FIGURE 1-1 Schematic of the factors influencing the pollutant mix in the atmosphere and the resultant impacts of pollution. Greenhouse gases and climate change impacts are not included because they fall outside the committee’s charge.

Air pollutants are often characterized by how they originate: pollutants emitted directly into the atmosphere are called primary pollutants; those formed as a result of chemical reactions within the atmosphere are called secondary pollutants. Control of secondary pollutants is generally more problematic than that of primary pollutants, because mitigation of second-

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