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109 What air could be more obviously toxic than the tailpipe emission of an automobile? Everyone knows automobiles produce hazardous air pollution, whether as the components of visible smog or by emitting invisible, deadly carbon monoxide, or as other less well-known emissions. Yet many adults in the industrialized world depend on or even love their cars; pollution has not frightened most drivers out of their vehicles. The challenge of reducing air pollution without reducing miles driven is real, and engineers have been addressing it since the 1950s. However, the story of emissions reduction is not a simple one in which scientists have explained the chemical phenomena and engineers designed the devices that intervene in producing the targeted chemicals. As Peter Brimblecombe’s chapter in this volume shows, scientific understandings of air pollution, particularly smog, have changed since the 1940s. Emissions control devices have coevolved with scientific explanations of pollution. This chapter examines the ways that changing understandings of air pollution have driven automobile engineers to address a long and changing list of chemical compounds emitted by internal combustion engines with a dynamic array of technological fixes with the goal of making a 6 CHASING MOLECULES Chemistry and Technology for Automotive Emissions Control Richard Chase Dunn and Ann Johnson 110–––Richard Chase Dunn and Ann Johnson less polluting car. Engineers have had great success in making vehicle emissions less hazardous, but in the twenty-first century have we reached the limits of making the internal combustion engine cleaner? Cars are real polluting machines; this is not a specious allegation. As early as 1966, the United States attributed 86 million tons of air pollutants to automobiles, out of a total of 146 million tons—over half of America’s identified air pollutants.1 Of the main categories of pollutants cars produce—carbon monoxide, hydrocarbons, and nitrogen oxides—automobiles produce the majority of all three, which have negligible nonanthropogenic sources. Automobile-related efforts to improve air quality since the late 1950s have focused primarily on devices to capture evaporating hydrocarbons, improve the combustion efficiency of the engine, and capture noxious tailpipe emissions. As a result, the three main pollutant classes have all been significantly reduced since 1965, when the first federal vehicle emissions standards were set. However, the vehicular emission of carbon dioxide—a seemingly harmless “natural” component of air—has increased due to the increased efficiency of the combustion process in cars. Whether carbon dioxide can be addressed as the other pollutants have been remains an open question and will most likely depend on reducing combustion as an energy source in the car, as hybrids and electrical vehicles do. Even with the carbon dioxide question open, it remains useful to understand how progress has been made in reducing the other noxious emissions of the internal combustion engine. The development of technologies to reduce carbon monoxide, hydrocarbons , and nitrogen oxides has been dependent on three general factors, all of which involve research that is only indirectly related to the design of emissions control technologies. First, scientific understandings of the thermodynamic and chemical processes taking place in the car’s engine and exhaust system have themselves been evolving since the 1960s; combustion cycles have become better understood. Secondly, atmospheric chemists began to investigate air pollution and understand what caused smog; this dynamic understanding fed into regulatory regimes which came to define what a pollutant was and what the penalties for its production would be. Thirdly, the automobile has never been a closed technology, and its development has depended on other technologies developed externally to the automobile, such as microprocessors, to manage and manipulate the production of a variety of combustion products and by-products. The interactivity of these three factors in engineering emissions control devices yields a complicated story of science, technology, and regulation. What’s more, emissions control technologies that have been developed since the late 1950s are also inter- Chasing Molecules–––111 active—they affect how each other works. This chapter shows many of these entanglements. The entangled story of science, technology, and regulation told here is different from a linear account of science begetting technology in many ways. The simple story that all these emissions control technologies were predicated on a particular chemical understanding of the photochemical process of smog formation is incomplete to the point of being inaccurate. What might be called sound science (but might better be called complete science) was neither necessary nor sufficient for the development of intervening technologies. In fact, chemical understanding of smog’s...


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