to be complete. Other definitions and examples of conceptual models are provided in Sections 1.8 and 9.2.3.

Given the multiple uses of “conceptual model” in the assessment, the committee struggled to determine how NARSTO intended to use the term. Looking to the goals and specific objectives that NARSTO set out for the assessment (see Box 1–1 in Chapter 1 of the present report), the committee notes that the second objective is to “Provide a comprehensive conceptual model of aerosol formation and particulate matter distribution for science-policy analysts and air quality decision makers.” However, although conceptual models may be comprehensive in the categories they include, they are by definition not comprehensive in detail, so this goal would be rather difficult to reach. A single conceptual model explicitly linking sources, chemical and physical processes in the atmosphere, and the environmental distribution of PM would be useful, but it is not identified in the assessment. What is provided in the assessment is a set of regionally specific “conceptual descriptions” that are neither comprehensive in the regions evaluated nor complete in the processes described. A single general conceptual model for airborne PM would go a long way toward meeting NARSTO’s second objective as well as its third objective, to provide a plain-language conceptual description of PM air quality for the public.

The committee recommends that a single general conceptual model for airborne-PM burden be explicitly introduced in the beginning of the assessment. The conceptual model could readily include emissions, atmospheric processing, and loss processes for airborne PM, all of which can be influenced by meteorologic factors and ultimately affect the distribution of PM in the atmosphere. To alleviate confusion, “conceptual model” should be clearly defined and consistently used throughout the document, and “conceptual descriptions,” “conceptual understanding,” and similar uses of “conceptual” should be eliminated. It should be made clear in the introduction of the conceptual model that it includes generalized concepts of major factors involved in the generation and distribution of PM.

NARSTO’s second objective also calls for the model to “accommodate changing knowledge about atmospheric processes, emission sources, emission control technology, exposure, and human health and environmental impacts.” Similarly, the fifth objective calls for the assessment to “provide a framework for atmospheric scientists which relates their work to standards, implementation and air quality management, and to health, exposure, and environmental impact research for standard setting.” Thus, effectively connecting the conceptual model of airborne-PM burden to the impacts and policy implications of increased PM requires placing the conceptual model in a larger framework for informing airborne-PM management. Figure 1.6 in the draft assessment illustrates part of the interaction between science and policy, but it is not detailed enough to meet the objectives.

The committee recommends that a clear framework for informing airborne-PM management, of which the conceptual model for airborne-PM burden is an important component, be explicitly introduced in Chapter 1 and near the beginning of the executive summary. The framework should be sufficiently fundamental for its components and their implications to be readily understood by lay readers (that is, it should be largely self-explanatory) and to serve as a basic paradigm for understanding variations that apply to any region over time. An example of such a framework is provided in Figure 2–1 of the present report, and the committee recommends that this framework or something similar of NARSTO’s choosing be used in the assessment. The

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