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117 9 N in the Air Nitrogen follows a complex path as it flows from the atmosphere through living systems and eventually back to the atmosphere, and any diagram that attempts to depict this route is bound to be confusing. The full biogeochemical cycle contains several inner loops, alternate pathways, and reservoirs that involve many chemical reactions and compounds. To represent activity in the soil, a tangle of arrows connects one set of bacteria to another. Add details of crops, animals, sewage-treatment plants, forest fires, groundwater, coal, and oceans, and the diagram becomes practically unintelligible. Thus, to keep things simple, most diagrams do not depict the movement of chemical compounds within the atmosphere. In most cases, they represent the atmosphere as a giant reservoir of inert molecular nitrogen (N2 ) capable of being converted into fixed nitrogen by one of four main processes: nitrogen-fixing bacteria, industrial fixation, lightning, and combustion . According to these depictions, nitrogen usually returns to the atmosphere through a single process: denitrification, in which denitrifying bacteria release inert molecular nitrogen, nitrous oxide (N2 O), and nitric oxide (NO) back into the atmosphere. Although diagrams sometimes note that combustion processes release nitric oxide and nitrogen dioxide (NO2 ), they do not depict the fate of these chemically active gases because their atmospheric concentrations are so small. However, as the citizens of Los Angeles discovered in the 1940s, these trace atmospheric gases cannot be ignored; and the city’s response to emissions of nitrogen oxides played a pivotal role in subsequent U.S. efforts to measure, monitor, and manage all types of emissions into the atmosphere. Framed in terms of a biogeochemical imbalance, emissions of nitrogen oxides might not have seemed to be particularly serious. After all, according to a 1970 advisory committee charged with recommending an 118 LE A R NING TO ESTA BLISH HUM A N-DEFINED LIMI TS ambient-air-quality standard for nitrogen dioxide, power plants, industrial facilities, and vehicles were releasing only about 50 million tons of nitric oxide and nitrogen dioxide (together referred to as NOX ) each year, an order of magnitude less than the 500 million tons circulated by nature in the same period. According to a parallel study funded by the American Petroleum Institute, the difference in these quantities suggested that combustion sources were in no danger of overwhelming the natural biogeochemical cycle. In practice, however, anthropogenic emissions of these gases were generating concern: the advisory committee found that about 40 percent of all nitrogen oxides released from vehicles and industrial facilities was coming from sources in the United States, and most of that amount was concentrated in several highly populated areas.1 The general process of developing a network to measure and monitor their presence in ambient air, along with the effort to manage their emissions, represented another step toward developing more sustainable interactions with Earth systems. Learning to See the Invisible It is no coincidence that the main constituents of the atmosphere, molecular nitrogen (78 percent) and oxygen (21 percent), are invisible to human eyes: eyeballs evolved in a way that allowed organisms to explore their environments using solar radiation that passes freely through the atmosphere. But humans are still adapting to their environments and still developing tools to explore their surroundings. In the twentieth century, for example, we learned to exploit the properties of light to “see” atmospheric gases present in extremely low concentrations, including nitric oxide and nitrogen dioxide. Such innovations are especially interesting because they were not driven solely by military needs or market forces; they were also encouraged by policy choices designed to prevent the degradation of air quality. Although the main constituents of the atmosphere, oxygen and inert molecular nitrogen, are transparent to a large band of solar radiation, other atmospheric gases (present in much smaller concentrations) absorb a portion of that radiation. To atmospheric scientists, this ability of trace gases to absorb certain wavelengths of electromagnetic radiation is an extremely valuable characteristic. By measuring the amount absorbed at different wavelengths, they can determine the concentration of each gas in a sample of air. Each gas, in a sense, has a different fingerprint, and analytical tools for detecting those fingerprints have been key to understanding how trace gases affect atmospheric dynamics.2 Development of this analytic capacity began in earnest with Robert Bunsen and Gustav Kirchhoff’s 1859 invention of the spectroscope. They had been studying phenomena that gave off light, such as the glow of an...

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