4
COMMENTS ON THE ASSESSMENT’S CHAPTERS AND RELATED APPENDIXES

CHAPTER 1: “PERSPECTIVE AND CONTEXT FOR MANAGING PM”

Chapter 1 introduces the reader to PM, the various factors that affect its composition and spatial distribution, how it affects humans, and what types of regulatory actions have been taken or proposed by the three North American nations. The right assortment of concepts is addressed, but they are sometimes poorly presented. In particular, a reader who is not already well informed on the subject matter would probably carry away incomplete concepts regarding the role of gaseous emission of organic substances, the relative roles and interactions of elemental and organic carbon, the present ability to dissect organic-carbon composition, and the nature and importance of ultrafine PM. Other specific problems are noted in Attachment B of the present report and can be readily clarified by modest editing.

A larger underlying problem with Chapter 1 is that it does not effectively provide a clear framework defining PM and the interconnections between the numerous processes that affect PM and its effects. Readers new to the subject need such a framework to understand PM and how the various chapters in the NARSTO draft assessment fit together. As mentioned in Chapter 2 of the present report, the committee recommends that a general framework for informing airborne PM management, similar to that in Figure 2–1 of this report, be explicitly introduced and defined in Chapter 1 of the NARSTO assessment. The discussion accompanying the figure should explain the processes contained in each box and the interconnections between the boxes. It should also be stated that the general conceptual model presented provides a paradigm of the factors affecting PM but that the complexity of PM makes it impossible to incorporate all the processes and their interactions in a single conceptual model. Indeed, an infinite number of conceptual models could be developed for different regions, times, and conditions, such as those prepared for the nine regions discussed in Chapter 10 of the draft.

Once the framework for informing airborne-PM management is clearly presented in Chapter 1, attention needs to be paid to doing a better job of linking to and being consistent with the rest of the document. Signposts that guide the reader to where in the other chapters one could find more discussion about each topic are essential. The signposts should be clearly associated with



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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter 4 COMMENTS ON THE ASSESSMENT’S CHAPTERS AND RELATED APPENDIXES CHAPTER 1: “PERSPECTIVE AND CONTEXT FOR MANAGING PM” Chapter 1 introduces the reader to PM, the various factors that affect its composition and spatial distribution, how it affects humans, and what types of regulatory actions have been taken or proposed by the three North American nations. The right assortment of concepts is addressed, but they are sometimes poorly presented. In particular, a reader who is not already well informed on the subject matter would probably carry away incomplete concepts regarding the role of gaseous emission of organic substances, the relative roles and interactions of elemental and organic carbon, the present ability to dissect organic-carbon composition, and the nature and importance of ultrafine PM. Other specific problems are noted in Attachment B of the present report and can be readily clarified by modest editing. A larger underlying problem with Chapter 1 is that it does not effectively provide a clear framework defining PM and the interconnections between the numerous processes that affect PM and its effects. Readers new to the subject need such a framework to understand PM and how the various chapters in the NARSTO draft assessment fit together. As mentioned in Chapter 2 of the present report, the committee recommends that a general framework for informing airborne PM management, similar to that in Figure 2–1 of this report, be explicitly introduced and defined in Chapter 1 of the NARSTO assessment. The discussion accompanying the figure should explain the processes contained in each box and the interconnections between the boxes. It should also be stated that the general conceptual model presented provides a paradigm of the factors affecting PM but that the complexity of PM makes it impossible to incorporate all the processes and their interactions in a single conceptual model. Indeed, an infinite number of conceptual models could be developed for different regions, times, and conditions, such as those prepared for the nine regions discussed in Chapter 10 of the draft. Once the framework for informing airborne-PM management is clearly presented in Chapter 1, attention needs to be paid to doing a better job of linking to and being consistent with the rest of the document. Signposts that guide the reader to where in the other chapters one could find more discussion about each topic are essential. The signposts should be clearly associated with

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter 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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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter mechanisms by which PM evolves in the atmosphere. Chapter 2 is the logical place to provide some discussion of this topic, and the committee recommends that such discussion be included. Several of the recommendations from Chapter 11 that are included in boxes in Chapter 2 (for example, 1.3, dealing with gas-particle conversion effects for organic aerosols and 3.4 dealing with modeling particle microphysics) will not be achievable without substantial innovative experimental laboratory work in heterogeneous chemistry and aerosol microphysics. The committee notes several problems with Sections 2.8 and 2.9, in which the discussion aims to link the PM problem with other pollutants, visibility, and climate. A number of weaknesses in Table 2.2 were identified in the review of the response to PQ5, where the same table is used. Not only is the table almost impossible to read in the print form, but it is not discussed at all in the Chapter 2 text, despite the clear need to explain it. The table should be either eliminated or better integrated into the discussion. The paragraph in Section 2.8 on PM and haze is both too terse and redundant with Chapter 9, which is not cited for a more thorough discussion. It could be shortened or even deleted with a suitable signpost to Chapter 9. Climate is addressed briefly in Sections 2.9 and 9.4; neither treatment is sufficient. The climate discussion in Section 2.9 is outdated by the publication of the most recent report of the Intergovernmental Panel on Climate Change (IPCC 2002). The specific forcing values presented should be updated with those in IPCC (2002). The discussion in both sections is a little too simplistic in concluding that the factors governing the direct climate effect and haze are the same. The direct climate effect is actually a balance between visible and near infrared scattering and absorption. Scattering, which leads to cooling, dominates for most aerosol particle types. In contrast, absorption, which leads to heating, can be important for some particle types. The committee recommends that the subject of PM climate effects be addressed well once rather than poorly twice. CHAPTER 3: “EMISSION INVENTORIES”; APPENDIX A: “EMISSION CALCULATIONS AND INVENTORY LISTINGS” Chapter 3 summarizes emission information available for Canada, Mexico, and the United States. The types of emission inventories and their uses, uncertainties, and limitations are presented. The chapter also discusses the spatial and temporal patterns of PM emission, and provides PM inventories for various locations and sources. In general, the chapter contains appropriate subject matter. However, one missing topic is the role of transient emissions from off-normal operating conditions (such as, startup of a waste incinerator) in overall emissions and uncertainties in their inventories. The presentation of material in this chapter is uneven. First, the descriptive text needs a thorough editing to reduce repetition and increase clarity. Second, the tables and the analysis of the data derived from the tables need to be checked for accuracy. Some obvious typographic errors and miscalculations are identified below and in Attachment B of this report. Finally, as discussed in Chapter 2 of this report, the recommendation boxes are confusing. In Chapter 3 of the draft assessment, the placement of the boxes sometimes appears to be arbitrary and has little relation to the surrounding text.

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter There needs to be a better crosswalk between this chapter and other chapters in the assessment to ensure consistency. The presentation of Mexico City’s emissions in Chapter 10 is much better than that in Chapter 3. The information contained in CAM (2001) and Molina and Molina (2002) may be useful for improving the discussion of Mexican emissions. It is unclear whether the research recommendations for emission-inventory improvements presented in Chapter 11 are adequately supported in Chapter 3. The discussion of using receptor-based and source-based models to validate emission inventories in Chapter 3 appears inconsistent with the discussion of these tools in Chapters 6 and 7. Chapters 6 and 7 seem to indicate that the modeling tools can be used in a “weight-of-evidence” mode to help to evaluate the accuracy of inventories. The authors should be consistent as to whether emission inventories can be definitively validated or merely evaluated against the results of source-based and receptor-based models. The comparison of Canadian and U.S. particulate inventories listed under Table 3.3 would be more informative if numbers were shown as intensities, that is, normalized by population or gross domestic product (GDP). For example, in comparison with Canada, the United States, with 9 times the population and 11 times the GDP, produces only 3 times as much industrial primary PM10 or PM2.5, 2 times as much industrial SO2 or VOC, 6 times as much NOx, and 7 times as much NH3. Tables 3.5 and 3.6 should also be normalized by population, number of vehicles, or other parameters to enable comparisons among these locations. This normalization will allow the authors to check whether the data are consistent and accurate. For example, comparing on-road emissions in Tables 3.5 and 3.6 on a per-vehicle basis shows that Los Angeles has much lower SO2 emissions than Atlanta or Mexico City, as would be expected from the lower sulfur in the fuel. However, the Toronto number appears to be inconsistently low, given the relatively higher concentrations of sulfur in today’s Canadian gasoline. The authors appear to be occasionally confusing the discussion of emission inventories, which are related to sources, with atmospheric concentrations, which is the relevant measure of human exposure. PM concentrations to which humans are exposed are the result of emissions that have undergone some amount of atmospheric processing, which can change the concentration, composition, and spatial distribution of PM. Indeed, airsheds can have widely different assimilation capacities for PM and lead to large differences in exposure for a given magnitude of emission. The differences between emission characteristics and ambient concentrations can be especially large for secondary pollutants and pollutants that can be transported over long distances. The text occasionally gives the impression that ambient concentrations and their resulting exposures can be understood merely through an understanding of emission inventories. CHAPTER 4: “GAS AND PARTICLE MEASUREMENTS”; APPENDIX B: “MEASUREMENTS” Overall, Chapter 4 is better written and edited than many others. As is the case with other chapters, a terminology box would help many readers with technical terms. The tone of the

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter introductory discussion (pages 4–1 to 4–4) is appropriate. The various reasons for making measurements, which are not widely known, are accurate, and the discussion of the types of things that need to be measured is clear and useful. Section 4.1, which forms the bulk of the chapter, is an accurate, comprehensive, well-written, and effectively organized description of the current state of research and monitoring instrumentation. Short descriptions of the measurement instruments and techniques for various target data types, and their strengths and weaknesses, are presented in the chapter’s subsections; and more detail, suitable for the measurement specialist, is provided in Appendix B. Both the chapter section and the appendix are well organized by measurement type; the two of them combined are a tour de force. A few overview points, however, are missing from Chapter 4. First, the research-grade instruments and deployment strategies used to gain scientific understanding are often much more sophisticated, accurate, and reliable than the instruments and techniques used to monitor compliance; to develop emission inventories, exposure assessments, and source attributions; and to evaluate program success. Thus, some measurements are of much higher quality than others. That point needs to be made more clearly. Second, the advancement of key technologies enabling improved instrument design and deployment strategies has been steady. However, for various regulatory, management, and economic reasons, the penetration of advanced measurement techniques into monitoring and compliance applications has been much slower. In fact, many monitoring networks and exposure- and emissions-assessment activities depend on instruments and measurement strategies based on 30-year-old technology. Thus, many of the measurements used to monitor exposure, visibility, and compliance, to develop source attributions, and to evaluate program success are far less comprehensive and reliable than possible. It would be useful to policy-makers to discuss the technologic limitations of current ambient monitoring, exposure and emission assessment, and program evaluation and any programmatic reasons for those limitations. Third, insufficient attention is paid in the draft assessment to the design of measurement strategies. For example, issues pertaining to the frequency of monitoring, duration of sampling, and number of monitoring sites are not extensively addressed but may be equally important with respect to the specific types of measurements that are made. Better-quality measurements are not necessarily helpful unless they are made in the context of well-designed (ideally, statistically based) networks or studies. In addition, except for the discussion of remote sensing instruments and some terse mention of aircraft-inlet issues, it seems to be assumed that most sampling instruments will be used only at fixed surface sites. However, the development of accurate, real-time (measurement time, of a few seconds or less) instruments opens up opportunities for both airborne instruments (on manned and unmanned aircraft and balloons) and ground-level instruments (on vans, boats, and trains) that can make measurements with better spatial resolution and coverage. The committee suggests that a short section on measurement strategies be added, motivated by the fact that current fixed-site deployments badly undersample the atmosphere. Current measurement data are almost always more sparse than the spatial resolution of analysis and assessment models, and this makes model validation extremely difficult. The recommendations drawn from Chapter 11 and interspersed in this chapter are sometimes oversimplified and neglect some of the related subtleties and implications. The

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter recommendations are not always fully supported by discussion of where abilities lie now and where they are expected to lie in the near future. Many of the recommended measurement goals will require enhanced versions of existing instruments, such as continuous chemical-speciation measurement and real-time single-particle chemical-speciation techniques. Chapter 5 should identify the research activities necessary to attain the recommended instrument innovations. The implications of the recommendation for more continuous-monitoring development are not fleshed out. Clearly, not all the components that can be measured with filter techniques can now be measured continuously, so replacement of filter-based measurements would result in an increase in temporal resolution at the expense of decreasing the specificity. It is not clear when this is warranted; it certainly depends on the ultimate uses of the measurement data. If the recommendation is not suggesting that real-time measurements of all the species are desirable, those of highest priority should be identified. On the other hand, real-time, hour-by-hour data could be useful to the health community (for example, to address hypotheses of spiked versus integrated exposure) and may be justified on that account. Section 4.2, on measurement uncertainties, is an excellent overview of individual measurement uncertainties. This topic is often neglected and has policy implications. However, the topic of uncertainties due to undersampling or otherwise inappropriate sampling of the atmospheric system are not mentioned and should be addressed. The section could be improved by adding a paragraph or two at the end to discuss the policy implications of all types of measurement uncertainties. In general, Chapter 4 could be improved if the policy implications of the scientific and technical information presented were more clearly delineated. The chapter should address how measurements are now used in air-quality management and what advances in measurement technology are necessary for management. Furthermore, the chapter would be more useful to air-quality managers if it provided information for choosing among the different measurement strategies and devices, perhaps via a table that compared the size, sample duration, specificity, sensitivity, and other characteristics of each. Finally, Chapter 4 needs to be clear about the accuracy and precision of each measurement technique and the resulting policy implications. The information presented in Appendix B is useful, but the presentation could be improved. Of particular concern is that the numerous tables are not discussed or referred to in the text. Many tables have titles and captions that are actually discussions that should be in the text. A number of the tables (such as, Table B.1) have several empty boxes; it should be clarified whether this means that the box is “not applicable” or the information required is “not available” or that the answers are “variable” and depend on different researchers. CHAPTER 5: “SPATIAL AND TEMPORAL CHARACTERIZATION OF PARTICULATE MATTER CONCENTRATION AND COMPOSITION”; APPENDIX C: “MONITORING DATA: AVAILABILITY, LIMITATIONS, AND NETWORK ISSUES”; APPENDIX D: “GLOBAL AEROSOL TRANSPORT” Chapter 5 is generally well done, presenting a host of PM10 and PM2.5 data from a wide variety of North American monitoring networks. Recent and historical data from a number of

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter measurement programs are summarized cohesively. These data are the best available representation of the scope and scale of the North American PM problem, including its seasonal variability and 10- to 15-year trends. The chapter is critical in setting up Chapters 6 and 10 and in providing the foundation for the responses to PQ1-PQ3. In short, the data presented in Chapter 5 form the basis for the most policy-relevant part of the report—the regional descriptions of the PM problem presented in Chapter 10. The tie-in between Chapters 5, 6, and 10 should be noted in the Chapter 5 introduction, and the authors should ensure that the discussion in Chapter 5 adequately supports that in Chapters 6 and 10. Many of the figures presenting time-series data for various monitoring sites or comparing data from multiple sites for the same timeframe are compelling because they illustrate well the high degree of spatial or temporal variability in the aerosol burden. Conveying the highly variable nature of the PM problem in the temporal and spatial dimensions is an important goal for Chapter 5. The data plots are generally effective, although several still fall short of the clarity and readability that are necessary, so it was difficult for the committee to evaluate them. Specific deficiencies are noted in Attachment B of the present report. To be fair, Chapter 5 starts with a note that some of the chapter figures are still under development. If properly done, the figures will be worth many words, so it is critical that they be done well. Many of the data in Chapter 5 are from the gray literature (publications that are not peer-reviewed, not easily accessible, or both) or personal communications. If the discussion in the chapter is going to rely heavily on gray literature, it is critical to tell the reader how to find the data. And if no widely distributed publication is available, more of the details of the data collection should be included in the chapter (or in an appendix). The most egregious example of this problem is the frequently referred-to Hansen (2000) document, which is a memo rather than a publication and should properly be cited as a personal communication. Because a large fraction of the data presented in this chapter are described in the memo, either it needs to be submitted to a peer-reviewed journal or the data-collection method needs to be detailed in the assessment. Other gray-literature documents that are heavily referred to are Vet et al. (2001), an internal report available on request from the Meteorological Service of Canada, and CARB (2001), a similar internal report. In general, the authors should include references to Web pages where the data can be found and make it clear to the reader that the information is based on work that has not been peer-reviewed. Chapter 5 does a good job of presenting the variability of the atmospheric PM burden, but it is largely silent on the variability imposed by measurement error in the data presented. There is some useful discussion of the analytic errors that affect data accuracy and the statistical sampling and measurement errors that affect data precision in Sections C.2 and C.3 of the supporting Appendix C. A statement about the general level of accuracy and precision of the data in the chapter needs to be included, and the interested reader should be directed to Appendix C for further information. The information on measurement quality could also be presented in a more coherent and complete manner in the appendix. Another aspect of variability in PM that Chapter 5 does not address sufficiently is spatial heterogeneity within urban areas. The health community is increasingly concerned about health effects on finer scales than the current monitoring network is able to resolve. For example, recent studies indicate that proximity to roadsides may be associated with PM health effects (e.g.,

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter Brunekreef et al. 1997); but the current network does not typically include any routine roadside monitoring. The committee recognizes that the assessment authors are limited by the available data but recommends that they augment the discussion of smaller-scale variability and its relation to PM health effects. That can be done, in part, by referring to the discussion in Chapter 8 of the assessment. The authors should also add discussion of how the atmospheric-science community might address this outstanding research question better. Mexico is not satisfactorily represented in Chapter 5. Just as a key PM-problem region crossing the U.S.-Canada border is identified in the Pacific Northwest, several areas with severe PM problems cross the U.S.-Mexico border. Those areas are the focus of much current research because expected human exposures to PM are probably very high. Many factors contribute to the high levels of particle concentrations that have been observed in those areas, including the nature and characteristics of the emission sources, land use and topographic characteristics, and meteorologic conditions (Mukerjee 2001; Mejia-Velazquez and Rodriguez-Gallegos 1997). Chapter 5 should discuss the areas. In addition, the figures in Chapter 5 that already include Mexican data often are inadequate (see Attachment B of the present report). A better attempt could be made to discuss Mexico’s PM characteristics more completely. The introduction of Chapter 5 tries to set up the theme of widely varied spatial and time scales and their effect on PM distributions and trends. The text is reasonably successful, but Table 5.1, which is supposed to summarize the message, is unsuccessful. First, the “global” scale does not start at 1000 km; in fact, for tropospheric aerosol, it is debatable whether there are any truly global effects—even aerosol climate forcing operates on a continental or ocean-basin scale that is semiglobal at most. Second, and more serious, by apparently mixing the time scales for aerosol processes with the time scales for such aerosol effects as health effects, which can be acute (hours to days) or chronic (years to decades), the table obscures all sense of the connection between atmospheric-process spatial and time scales. Table 5.1 needs much more thought so that it will support rather than apparently contradict or confound, the text of Section 5.1. Section 5.2, “The Influence of Global Aerosol Transport on PM Mass Concentrations in North America,” is poorly positioned in the chapter in that the data show that global transport is not significant. Although this section is scientifically accurate and interesting, starting the chapter by discussing an insignificant determinant of observed aerosol variability will draw the reader away from factors that are important. The committee suggests shortening the section and making it a subsection with the same title right after the current subsection 5.4.4., “Regional Transport.” As noted above, transport of aerosols is still at most semiglobal or supraregional; dust can be transported across an ocean but generally not around the entire globe. Most of Section 5.4, “Regional and Urban Contributions to Particulate Matter,” is well motivated and well done. The discussion of weekly cycles is interesting, especially the difference between PM2.5 and PM10. The data showing the weekly pattern is potentially powerful and needs to include references to published work to allow the reader to be convinced that it is statistically relevant. The section on visibility in the Grand Canyon is redundant with, and in some cases contradicts, material on the same topic in Chapters 6 and 9. The committee strongly suggests integrating the material from page 5–37, line 31, through 5–39, line 10, into the relevant material in Chapter 9. Chapter 5 can mention the regional-transport evidence from the Grand Canyon study in one short paragraph and refer the reader to Chapter 9 for details, thus reducing a

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter serious redundancy in the overall assessment. Furthermore, special attention should be paid to making all discussion of the Grand Canyon study consistent. Section 5.6, “Covariation of PM with Ozone and Other Air Pollutants,” successfully makes the important point that ozone correlates with only part of the PM burden—and then only part of the time. However, it could be made more succinctly but just as forcefully. In addition, the only data presented compare ozone with various PM measures, so the “other pollutants” in the section title is misleading. In general, this section could use some copyediting to tighten the text. The current “Conclusion” section is done well from a scientific-summary point of view. However, it begs to be rewritten so that its many important policy implications are explicitly identified. As is the case with the other technical chapters, the committee recommends that this section be rewritten in this fashion and relabeled “Policy Implications.” In particular, the policy implications of Section 5.5, “Trends and Their Implications,” one of the most policy-relevant sections in the assessment, should be more definitively expressed here, and another signpost to Chapter 10, where many of the policy implications are further illuminated, should be planted. CHAPTER 6: “RECEPTOR METHODS FOR SOURCE APPORTIONMENT— BEYOND THE EMISSION INVENTORIES” Chapter 6 presents a comprehensive review of receptor-modeling methods. The authors have done an excellent job of reviewing the science. In particular, they have tied together disparate approaches that have been developed over the years and have explained the basic concepts in lay terms while providing some seminal references for the reader. Nonetheless, a number of issues should be improved before publication. Better coordination between Chapters 6 and 7 is needed. A consistent approach to framing and contextualizing Chapters 6 and 7 would enhance the presentation particularly because the two categories of tools are complementary and can be used to address overlapping sets of questions. Furthermore, the two chapters would benefit from being more consistent with the rest of the assessment. The committee recommends that each chapter begin by explaining how the receptor- or source-based modeling tools fit into the framework for airborne-PM management, which was ideally presented in Chapter 1. How the tools in Chapters 6 and 7 are similar and different should also be explicitly described. Each chapter should present the policy relevance of the material in a parallel manner in a final “Policy Implications” section, as recommended for all technical chapters. The policy-relevant material in Chapter 6 is now found in Section 6.5, which addresses how modeling tools are applied in PM policy development, and in Section 6.6, which includes answers to “science questions” that are similar to but not exactly the same as the PQs used in the executive summary. Chapter 6 would benefit from heavy editing to eliminate sections that are presented and discussed elsewhere and to improve the consistency between it and the rest of the assessment. There seems to be substantial overlap with other chapters, including the discussions of single-particle mass spectrometers and of haze in the Grand Canyon. The definition of manageable and unmanageable source contributions is introduced more successfully in Chapter 1 than in Chapter 6. Definitions of terms in Chapter 6 are not entirely consistent and should be better aligned with

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter those in Chapter 1. For example, in Chapter 6, “Saharan dust” is listed as an unmanageable source, but “wind-blown dust from natural areas (e.g., deserts)…that is accentuated due to land-use or industrial practices is at least partially manageable.” Likewise, the idea of “weight-of-evidence,” introduced on page 6–49 as the “agreement between receptor and source models,” needs to be treated consistently and introduced in Chapter 1 of the assessment. The only previous mention of weight-of-evidence is in the executive summary, where it is defined as the “use of integrated information from emissions inventories, ambient concentration measurements, and air quality models.” The idea of a conceptual model is defined and used in the most detail in Chapter 6, but it still is not very clear. As discussed at length in Chapter 2 of the present report, the committee recommends that the treatment of conceptual models be consistent throughout the assessment. Despite the problems with repetitiveness and consistency, Chapter 6 is very readable, although possibly at the expense of a number of details and recent results that would provide more specific information. For example, the discussion of source tracers is informative but would benefit from being more quantitative. Such species as vanadium are good for identifying some fossil-fuel combustion sources, but their utility often depends on measuring a sufficient quantity relative to other species to obtain a clear signal. In addition, although some sources have a number of elemental tracers, not all have them in sufficient quantities to be useful for identification. The graphs in Figure 6.5 illustrating several source signatures are useful for making that point, but the discussion in the text needs to link to the figures better. Another example is the diesel-emissions discussion (page 6–39), which neglects the possibility that these emissions may produce ultrafine particles not captured by PM2.5 mass measurements at very high number concentrations or fluxes. Although few measurements of ultrafine particles exist, it would be worth pointing out the potential health effects associated with them, especially considering that other currently unregulated factors (for example, cold starts and vehicle maintenance) have been discussed. A number of the examples chosen to illustrate points made in Chapter 6 are not appropriate or effective, possibly because not many case studies that assess PM-management strategies are available. The discussion of sulfur reductions in Canadian gasoline, which is intended to give an example of how to use a conceptual model and receptor-oriented methods to identify emission-reduction targets, is par-ticularly problematic. It is a poor example of PM management because fuel sulfur reductions were not intended to reduce primary emission of PM. In this example, the chemical mass balance model indi-cates that vehicle emissions are responsible for 50% of PM2.5 in Toronto and 42% of PM2.5 in the Lower Fraser Valley of British Columbia. The motor-vehicle source is not speciated in Figure 6.7, so it is unclear whether the PM2.5 from motor vehicles is due mainly to sulfates, organic compounds, or nitrates. The accompanying text indicates that reducing emissions of fuel sulfur by more than a factor of 20 was estimated to decrease PM2.5 by only 1.8%. That ambient PM2.5 was predicted to be largely unaffected by fuel sulfur reductions suggests that perhaps other sources might be better emission-reduc-tions targets. It seems that one could conclude from the modeling activities that targeting the large amount of nonvehicular ammonium sulfate or vehicular emissions of organic material and nitrate may be more effective for reducing PM2.5. One might even conclude that reducing fuel sulfur is not an effective strategy

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter for PM control. It would be better to choose an example in which the receptor-based modeling methods are used to reduce PM2.5 more dramatically. Further muddling that example is Figure 6.8, which is confusing despite seeming to be central to the discussion. It is unclear how to read the diagram: Does the bottom right corner indicate that changing fuel causes emission changes in the top left? If so, the directionality of the arrows and having some of the arrows go in two directions do not make sense. And, the intent of the boxes labeled “Approach…” and “Responds…” is unclear. For example, some boxes under the “Approach” label appear to indicate emissions. The discussion of Figure 6.8 refers to colored and numbered boxes, but the figure is not in color, and no boxes are numbered. A second problematic example is the discussion of “Haze in the Grand Canyon.” The modeling activities indicated that emissions of SO2 from the Mohave Power Project did not contribute much to the sulfate observed over the Grand Canyon, but a decision to install sulfur scrubbers was made nonetheless. Again, the modeling results did not seem to guide the policy decision. The comment on page 6–47 about how the results “were most influential in determining the timing of implementation rather than whether or not to implement sulfur reductions” is confusing. Were the pre-existing plans to install sulfur scrubbers delayed because the modeling results indicated that the effect would be small? Perhaps the example should be presented as a study that should have been done before the decision to implement scrubbers was made. As it stands, the authors seem to have concluded that the money spent on installing scrubbers will have a negligible effect on visibility and that the investment in the scrubbers will be shown to have been an unwise policy decision. If that was not the point of the example, it needs to be better clarified in the text. Understanding the example is complicated by the confusing nature of Figure 6.11 and its caption, which together provide the basis for the assertion that the receptor models indicated that sulfur from the Mohave Power Project contributed only a small fraction to sulfate over the Grand Canyon. If that is the key result of the study, the figure and the caption should be made abundantly clear, especially because not all readers will be familiar with cumulative-frequency plots; for example, it may be unnecessary to include the left panel for this purpose. CHAPTER 7: “USING MODELS TO ESTIMATE PARTICLE CONCENTRATIONS AND EXPOSURE” The role of chemical transport models (CTMs) in calculating particle concentrations and exposure is addressed in Chapter 7. Overall, this chapter is well written and comprehensive and constitutes a useful summary. Relative to the other chapters, Chapter 7 has a large number of specific references. That is not totally consistent with the style of the other chapters, but it is a better model, and the other chapters could include more specific citations. As discussed in the comments on Chapter 6, Chapters 6 and 7 should be framed more consistently to draw more attention to how the tools presented in each chapter complement each other. Chapter 7 should be made more consistent with the rest of the assessment, via the framework for airborne-PM management and a final “Policy Implications” section, as recommended for all technical chapters. Because CTMs try to represent all the factors

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter influencing PM discussed in previous chapters of the assessment (such as atmospheric aerosol processes and emissions), the authors should ensure that the discussion in Chapter 7 is consistent with previous treatments of the topics and try to avoid repetition. Likewise, how CTMs are used to elucidate the aspects of the framework for airborne-PM management, particularly the conceptual model for airborne-PM burden and the management scenarios, should be described. The policy-relevant material in Chapter 7 is in Section 7.4, “What Questions Can Chemical Transport Models Address and How Well?”; Section 7.9, “Policy-relevant results from CTM applications”; and Section 7.10, “Critical Uncertainties.” The summary in Section 7.11 is adequate from a technical perspective but could do a better job of highlighting policy-relevant contributions. For example, the discussion of uncertainties is useful for guiding policy, but that connection is not made in this section. Therefore, a concluding section should be added that lists the highest uncertainties that policy-makers could address by requiring specific measurements. Chapter 7 includes many references to uncertainties associated with CTMs, both in Section 7.10, “Critical Uncertainties,” and in the sections preceding it. To provide effective guidance to decision-makers, it is important to provide, if possible, some quantification or at least qualification of the uncertainty. Indeed, the authors should ensure that a reader does not conclude that CTM results should be discarded altogether. It should be made clear in the introductory material that CTMs will never be able to represent all the processes in the atmosphere precisely and that approximations are therefore necessary and often reasonable if based on good assumptions and data. Ideally, the discussion should relate clearly how CTMs can be effective tools for air-quality management despite their limitations. The important point needs to be emphasized that although CTMs function reasonably well under normal, or the most typical, meteorologic conditions (that is, most of the time, in most places), they do not function well under unusual meteorologic conditions. Not only might the meteorological model be unable to reproduce unusual conditions, but model components which use meteorological data as input (for example, chemistry modules use temperature and solar intensity) may include approximations or assumptions that do not apply under extreme meteorological conditions. Because pollution episodes often occur as a result of unusual meteorological conditions, models may have limited ability to predict such episodes. That point could be reinforced in both the sections describing the models and in the summary. It would provide a helpful context to include more information on the accuracy of model predictions. The treatment of this topic in Section 7.6, “Current Status of CTM Performance and Intercomparisons,” is meager. The only measures of actual model performance given are in Table 7.1, a rather dense summary of model evaluation that is not immediately accessible to an uninformed reader. The section would benefit from figures giving specific examples of how model results compare with observations. The summary record (or at least examples) of the accuracy of predictions of both episodic and longer-term averaged PM concentrations (or those of other pollutants if there are no PM data) would be useful to portray the state of the science (and art). The model intercomparison in Table 7.1 is limited by the fact that the models were not all run for the same situation or with common inputs. The statistics used to compare them are not even uniform. Some discussion of the prerequisite conditions for meaningful model comparisons should be included. For models to be compared effectively, they would ideally simulate the same

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter episode, have identical inputs for emissions and meteorology, use common formulations for basic physical and chemical processes, have a standard set of rate constants, and produce output in the same statistical format. The extent of use and congruence of CTMs among the three nations should be discussed. Are CTMs primarily research tools, or are they being used routinely for policy-evaluation studies (and are there differences among the countries)? To what degree is there uniformity of models and model use among the countries? If they are not standardized, what advantage, if any, would there be if models for specific tasks were standardized among the countries? Should the atmospheric-science community be trying to develop standardized process submodels or input datasets (as the stratospheric-chemistry community has done)? Chapter 7 does not give a balanced account of how CTMs can be used to estimate exposure. The difference between the information needed to estimate concentrations and exposure is not recognized in most of the chapter; in fact, Figures 7.2 and 7.3 deal only with the steps for obtaining concentrations. If estimating exposure is intended to be a central topic in Chapter 7 (as the title indicates), discussion of it should not be relegated to vague comments in Section 7.8. Figures 7.2 and 7.3 should be modified to show the additional inputs and steps needed to determine exposure on the basis of concentration (regardless of whether they have been implemented). Furthermore, the discussion in Section 7.8 should be more specific and provide examples of exposure estimates. Since no real analysis of exposure to ambient PM is provided, the authors may want to remove exposure from the chapter title. CHAPTER 8: “HEALTH EFFECTS OF PARTICULATE MATTER” Chapter 8 is the most problematic chapter in the draft assessment. The committee debated the utility of retaining this chapter in the assessment or of starting with the assumption that sufficient health “drivers” exist and referring in the introduction to other summaries of PM health issues (e.g., NRC 1998, 1999, 2001; EPA 2002; Molina and Molina 2002). Removing the chapter, however, would eliminate the little emphasis on health now included in the assessment, detract from meeting objective 5, and contradict assumption 2 as listed in the NARSTO charge (reproduced in Box 1–1 of the present report). Indeed, the committee feels that a chapter devoted to health effects of PM and opportunities for linking health and atmospheric-science research could be very useful, if written well, and recommends that a chapter discussing PM health effects be retained. The committee recommends that Chapter 8 undergo major revision to correct conceptual errors, improve readability, reduce reliance on technical terminology, and highlight opportunities for interaction between health and atmospheric scientists. Ideally, this chapter should provide atmospheric scientists with a highly accessible summary of PM health effects that helps them to frame their work better. References to more-detailed assessments should be provided. The summary of PM health effects should also be highly accessible to decision-makers; this assessment may be their only exposure to the topic, and it is critical that the summary be conceptually correct. In addition to providing a current, readable summary of the health effects of PM, the committee recommends that Chapter 8 be rewritten to focus more on the interface between

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter atmospheric and health sciences, on how to facilitate flow of information between the communities in both directions, and on any resulting policy implications. In particular, it would be highly useful to discuss information known or being investigated by the health community that would help atmospheric scientists. Likewise, a discussion of research avenues for atmospheric scientists to pursue to assist health scientists would be useful. For example, epidemiologic analyses indicate spatial heterogeneity in PM health effects, but the extent to which the spatial variability can be explained by PM composition or atmospheric chemistry is unknown. As another example, the importance of understanding exposure to a putative agent (for example, PM or a sub-fraction of PM) for confirming the relationship between a health outcome and that agent points toward the need for coordinated involvement of atmospheric and exposure scientists. Numerous specific corrections for Chapter 8 are provided in Attachment B of the present report. In general, the material is too technical and specific for a target audience of air-quality managers, policy-makers, and atmospheric scientists. Many in the intended audience are not likely to understand some of the terms without further explanation, such as spline curves, lag, interquartile, log of mortality-days, andAkaiken Information Criteria. The chapter should be written so that readers only modestly informed about health issues and the technical aspects of dosimetry and risk estimation can understand it. It should focus on selected key concepts and avoid being an abstract of technical information. In addition, the presentation is uneven: the personal-exposure section needs substantial editing, has several factual errors, and is out of date in places; the epidemiology section is in better shape. Chapter 8 needs editing to provide the reader with an appropriately broad and accurate overview of health issues. The approach of paraphrasing and excerpting text from the U.S. Environmental Protection Agency (EPA) criteria document for PM (EPA 2002), which is intended to be a comprehensive survey of the health-effects literature, is not the best for the NARSTO assessment. Chapter 8 relies so heavily on the EPA criteria document that the reader would become much better informed by reading summary sections of that document. Indeed, the reader may not be able to grasp the information in this chapter without having the EPA document close at hand and referring to it often to place information in context, get more background on a subject, or read a better explanation. Many recommendations included in Chapter 8 are not specific and therefore not very useful—for example, “Comprehensive, directed study of the factors [that influence exposure] would also be of tremendous value” (page 8–5) and “All of the epidemiological studies of PM and other air pollutants will benefit from close collaboration between atmospheric and health scientists” (page 8–12). In several places, the chapter notes that closer interaction between the health-science and atmospheric-science communities is needed but gives no specific examples of beneficial interactions. The committee agrees that there ought to be more communication between the communities, but mentioning the issue is just lip service unless the nature and potential results of specific types of interactions are described. CHAPTER 9: “VISIBILITY EFFECTS” Chapter 9 gives the reader a basic understanding of visibility impairment. The committee

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter notes that visibility and regional haze are also discussed in Chapters 1, 2, 5, and 6. Efforts should be made to reduce repetition by concentrating the discussion in Chapter 9, having other chapters refer to the discussion in Chapter 9, and ensuring that all treatment of the topic is consistent. Of particular importance is a consistent treatment of the roles of light absorption and scattering and of how water uptake by PM affects visibility. The issue of managing visibility is taken up in several places in Chapter 9, including Sections 9.1.4, 9.1.5, and 9.5. The committee recommends that the chapter be reorganized to address management in a single section. Current or proposed rules, regulations, and programs that address visibility, which are absent from the draft, should be introduced and described in this section. To the extent practicable, the text should address how and why each standard was set, what measurements they are based on and how the measurements are obtained, and whether the standards have been effective in improving visibility. Some discussion of the recently proposed regional haze rules in the United States should be included. The discussion in Section 9.5, which logically would be in the new section, would also be improved if more specific information— including details about the modeling tools, control strategies, or lessons learned—were given about the various examples (such as the Grand Canyon Visibility Transport Commission and the Southern Appalachian Mountains Initiative). At least, references should be provided to guide the interested reader to such detailed information. Finally, the new section devoted to visibility management should address the economic and societal effects of visibility impairment, particularly explaining why this effect of PM is of concern. The first section of Chapter 9 introduces the topic by describing how visibility is linked to PM. Extensive data are presented for the United States, but there is not much information on Mexico or Canada, even though some such information is available. For example, there is some evidence that visibility has decreased markedly in Mexico City from an average of 4–10 km in 1940 to an average of 1–2 km in recent years (Ciudad de Mexico 1999). Sections 9.1.2 and 9.1.3 describe many of the technical relationships between particles and visibility. How particle composition may increase or decrease visibility is discussed, as is how relative humidity may affect visibility by increasing scattering efficiency. Relationships among PM concentrations, PM type, measurement methods, humidity, and visibility could be presented in more depth. For example, the present discussion does not indicate that measurements of PM on filters, which are dried to some moisture specification before weighing, will not necessarily give an accurate indication of visibility, which is strongly influenced by water uptake. Section 9.1.4, “What are some unique aspects of visibility management?”, needs to be revised or integrated into other sections of the chapter, such as the proposed section dedicated to visibility management. It is not evident from the points presented why visibility management is unique. For example, the authors mention that “visibility is perceived instantaneously, so there is no averaging time.” Does that mean that there is no standard or that visibility cannot be modeled (which is not correct), or are the authors trying to make some other point? A good reason to integrate the material in this section into other parts of Chapter 9 is to reduce redundancy. Much of the information presented in the previous section is restated here, in particular that composition, size, and humidity are factors that may modify the effect of PM on visibility. Other points made here fall nicely into the discussion of visibility management, including comments on how visibility is perceived instantaneously and the implications for visibility management in clean and dirty areas. The summary nature of some of the comments also makes them

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter appropriate for a final section of the chapter in which the policy implications of the scientific information are described (as has been suggested previously for all chapters). Sections 9.2.1 and 9.2.2 provide a summary of the different visibility inventories. This section would benefit if the authors provided references for the studies mentioned. Section 9.2.3 presents a case study of the Colorado Plateau. Although the section is interesting, why this case is presented as a conceptual model and what can be learned from it are not clear. Are there lessons from this case study for air-pollution control offices in Mexico or Canada? Section 9.2.4 poses an important question about using PM studies designed to address health questions to learn about visibility, but the response to it is not complete. The authors should flesh out the response by discussing, for example, the additional benefit of speciation data for understanding the visibility problem in rural, scenic areas and in urban areas. Section 9.3 starts with a long introduction regarding models available to estimate visibility. The authors should attempt to condense this section, focusing on describing the available models, the information they can provide, measures of their performance, and how they can be used in visibility management. A table that summarizes that information and advantages and disadvantages of each modeling tool may be a more efficient way to present the material. The authors should keep in mind the audience of visibility managers who need to use and develop models to assess the effects of distant sources of PM; in this context, references to specific software and documentation would be useful. At first glance, Section 9.4, “Atmospheric Aerosols Affect the Global Radiation Balance,” seems out of place in a chapter titled “Visibility Effects.” However, the discussion about how efforts to understand the role of PM in climate and visibility complement each other and how mitigation strategies could be developed to address both problems is interesting and appropriate for this chapter. The subject is more logically probably a subsection of the proposed section on visibility management. Indeed, the existing Section 9.5 has a parallel discussion of aligning visibility and health-based PM-control programs. CHAPTER 10: “CONCEPTUAL DESCRIPTIONS OF PM FOR NORTH AMERICAN REGIONS”; APPENDIX E: “CONCEPTUAL DESCRIPTIONS OF SELECTED NORTH AMERICAN SITES” Chapter 10 offers a summary of the PM problem in nine regions in North America and comprehensive descriptions of the PM problem in the San Joaquin Valley of California, the Windsor-Quebec City Corridor, and Mexico City. Appendix E provides details on the other six regions. The material presented in the chapter is the most original contribution of the PM assessment and provides the most useful information for decision-makers. In particular, the information in the tables provides an excellent summary and deserves greater prominence in the assessment because it clearly links recent research to policy suggestions in a concise way. For those reasons, the committee recommends that all nine of the regional descriptions be presented in Chapter 10 rather than having six relegated to Appendix E. In general, the approach is clearly explained, and the chapter is well written. The committee, however, has identified a number of ways in which this policy-relevant information could be better communicated.

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter As discussed previously in Chapter 2 of the present report, the conceptual model discussion in Chapter 10 of the draft assessment is not consistent with the other chapters. The committee recommends that the same framework for informing airborne-PM management (Figure 2–1 of the present report) be applied here but in a region-specific manner. Using a consistent definition of the framework and the conceptual model that is part of it would alleviate the confusion introduced by having a “schematic diagram of a conceptual model” presented in Figure 10.1, a somewhat different conceptual model identified in Figure 10.2 (note that the processes included in the conceptual model in Figure 10.1 are inputs to the conceptual model in Figure 10.2), and a third variation of the conceptual model for the nine regional descriptions. Presenting the nine region-specific conceptual models in the same figure format as the framework for informing airborne-PM management, but with information in each box reflecting the regional characteristics, may be a more effective way to communicate a coherent story about each region. The figures could replace or augment Tables 10.1, 10.2, 10.3, 10.4, and 10.6, which are tedious and informal in their current form (and contain a number of grammatical and spelling errors). The tables map fairly easily into the conceptual-model diagram in Figure 2–1 of the present report: Tables 10.1 and 10.2 go into the “Airborne-PM Burden” box. Table 10.3 goes into the “Meteorology” box. Table 10.5 goes into the “Source attributions” and “Emissions” boxes. Table 10.6 goes primarily into “Policy Implications”; some information goes into the “Atmospheric Processing” box. A second problem with the presentation in Chapter 10 is that the discussion in the text needs to support the tables better. There should be a discussion of how the source attributions (Table 10.5) and the policy implications (Table 10.6) were determined from the information in Tables 10.1–10.3. In addition, the nine sections titled “Implications for Policy Makers” in Chapter 10 and Appendix E should be rewritten to mirror and augment the equivalent sections in Table 10.6. In rewriting those sections, the authors should keep in mind that decision-makers are typically looking for scientists to provide a range of options for addressing the policy issue and, wherever possible, articulate the respective strengths and weaknesses of competing options. By and large, that has not been done in Chapter 10, and the authors should attempt to provide at least some such material. In discussing policy implications, the authors should carefully consider how they present information that is highly uncertain or demonstrates adverse collateral effects associated with particular control strategies. An example of a discussion of uncertain information can be found in Section 10.4.6, where the emission inventory for Mexico City is said to be “highly uncertain.” No comment is made in the section about the importance of reducing the uncertainty, what specifically is uncertain about the inventory, or what causes the uncertainty. Without such additional discussion, a policy-maker may be inclined to ignore any policy implications stemming from the uncertain information. Likewise, wherever tradeoffs are identified in the analysis of possible actions (for example, where solving one problem will or may exacerbate another), it is important to provide further guidance to the policy-maker on the implications of action and inaction in the face of such tradeoffs. For example:

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter Are the two problems of comparable significance or magnitude? Why or why not? Is there a net benefit in risk reduction of acting one way or the other? Are there other aspects to each of the two problems (such as effects on sensitive populations) that would affect how they are balanced against each other? Is the tradeoff recommended on cost-effectiveness grounds in any case? In short, it is critical to provide information that puts the tradeoffs into context. A final aspect of Chapter 10 that might compromise its ability to speak to decision-makers is associated with the five questions used for organizing the regional descriptions. The five questions bear some resemblance to the eight policy questions (PQs) presented in the draft assessment’s executive summary. However, they are not identical and are trying to elicit different responses; the eight PQs are intended to provide a general understanding of PM and its management, whereas the set of five questions in Chapter 10 are intended to provide an organizing framework for discussing the regional descriptions. The wording should be modified to distinguish the objectives of the two sets of organizing questions. Furthermore, the five questions do not exactly align with the five tables presented in Chapter 10 or with how each region is described in the text. The regional descriptions should be presented in a consistent manner within Chapter 10. For example, the section headings should not vary among regions. CHAPTER 11: “RECOMMENDATIONS” Chapter 11 does a good job of presenting a detailed discussion of the high-priority scientific research that should be undertaken to improve the understanding of PM in North America. The scientific targets of the research are described in detail elsewhere in the draft assessment, the recommendations are specific and accompanied by their rationale, and they focus on questions that are clearly important for policy decisions and uncertain given current findings. The link of each scientific subject to policy is discussed, but these sections generally need to give decision-makers more rationale for why investments will give the most benefit in addressing the difficulties in making policy choices. It would also be useful for the decision-maker community to identify what aspects of the recommendations are practical, on the basis of current abilities, and on which fronts progress is expected in the near term or the long term. The committee finds that the recommendations are generally appropriate, but it also feels that the context and framing of the recommendations could be improved. The introductory comments should mention that advocates on all sides of the environmental-science debate agree that good decisions should be informed by a sound understanding of science and its uncertainties, and these recommendations will allow the uncertainties to be reduced. The title and introductory discussion should better reflect the thrust of the discussion in the chapter, that is, what improvements in data, tools, and knowledge are needed for a better assessment of the science and better answers to the policy questions in the future. Indeed, the recommendations of most interest for air-quality policy are in Chapter 10. A more specific chapter title would make it clear that the recommendations pertain only to scientific research that should be pursued, not to PM-management options. The committee recommends that the title of Chapter 11 be changed to

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter “Future Research Directions,” “Science to Inform Future Policy,” or something similar of NARSTO’s choosing. In the discussion of research needs, it would be useful to call attention to the fact that some gaps in scientific knowledge for policy-making are due to true uncertainty (for example, magnitude of a local source’s contribution to ambient-PM concentrations at a specific location). However, other knowledge gaps faced by decision-makers are due to variability (for example, differences in the susceptibility of individuals to similar particle exposures). These general concepts and terms are discussed extensively in the National Research Council report Science and Judgement in Risk Assessment (NRC 1994). That report discusses uncertainty in terms of knowledge that does not extend beyond a certain degree of precision because of measurement or estimation error. On the other hand, variability is an attribute of a factor, such as regional weather patterns or human characteristics, that does not allow the factor to be represented by a single value. Both uncertainty and variability pertain to issues discussed in the NARSTO assessment, and a clearer exposition of their differences would facilitate a better and more realistic understanding of the issues for deicision-makers. To frame Chapter 11 better with respect to the material presented in other chapters, the authors should explicitly relate the recommendations to the policy questions, the policy-implications sections to be added at the ends of other chapters, and the limitations identified in Chapter 10. A table may be an effective way to present such information, or it should be addressed in the short “policy-relevance” paragraphs. Those paragraphs currently provide the rationale for the recommendations. They should also indicate how the recommended research would affect the decision-maker’s choices. An example of where this link is made clear is the recommendation about carbonaceous aerosols, which states that a better understanding of organic compounds will help in deciding whether reductions in volatile organic chemicals will be effective in reducing PM mass. There needs to be attention to articulating summary statements and expressing research needs as clearly as possible. Granted, many of the statements are intentionally brief and in bullet form (especially in the tables), but a recommendation is not effective if the reader cannot readily grasp what it means. Many readers will read Chapter 11 without having read the preceding chapters, so it needs to be able to stand largely on its own. Likewise, many readers may read the tables without reading the text, but the concepts are much more clearly conveyed in the text than in the tables. The explicit intent of the summary recommendations in the tables also needs to be understandable, even though not all the details surrounding them can be conveyed there. The term harmonization is used often in Chapter 11, but it is ambiguous and overused. In some instances, the authors seem to be calling for standardization (that is, everyone using the same approach or model). Harmony does not mean everyone doing the same thing; rather, it means different people doing different things that are complementary and that make a whole that is greater than the sum of the parts (as in musical harmony). In a scientific context, the term could refer to two adjacent measurements that may not match exactly but differ in an explainable manner. Or the term could refer to standards that differ but do not conflict. To alleviate the confusion, the wording could be made more explicit when the term is used and examples of harmonization could be given. An important research need seems to have been overlooked. A much better understanding of PM surface chemistry and morphology is needed. Unless the PM is soluble, cells are not

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Review of the Narsto Draft Report: Narsto Assessment of the Atmospheric Science on Particulate Matter sensitive to bulk chemistry; they are affected mostly by surface chemistry. The atmospheric-science community has made progress in analyzing the bulk composition of PM, either by using collected samples or single-particle analysis. However, little (if any) progress is being made in the atmospheric-science or health-science community in understanding PM surface composition or interactions between PM surface composition and biologic fluids. Whether or not a specific priority-setting was intended, the order of the five recommendations and their subrecommendations will suggest priorities to the reader. Thus, the order is important and should be the same everywhere it is discussed (that is, on page 11–1, in the tables, in Section 11.1, and in the synthesis of key issues). Likewise, the same wording should be used everywhere the recommendations are presented. Because priorities are useful for decision-makers, the committee recommends that NARSTO attempt to assign priorities to the recommendations for future research in terms of when each activity needs to be undertaken and what should receive more funding. The committee recognizes the difficulty in assigning uniform priorities for all countries and situations. Thus, based on the basis of the level of understanding in different areas, regional priorities may be appropriate. The committee also identified a number of specific problems with the recommendations themselves. Recommendation 2.8 should mention the relationships between sources and health effects, rather than only constituents and health effects. A recommendation for continued and enhanced laboratory studies of aerosol atmospheric chemistry and microphysics is missing. And there is no recommendation for a better understanding of PM surface area and surface composition, which may be more closely associated with health effects.