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Environ. Sci. Technol., Vol. 28, No. 2, 1994
0013-936X/94/0927-70A$04.50/0 © 1994 American Chemical Society
ES&T
The O u e s t
FOR AN ADVANCED
REGIONAL AIR QUALITY MODEL e take the Earth's atmosphere for granted. Why shouldn't we? It is a literal fact of life: it is as it is because of life. We and all other living creatures have spent some 3 billion years evolving beneath its protective and sustaining umbrella. For all we know this may be a universal record for sustainability, and it is our nature to wish it extended far into the future for our descendants. For that to happen, humans must not perturb the atmospheric composition beyond the bounds of their sustainability. Not too long ago, the atmosphere seemed so vast and we so puny that the very idea of humankind having any influence on the mighty natural forces at play within it was surely in the realm of fiction. But as our population has soared with its concomitant demands for energy, space, shelter, and food, so has the quantity of industrial and agricultural emissions we pour into the atmosphere. We have come to understand that these emitted materials can impair, or at least shift, the health of not just compartments but the whole global ecosystem—not to mention our aesthetic appreciation of it. Our emissions increasingly are doing just that. Accordingly, air
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pollution and climate alteration are issues of immediate concern worldwide. This article describes an international initiative that addresses regional and smaller scale air pollution. It should provide an advanced set of tools for air quality managers to use in making scientifically
D. A L A N
HANSEN
Electric Power Research Institute Palo Alto, CA 94304
R O B I N L. D E N N I S EPA Research Triangle Park, NC 27711
ADOLF
EBEL
Universität Zu Koln D-5000 Koln1,Germany
S T E V E N R. H A N N A Sigma Research Corporation Concord, MA 01742
JACK
KAYE
NASA Headquarters Washington, DC 20546
RICHARD
THUILLIER
Pacific Gas and Electric Company San Ramon, CA 94583
based policy decisions. We will refer to these tools as comprehensive modeling systems (CMSs) because of the depth and breadth of their formulations and applications. The initiative is being carried out by the Consortium for Advanced Modeling of Region Air Quality (CAMRAQ). This article explains why the consortium feels the time for a CMS is opportune, what CAMRAQ's objectives are, how we are organized, what our roots are, how we are approaching coordinated research, where we currently are collaborating on projects, and what opportunities we see for future collaboration. Background Numerous issues confront those who must make and those who must respond to air quality management decisions. A particularly nettlesome issue at present is tropospheric ozone. For example, in North America dozens of urban areas are not in compliance with federal and other ambient air quality standards and guidelines for ozone. Similar conditions exist throughout the world near urban and industrial complexes. Elevated ozone concentrations are usually the result of photochemical reactions involving nitrogen oxides and organic gases. Environ. Sci. Technol., Vol. 28, No. 2, 1994
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(We s a y " u s u a l l y " b e c a u s e tropopause folding can allow stratospheric ozone to mix downward to the ground; 1-3). What makes this problem especially perplexing is that the reactions are extremely complex and nonlinear; reducing one or the other of these ozone precursors does not necessarily lead to ozone reduction. Further, because of atmospheric transport, ozone or emissions of its precursors in one area can influence air quality hundreds of kilometers downwind. These and other scientific and technological complications—as well as questions of equity regarding whose emissions should be reduced (because such reductions can require substantial capital outlays)—make the formulation of ozone attainment strategies very difficult. Other issues demanding the attention of air quality regulators are toxic airborne materials and visibility impairment. And even though the immediacy of the acid rain issue has diminished as precursor emission reductions have been mandated and other competing issues have arisen, it is still with us. In the final analysis, these issues are always addressed in the regulatory arena by requirements for emission controls. To estimate how exposures derived from concentrations in the atmosphere will respond to changes in emissions of undesirable materials and their precursors, we must understand where the emissions occur, how they are transported and dispersed through the atmosphere, what chemical and physical transformations occur, how they are removed, and the rates of all these processes. In the case of visibility, we must understand the optical properties of airborne materials and the response of human vision to them. As with ozone, this understanding is required to formulate strategies to ameliorate conditions. Some important nonregulatory issues include real or hypothetical emergencies in which we need to estimate exposures and effects resulting from the release of undesirable (or worse) substances into the atmosphere, such as occurred at Chernobyl, from the Kuwaiti oil fires, or from the use of chemical or nuclear weapons. And finally there is the issue of enhancing our theoretical understanding of the highly interactive chemical and physical d y n a m i c s of t h e t r o p o s p h e r e through a systematic analysis of the 72 A
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processes involved. Each of these issues can be addressed wholly or in part through the use of numerical simulation models. We in CAMRAQ believe that these models are fundamentally the best available type of tool for establishing a workable link between scientific understanding and policy analysis. Other advantages of numerical models are their intrinsic ability to integrate all the relevant scientific knowledge we care to include, the control we have over
Modeling, computer science, and scientific understanding of the troposphere will soon allow convenient and skillful computer simulations to be made that address all air quality issues in concert. their governing conditions (i.e., their initial and boundary conditions and other inputs), and the ability they give us to examine details of complex, interacting systems. With models we have a degree of control and a capability for exploring virtual realities beyond those available to us through field and laboratory experimentation, particularly when dealing with atmospheric phenomena. Further, we in CAMRAQ have concluded that modeling, computer science, and scientific understanding of the troposphere will soon allow convenient and skillful com-
puter simulations to be made that address all the above issues in concert. We desire this capability for simultaneity because many of the issues are interrelated and should not be addressed in isolation. We also are aware that numerous high-quality environmental data sets are or soon will be available to support intensive and diverse analyses as well as comprehensive evaluations of model performance—an absolute prerequisite for establishing the reliability of models slated for regulatory or emergency response applications. To package all this science, technology, and information in a practical form requires a CMS. Further, a program designed to acquire a CMS (hardware, software, and supporting infrastructure) and to ensure access to the vast quantity of data needed for its applications would strain the resources of any single organization. Only through the collaboration of many organizations could a CMS capability be realized in a reasonable time frame. So where do we stand today? Many of the agencies participating in CAMRAQ develop or use a large variety of air quality models in fulfilling their missions. Unfortunately, most of these models have very different genealogies and target different issues. They vary in level of explicitness, parameterization of process representations, and underlying assumptions. Thus, when different models are applied to the same issue, for example, regional ozone, they often yield different results. Further, single or limited-issue models, such as those labeled "acid rain models," may perform very poorly if applied to other issues, such as airborne particulate matter. Models typically have evolved over many years, have been designed and programmed by individuals having a wide range of skills and following no standardized methods, have not been adequately evaluated or compared, are not well documented, and are not computationally efficient. We believe the CMS approach we have adopted will avoid these problems. Objectives Our overall objective is to make available to our community a convenient set of tools that will allow us to reliably project what will happen to specific air quality indices over a broad range of temporal and spatial scales if the source term for
virtually any substance of interest is changed. A CMS that will provide this set of tools will have to have at least the following attributes: • Regulatory approval. Many in the consortium either include air quality regulation in their mission or represent interests that must respond to such regulation. Thus, we require a CMS to be suitable for regulatory applications and endorsed by the pertinent agencies for such use. • Accessibility. Convenient access to a CMS, including its supporting data files, must be assured for all CAMRAQ members. In all likelihood a CMS will reside in a distributed data-computing environment, and access would be via a workstation in this network. • Versatility. A CAMRAQ CMS would be a hierarchy of user-selectable models that have at the high end the most complete and explicit schemes for estimating emissions, representing all relevant tropospheric processes and estimating initial and boundary conditions, that our scientific knowledge and computational capability will allow. Lower in the hierarchy, process representations would become more highly parameterized, and, in the absence of observational data, model inputs could be selected from pretabulated data sets that define conditions "typical" for the modeling domain selected. A CMS can therefore be used for anything from rapid screening exercises to full blown, research quality, multiissue assessments. • Reliability. The performance of each selectable configuration would be evaluated through sensitivity analyses and against field data sets of defined uncertainty to establish the confidence that can be placed in the individual simulations produced. Because a criterion for inclusion of a component model in a CMS would be that it has been evaluated against field data, a high priority among CAMRAQ members would be the acquisition of appropriate data sets. The computer code itself would conform to modern standards and be thoroughly documented. • Self consistency. As far as practical, consistent underlying assumptions would be maintained throughout the hierarchy to achieve as much comparability among CMS configurations as possible. • Convenience. Advanced, menudriven user interfaces for input and output data access, generation,
management, display, analysis, and interpretation are required. Process representations would be modular for ease of replacement, substitution, and upgrading. An example of such a user interface and its display possibilities can be seen in Figure 1. These pictures were produced by a prototype CMS system currently under development at Carnegie Mellon University (Bruegge, B. et al. "GEMS: A Geographic Environmental Modeling System," unpublished manuscript).
Only through the collaboration of many organizations could a comprehensive modeling system capability be realized in a reasonable time frame. • Efficiency. Process algorithms and numerical solvers would be selected for computational speed without sacrificing accuracy. A CMS should be exercisable in a computing environment having a combination of massively parallel, specialized, and vector-processing capabilities. Its software and hardware should be compatible and designed to allocate the computing problem according to the hardware and software combination yielding the optimum performance. CAMRAQ's secondary objectives relate to facilitating CMS acquisition, characterization, and application through coordination of re-
search and collaboration on highly focused projects and encouraging advancements in the relevant atmospheric and computer sciences. In carving out our niche, we understand that many of our agencies have substantial research programs on global issues as part of the international effort to elucidate the causes and consequences of climate change. CAMRAQ intends to complement, not duplicate this effort and to focus on regional and smaller scale applications of air quality models—including those dealing with climate change impacts. In addition, in response to the ozone-nonattainment issue intensified by the 1990 Clean Air Act Amendments, a North American Research Strategy for Tropospheric Ozone (NARSTO) is under way that involves many of the same organizations participating in CAMRAQ and embraces the same philosophy that we all benefit through cooperation. Although these activities are fueled by a single issue, we view them as contributing to the CAMRAQ objectives. Organization CAMRAQ members are symbolically bound by a memorandum of understanding that articulates our goals and willingness to cooperate in achieving them. A steering committee of one individual from each organization desiring representation guides CAMRAQ's course (presently about 20, from North America and Europe). Very large agencies having several modeling groups with substantially different missions within "regional modeling," such as the National Océanographie and Atmospheric Administ r a t i o n , may have each g r o u p represented. Ad hoc subcommittees are formed to address programmatic issues as they arise. Roots CAMRAQ's roots are in the spirit of cooperation nurtured for many of the participants by the success of consortia on related topics. The specific concept, however, began as an initiative of EPRI. EPRI was encouraged by its participation in two collaborative studies [the Eulerian Model Evaluation and Field Study (EMEFS) and the Subregional Cooperative Electric Power, National Park Service, Environmental Protection Agency, and Department of Defense Study (SCENES)] (see Table 1) and was convinced of the concept's feasibility by the existing Environ. Sci. Technol., Vol. 28, No. 2, 1994 73 A
FIGURE 1
(a) Example of the analysis that can be performed using the GEMS modeling system overview Fulton County
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