Chap. 4: Emissions & Urban Air
Plants, animals, and machines all contribute chemicals to the air we breathe. Over hundreds of millions of years, the atmosphere changed in response to biological and geological processes, and, in turn, organisms evolved in response to a changing atmosphere. In this chapter I look at the issues involving air, covering the causes and consequences of contributions from both human and nonhuman organisms.
Humans usually dominate emissions in urban areas. Air quality studies categorize emission sources as either point or nonpoint, roughly corresponding to fixed or moving, respectively. Both types of emissions roughly match maps of population density, at least in the United States. A number of studies demonstrate the clear connection between transportation and urban air quality, including an opportunity presented by the 1996 Summer Olympics in Atlanta, Georgia. Looking on the positive side, the United States undertook serious efforts to improve air quality, with good reductions measured and reported by the Environmental Protection Agency. Still, the United States measures some of these emissions in pounds per person per day.
Vegetation and soils also emit chemicals, often as a by-product of evolution in the face of herbivory, competition, and environmental stresses that yielded chemical responses and defenses. These emissions can reduce air quality in urban areas, with some tree species being worse “violators” than others. Using various information sources, biogenic emissions across North America shows some regions with particularly high emissions, one area being the mountains of the southeastern United States, just upwind of Durham, North Carolina. Though biogenic emissions fall well below fossil-fuel emissions, considering their propensity to interact with other chemicals, they have important air quality implications.
These emissions feed into, among other pollutants, ozone formation in the hot summer months. For a more complete understanding of urban pollution, I provide an overview of the chemical reactions that link volatile organic compounds (VOCs), reactive nitrogen, sunshine, ozone, and eye-stinging pollutants. A particularly revealing example follows the emissions plume from a coal-burning electrical power plant, in this case, with emissions producing ozone just as the plume sweeps over Nashville, Tennessee. This example helps explain several urban situations where transportation produce emissions. It would be convenient to blame some regulatory agency for falling down on the job, but these ozone-producing reactions have complicated dynamics depending on the ratio of VOCs to reactive nitrogen. Sometimes one thing should be reduced, sometimes the other, and sometimes climate plays a major role. Given that these emisssions vary greatly over the United States, as we saw in the previous chapter, nationwide regulations seem elusive.
Still, cities have higher ozone levels than rural areas: in the worst of times more than double the ozone levels. Although in a later chapter we examine ozone’s health effects, here I show that vegetation also suffers from high ozone levels. Indeed, high ozone levels reduce wheat and potato yields by about 30%, meaning that farms surrounding cities might have problems beyond urban sprawl.