the framework (for example, expanded discussion of “Airborne-PM Burden” can be found in Chapter 5 of the draft), thereby enabling readers to understand how the chapters interconnect in this consistently used context.

Chapter 1 is also the place where the PM standards adopted or proposed in the three countries should be presented, especially because these standards are taken as the starting point for the assessment. The current discussion of the standards does not provide enough information about the rationale for the concentrations and averaging times chosen by each country. Ideally, a table that clearly indicates the standards, when they became or will become effective, and whether or when they will be reviewed should be presented. And, it would be useful if some context for comparing standards that have different concentrations, averaging times, and exceedance allowances could be provided.

PM health effects are mentioned briefly in Section 1.4. This discussion would be improved by acknowledging that effects of long-term exposure are perhaps more important than effects of short-term exposure. Section 1.6 very briefly mentions impacts other than health and visibility, but notes that the assessment focuses on those impacts as the primary current drivers of air quality management. Although this limitation of scope may be reasonable, it would provide a useful perspective for readers to expand this section by including at least a paragraph describing other impacts more explicitly and giving one or two key references for each. Such impacts might include those listed in the “Impacts” box of Figure 2–1 of this report. Perspective might also be improved by mentioning that although the health effects are currently the primary driver for concern over PM, it is plausible that other effects, such as climate, might possibly become more significant drivers in the future.

CHAPTER 2: “ATMOSPHERIC AEROSOL PROCESSES: HOW PARTICLES CHANGE WHILE SUSPENDED IN THE AIR”

Chapter 2 is a good tutorial on the chemistry and microphysics of atmospheric aerosols and should be generally accessible to someone who has taken classes in undergraduate chemistry and physics. Jargon is not too dominant and is reasonably well defined when necessary. Nevertheless, a terminology box, as previously suggested for all technical chapters, would be helpful to many potential readers. In addition, there are quite a few awkward sentences (some have been flagged in Attachment B of this report), and the chapter would benefit from a thorough and professional copyediting. As mentioned in Chapter 2 of this report, the recommendation boxes are confusing because they are listed in no apparent order and some numbers are duplicated.

The discussion in Chapter 2 emphasizes science, with the policy implications of the science largely confined to Sections 2.6, 2.8, and 2.9. The policy-relevant points should be summarized and re-emphasized in a concluding policy implications section, as suggested for all technical chapters. Ending the chapter with a section on PM climate effects tends to be a diversion rather than providing a focusing summary.

The draft PM assessment does not describe in enough detail the key role that laboratory experiments play in developing an understanding of the specific chemical and physical



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