c=o sunlight, NO free radicals

an aldehyde has been greatly reduced. This happens because residual nitric oxide reacts with and destroys ozone formed in the morning to re-create nitrogen dioxide and oxygen, a reaction that also occurs in the stratosphere:

The sum of the last three reactions above constitutes a null cycle, whereby there is no net buildup of ozone or oxidation of NO to NOz by this mechanism.

In fact, the oxidation of NO to NOz does occur rapidly, before the pollution has diffused away, partly because of weather conditions and partly because of the high concentration of catalytic free radicals that are generated during a smog episode. More free radicals are produced than are consumed in

FIGURE 3-4 Time-of-day (diurnal) variation in the concentration of gases during days of marked eye irritation in Los Angeles in the 1960s. [Source: Redrawn from D. J. Speeding, Air Pollution (Oxford: Oxford University Press, 1974).I


— Aldehydes Ozone N02 -NO

10 12 noon

Diurnal time scale

10 12 noon

Diurnal time scale


— Aldehydes Ozone N02 -NO

smog because of the reactions of VOCs, especially those containing highly reactive bonds such as C=C and C=0. For example, ethene and its derivatives react in a complex sequence of reactions with NO, free radicals, and atmospheric oxygen to produce nitrogen dioxide and aldehydes, the concentrations of which rise rapidly in the morning (see Figure 3-4). The aldehydes absorb sunlight with A < 350 nm, i.e., UV-B and some UV-A light; some of them photochemically decompose in sunlight to produce additional free radicals, thereby increasing their concentration (see Figure 3-3). Once produced in significant amounts by decomposition of nitrogen dioxide—and not quickly destroyed since the level of NO has abated—some of the ozone also reacts with VOCs to yield more hydroxyl radicals, further accelerating the smog reaction process.

Although our analysis above has identified ozone as the main product of smog, the situation is actually more complicated, as a detailed study in Chapter 5 indicates. Some of the nitrogen dioxide reacts with hydroxyl radical to generate nitric acid, HN03, and some reacts with organic free radicals to produce organic nitrates.

Governmental Goals for Reducing Ozone Concentrations

Many countries individually, as well as the World Health Organization (WHO), have established goals for maximum allowable ozone concentrations in air of about 100 ppb or less, averaged over a one-hour period. For example, the standard in Canada is 82 ppb, and that of WHO is 75-100 ppb. The United States has adopted a standard in which the ozone level over an eight-hour period is what is regulated, rather than the one-hour average; the average eight-hour limit was set at 80 ppb in 1997 for the United States, compared to the WHO eight-hour standard of 50-60 ppb. Generally speaking, the longer the period over which the concentration is averaged in a regulation, the lower the stated limit, since it is presumed that exposure to a higher level is acceptable only if it occurs for a short time.

The ozone level in clean air amounts to only about 30 ppb. By way of contrast, the levels of ozone in Los Angeles air used to reach 680 ppb, but peak levels have now declined to 300 ppb. Many major cities in North America, Europe, and Japan exceed ozone levels of 120 ppb typically for 5 to 10 days each summer.

The electrical power blackout that occurred in August 2003 in eastern North America yielded some interesting information concerning the contribution of power plants to air pollution in that region. Measurements over Pennsylvania taken 24 hours after the blackout began found that S02 levels were down 90%, and ozone levels down about 50%, compared to a similarly hot, sunny day a year earlier, and that visibility increased by about 40 km because haze from particulates had decreased by 70%.

Photochemical Smog Around the World

The air in Mexico City is so polluted by ozone, particulate matter, and other components of smog, and by airborne fecal matter, that it is estimated to be responsible for thousands of premature deaths annually; indeed, in the center of the city residents can purchase pure oxygen from booths to help them breathe more easily! In 1990, Mexico City exceeded the WHO air guidelines on 310 days, though peak levels have steadily declined since then. In contrast to temperate areas where photochemical smog attacks occur almost exclusively in the summer—when the air is sufficiently warm to sustain the chemical reactions—Mexico City suffers its worst pollution in the winter months, when temperature inversions prevent pollutants from escaping. Some of the smog in Mexico City originates from butenes that are a minor component of the liquefied gas that is used for cooking and heating in homes, some of which apparently leaks into the air.

Athens and Rome, as well as Mexico City, attempt to limit vehicular traffic during smog episodes. One strategy used by Athens and Rome is to allow only half the vehicles to be driven on alternate days, the allocation being based upon the license plate numbering (odd or even numbers).

Due to long-range transport of primary and secondary pollutants in air currents, many areas which themselves generate few emissions are subject to regular episodes of high ground-level ozone and other smog oxidants. Indeed, some rural areas, and even small cities, that lie in the path of such polluted air masses experience higher levels of ozone than do nearby larger urban areas. This occurs because in the larger cities, some of the ozone transported from elsewhere is eliminated by reaction with nitric oxide released locally by cars into the air, as illustrated previously in the reaction of NO with 03. Ozone concentrations of 90 ppb are common in polluted rural areas.

When hot summertime weather conditions produce large amounts of ozone in urban areas but do not allow much vertical mixing of air masses as they travel to rural sites, elevated ozone levels are often observed in eastern North America and western Europe in zones that extend for 1000 km (600 miles) or more. Thus, ozone control is a regional rather than a local air quality problem, in contrast to what was usually assumed in the past. Indeed, on occasion, polluted air from North America moves across the Atlantic to Europe, northern Africa, and the Middle East; that from Europe can move into Asia and the Arctic; and that from Asia can reach the west coast of North America. Some analysts believe that by 2100, even the background level of ozone throughout the Northern Hemisphere will probably exceed current ozone standards.

A plot of ozone concentration contours for summer afternoon smog conditions in North America is shown in Figure 3-5a. At each point along any solid line, the concentration of ozone has the same value; hence contours connect regions having equal levels of ozone. The highest levels (100 ppb)

FIGURE 3-5b Maximum surface ozone levels, in ppb, for 1996-1998 in eastern North America. ISource: Environment Canada, "Interim Plan 2001 on Particulate Matter and Ozone/' Government of Canada Publication (Ottawa: 2001). I

FIGURE 3-5a Ninetieth percentile contours of summer afternoon ozone concentrations (ppb) measured in surface air over the United States. Ninetieth percentile means that concentrations are higher than this 10% of the time. fAdaptcd from A. M. Fiore, D. J. Jacob, J. A. Logan, and J. H. Yin, "Long-Term Trends in Ground-Level Ozone over the Contiguous United States, 1980-1995/'}. Ceo-phys. Res. 103 (1998): 1471-1480.)

FIGURE 3-5b Maximum surface ozone levels, in ppb, for 1996-1998 in eastern North America. ISource: Environment Canada, "Interim Plan 2001 on Particulate Matter and Ozone/' Government of Canada Publication (Ottawa: 2001). I

occur in the Los Angeles and New York-Boston areas, but note the 80-ppb contour over a wide area south of the Great Lakes and into the Southeast, as well as one surrounding Houston. Ozone levels are particularly high over Houston—reaching 250 ppb on occasion—because of emissions of highly reactive VOCs containing C=C bonds from the region encompassing the petrochemical industry. Indeed, as of the late 1990s, Houston had overtaken Los Angeles for the number of days per year in which ozone standards were exceeded.

Considerable ozone is transported from its origin in the U.S. Midwest to surrounding states and Canadian provinces, especially around the Great Lakes (see Figure 3-5b). An example of a rural area subject to high ozone concentrations is the farmland in southwest Ontario, which often receives ozone-laden air from industrial regions in the United States that lie across Lake Erie.

Elevated levels of ozone also affect materials: It hardens rubber, reducing the useful lifespan of consumer products such as automobile tires, and it bleaches color from some materials such as fabrics.

The photochemical production of ozone also occurs during dry seasons in rural tropical areas where the burning of biomass for the clearing of forests or brush is widespread. Although most of the carbon is transformed immediately to C02, some methane and other hydrocarbons are released, as is some NOx. Ozone is produced when these hydrocarbons react with the nitrogen oxides under the influence of sunlight.

Limiting VOC and NO Emissions to Reduce Ground-Level Ozone

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 C^C bonds plus other reactive VOCs, emitted into the air must be reduced. The control strategies in place in the United States have resulted in some ozone level reduction over the past few decades, notwithstanding the huge increase in total vehicle-miles driven—up to 100% more in the last 25 years.

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 achieved usually has been much less than the percentage reduction in hydrocarbons. This happens because usually there is initially an overabundance 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 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 functional dependence of smog production upon reactant concentration is complicated, and the net consequence of making moderate decreases in primary pollutants is difficult to deduce without computer simulation. Computer

FIGURE 3-6 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).!

modeling indicates that NOx reduction, rather than VOC reduction, would be much more effective in reducing ozone in almost all of the eastern United States. An example of the predictions that arise from the modeling studies is shown in Figure 3-6. The relationships between the NOx and the VOC concentrations that produce contours for three different values for the concentration of ozone are shown. Point A represents 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 curve 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 predicted 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 NOz:

The nitrogen dioxide then photochemically decomposes to produce the free oxygen atoms that react with 02 to produce ozone, as previously discussed (see Figure 3-3).

In the portion of the VOCAimited region that lies to the left of the diagonal dashed line of Figure 3-6, there is a large excess of NOx; under such

Ozone =


-80 ppb

/ /

160 ppb

/ • B

240 ppb



limited I

~~ region j

_ /

f X'c

A NOx-limited region

1 1 1 1 1 ! 1 1 1 1

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VOC (ppm of carbon)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VOC (ppm of carbon)

conditions, the OH radical tends to react with NQ2, so less of it is available to initiate the reaction of more VOCs:

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