In order to improve the air quality in urban environments that are subject to photochemical smog, the quantity of reactants, principally NOX and hydrocarbons containing CRC bonds, plus other reactive VOCs, emitted into the air must be reduced. The control strategies that have been put in place in the United States have resulted in some reduction in ozone levels in the past few decades, notwithstanding the huge increase in total vehicle-miles driven—up to 100% more in the last 25 years—that has occurred.
For economic and technical reasons, the most common control strategy has been to reduce hydrocarbon emissions. However, except in downtown Los Angeles, the percentage reduction in ozone and other oxidants that is thereby achieved usually has been much less than the percentage reduction in hydrocarbons. This happens because usually there is initially an over- abundance of hydrocarbons relative to the amount of nitrogen oxides, and cutting back hydrocarbon emissions simply reduces the excess without slowing down the reactions significantly. In other words, it is usually the nitrogen oxides, rather than reactive hydrocarbons, that are the species that
Review Questions 5–7 are based on material in the sections above.
Nitrogen oxides are the limiting reactants in these areas.
determine the overall rate of the reaction. This is especially true for rural areas that lie downwind of polluted urban centers.
Due to the large number of reactions that occur in polluted air, the func- tional dependence of smog production upon reactant concentration is com- plicated, and the net consequence of making moderate decreases in primary pollutants is difficult to deduce. Computer modeling indicates that NOX reduction, rather than VOC reduction, would be much more effective in reducing ozone in most of the eastern United States. However, Mexico City’s ozone is limited by VOCs, even though there are numerous sources of them.
An example of the predictions that arise from the modeling studies is shown in Figure 3-8. The relationships between the NOX and the VOC con- centrations that produce contours for three different values for the concen- tration of ozone are shown. Notice that the same concentration of ozone results from many different combinations of VOC and NOX. Point A repre- sents a typical set of conditions in which the ozone production is NOX limited.
For example, reducing the concentration of VOCs from 1.2 ppm to 0.8 ppm has virtually no effect on the ozone concentration, which remains at about 160 ppb since the contour in this region is almost linear and runs parallel to the horizontal axis. However, a reduction of the NOX level, from about 0.03 ppm at point A to a little less than half this amount, which corresponds to dropping down to the curve directly below it in the figure, cuts the pre- dicted ozone level in half, from 160 ppb to 80 ppb. Chemically, NOX-limited conditions occur when, due to the high concentration of VOC reactants, an abundance of peroxy free radicals HOO and ROO are produced, which quickly oxidize NO emissions to NO2:
HOO NO 9: OH NO2
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.28
NOX (ppm)
VOC (ppm of carbon) 0.24
0.20 0.16 0.12 0.08 0.04 0
80 ppb 160 ppb 240 ppb Ozone =
VOC- limited region
A C
B
NOX-limited region FIGURE 3-8 The
relationship between NOX
and VOC concentrations in air and the resulting levels of ozone produced by their reaction. Points A, B, and C denote conditions discussed in the text. [Source: Redrawn from National Research Council, Rethinking the Ozone Problem in Urban and Regional Air Pollution (Washington, DC:
National Academy Press, 1991).]
Improving Air Quality: Photochemical Smog 89
The nitrogen dioxide then photochemically decomposes to produce the free oxygen atoms that react with O2 to produce ozone, as previously discussed (see Figure 3-4).
In the portion of the VOC-limited region that lies to the left of the diago- nal dashed line of Figure 3-8, there is a large excess of NOX; under such conditions, the OH radical tends to react with NO2, and so less of it is avail- able to initiate the reaction of more VOCs:
OH NO29: HNO3
Consequently, lowering the NOX concentration actually produces more ozone, not less, since more OH is thereby 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 3-8—even by two-thirds of this amount is predicted to increase slightly the ozone level beyond 160 ppb; further reductions do not begin to decrease ozone until NOX reaches about 0.05 ppm. Indeed, ozone concentrations on weekends, when there is much less truck traffic, are higher in Los Angeles and many other cities, since the very high weekday levels of NO2 are lower then and so less of it is available to combine with and provide a sink for OH.
In situations where the NOX is less abundant but VOCs are relatively plentiful, i.e., to the right side of the dashed line in Figure 3-8, reducing NOX does reduce ozone. Thus, when the VOC level is 0.5 ppm, the ozone concen- tration falls back to 160 ppb when the NOX is reduced to 0.04 ppm (point C), and declines more with further decreases of NOX.
PROBLEM 3-5
Using Figure 3-8, and assuming a NOX 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? ●
PROBLEM 3-6
Using Figure 3-8, 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, incorporate 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,
PROBLEM 3-5
PROBLEM 3-6
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 RC bonds.
In urban atmospheres, the concentration of these natural reactive hydro- carbons 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 pres- ence 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 NOX limited, since there are few sources of nitro- gen oxides, but often substantial ones of reactive VOCs, outside of cities, and since the reactions that consume nitrogen oxides occur more quickly than do those that consume VOCs.
Although hydrocarbons with C RC bonds and aldehydes are the most reactive types in photochemical smog processes, other VOCs play a signifi- cant 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 con- tribute significantly to atmospheric concentrations of hydrocarbons.
The control of VOCs in air is discussed in more detail in Chapter 11.
Regulations in California (with Los Angeles especially in mind) limit the use of hydrocarbon-containing products such as barbeque-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 non-transportation 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 unreactive VOC.
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 natural hydrocarbons in sunlight—in the absence of much NOX and hence largely without its involvement—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 RC bond in the natural hydrocarbons to first pro- duce aldehydes, which then are 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.
Recall that carboxylic acids have the general formula RCOOH.
Improving Air Quality: Photochemical Smog 91