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Air Quality Management in the United States
ary pollutants requires identification of the precursor compounds and their sources as well as an understanding of the specific chemical reactions that result in the formation of the secondary pollutants. Control can be further complicated when the chemical reactions resulting in secondary-pollutant formation involve complex, nonlinear interactions among the precursors. Under those conditions, a 1:1 relationship might not exist between a reduction in precursor emissions and reductions in secondary-pollutant concentrations. Ground-level O3 is an example of such a secondary pollutant; it is formed by reactions of nitrogen oxides (NOx) and volatile organic compound (VOC) species3 in the presence of sunlight. In some circumstances, O3 concentrations are most effectively controlled by lowering both VOC and NOx emissions. For other circumstances, lowering VOC or NOx emissions may be most effective (NRC 1991).
Similar complications arise in the mitigation of suspended particulate matter (PM), which refers to a heterogeneous collection of solid and liquid particles that include ultrafine particles (diameters of less than 0.1 micrometers [μm]); fine particles (diameters of 0.1 to a few micrometers), which are commonly dominated by sulfate, nitrate, organic, and metal components; and relatively coarse particles (diameters of a few micrometers or more), which are often dominated by dust and sea salt. PM can be a primary or secondary pollutant. As a primary pollutant, PM is emitted directly to the atmosphere, for instance, as a result of fossil fuel combustion. As a secondary pollutant, PM is formed in the atmosphere as a result of such processes as oxidation of sulfur dioxide (SO2) gas to form sulfate particles. Because the reactions that result in the formation of secondary PM often depend on the concentration and composition of preexisting airborne PM, control strategies that lower the emissions of one chemical constituent of airborne PM might not affect or might in some cases increase the concentrations of other components of PM. Even though pollutants have been typically treated independently in many of the air quality regulations in the United States, pollutants are often closely coupled. For example, most pollutants are emitted into the atmosphere by the same source types (see Figure 1-2). They also often share similar precursors and similar chemical interactions once in the atmosphere. For example, many of the VOCs that react to form O3 are also identified as hazardous air pollutants
Nitric oxide (NO) and nitrogen dioxide (NO2) are referred to together as NOx. VOCs are organic compounds present in the gas phase at ambient conditions. Several other terms are used operationally to refer to and classify organic compounds. For example, reactive VOCs are sometimes designated as reactive organic gases (ROG); however, because hydrocarbons make up most of the organic gas emissions, this category is also called reactive hydrocarbons (RHC). Moreover, because methane dominates the unreactive category, the term nonmethane hydrocarbons (or NMHC) is often used. Unless noted otherwise, VOCs will be used in this report to represent the general class of gaseous organic compounds.