Environmental Chemistry 5 Reactions That Produce And Consume Ozone Naturally

Consequently, lowering the NOx concentration actually produces more ozone, not less, since more OH is available to react with the VOCs, although production of other smog reaction products such as nitric acid is thereby reduced. Thus, for example, when the VOC concentration is about 0.5 ppm, lowering the NOx concentration from 0.21 ppm—corresponding to point B on Figure 3-6—even by two-thirds of this amount is predicted to increase the ozone level slightly beyond 160 ppb; further reductions do not begin to decrease ozone until NOx reaches about 0.05 ppm. In situations where VOCs are relatively plentiful, that is, to the right side of the dashed line in Figure 3-6, reducing NOx also reduces ozone. Thus, when the VOC level is 0.5 ppm, the ozone concentration falls back to 160 ppb when the NOx is reduced to 0.04 ppm (point C) and declines mote with further decreases of NOx.


Using Figure 3-6 and assuming a MOx concentration of 0.20 ppm, estimate the effect on ozone levels of reducing the VOC concentration from 0.5 to 0.4 ppm. Do your results support the characterization of that zone of the graph as VOC-limited?


Using Figure 3-6, again with an initial VOC concentration of 0.50 ppm, estimate the effect on ozone levels of lowering the NOx concentration from 0.20 to 0.08 ppm. Explain your results in terms of the chemistry discussed above.

Some urban areas such as Atlanta, Georgia, and others located in the southern United States include or border upon heavily wooded areas whose trees emit enough reactive hydrocarbons to sustain smog and ozone production, even when the concentration of anthropogenic hydrocarbons, i.e., those that result from human activities, is low. Deciduous trees and shrubs emit the gas isoprene, whereas conifers emit pinene and limonene; all three hydrocarbons contain C=C bonds. The blue hazes that are observed over forested areas such as the Great Smoky Mountains in North Carolina and the Blue Mountains in Australia result from the reaction of such natural hydrocarbons in sunlight to produce carboxylic acids that condense to form suspended particles of the size that scatter sunlight and thereby produce a haze. Some of the ozone molecules present above the forests react with the C=C

bond in the natural hydrocarbons to first produce aldehydes, which are then further oxidized in air to the corresponding carboxylic acids. Eventually, the acids in the aerosol are attacked by hydroxyl radicals, which initiate their decomposition, if the haze is not rained out of the air beforehand.

In urban atmospheres, the concentration of these natural reactive hydrocarbons normally is much less than that of the anthropogenic hydrocarbons, and it is not until the latter are reduced substantially that the influence of these natural substances becomes noticeable. In areas affected by the presence of vegetation, then, only the reduction of emissions of nitrogen oxides will reduce photochemical smog production substantially. As an air mass moves from an urban area to a rural one downwind, it often changes from being VOC-limited to being NO^'limited, since there are few sources of nitrogen oxides, but often substantial sources of reactive VOCs, outside cities and since the reactions that consume nitrogen oxides occur more quickly than do those that consume VOCs.

Although hydrocarbons with C—C bonds and aldehydes are the most reactive types in photochemical smog processes, other VOCs play a significant role after the first few hours of a smog episode have passed and the concentration of free radicals has risen. For this reason, control of emissions of all VOCs is required in areas with serious photochemical smog problems. Gasoline, which is a complex mixture of hydrocarbons, is now formulated in order to reduce its evaporation, since gasoline vapor has been found to contribute significantly to atmospheric concentrations of hydrocarbons. The control of VOCs in air is discussed in more detail in Chapter 16. New regulations in California (with Los Angeles especially in mind) limit the use of hydrocarbon-containing products such as barbecue-grill starter fluid, household aerosol sprays, and oil-based paints that consist partially of a hydrocarbon solvent that evaporates into the air as the paint dries. The air quality in this region has improved because of current emission controls, but the increase in vehicle-miles driven and the hydrocarbon emissions from nontransportation sources such as solvents have thus far prevented a more complete solution. Research has also indicated that any substantial increase in the emissions of methane to the atmosphere could prolong and intensify the periods of high ozone in the United States, even though CH4 is usually considered to be a rather unre-active VOC.

Continue reading here: Technological Control of Emissions Catalytic Converters

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