Many major cities in North America, Europe, and Japan exceed ozone levels of 120 ppb typically for 5 to 10 days each summer. The levels of ozone in Los Angeles air used to reach 680 ppb. Figure 3-6 illustrates the decline in
PROBLEM 3-3
PROBLEM 3-4
300
100 200
1970
1960 1990 2000
Year
2010 0
O3 (ppbv) Population (millions)
1980 Ozone
Population
10
5 15
0 FIGURE 3-6 Maximum
atmospheric ozone concentration (left axis) and human population (right axis) in Los Angeles over a half-century. [Source: D. D.
Parish and T. Zhu, Science 326 (2009): 674–675.]
Urban Ozone: The Photochemical Smog Process 85
maximum ozone levels (dark green curve) in Los Angeles over time—by more than a factor of two over the last half-century—notwithstanding the continual increase in population of the area over that time period (light green curve). Presumably the Los Angeles pollution level would have increased greatly over the half-century, given the increase in the number of vehicles, if pollution controls had not been instituted.
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 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, although peak smog levels have steadily declined since the 1990s and never reached the maximums attained in Los Angeles. 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 reac- tions—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 traf- fic 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). France plans to ban access in cities including Paris to vehicles made before 1997, when strict emis- sion standards were instituted in Europe. Low-emission zones of various types are already in existence in parts of Sweden, Germany, Italy, and London, England and have succeeded in reducing urban air pollution.
Owing to the rapid increase in the number of vehicles on their roads and their generally warm climates, ground-level ozone levels and episodes of pho- tochemical smog occur more and more frequently in developing countries.
Peak ozone levels have been increasing in Beijing since the 1990s, especially from May to October. Other Chinese megacities such as Shanghai and Guang- zhou also now suffer from high ozone episodes.
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. 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 especially in springtime, that from Asia can reach the west coast of North America and contribute to increasing ozone levels there. Some analysts believe that by 2100, even the background level of ozone throughout the North- ern Hemisphere will exceed current ozone standards.
Butenes contain a CRC bond and consequently are very reactive.
Due to the 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 O3. Ozone concentrations of 90 ppb are common in polluted rural areas. This phenomenon occurs in northern European countries such as Denmark, where ozone transported from southern countries reacts with NO emitted from vehicles in the Danish cities, but remains unreacted over rural areas, which consequently are more polluted by ozone than are urban areas.
Considerable amounts of ozone are transported from its origin in the U.S. Midwest to surrounding states and Canadian provinces, especially around the Great Lakes, such as the farmland in southwest Ontario, which receives ozone-laden air from industrial regions in the United States that lie across Lake Erie.
A plot of ozone concentration contours for summer afternoon smog condi- tions in North America in about 1990 is shown in Figure 3-7. 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) occurred 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.
Imported O3 local NO !:
NO2 O2.
Contour diagrams like that in Figure 3-7 are similar to geographic maps in which adjacent locations of equal altitude are connected by lines.
60
60 80 60
80
80 60 60
60 80 100
100
60 FIGURE 3-7 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.
[Adapted 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,” J. Geo-phys. Res.
103 (1998): 1471–1480.]
Improving Air Quality: Photochemical Smog 87
Ozone levels were particularly high over Houston—reaching 250 ppb on occasion—because of emissions of highly reactive VOCs containing C R C bonds from the region encompassing the petrochemical industry. By the late 2000s, the most ozone-polluted cities in the United States were
• Los Angeles, and seven other California cities ranging from Bakersfield north of it to San Diego to its south
• Houston, Texas
• Charlotte, North Carolina
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 immedi- ately to CO2, 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.
Improving Air Quality: Photochemical Smog
The history of attempts to improve the quality of the air that we breathe stretches back many centuries. In the developed world, episodes of the type of smog (mainly soot and sulfur dioxide) produced by unregulated coal burn- ing began to be reduced in the mid-twentieth century and have largely disap- peared there, as discussed further in Chapter 4. As we shall see below, the conquest of photochemical smog is proving to be a harder objective.