TECHNICAL DISCUSSION ON THE INTEGRATION OF OZONE, FINE PARTICLES AND REGIONAL HAZE AIR QUALITY MANAGEMENT

The Base Programs and Regional Strategies Work Groups of the Federal Advisory Committee Act (FACA) Subcommittee have asked several questions regarding the technical basis and issues underlying the integration of regulatory programs for ozone, fine particles and regional haze, and the specification of geographic scales required for air quality management. The Scientific and Technical Support Work Group (STSWG) of the FACA Subcommittee is responding to these and other inquiries by developing a conceptual model framing our current scientific understanding of ozone fine particles and haze, the associated gaps and uncertainties, and responses to questions posed by Work Groups. This notice provides preliminary commentary on the subjects of integration and Geographic scales, in advance of more comprehensive discussion on these topics is being developed in the STSWG's conceptual model.

Regarding the rationality of integration, the initial response of the STSWG is a qualified yes, given the regionality, spatial patterns of air quality indices, precursors, sources, atmospheric chemistry and meteorological processes which affect more than one pollutant, and control options. However, while reading this discussion, it is important to recognize and distinguish those attributes where there is little linkage. Moreover, many examples and inferences presented here tend to reflect what is known about Eastern U.S. air quality issues (e.g., ozone), with possibly little relation to Western U.S. phenomena. At the risk of generalizing (and simplifying) air quality descriptions for illustrative purposes, recognition that a generalized approach cannot operate effectively everywhere must be retained. The discussion focuses on the relationships between ozone and fine particles (FP), given the close linkage between FP levels and regional haze (the widespread impairment of visibility in every direction, mostly attributed to fine particle light scattering and absorption).

Some sections of this discussion are fairly technical and may only be understood by the more scientifically-inclined reader. However, a scientific background is not necessary to embrace this discussion's primary points:

understanding the emission sources and atmospheric processes which are responsible for elevated air pollutant levels requires an examination of urban and regional geographical scales;

ozone and fine particles may exhibit similar spatial patterns, although the frequency (and importance) of co-occurring patterns is not well understood;

many of the emission precursors (and sources of precursors) to ozone, fine particles and regional haze are the same;

many of the atmospheric processes (chemistry and meteorology) affecting ozone, fine particles, and regional haze are the same, and

several critically important information gaps exist which create very difficult challenges for air quality management of these pollutants.

1.	Interacting Spatial Scales of Emissions, Atmospheric Processes and Air Quality Indices

As explained in greater detail below, there are a variety of emissions that are precursors to elevated levels of ozone, fine particles, and regional haze and of sources to these emissions. Historically, attempts at air quality management of these problems focused on local sources in the context of an anonymous background term quantifying imported air quality. The evolution in our understanding of the spatial and temporal scales of the effects on ozone, fine particles, and regional haze of the emissions from all sources has, however, spawned the recognition of the need for a larger geographical perspective. This larger geographical perspective, which considers individually sources over regional, as well as local, scales, is needed to support quantitative analysis of the relative contribution of the various source types and of their emission types (species) that contribute to "nonattainment" levels. The need for an altered perspective has been recognized by the establishment of the Ozone Transport Commission (OTC), the Ozone Transport Assessment Group (OTAG), and the Grand Canyon Visibility Transport Commission (GCVTC).

Air quality management on the Metropolitan Statistical Area or Consolidated Metropolitan Statistical Area (MSA or CMSA) has worked well historically to control the local source effect on "nonattainment" problems. This is evidenced by the significant decrease in the number of ozone nonattainment areas over the past decade. As these controls have reduced emissions and as modeling tools have progressed, the role of the effect of sources beyond the MSA or CMSA, and the varying spatial scales of air quality indices and atmospheric processes, continue to be investigated and supported by a strong body of scientific evidence:

the 1991 National Academy of Science (NAS) Report, Rethinking Ozone in Urban and Regional Scales (NRC, 1991),

the 1993 NAS Report, Protecting Visibility in National Parks and Wilderness Areas (NRC, 1993);

the National Acid Precipitation Assessment Program (Trijonis et al., 1990); and

the Southern Oxidant Study (Chameides and Cowling, 1995).

Recent analyses based on ambient air monitoring data (Rao, 1995) and Regional Acid Deposition Model (RADM) air quality modeling (Appleton, 1995) suggest a very broad spatial air pollution region covering the greater part of the Eastern U.S. These studies indicate that, while sources still have their largest influence in the near field, the zones of potential influence of source regions (e.g., an urban city) can under certain conditions extend out 100s of km for ozone, fine particles, and regional haze. Moreover, these scales appear to be similar for ozone and FP. In other words, sources once thought to be remote with respect to "nonattainment" levels of ozone, fine particles, and regional haze are seen as potential contributors to those levels. The analyses suggest that chemical and meteorological processes which influence pollutant generation, air mass movement and pollutant removal (e.g., clouds and precipitation) are key factors in delimiting regional zones of influence. When the various "nonattainment' areas of the Eastern US are surrounded by even conservative estimates of the zones of influence of these other sources, what results is a modeling domain that may span the greater part of the Eastern U.S. Accordingly, efficient air quality management requires addressing these additional sources, atmospheric processes and related impacts as scales of interactions over multiple spatial and temporal frames.

In air quality management practice, the term "transport" has been used in a very broad context beyond the strict meteorological definition of the term. This broad context includes the (1) overall regionalization of both the scale of pollutant distributions and zone of influence of sources, (2) interaction (or effect of one area on another) among local, urban and regional source scales, and (3) meso and large-scale meteorological phenomena such as recirculation due to stagnant high pressure systems and land-sea interactions; "transport", which refers to large scale movement of air masses with fairly uniform motion; and other events perhaps as simple as widespread elevated temperatures. The prevalence and importance of biogenic volatile organic compound (VOC) emissions (e.g., emissions from trees) in the Eastern U.S. is "region wide", as are many other area source emissions such as motor vehicles. All of these regional attributes are enhanced by the relatively flat and consistent terrain in the East and Midwest, contrasting the greater topographic and meteorological effects in the Western U.S.

Several physical and chemical events act together in determining pollutant concentrations over multiple space and time scales. Moving air masses carry all chemical species; including precursors, fast reacting intermediates, and chemical sinks, as well as the specific pollutant species of interest (e.g., FP and ozone). Removal of pollutants occurs continuously through deposition. Also, the impact of these pollutants is not simply additive. Ozone (or precursors) transported from one location can affect ozone levels "downwind" by indirectly accelerating atmospheric chemistry reactions through the production of chemical intermediates (e.g., hydroxyl radicals). Clouds play several roles in modifying concentrations by (1) dissolving soluble gases (e.g., nitric acid, sulfur dioxide, hydrogen peroxide) and generating aerosols through aqueous phase reactions, (2) circulating and venting pollutants to high altitudes where strong winds promote large horizontal transport, and (3) removing pollutants through precipitation. Cloud related dissolution and transport also contribute to pollutant removal, depending on one's reference frame. Vertical air mass movements, or phenomena as basic as the daily mixed layer growth, affect air concentrations on various scales. Superimposed on these processes are a variety of emission sources with their own spatial, temporal and component (speciation) scales. Depending on location, pollutant and season, one particular spatial scale (e.g., urban) may (or may not) exert a dominating influence on air quality relative to another scale (e.g., regional). Even in cases where local and urban sources are responsible for most of the "local" air quality, an assessment of the contribution of distant sources to local air quality is required to reach such a conclusion. Thus, to avoid the exclusion of potentially important considerations in air quality analysis, Regionality or Interacting Scales is a more descriptive term (than transport) which encompass the broader meaning and effects of several complex interacting phenomena operating over extensive and multiple time and space scales.

The Eastern U.S. differs markedly from the West, so any extension to the West based on Eastern analyses (or vice-versa) is not appropriate (important differences exist between Northern and Southern regions as well). The monitoring data and modeling analyses of the GCVTC process highlight the challenge of identifying and quantifying specific sources, some at great distances in order to estimate their effects in Western national parks and wilderness areas. The variations in topography, meteorology and source distribution across regions require that area and case specific differences be accounted for in any air management approach. The effects of emission reduction strategies should be viewed through multiple scales, considering regional and urban scale consequences (i.e., health and welfare protection).

A few points summarizing "interacting scales" and "regionality" should be considered in air management practices:

A.	Analyses of observations in the Eastern U.S. reveal the existence of very broad multistate regions (interacting scales approaching linear extents of 1000 km or more) of elevated pollutant levels and zones of source influence (Rao, 1996).

B.	Air quality modeling for the East suggests that similar regions of influence exist for ozone and FP (Dennis, 1996), although only sparse monitoring data exist to support these similarities .

C.	Modeling analyses for the Grand Canyon National Park (and other) Class 1 Areas show that FP and precursors causing visibility impairment episodes are derived from both nearby (less than 50 km) and more distant (up to 1000 km) regions of influence (NRC, 1993; GCVTC, 1996).

D.	Area and case-specific analyses are required to delineate reasonable geographic areas for air quality planning purposes, because of the wide regional variations in meteorology, topography and source distribution.

E.	The use of terms such as "transport" or "background" inadequately describes the complex set of emissions, chemistry, and meteorological processes and interacting scales which contribute to the regionalization of air pollution

F.	Because of broad spatial extents and gradations of interacting scales ranging from regional down to sub-grid cell scales, an air quality assessment focusing on a particular scale (e.g., urban) must consider effects due to interactions across various space and time scales. The concept of a single MSA/CMSA nonattainment area is inconsistent with the spatial and temporal scales for ozone, FP and haze problems.

2.	Technical Basis and Considerations for integrating ozone, fine particles and regional haze implementation programs.

The technical and scientific rationale for underlying the integration of ozone, fine particle and regional haze air quality management practices is based on a mix of empirical observations, atmospheric processes and practical administrative concerns. While this discussion focuses on common attributes across pollutant groups, it is important to recognize and distinguish those attributes where there is little linkage. Many examples and inferences presented here tend to reflect what is known about Eastern U.S. air quality issues (e.g., ozone), with possibly little relation to Western U.S. phenomena. At the risk of generalizing (and simplifying) air quality descriptions for illustrative purposes, recognition that a generalized approach can not operate effectively everywhere must be retained. The discussion focuses on the relationships between ozone and fine particles (FP), with the implicit assumption that FP levels and chemical compositon directly relate to regional visibility impairment, given the strong relationship between the constituents of FP and the manmade portion of visibility impairment. Regional haze is a widespread impairment of visibility in every direction, mostly attributed to light scattering from fine particles.

Empirical evidence for integration.

Ozone and PM-10 concentrations in the Eastern U.S. can exhibit similar spatial patterns during summer time episodes (NESCAUM, 1995). Analysis of particulate data consistently indicates that FP constitutes the majority mass fraction of PM-10 in the summertime East (EPA, 1996). In combination, these observations qualitatively imply co- occurrence of elevated ozone and FP. However, quantification of the similarity and frequency (very common or quite unusual?) of such events is severely restricted by a lack of a FP data base in the East. While more data exists in certain Western locations, the episodic relationships between ozone and particulate matter appears to be more complex than in the East. For example, a major component of the FP problem in Los Angeles (as well as the San Joaquin Valley, Salt lake City and Denver) is wintertime formation of ammonium nitrate, which is not stable at the high temperatures associated with elevated ozone. High levels of FP in western nonattainemnt areas can impair visibility when high ozone concentrations are not observed. Nevertheless, "smog" events in LA almost always are accompanied by impaired visibility, and visibility is directly associated with FP levels. Although some limited empirical evidence is highly suggestive of area specific co-occurring events, other considerations as described below provide a stronger rationale for the appropriate level of integration across ozone, FP and regional haze control programs.

Emissions and Atmospheric Process Linkages Across Ozone, FP and Regional Haze

Several connections exist among the three pollutant categories. The linkages are based on the existence of common emission precursors, source categories and atmospheric chemistry and meteorological processes which affect more than one pollutant. For example, emissions of oxides of nitrogen (NOx) potentially can lead to both ozone and FP formation. A combustion source often emits both sulfur dioxide (a FP precursor) and NOx (an ozone precursor). The sequence of atmospheric chemistry reactions underlying ozone formation is in part responsible for FP formation. Similar meterological processes affect the movement, mixing and removal of ozone, FP and precursors. Some of these connections are complicated and explained more completely in forthcoming FACA science documents. The following are very brief, more technical, descriptions of the connections across pollutant categories, provided for those interested in more detail.

1. Common "direct" precursor emissions. Emissions of nitrogen oxides (NOx), volatile organic compounds (VOC) and carbon monoxide (CO) are considered precursors for ozone formation. NOx, VOC and sulfur (SOx, mostly as sulfur dioxide) emissions also can lead to FP formation through "secondary" atmospheric chemistry reactions. Both ozone and a substantial fraction (which can vary greatly with season and location) of FP are the result of secondary formation processes. The major components (which also are highly variant) of secondary FP include sulfates, carbon (elemental and organic) and nitrates. The fraction of FP due to secondary processes is highly variant in space and time. During certain conditions (e.g., available ammonia, negligible sulfate, low temperatures), NOx emissions can lead to fine particulate ammonium nitrate formation. Several directly emitted organic compounds contribute to fine particle organic aerosols. These organic compounds may contribute as "primary" organic aerosols, that is, they almost immediately condense to the aerosol phase during the emissions process or shortly downstream. Or, certain volatile organic compounds (e.g., toluene) which exist as gases under most conditions can undergo atmospheric reactions and transform into condensible "secondary" organic aerosols. Thus, a VOC like toluene can contribute to either ozone or FP formation as a precursor emission.

2. Common source categories. Based on the multiple roles of precursors, a particular source (natural or anthropogenic) emitting one precursor (e.g., Nox or VOC) can affect ozone and FP, and a single source emitting multiple precursors (e.g., combustion process releasing NOx, VOC, CO and SOx) can affect one pollutant category. In this case integration is not dependent on atmospheric chemistry linkages. This commonality among sources should lead to a more consistent approach in estimating emissions of multiple precursors within a specific source category. For instance, a consistent approach needs to be applied for estimating and projecting both NOx and SOx emissions from a combustion source.

3. Interaction of atmospheric chemistry reaction cycles, and "indirect" precursors. Much of the general atmospheric chemistry involved in ozone formation can affect FP formation, as alluded to above, in certain instances. For example, ozone is the major initiator of hydroxyl radicals, a chemical intermediate which converts sulfur dioxide and nitrogen dioxide to more oxidized sulfate (e.g., sulfuric acid) and nitrate (nitric acid) forms. Both sulfates and nitrates can contribute to FP formation. Clearly, a linkage between ozone and FP exists through the role of ozone in generating hydroxyl radicals. Note that this linkage between ozone and FP is at the process level and does not require coexisting "high" ozone and FP levels. Many other important linkages involving oxidizing chemical species (radicals and peroxides) exist within the NOx, VOC, SOx, ozone chemistry system. A correct characterization of the basic ozone chemistry and the associated linkages among the precursors is needed to predict the affect of changing emissions on air quality indices. Consequently, the predictive air quality models used to assess ozone and FP impacts should include a basic core set of atmospheric chemistry reactions (i.e., a gas phase ozone chemistry mechanism).

Because of their common atmospheric chemistry linkages, many precursors associated with one pollutant might be considered as an "indirect" precursor for another pollutant as well. Virtually all precursor emissions (NOx, SOx, VOC, CO) undergo initial attack by hydroxyl radicals and participate in in the general cycling of various chemical intermediate species. Therefore, precursors that typically may not be associated with a particular secondary pollutant, such as the effect of VOC on either sulfate or nitrate, indirectly participate through their roles in atmospheric chemistry. In this general context, the term precursor does not imply a positive effect on an associated secondary species as the emission precursor may only share in certain atmospheric chemistry processes without leading to increases in a secondary pollutant. Multiple possibilities exist. For example, NOx, which affects the cycling of hydroxyls radicals (which convert SOx to sulfate), could act indirectly as a sulfate particle precursor. The majority of VOC species that do not transform into organic aerosols could nevertheless be FP precursors through their general role (i.e., cycling of radicals) in atmospheric chemistry. Nitrogen oxides could serve as indirect precursors for aerosol sulfate formation. This "universal" pool of precursors does not imply that reductions of any specific precursor lead to reductions of every pollutant. Just as reductions in NOx potentially can raise local ozone levels, a reduction of a FP precursor possibly can increase ozone or increase a different FP component (e.g., SOx reductions leading to increased ammonium nitrate, or NOx reductions increasing sulfate formation). These examples are some of several conceivable indirect precursor relationships. Many other relationships with similarly unknown degrees of effect exist. Thus, integrated implementation is far from a straightforward exercise. Complex air quality simulation models (in combination with simpler models and receptor/observational methods) which include approximations of these process linkages will`need to be exercised to account for the multiple nonlinearities and positive and negative feedbacks. This complexity demands that high quality emission inventories, technically credible models and spatially and temporally representative monitoring data will be needed in predicting pollutant concentrations and control strategies.

Integrating Control Strategy Development through an Air Quality Modeling Approach

What does integration mean from an implementation perspective? Given the complex mechanisms for and linkages between ozone and FP formation, the formulation of control strategies should acknowledge the need to optimize control options; control of one precursor might affect both ozone and FP or might be detrimental for one or both. For example, one might start with ozone management strategies being developed as part of ongoing urban and regional studies, and attempt to quantify the future impact on secondary aerosols. On the other hand, because NOx controls might increase ozone levels in certain localized urban areas or because SO2 reductions might lead increased concentrations, efficient air quality management would attempt to optimize the system in relation to VOC, NOx and SOx emission reductions.

The real benefit of integration is the prospect of a more systematic, efficient, and comprehensive treatment of emission inventories, episode selection, and atmospheric physics and chemistry that might empower the air quality manager to characterize source to receptor effects in an orderly way. The addition of data on the costs and effectiveness of control options would enable the air quality manager to identify the cost-effective means for attaining a variety of air quality goals.

To this end, emission bases underlying most current ozone modeling efforts include most of the sources for aerosol formation (but not necessarily the aerosol specific emissions such as organic aerosols from motor vehicles). Notable exceptions include many of the fugitive primary particle sources and most sources of ammonia emissions. The result of this exercise would produce the residual aerosol (and regional haze) related air quality benefits from an ozone precursor control perspective. [Additional analysis directed at the specific needs for meeting FP and visibility concerns would follow this ozone oriented approach. Ideally, an objective (and likely iterative) ability to assess the benefits and tradeoffs associated with managing all three pollutant categories would evolve.] Although this example does not represent "full" integration given the unidirectional information flow (ozone to particles), it does acknowledge similarities among programs, avoids mistakes and inefficiencies incurred from independent analyses. Aside from any direct regulatory policy, the linkages across pollutants and emissions are reasons by themselves for planning for more efficient development and use of emissions, air quality models and monitoring networks which address sometimes confounding multiple pollutants and their related health/welfare effects and control options.

Distinctions among ozone, fine particles and regional haze

Coincident ozone and FP episodes may be expected to occur given similarities in the meteorological and atmospheric chemistry processes underlying ozone and FP formation, maintenance and destruction. As discussed above, the linkages associated with emission source categories and physical and chemical processes exist more frequently than the occurrence of coepisodic events. For example, several basic atmospheric chemistry reactions involved in ozone and FP formation occur whether or not high ozone and FP levels are generated in the atmosphere. Nevertheless, several distinctions among the pollutants persist. These differences include the contribution of primary particles to total FP (and especially PM10), and wintertime (actually non summertime) FP events. Some primary particles are generated by strong wind conditions (e.g., soil/geologic material) and other mechanical processes (e.g., roadway fugitives). The fraction of primary PM peaks in summer in most of the Western third of the country where there is little precipitation for 6-8 months per year, leading to dry, windy conditions for the generation and movement of geologic materials. As discussed earlier, ammonium nitrate, a significant FP component in the West, is stable at relatively low temperatures and therefore does not form significant levels during the summer. Meteorological effects which influence the creation, maintenace or removal of high levels of ozone and FP may be significantly different between pollutants, regions of the country, and times of the year. Other specific emissions driven events such as forest burning and wintertime woodsmoke (a major wintertime source of urban PM) bear virtually no relation to ozone. Many of these PM episodes can be dominated by either primary or secondary FP components, or by primary anthropogenic coarse PM emissions. Research exploring the frequency and characterization of coepisodic and uni-episodic events would yield further insight into underlying causes of events and provide direction for integrated implementation opportunities.

Visibility protection presents several additional considerations beyond the scope of topics covered under ozone and FP. First, FP concentrations that are far below a NAAQS can adversely affect visibility in a significant manner, particularly in more pristine environments, such as Class I areas in the rural West. For this reason, visibility management needs to consider the protection of "clean" days separately from assessments focusing on highly impaired days. The meteorology and emissions characteristics during "clean" days differs from those common during high pollution episodes. This concern raises complex technical issues related to the ability of models and monitoring instruments, which often have been designed or tested for meeting "high" concentration requirements, to characterize "low" level conditions. Second, relative humidity plays a significant role in enhancing visibility impairment, particularly in the East. In humid conditions, particularly above 70% RH, sulfates, nitrates, and certain organics readily take on water and expand to sizes comparable to the wavelength of light. Particles in this size range (e.g., 0.1 to 1.0 micron in diameter) are efficient scatterers of light. Third, unlike the NAAQS approach of setting a national standard, the regional haze program under section 169A of the Act has as its goal "the prevention of future, and the remedying of any existing, impairment of visibility in mandatory class I Federal areas which impairment results from manmade air pollution." States are required to make "reasonable progress" toward this goal. The notion of background versus manmade air pollution raises several technical and policy challenges, particularly in the protection of visibility in "cleaner" environments, where small increases of FP can lead to significant visibility changes.

Generally, PM10 is not considered in the integration discussions of ozone, FP and regional haze. This is because the coarse fraction (e.g., greater than 2.5 ) typically is derived from primary emissions (e.g., fugitives and geologic material) with little association to ozone from a process (or episodic) perspective. In addition, visibility impairment leading to regional haze is overwhelmingly associated with the FP fraction of PM10.

3.	Major Technical Issues

The principal technical issues associated with integrated air quality management involve the adequacy of data bases and models (including specific process formulations) on which to base credible assessments. While it is impossible to ascribe consensus opinion regarding clear acceptable limits on the available information, gradations of "acceptability" or "comfort-level" can be associated with various pollutants. Generally, the tools (ambient data, models and emissions) underlying ozone analyses are far more mature than those for FP. Major efforts in chemical mechanism development, ambient monitoring methods and establishment of national and special study efforts for monitoring, emissions and modeling have resulted in a wealth of information and familiarity with these tools. This relative abundance of knowledge for ozone should not be construed as a science lacking uncertainty, as significant technical issues remain (e.g., the current NARSTO effort) and even more are yet to be defined. A sampling of these issues include the representativeness of emission inventories, particularly biogenic emissions; uncertainties in the modeling system (e.g., chemical characterizations of aromatics and biogenics; treatment of vertical mixing processes); difficulties in monitoring techniques (carbonyls, NOx-NO2, polar VOCs); and lack of measurements (e.g., total reactive nitrogen, NOy; upper air data). These gaps are significant and compromise our ability to perform highly credible ozone analyses and to ascribe confidence levels in our results.

Consideration of FP and regional haze presents several additional issues, a result of (1) a very complex multiphase, multicomponent, multiseason aerosol system , (2) the complex covariance of these data, and (3) the present PM10 form of the NAAQS which has resulted in few regulatory pressures to drive an improved characterization. Significant concerns include major positive and negative measurement artifacts (related to gas-particle phase changes); a simple lack of ambient data, especially urban FP measurements; poor quality assurance/control of ambient sampler data; emissions data with poor general spatial applicability; very limited availability and nearly nonexistent application and evaluation of regioanlly-accurate air quality models; and highly empirical treatment of organic aerosols within the available models. These gaps are interconnected, in the sense that quality model evaluation and improvement rely on available quality measurements. The issue is further complicated by difficulties (due to complexities, lack of precedence and resource constraints) in designing a data collection program to evaluate a gridded model's ability to characterize FP covering wide scales of time (annual, seasonal, daily) and spatial resolution (regional, urban, local). On the positive side, a strong history of using ambient data for PM source apportionment probably are more adoptable to fine particle analyses than ozone, given that the measurable components of secondary FP (e.g., sulfate) have some direct linkage to precursors, whereas an ozone measurement by itself provides no inference regarding contributing precursors.

Several interesting atmospheric chemistry questions remain to be answered, two examples include nitrate FP formation and organic aerosols. Where and when do ammonia and sulfate become limiting factors in ammonium nitrate formation? The relatively abundant nitrate FP at sites in the urban West contrasts with abundant regional sulfate FP in the East. Substantive decreases in sulfur dioxide emissions could lead to increased nitrate FP formation in the East, if sufficient ammonia (a highly uncertain emissions category) is available. What impacts will NOx emission reductions have on FP? Many possibilitires exist. If nitrate is significant, one would expect a reduction in FP. However, if sufficient sulfur remains available, NOx reductions could increase or decrease sulfate formation ( and therefore FP) depending on a complex cycling of oxidizing species. Reductions in NOx emissions could actually lead to sulfate increases by reducing competition (between SOx and NOx) for gas phase oxidizing radicals, or by increasing peroxide levels leading to greater aqueous phase sulfate production. Or, NOx reductions could slow down sulfate formation through overall reductions in ozone and other oxidants. This relationship is very complex, and we must exercise caution in associating FP benefits with NOx reductions in the Eastern U.S.

What are the relative contributions of primary and secondary organic aerosols across varying spatial (and time) scales? The potential for large secondary organic aerosol production from biogenic sources (e.g., pinene emissions) exists throughout the East. How significant are biogenic derived aerosols compared to local/urban contributions from primary anthropogenic organic aerosols? How different are these relative contributions across seasons, given that secondary organic aerosol formation increases during the summer? Many uncertainties underlie the integration of primary and secondary particles, aside from integrating particles and ozone. For instance, what are the interactive roles that elemental carbon emissions, other products of incomplete combustion and geologic materials exert in both primary contribution to PM and as formation nuclei for highly complex secondary PM? On balance, the ability to perform ozone air quality assessments far exceeds that of FP. However, the infrastructure for conducting FP analyses appears to be in place as a result of progress gained from ozone and acid deposition modeling and existing monitoring programs for ozone and visibility (i.e., the IMPROVE program). Finally, although uncertainties remain in transforming particles into visibility impairment within short averging times, the IMPROVE methodologies for particle and visibility measurements (and the relationships between particles and visibility) are widely accepted.

Specific issues across PM and ozone include the ability to formulate fully integrated models accounting for multidirectional effects on several pollutants. For example, the formation of secondary organic aerosols is a loss mechanism for VOC, which presently is not accounted for in ozone modeling efforts. Many other integration topics exist, and collectively there is uncertainty regarding the overall importance of one pollutant imparting an effect on another.

Two basic issues span the gap between science and policy: (1) the manner in which tools are applied, and (2) accommodating scientific findings and uncertainties in air quality management decision making. The first topic reflects the concerns of how one applies deterministic (i.e., models that establish exact cause and effect relationships) and uncertain air quality models to probabilistic forms of the standard in ascribing rigid control requirements. The selection of "severe" meteorological episodes versus "prototypical" episodes for ozone and PM10 modeling has been controversial, and remains a difficult model application issue. Equally complicated is the emerging need to model seasonal and annual cases. The debate on the credibility of models is fueled by the manner in which they are applied as much as the valid concerns about their formulations and supporting data bases. The second topic acknowledges the need for conducting policy-relevant as opposed to policy-driven research, and recognizing the different time scales operating in research and policy arenas (where the time frame demands move much faster than research results). Extremely useful information emerges continuously from research programs, yet a separate, sometimes very significant, time-lag occurs before information is considered in the policy setting process. Hence, opportunities must be available to incorporate the latest science into policy.

4.	Integrating Models and Observations for Sound Air Quality Management Practice

Much emphasis has been placed on the complementary and integrated use of models and ambient data in air quality management practice (Rao et al., 1996). Several facets are associated with this topic, ranging from the need to evaluate models with sound data bases to conducting fully integrated analysis optimized through the separate, strong attributes of data and models. As the technical debate on the use of models and data continues to mature, perceptions such as "model" or "data" are replaced by the intelligent and integrated use of "models and data." Clearly, the demand for measurements as ground-truthing and feedback information loops initiated by the National Academy of Sciences Ozone Report (NRC, 1991) has been adopted by large segments of the air quality community and reflected in major efforts such as the Photochemical Assessments Measurement Stations (PAMS) and North American Research Strategy for Tropospheric Ozone (NARSTO).

An appreciation of the stengths of models and observations can assist the undestanding of current analyses and lead to improved techniques. A models strength is its ability to (1) integrate an enormous spectrum of data (e.g., emissions and meterological variables) and process understandings (e.g., chemical mechanisms and flow phenomena), and (2) serve as an exceptional space and time mapping tool. This latter attribute reflects the model's unique ability to predict into the future and to supplement (or fill in) present gaps in observed data. The process formulations imbedded in models enable the addressing of so many "what if" questions related to emissions control. However, models are engineering tools that invoke substantial approximations of scientific understandings of natural phenomena, both their formulations and application methods reflect engineering principles more than fundamental science. Observations provide a basis for testing and diagnosing models, but in some instances can capture process type relationships by themselves (e.g., the emergence of observational based models for defining NOx and VOC control preferences). However the observations are very spaarse. Hence, applied in isolation, the use of models or observations is not acceptable. Space and time constraints often bias the interpretation of observational analyses (i.e., analysis results reflect time and space of monitors which may or may not reflect the scales of concern). Models suffer from a very large spectrum of weaknesses, because they attempt to portray so many phenomena. Most critical though is the risk of using a potentially biased model that is assumed bias free. The integrated use of observations and models mitigates the individual weaknesses of both approaches and produces a powerful air quality management tool, especially when applied in an iterative (even retrospective) manner to continually assess model results and related implementation strategies.

5.	Summary

Air quality assessments for FP, ozone, and regional haze must consider emissions, meteorological processes, atmospheric chemistry, and deposition all of which interact over multiple spatial and temporal scales. Examining in detail the sources only from the MSA/CMSA surrounding the monitor reporting "nonattainment" levels of air quality may need to be augmented (on a space and time basis) for responsibly allocating those levels to the sources causing them. When examining the issues on expanded time and space scales, the air quality management should also take into account, the similarities of these air quality indices such as their common precursor emissions (e.g., NOx, VOC); common emissions sources (e.g., mobile sources, stationary and area source combustion emissions, biogenics), and shared chemical and meteorological processes (e.g., transport, transformation, precipitation, and removal).

The principal technical issues associated with integrated air quality management involve the adequacy of data bases and models (including specific process formulations) on which to base credible assessments. Many of these gaps are interconnected, since model evaluations rely on available high quality measurements of emissions, atmospheric processes (such as wind fields), and ambient concentrations. On balance, the ability to perform ozone air quality assessments far exceeds that of FP, due mostly to the maturity of ozone research as well as lack of urban FP measurements and important emissions components. However, many of the components of the infrastructure for conducting FP analyses appears to be in place as a result of progress gained from ozone, acid deposition, and visibility modeling and monitoring programs.

The integrated application of models and observed data is strongly encouraged. In combination, both approaches help to mitigate the weakness of an isolated approach, producing a powerful tool for air quality management.

References:

Appleton, E.L., "A Cross-Media Approach to Saving the Chesapeake Bay," Environ. Sci. Technol., 1995, 29, 550A-555A.

Dennis, R.L., Personal Communication, 1996.

NECAUM, "Preview of 1994 Ozone Precursor Concentrations in the Northeastern U.S.," 1995 Northeast States for Coordinated Air Use Management, Boston, MA.

EPA, 1996, "PM Criteria Document."

Rao, 1996, Personal Communication, 1996

Rao et al., "Dealing with the ozone nonattainment problem in the eastern United States", 1996

NRC, "Rethinking the Ozone Problem in Urban and Regional Air Pollution, National Academy Press, 1991.

NRC, "Protecting Visibility in National Parks and Wilderness Areas," National Academy Press, 1993.

Rao, S.T. E. Zalewsky and I.G. Zurbenko, "Determining Temporal and Spatial Variations in Ozone Air Quality," J. Air & Waste Manage. Assoc.1995,45,57-61.

Trijonis, J. et al., "Report 24 - Visibility: Existing and Historical Conditions - Causes and Effects," from Acidic Deposition: State of Science and Technology," Volume III, National Acid Precipitation Assessment Program, 1990.