A principal lesson learned from the twentieth century air pollution episodes is the importance of meteorological conditions. The same emissions in terms of absolute mass released in the same time period can result in very different air quality, depending on winds, ambient temperature, atmospheric pressure, and, most importantly, inversion conditions. This is particularly important when conditions lead to elevated inversions above valleys, such as the Meuse River valley in Belgium and the Los Angeles Basin. Elevated inversions can cap pollutants in a stagnant air stratum when vertical air circulation occurs above the valley, so that pollution levels build up over days (see Figure 3.4). This indicates that pollution controls need to be targeted toward worst-case scenarios, rather than average meteorological conditions.
Another lesson is the importance of paying attention to evolving knowledge. The increasing awareness of the importance of certain conventional pollutants, like PM, SOx, and NOx, and their links to health effects should have been heeded and incorporated into city planning and pollution control decision making processes. Also, the toxic cloud episodes could have been expected from these more conventional episodes, given that toxic substances like methylsiocyanate are much more toxic than oxides of sulfur and nitrogen. Thus, a release even at low concentrations should have been provided for in contingency plans. Even worse, shortly after the Bhopal disaster, engineers and regulators were given a test on what they had learned about toxic releases and communicating risk when a release of another toxic gas (this time, aldicarb oxime) occurred at the Institute, West Virginia, pesticide plant. The same company implicated at Bhopal, Union Carbide, owned the Institute plant and, in the minds of many, once again failed the test. There should have been significant progress made, since in many ways the two plants and situations were so very similar, but some of the same weaknesses remained even after the horrible consequences of Bhopal.
The Bhopal disaster again reminds us that forgetting the past can be deadly.
Contaminant of Concern: Photochemical Oxidant Smog
The term smog is a shorthand combination of “smoke-fog.” However, it is really the code word for photochemical oxidant smog, the brown haze that can be seen when flying into Los Angeles, St. Louis, Denver, and other metropolitan areas around the world. Smog is made up of at least three ingredients: light, hydrocarbons, and radical sources, such as the oxides of nitrogen. Therefore, smog is found most often in the warmer months of the year, not because of temperature, but because these are the months with greater amounts of sunlight. More sunlight is available for two reasons, both attributed to the earth’s tilt on its axis. In the summer, the earth is tilted toward the sun, so the angle of inclination of sunlight is greater than when the sun is tipped away from the earth leading to more intensity of light per earth surface area. Also, the days are longer in the summer, so these two factors increase the light budget.
Hydrocarbons come from many sources, but the fact that internal combustion engines burn gasoline, diesel fuel, and other mixtures of hydrocarbons makes them a ready source. Complete combustion results in carbon dioxide and water, but anything short of combustion will be a source of hydrocarbons, including some of the original ones found in the fuels, as well as new ones formed during combustion. The compounds that become free radicals, like the oxides of nitrogen, are also readily available from internal combustion engines, since the ambient air is more than three-quarters molecular nitrogen (N2). Although N2 is relatively not chemically reactive, with the high temperature and pressure conditions in the engine, it does combine with the O2 from the fuel/air mix and generates oxides that can provide electrons to the photochemical reactions.
The pollutant most closely associated with smog is ozone (O3), which forms from the photochemical reactions just mentioned. In the early days of smog control efforts, O3 was used more as a surrogate or marker for smog, since one could not really take a sample of smog. Later, O3 became recognized as a pollutant in its own right since it was increasingly linked to respiratory diseases.
Cities that failed to achieve human health standards as required by the Clean Air Act’s National Ambient Air Quality Standards (NAAQS) were required to reach attainment within six years of passage, although Los Angeles was given 20 years, since it was dealing with major challenges in reducing ozone concentrations. Almost 100 cities failed to achieve ozone standards and were ranked from marginal to extreme. The more severe the pollution, the more rigorous controls required, although additional time was given to those extreme cities to achieve the standard. Measures included new or enhanced inspection/maintenance (I/M) programs for autos; installation of vapor recovery systems at gas stations and other controls of hydrocarbon emissions from small sources; and new transportation controls to offset increases in the number of miles traveled by vehicles. Major stationary sources of nitrogen oxides also have to reduce emissions.
The ozone threshold value is 0.12 parts per million (ppm), measured as a one-hour average concentration. An area meets the ozone NAAQS if there is no more than one day per year when the highest hourly value exceeds the threshold. (If monitoring did not take place every day because of equipment malfunction or other operational problems, actual measurements are prorated for the missing days. The estimated total number of above-threshold days must be 1.0 or less.) To be in attainment, an area must meet the ozone NAAQS for three consecutive years.
Calculating compliance can be tricky. Air quality ozone value is estimated using a calculation usually based on the fourth highest monitored value with three complete years of data and is selected as the updated air quality value because the standard allows one exceedance for each year. It is important to note that the 1990 Clean Air Act Amendments required that ozone nonattainment areas be classified on the basis of the design value at the time the Amendments were passed; generally the 1987–1989 period was used.
The strong seasonality of O3 levels makes it possible for areas to limit their O3 monitoring to a certain portion of the year, termed the O3 season. Peak O3 concentrations typically occur during hot, dry, stagnant summertime conditions; that is, high temperature and strong solar insolation (i.e., incoming solar radiation). The length of the O3 season varies from one area of the country to another. The months of May through October are typical, but states in the south and southwest may monitor the entire year. Northern states have shorter O3 seasons, for example, May through September for North Dakota. This analysis uses these O3 seasons to ensure that the data completeness requirements apply to the relevant portions of the year.
Children have higher health risks associated with exposure to ozone than do most adults. The average adult breathes 13,000 liters of air per day, but on an air-per-kilogram basis, children breathe even more air than do adults. Because children’s respiratory systems are prolific and still developing, they are more susceptible than adults to many environmental threats. Children are outside playing and exercising during the summer months more frequently than in the less warm months. Unfortunately, this is also the time of year with elevated O3. In addition, asthma is a growing threat to children and adults. Children make up 25 percent of the U.S. population, but comprise 40 percent of the asthma cases. The asthma death rate has increased three-fold in the past 20 years, and African Americans die at a rate six times that of Caucasions. Even moderately exercising healthy adults can experience 20% or greater reductions in lung function from exposure to low levels of ozone over several hours. These factors make smog an important public health concern.
The principal lesson from the history of air pollution episodes, as from every case in this book, is that the atmosphere is not infinite in its ability to absorb wastes. Although this appears obvious to the twenty-first century scientist, it is actually a fairly recent realization.