) electronic state, singlet oxygen. This is important in the troposphere too, as 0( ) reacts with water vapor to produce chemically reactive hydroxyl radicals, -OH  

The hydroxyl radical is normally present only in low concentrations in the troposphere, as it reacts with further ozone to form the hydroperoxy radical HOO- which in turn gives hydrogen peroxide H2O2. Ozone, the hydroxyl radical, and hydrogen peroxide are the main oxidizing species in the troposphere, from the standpoint of environmental chemistry. The hydroxyl radical in particular performs an important function as a natural cleansing agent for the atmosphere. In elevated concentrations, however, 

Ozone is produced in substantial concentrations by industrial activity and, indirectly, from automobile exhausts. The most important sequence of reactions producing tropospheric ozone begins with hydrocarbon vapors, nitric oxide, and sunlight  

rather surprisingly, the key to reducing tropospheric ozone pollution is to minimize the release of hydrocarbon vapors from such sources as unburnt fuel in automobile exhausts, vaporization of fuel at service station pumps (modern pumps recover fuel vapors from automobile fuel tanks while refilling them), kitchen exhausts from fast-food restaurants and, as in Mexico City, leakages of liquefied petroleum gas (mainly butane) used for domestic heating and cooking.  

Ozone consists of three oxygen atoms bound together in a molecule with the formula O3. It is an allotrope of molecular oxygen. It has a pungent odor that we often smell near sparking electrical appliances or after a thunderstorm when rainfall washes hghtning-produced ozone out of the troposphere. 

Although there has been an overall improvement in air quality in the United States in recent years, there are still problems with ozone in many urban areas and even in some remote areas such as Big Bend National Park deep in southwest Texas. Federal, state, and local efforts to control emissions of nitrogen oxides are constantly discussed and debated in the local newspapers and by political action groups in many areas of the United States. The popularity of the automobile in American culture has had a dramatic impact on air quality. In an approach not yet tried in the United States, there have been efforts in some major cities in other parts of the world to limit the number of automobiles allowed into those cities on a given day or during a given time period through the use of permits or taxes. There have been noticeable 

Allotropes Different molecular forms of the same element in this case the two molecular forms are diatomic Oj and triatomic 0, 

In the United States, the EPA s efforts to control ozone concentrations in our ground-level air have had some success. In 2004 the EPA reported that, thanks to their emission control programs, ozone levels had continuously decreased during the period 1980—2003. The result was the lowest national ozone level since 1980, making 2003 one of the cleanest years on record. However, concerns about the prospect that the downward trend in ozone levels may be slowing, caused regulators to propose additional national rules designed to decrease vehicle exhaust and industrial emissions. 

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Heck and Brandt discussed the effectiveness of green belts in relation to vegetation as a pollutant sink and concluded that vegetation probably acts as a major sink for air pollutants, including oxidants and ozone, over time, but has a relatively minor effect on oxidant concentrations during high-pollution episodes is more effective in some seasons than others or with some cultural and management practices than others and should not be considered an important contributor to short-term reductions in oxidant or ozone concentrations. 

In most research on lower organisms, there has been an attempt to use ozone as a germicide or to understand the interactions of pollutants and pathogens on the responses of higher plants. In few studies has the interest been on the effects of ozone on the organisms themselves, except in the studies of effects on algae. 

Biological consequences of ozone as a constituent of atmospheric pollution are discussed in Section 4.16.10.3. Atmospheric ozone can also react with alkene pollutants present in the atmosphere, giving rise to 1,2,4-trioxolane components. 

In this section, we use another chain reaction to show the relation between the steady-state treatment and the quasi-equilibrium treatment. The former is more general than the latter, and leads to more complete but also more complicated results. Ozone, O3, is present in the stratosphere as the ozone layer, and in the troposphere as a pollutant. Ozone production and destruction in the atmosphere is primarily controlled by photochemical reactions, which are discussed in a later section. Ozone may also be thermally decomposed into oxygen, O, although... 

At this level, ozone is a pollutant with a harsh odour. In humans and animals, it causes respiratory problems, including coughing, wheezing, and eye irritation. It retards plant growth, reduces the productivity of crops, and damages forests. Concentrations of ozone as low as 0.1 ppm (parts per million) can decrease photosynthesis by 50%. In addition, it damages plastics, breaks down rubber, and corrodes metals. 

As a pollutant moves in the liquid phase from the substomatal regions to the cellular sites of perturbation, it may encounter many obstacles along the pathway. Scavenger reactions between endogenous components and the pollutant may occur, influencing the toxicity of the pollutant. For example, ascorbate, which occurs widely in plant cells, may absorb or neutralize a pollutant. On the other hand, an oxidant such as ozone may react with membrane material to form other toxic substances such as aldehydes, ketones, and various free radicals which in turn adversely affect the cell. 

Because of the catalytic effect of some impurities, the atmospheric ozone equilibrium is very sensitive to pollution of the atmosphere. Therefore, taking into account the role of ozone as a protective shield to UV-damaging, air pollution could become a serious danger for all biological systems living on the earth16. ... 

Under polluted urban air conditions, where sufficient NO is present to reduce HO2 rapidly, oxidation of CO produces ozone as a by-product, additional to that formed by pseudo-equilibrium reaction (11) alone. 

A primary role of nitrogen dioxide as a pollutant is in the formation of the secondary pollutant ozone (Section 4.7) and photochemical smog. Nitrogen dioxide can also react with water to form nitric acid and nitrous acid. 

Ozone, known for its beneficial role as a protective screen against ultraviolet radiation in the stratosphere, is a major pollutant at low altitudes (from 0 to 2000 m) affecting plants, animals and human beings. Ozone can be formed by a succession of photochemical reactions that preferentially involve hydrocarbons and nitrogen oxides emitted by the different combustion systems such as engines and furnaces. 

Although the naturally occurring concentration of ozone at the earth s surface is low, the distribution has been altered by the emission of pollutants, primarily by automobiles but also from industrial sources which lead to the formation of ozone. The strategy for controlling ambient ozone concentrations arising from automobile exhaust emissions is based on the control of hydrocarbons, CO, and NO via catalytic converters. As a result, peak ozone levels in Los Angeles, for instance, have decreased from 0.58 ppm in 1970 to 0.33 ppm in 1990, despite a 66% increase in the number of vehicles. 

In 1966, the Los Angeles Air Pollution Control Board designated trichloroethylene as a photochemically reactive solvent that decomposes in the lower atmosphere, contributing to air pollution. In 1970 all states were requited to submit pollution control plans to EPA to meet national air quaUty standards. These plans, known as State Implementation Plans (SIPS), controlled trichloroethylene as a volatile organic compound (VOC). They were designed to have each state achieve the National Ambient Air QuaUty Standard (NAAQS) for ozone. The regulations were estabUshed to control the emission of precursors for ozone, of which trichloroethylene is one. 

This interaction between airborne acid components and the tree-soil system may alter the ability of the trees to tolerate other environmental stressors such as drought, insects, and other air pollutants like ozone. In Germany, considerable attention is focused the role of ozone and acid deposition as a cause of forest damage. Forest damage is a complex problem involving the interaction of acid deposition, other air pollutants, forestry practices, and naturally occurring soil conditions. 

Burning fossil fuels can release air pollutants such as carbon dioxide, sulfur oxides, nitrogen oxides, ozone, and particulate matter. Sulfur and nitrogen oxides contribute to acid rain ozone is a component of urban smog, and particulate matter affects respiratory health. In fact, several studies have documented a disturbing correlation between suspended particulate levels and human mortality. It is estimated that air pollution may help cause 500,000 premature deaths and millions of new respiratory illnesses each year. 

Again, the myriad influences of human activity are usually viewed as separate effects (global warming, acid rain, ozone loss, urban pollution, etc.) However, these individual symptoms clearly have major interdependencies that must be understood if humans are to learn how to coexist with a stable Earth system. 

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