Conclusions and Recommendations
The preceding chapters described the scientific and legal framework of efforts to protect and improve visibility, as well as methods for attributing visibility impairment to sources and for assessing alternative control measures. This chapter discusses the implications of current knowledge for future regulatory and research efforts. The committee did not presuppose a particular form for a visibility program because the design of a program involves many policy issues outside the bounds of science and the committee's expertise. However, present scientific knowledge about visibility impairment in Class I areas has several implications for policy makers.
Any effective strategy to accomplish the statutory goal of remedying and preventing anthropogenic visibility impairment in Class I areas must limit emissions of pollutants that can cause regional haze.
Incontrovertible scientific evidence links emissions of air pollutants to the formation of haze that limits visibility and degrades the visual environment. Almost all the effects of air pollution on visibility are caused by airborne particles. In most cases, visibility degradation is caused by five kinds of particulate substances (and associated particulate water): sulfates (SO42-), nitrates (NO3-), organic matter, elemental carbon, and
soil dust. Although some of these particles are emitted directly into the atmosphere, others, such as SO42-, are the result of the transformation in the atmosphere of gaseous emissions such as sulfur dioxide (SO2). Because of their physical properties, airborne particles and their gaseous precursors can exist in the atmosphere for several days; during this time, winds can carry these materials great distances (e.g., typically hundreds of miles). This leads to the formation of a widespread regional haze.
Visibility problems in Class I areas are predominantly the result of regional haze from many sources, rather than individual plumes caused by a few sources at specific sites. Therefore, a strategy that relies only on influencing the location of new sources, although perhaps useful in some situations, would not be effective in general. Moreover, such a strategy would not remedy the visibility impairment caused by existing sources until these sources are replaced.
Progress towards the national goal of remedying and preventing anthropogenic visibility impairment in Class I areas will require regional programs that operate over large geographic areas.
Because most visibility impairment in Class I areas results from the transport by winds of emissions and secondary airborne particles over great distances, focusing only on sources immediately adjacent to Class I areas—as under the current program—is unlikely to improve visibility effectively.
A program that focuses solely on determining the contribution of individual emission sources to visibility impairment is doomed to failure. Instead, strategies should be adopted that consider many sources simultaneously on a regional basis.
Nonetheless, the assessment of the contribution of individual sources to haze will remain important in some situations. For instance, a regional emissions management approach to haze could be combined with a strategy to assess whether locating a new source at a particular location would have especially deleterious effects on visibility. The committee has set out working principles in Chapter 5 and Appendix C for attributing visibility impairment to single sources.
The committee doubts, however, that such attributions could be the basis for a workable visibility protection program. It would be extremely time-consuming and expensive to try to determine the percent contri-
bution of individual sources to haze one source at a time. For instance, the efforts to trace the contribution of the Navajo Generating Station to haze in the Grand Canyon National Park took several years and cost millions of dollars without leading to quantitatively definitive conclusions. Moreover, there are (and probably will continue to be) considerable uncertainties in ascertaining precise relationships between individual sources and the spatial pattern of regional haze.
Visibility policy and control strategies might need to be different in the West than in the East.
Haze in the East and in the West differ in important ways. Haze in the East is six times more intense than in the West because of the much higher level of pollution in the East. Were all anthropogenic pollution to disappear, visibility would remain greater (by about 50 %) in the West. In relatively clean areas, small increases in pollutant concentrations can markedly degrade visibility; increases of the same magnitude are less noticeable in more polluted areas. Hence, visibility in Class I areas in the West is particularly vulnerable to increased levels of pollution. Moreover, the West contains most of the nation's large national parks and wilderness areas, which can be fully appreciated only when visibility is excellent. The East, however, contains a large population to enjoy the benefits of any improvement in visibility in that region.
In the East, SO2 emissions from coal-fired power plants account for about one-half of all anthropogenic light extinction. Reductions of these emissions are expected to occur in the next two decades as a result of the 1990 Clean Air Act amendments' acid rain control program. In the West, no single source category dominates; therefore, an effective control strategy would have to cover many source types, such as electric utilities, gasoline-and diesel-fueled vehicles, petroleum and chemical industrial sources, forest management burning, and fugitive dust.
Efforts to improve visibility in Class I areas also would benefit visibility outside those areas.
Because most visibility impairment is regional in scale, the same haze that degrades visibility within or looking out from a national park also degrades visibility outside it. Class I areas cannot be regarded as potential islands of clean air in a polluted sea.
Reducing emissions for visibility improvements could help alleviate other air-quality problems, just as other types of air-quality improvements could help visibility.
The substances that contribute to regional haze also contribute to a variety of other undesirable effects on human health and the environment. For example, SO2 is a precursor of sulfuric acid in acid rain; oxides of nitrogen (NOx) and volatile organic compounds (VOCs) are precursors of lower-atmosphere ozone; and fine atmospheric particles are a respiratory hazard. Such particles can influence climate by interacting with incoming solar radiation and by modifying cloud formation. Policy makers should consider the linkages between visibility and other air-quality problems when designing and assessing control strategies.
Achieving the national visibility goal will require a substantial, long-term program.
As shown in Chapter 6, the application of all commercially available control technology would reduce, but not eliminate, visibility impairment in mandatory Class I areas. Policy makers might develop a comprehensive national visibility improvement strategy as the basis for further regulatory action. Policy makers also might establish milestones against which progress toward the visibility goal could be measured.
Various indices can be used to characterize visibility and measure progress. Of these, the extinction coefficient (which includes scattering and absorption as components) is the most fundamental and can be linked most closely to the chemical composition and physical properties of the atmosphere. In addition, the extinction coefficient can be quantitatively related to other indexes, including human judgment of visibility.
Current scientific knowledge is adequate and control technologies are available for taking regulatory actions to improve and protect visibility. However, continued national progress toward this goal will require a greater commitment toward atmospheric research, monitoring, and emissions control research and development.
The slowness of progress to date is due largely to a lack of a commitment to an adequate government effort to protect and improve visibility and to sponsor the research and monitoring needed to better characterize the nature and origin of haze in various areas. As discussed in Chapter 3, the federal government has accorded the national visibility goal less priority than other clean-air objectives. Even to the extent that Congress has acted, the Environmental Protection Agency (EPA), the Department of Interior (DOI), and the Department of Agriculture have been slow to carry out their regulatory responsibilities or to seek resources for research.
This lack of commitment can be seen by reviewing this committee's recommendations, as well as those drafted in 1985 by EPA's Interagency Visibility Task Force. The task force (now defunct) consisted of representatives from EPA, DOI, the Departments of Agriculture and Defense and was formed to develop long-term strategies for remedying regional haze and to recommend a long-range program to restore visibility in the national parks. The task force's research recommendations include many of those highlighted by this committee, such as the study of human perception, airborne particle characteristics, and atmospheric optics. Although several task force recommendations were implemented (e.g., establishment of an eastern visibility monitoring network and expansion of monitoring into more wilderness areas managed by the Forest Service), many were not. The gaps in scientific understanding of visibility impairment are a symptom of the absence of a strong national commitment to enforcing the visibility protection provisions of the Clean Air Act.
SUMMARY OF CONCLUSIONS
To accomplish statutory goals on visibility, emissions of pollutants that cause visibility impairment must be limited.
Visibilty programs must take large geographic areas into consideration because the visibility problem is regional.
Many sources must be considered simultaneously for visibility improvement.
Visibility policy and control strategies may need to be different in the East and West.
Improving visibility in Class I areas will also improve visibility in other areas.
Visibility improvements will help alleviate other air quality problems.
A long-term program is required to achieve the national visibility goal.
Current scientific knowledge is adequate and controls are available for improving and protecting visibility, however, continued national progress requires a greater commitment toward atmospheric research, monitoring, and emission control research and development.
RECOMMENDED RESEARCH STRATEGIES
The committee was charged with recommending strategies for alleviating critical scientific and technical gaps in the areas of visibility and aerosol monitoring, source apportionment, and emissions control technology. The committee considered what measures might be taken to understand better sources of haze, possible means of reducing emissions from those sources, and alternative ways of preventing future visibility impairment in Class I areas. The remainder of this chapter summarizes the committee's recommendations to fill technological and data gaps in research and program implementation.
The committee emphasizes that the existence of technological and data gaps does not imply that it would be premature to take further regulatory action, if otherwise warranted, to protect and enhance visibility in the nation's Class I areas. Regulatory action in the environmental field typically is taken without complete scientific knowledge; this is inherent in a statutory scheme like the Clean Air Act that is based on a philosophy of prevention. A decision as to whether or how to take regulatory action is therefore not wholly one of science but also involves policy considerations, such as the relative risks of overprotection and underprotection, which were outside the committee's purview.
Although the causes of visibility impairment are reasonably well understood, additional research is still necessary in some areas.
These areas include research on atmospheric transport and transformations of visibility-impairing pollutants, the development of models integrating these complex processes to relate source emissions to effects, and improved instrumentation for routine monitoring and for obtaining data that can be used to evaluate model performance. Such research would make possible a more effective and efficient visibility improvement program. Monitoring and research must be closely coordinated so that progress in research can be implemented in the monitoring program. However, better models are not enough. Any model, even the simplest or most refined, depends on good empirical data on the airborne particles that cause haze and on their sources. Greater resources are needed to develop these data.
Steps must be taken to ensure that visibility-related research is of high quality.
Resources are limited; therefore, the committee believes that precautions should be taken to ensure that the visibility protection activities in the federal land management agencies, EPA, the Department of Energy, and state air agencies are of the highest possible quality. In addition, a greater effort is needed for formal publication of scientific work in independent, peer-reviewed literature.
EPA should build upon and expand its efforts to track the success of the PSD program. In particular, information is needed about the control of new sources with the potential to damage visibility in Class I areas and about the effects on such areas of the new emission trading programs under Title IV of the Clean Air Act.
EPA, the states, and the public need to be able to track the implementation of programs for controlling emissions of new, modified, and existing sources of visibility impairment. For instance, the present PSD program requires that new or modified major emitting facilities in clean air areas install the best available control technology and comply with a system of increments intended to limit pollution degradation in national parks and wilderness areas. In the early 1980s, EPA sponsored a series of studies that furnished much valuable information about the extent to which the PSD program resulted in additional control of new sources. This data base, however, has not been updated since 1984. In addition, the new permitting program established by Title V of the Clean Air Act creates a promising data base for tracking emission changes near national parks, especially those resulting from Title IV's acid rain control program.
EPA's visibility screening model needs to be revised to consider a proposed source's contribution to regional haze.
The new source review provisions of the PSD program require the states and federal land managers to complete an analysis of Class I area visibility impairment of the proposed source. Unfortunately, the EPA's
VISCREEN model used in this analysis is inadequate, because it ignores SO2-to-SO42-conversion and considers the visibility impairment caused by the NO2 and primary particulate plume only as viewed from clean air. EPA's guidance on the use of the model is also outdated. In addition, VISCREEN is not a cautious model for screening haze effects and therefore is not consistent with EPA's PSD impact assessment policies.
Research on relating human judgments of visibility to objective measures, such as light extinction, should continue, with the results used to inform decision makers and the public about the perceptibility of forecasted visibility changes.
Development of control strategies will require communication among the scientific community, decision makers, and the public. During this process, it would be useful to relate light extinction to human perception. Considerable progress has been made by researchers in developing techniques that produce photographic simulations of visibility changes. Over the past decade, progress also has been made in relating human judgments of visibility and various perceptual cues, such as contrast and visual range, to visibility changes measured by light extinction.
An independent science advisory panel with EPA sponsorship should be established to help guide the research elements of the national visibility program.
Such a board could address the need for wider participation among the scientific community in visibility issues.
SUMMARY OF RECOMMENDED RESEARCH STRATEGIES
Additional research is necessary in several areas of visibility impairment.
Visibility-related research must be of the highest quality.
The success and future of the PSD program must be determined.
The contribution of individual sources to regional haze must be included in the EPA visibility screening model.
Research relating human judgments of visibility to objective measures should be continued.
EPA should form an independent science advisory panel for the national visibility program.
RECOMMENDED MONITORING STRATEGIES
A consensus should be developed on the specific instrumentation to be used for monitoring light extinction. Standards should be established for the performance characteristics of the instrumentation.
Progress toward the visibility goal should be measured in terms of the extinction coefficient, and extinction measurements should be routine and systematic. Extinction either can be measured directly using transmissometry or inferred from measurements of absorption and scattering. Instruments for making such measurements have been developed and can yield reasonably accurate results. Such instrumentation can be cost-effective, reliable, and relatively easy to use. Desirable refinements in optical instrumentation are summarized below.
The monitoring program should be able to relate visibility degradation to the sources of the impairment on a scale commensurate with regional haze events and the distribution of major emissions sources.
The monitoring program should be designed to characterize the existing patterns of haze, trends in those patterns, and the pollutants that contribute to visibility degradation on a regional scale.
National visibility monitoring networks, notably the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, have been established to assess visibility impairment in the nation's Class I areas, to identify sources of haze, and to track trends in impairment conditions. The committee identified several scientific, technical, and data gaps in existing networks that should be addressed.
If national visibility monitoring networks are to accomplish their goals, a long-term commitment to establishing and financially supporting monitoring programs is essential.
This commitment is critical in view of the adoption of Title IV of the Clean Air Act Amendments of 1990 (acid deposition control), which is expected to result in an annual 10-million-ton reduction in SO 2 emissions
from 1980 levels. This amounts to an estimated 36% reduction in SO2 emissions in the eastern states, with parallel reductions in SO42concentrations. These reductions are expected to result in perceptible improvements in visibility.
The SO42- reductions achieved through Title IV offer an exceptional opportunity to track visibility improvements associated with reductions in SO2 emissions from coal-fired power plants. To document adequately the visibility benefits from Title IV, monitoring networks in the eastern states need to be expanded and long-range funding ensured. Otherwise, EPA will lack information needed to measure whether reasonable progress is being made toward the national visibility goal and will be unable to meet its responsibility under Section 169A(a)(4) of the act to ensure such progress.
Future measurement programs should devote increased attention to quality assurance and control.
Because of the importance of the IMPROVE network to the regulatory process, strengthening IMPROVE's quality assurance and control program should be a high priority. Most of this effort should go into quality-control procedures integrated into routine monitoring and should be designed to characterize and document the performance of IMPROVE's measurements.
Any quality-assurance program should include, at the management level, periodic systems and performance audits by independent outsiders. External audits are not a substitute for internal quality control but should be designed to provide timely information on potential problems and on departures from quality-control specifications.
Greater attention should be given to the implications of planned changes in airport visibility monitoring for research on visibility impairment.
The transition from human to automated airport visibility monitoring that is planned by the National Weather Service, the Federal Aviation Administration, and the Department of Defense has unfortunate implications for monitoring haze. The Automated Surface Observing System (ASOS) will provide little or no information on the magnitude and extent of haze. In particular, this system cannot quantify haze levels corre-
sponding to visual ranges exceeding 10 miles; prevailing visual range is typically much greater than this.
The historical record of ambient haze levels is based largely on airport visibility data from human observers. There is no other source of such information. If airport visual range measurements cease, no information will be available for assessing temporal and spatial trends. It is especially important that such trends be documented during the coming decade, so that the effect of acid rain controls on visibility can be determined.
Airports should be equipped with integrating nephelometers that are sensitive enough to measure the range of haze levels encountered in the atmosphere.
Automated and human observer measurements should overlap for approximately 1 year to ensure consistency between the measurements. In this way, the collection of valuable data on visibility trends could continue.
SUMMARY OF MONITORING RECOMMENDATIONS
Agreement must be reached on the instrument to be used for monitoring light extinction.
Monitoring programs must relate regional visibility degradation to the sources of impairment.
A long term commitment to a national visibility monitoring program is required.
Quality assurance and control must be given increased attention.
The implications of changes in monitoring airport visibility on visibility research must be determined.
Airports should be equipped with integrating nephelometers.
RECOMMENDED MEASUREMENT METHODS
Current measurement techniques permit useful estimates of the average contributions of major aerosol constituents to atmospheric visibility impairment. However, several measurement methods must be developed or improved to refine present understanding of visibility impairment.
An accurate method for measuring particulate organic and elemental carbon, particularly at the low concentrations found in and near national parks, needs to be developed.
Although organic and elemental carbon often are major constituents of atmospheric particles, atmospheric measurements of particulate carbon using different sampling and analytical methods can disagree by as much as a factor of 5. Discrepancies tend to be larger in national parks and wilderness areas with relatively low particle concentrations. This uncertainty limits quantitative understanding of the contribution of organic and elemental carbon to visibility impairment. Methods that can characterize the organic aerosol composition in more detail also are needed. Such methods could provide important insights into the origins and properties of organic particles.
A method for the routine measurement of water content of airborne particles needs to be developed.
Water is another important particulate constituent that is difficult to measure. Hygroscopic species such as SO42-, NO3-, and possibly certain organic acids absorb water and significantly reduce visibility, especially at high relative humidities. At present, there is no routine method of measuring particulate water content.
Greater attention should be given to measuring airborne particle size distributions.
Significant progress has been made during the past decade in measuring aerosol composition as a function of particle size. This work has shown that variations in size distributions can lead to differences of a factor of 2 in scattering and absorption efficiencies. Unfortunately,
measurements of aerosol chemical composition typically are based on filter samples that provide inadequate information on size-dependent variations in extinction efficiencies. Fine and coarse particle filter samples should be supplemented with measurements of size-resolved chemical composition to address the importance of particle size in atmospheric optical phenomena.
Additional research on inlet sampling efficiencies should be conducted especially for submicron-size particle sampling from aircraft.
While it has long been recognized that inlet sampling losses for coarse particles can be significant, recent work suggests that loses of submicron-size visibility-impairing particles also can be important during sampling from aircraft. Aircraft sampling is often required to characterize the extent of regional haze, and to study phenomena including secondary airborne particle production. It is important to have sampling inlets that are known to be accurate for visibility-impairing particles.
Commercially available instruments are needed for continuous measurements of particulate sulfates, organic and elemental carbon, nitrates, and elemental composition.
Such instruments would be valuable for visibility research and monitoring as well as for work on other atmospheric problems involving airborne particles. Information on temporal variations in particulate composition can provide further clues about the sources, transport, and atmospheric processing of airborne particles. Some progress has been made toward the continuous measurement of particulate sulfur and carbon loadings, and elemental composition can be measured with a time resolution of several hours. However, the instruments used for such measurements usually are custom built and not readily available.
The development of self-powered instrumentation for visibility monitoring and research should be accelerated.
Visibility monitoring is often carried out in remote regions where electric power is not available, which limits the range of measurements that can be conducted. In recent years, significant progress has been made in the development of solar- or battery-powered instrumentation
for visibility-related measurements, but major limitations remain, particularly with regard to measurements of gas and particle composition.
High-sensitivity integrating nephelometry should be used for routine visibility monitoring.
Factors considered in making this recommendation include reasonable cost, potential wide dynamic range in sensitivity, and ease of installation, operation, and calibration. In addition, nephelometer data can be compared with airborne particle measurements at the same point to determine the relative contribution of different pollutants to haze.
The fact that nephelometers measure only scattering and not total extinction (scattering plus absorption) is both a strength and weakness of this method. By supplementing nephelometer measurements with independent data for absorption coefficients, the separate contributions of scattering and absorption to extinction can be determined. This can be useful for evaluating the relative contributions of various types of particulate matter (e.g., soot versus SO42-, NO3-, and organics) to haze. The widespread use of integrating nephelometers has been hindered by the lack of a readily available, easily serviced, and electronically up-to-date instrument. The committee recommends the development of an instrument with adequate sensitivity for good and poor visibility conditions. Issues that need to be considered in the design of such a nephelometer include inlet losses of coarse particles and decreases in aerosol relative humidity associated with heating by the instrument.
Efforts should be made to develop instrumentation for continuous measurements of particle absorption coefficients.
This instrumentation should be sensitive to the range of absorption coefficients obtained from atmospheric observations. Although instrumentation for routine, continuous, real-time monitoring of scattering and extinction coefficients is available, similar instrumentation for absorption coefficients is not. Instead, particle absorption coefficients usually are determined from filter samples. Photoacoustic spectrometers can provide real-time absorption data, but sensitivities are limited under good visibility conditions, and those one-of-a-kind instruments are expensive
to build and difficult to operate. It would be desirable to have an instrument for routine, continuous measurements of absorption coefficients to characterize atmospheric optical properties more completely.
SUMMARY OF MEASUREMENT METHODS RECOMMENDATIONS
Accurate methods for measuring particulate organic and elemental carbon must be developed.
A method for the routine measurement of aerosol water content must be developed.
Increased emphasis should be given to measuring aerosol particle size distributions.
Research should be conducted inlet sampling efficiencies, especially for submicron-size particle sampling from aircraft.
Continuous measurements are required for aerosol sulfate, organic and elemental carbon, nitrate and trace elements.
Development of self-powered instrumentation for visibility measurements must be accelerated.
The integrating nephelometer should be used for routine visibility monitoring.
Instrumentation for continuous particle absorption measurements must be developed.
RECOMMENDED SOURCE APPORTIONMENT MODELING RESEARCH
In Chapter 5 and Appendix C, the committee reviewed methods for identifying and quantifying source contributions to visibility impairment. These methods include simple source identification techniques, speciated rollback models, receptor models, and mechanistic models. The committee found that there is no single best source apportionment technique for a given visibility problem and recommended a nested progression from simple through more advanced and precise techniques. This section discusses research and data collection needed for the development and validation of source apportionment methods.
Better data on source emissions are needed.
Source emissions data are essential to most haze apportionment models. Speciated rollback models require data on aggregate emissions of SOx, NOx, VOCs, NH3, and primary airborne particles; chemical mass balance (CMB) receptor models require additional information on endemic tracers such as key trace metals found in source exhaust; and mechanistic models require further data on the particle size distributions in the primary source exhaust.
Unified procedures are needed for testing individual sources that will simultaneously meet the input data requirements of the modeling systems recommended in Chapter 5. More source tests should include collection of the aerosol chemical data required by speciated rollback and CMB models and the size distribution data for fine airborne particles required by mechanistic visibility models (in addition to the usual particle mass and gaseous pollutant emission data). Efforts should be directed at identifying and measuring additional endemic tracers that might be useful in detecting the contribution of one or more key sources at a particular ambient sampler. Such tracers would be identified using molecular or isotopic characteristics rather than using those of elements.
Cheaper and more reliable vehicle exhaust emission measurement techniques are needed.
Source emissions data must be integrated accurately into overall emission inventories.
Despite major advances made through the National Acid Precipitation Assessment Program (NAPAP), the integration of improved single-source emissions data into national or regional inventories is not yet satisfactory. The committee believes that further thought and planning need to be directed at this issue.
As discussed in Chapter 4, the most important anthropogenic emissions from a visibility standpoint are SO2, primary organic particles, gaseous VOCs, primary elemental carbon particles, soil dust materials, ammonia, and nitrogen oxides. The national inventories for gaseous sulfur and nitrogen oxides are of good quality. The inventories for ammonia and soil dust are of lesser quality and need further attention. Inventories for ammonia and soil dust are less critical than those for some other pollutants because they usually are only minor contributors to regional haze.
From a visibility standpoint, the national inventories needing the most improvement are those for primary elemental and organic carbon particles and for VOCs that lead to secondary organic particle formation.
Inventories for elemental carbon and primary organic particles essentially are nonexistent except for Los Angeles.
Model Validation and Field Studies
Model validation studies employing existing data sets should be supported. Mechanistic models as well as hybrid receptor-oriented methods should be included in future model validation studies.
The most convincing tests of air-quality model performance are those directly based on actual measurements. Comprehensive field studies are an expensive but indispensable step toward the collection of atmospheric data sets against which new and existing models can be tested. Several
measurement programs and mechanistic-model development programs provide opportunities to test model performance at little additional expense. Possible candidates are the 1990 winter study of the Grand Canyon sponsored by the Salt River Project and project MOHAVE, which included comprehensive atmospheric chemistry, aerosol optics, and wind measurements. The redundancies and cross checks available in the resulting data should facilitate the comparison of differing source apportionment approaches. More potential test data will come from the application of the NAPAP's Regional Acidic Deposition Model to the comprehensive data base from the Denver Brown Cloud study.
This section focuses on research needs specific to haze in national parks and wilderness areas. The essential inputs to any receptor-oriented haze apportionment model are a collection of source signatures and estimates of chemical transformation mechanisms and reaction rates. The study of atmospheric reactions is motivated by a broad array of scientific and policy issues, ranging from the global cycling of sulfur, nitrogen, and carbon to the photochemistry of urban smog. This large complex of issues can be expected to drive the evolution of reaction models, which receptor methods could exploit with little additional work. This discussion therefore focuses on strategies for increasing the availability and quality of source signatures.
The chemical mass balance model is widely accepted in the management and study of local particulate air quality. Regression analysis, although not ready for operational use, is an accepted research tool. Both approaches have been tested successfully on simulated data sets for primary urban airborne particles. However, those endorsements are largely irrelevant to most haze applications, which involve mostly secondary particles at regional scales. Hybrid models have been proposed by several investigators but have not been validated. Such models should be tested on simulated haze data sets incorporating realistic operational factors, such as measurement uncertainty, variable emissions, and covarying source strengths. The results from model comparison studies would greatly increase present understanding of modeling errors, sensitivities to uncertainties, and the relative merits of differing receptor modeling approaches.
Substantial source testing as well as measurements of ambient concentrations resulting from particulate and gaseous emissions are needed to improve emissions profiles for sources of haze.
The development of source signatures has been driven largely by the apportionment of total suspended particles and PM10 in urban, suburban, and industrial areas. Much of the success demonstrated by chemical-signature methods has been in identifying coarse primary dusts. These dusts, although major contributors to total particle mass, are minor contributors to light extinction. Moreover, some of the most reliable tracers, such as lead for motor vehicles, are disappearing from emissions as the result of control measures. A major data gap exists in present knowledge of source emissions profiles that would be useful in distinguishing types of sources, including domestic wood burning, agricultural and prescribed burns and wildfires, gasoline-and diesel-fueled motor vehicles, and coal combustion.
Standard protocols for the release and sampling of tracers should be developed, along with formal field studies to verify these protocols.
The identification of haze sources in remote locations may require the use of injected tracers which can be measured at very low ambient concentrations. Artificial tracers have been widely used to trace airflows in meteorological studies. Their use in quantitative haze apportionment studies has received less attention.
Resources should be devoted to the development of novel, inexpensive, and relatively short-lived tracers for use in multiple-tracer studies.
In many studies, distinct artificial tracers are needed for several different sources. The suite of tracers now used in regional applications is very limited, comprising only heavy methane and certain perfluorocarbons. Those tracers have tropospheric lifetimes of decades; thus, large-scale operational use would degrade their value by increasing background levels. Useful tracers need not have such long lifetimes; a lifetime of weeks is adequate for haze studies.
Research efforts should continue toward the completion of advanced mechanistic models for the calculation of source effects on visibility. The two highest priorities are for: (1) an advanced reactive plume aerosol processes model for use on single-source problems, and (2) a grid-based multiple source regional model for use in analysis of regional haze problems.
Recent advances in mechanistic models (see Chapter 5) might greatly improve simulations of atmospheric physical and chemical processes that affect secondary particle formation, particle size distributions, dispersion, and deposition over long distances. As a result, future models may provide far better estimates of the impairment caused by multiple sources in complex airsheds than do models now available.
Research is needed to improve the understanding of phenomena that determine atmospheric particle size distributions.
This is the most serious barrier to the development of advanced mechanistic models. For example, better understanding is needed of particle emissions and of atmospheric aerosol processing phenomena including nucleation, gas-to-particle conversion, and cloud processing to complete the development of reliable, advanced models. Such information can only be obtained through field measurements, but little work of this kind has been done during the past decade. Field studies need to be designed carefully to advance the understanding of aerosol processes that affect size distributions.
SUMMARY OF SOURCE APPORTIONMENT MODELING RECOMMENDATIONS
Better data on source emissions are needed.
Source emissions data must be accurately integrated into overall emission inventories.
Inventories for elemental and organic carbon particles and VOCs that lead to particles require improvement.
Model Validation and Field Studies
Existing data sets must be used in model validation studies.
Mechanistic models and hybrid receptor-oriented methods should be subjected to further validation studies.
Source testing and ambient concentration-measurements are required for improved emissions profiles.
Inexpensive and short-lived tracers should be developed.
Standard protocols and field validation studies are required for the release and sampling of tracers.
An advanced reactive plume aerosol processes model is required for single-source problems.
A grid-based multiple source regional model is required for the analysis of regional haze.
A comprehensive understanding of the factors that determine particle size distributions is required.
RECOMMENDED CONTROL TECHNOLOGY RESEARCH
Better economic modeling techniques are needed to assess the most cost-effective methods to reduce emissions of relevant pollutants from their different sources.
A variety of control technologies for point and area sources will have to be applied to achieve the emission reductions required to restore clear air to the nation's Class I areas. The emissions reduction capabilities and the costs of commercially available control technologies for stationary and mobile point sources are well documented. However, it is difficult to translate unit costs for a specific technology into aggregate costs for overall emissions reductions for an urban area or region.
Cheaper and more effective pollution-control technologies should be developed.
Technological advances that reduce the cost and improve the effectiveness of controls could not only improve visibility but also alleviate other environmental problems such as acid rain and tropospheric ozone. More cost-effective controls for power plants and motor vehicles could be especially beneficial. Areas where further research and development are desirable include:
Combined NOx and SO2 control technologies for power plants and industrial boilers, at a cost less than the aggregate cost of flue gas desulfurization for SO2 control and selective catalytic reduction (SCR) for NOx control;
Low-cost, low-temperature SCR for NOx control;
Better particulate control for diesel vehicle engines;
More efficient batteries for electric vehicles and innovative methods for increasing vehicle range before recharging. (Electricity is potentially a very clean motor vehicle energy source relative to most alternatives even when the environmental effects of its production and delivery are taken into account.)
Important research efforts already are under way, for example, in the Clean Coal Technology Demonstration Program administered by the Department of Energy.
For longer-term visibility improvements, an integrated assessment of industrial, economic, and social changes that would minimize emissions of haze-causing pollutants should be carried out.
Promising prospects for emission reduction include the following: replacement or repowering of existing plants by high-efficiency and low-emission technologies, such as coal gasification and fuel cells, or by nonfossil generation systems; increased emphasis on efficient energy use in all sectors; industrial process modification to minimize aggregate emissions; and development of low-emission transportation systems.
SUMMARY OF RECOMMENDED CONTROL TECHNOLOGY RESEARCH
Better economic models are needed to assess the most cost-effective methods to reduce pollutant emissions.
Cheaper and more effective control technologies should be developed.
An integrated assessment of industrial, economic, and social changes that would minimize emissions of haze-causing pollutants should be carried out.
Present scientific knowledge has important implications for the design of programs to protect and improve visibility. What is needed, overall, is the recognition that any effective visibility protection program must be aimed at preventing and reducing regional haze. An effective program must, therefore, control a broad array of sources over a large geographic area. Such a program would mark a considerable break from the present approach of focusing on visible plumes from nearby sources and of attempting to determine the effects of individual sources on visibility impairment.
Although visibility impairment is as well understood as any other air-pollution effect, gaps in knowledge remain. Filling these gaps will require an increased national commitment to visibility protection research. We believe that the time has come for Congress, EPA, and the states to decide whether to make that commitment.