4.3	MODELS AND RELATED APPLICATION STUDIES

4.3.1	Introduction.

Basics. This section addresses urban, regional and super-regional scale modeling systems, most of which use gridded Eulerian frameworks. Gridded models utilize a fixed-frame reference with respect to moving air masses and can accommodate a physically realistic treatment of atmospheric mixing processes. In contrast, trajectory or Lagrangian systems utilize a moving reference frame perspective tracking the movement (and retaining the identity) of individual plumes. Trajectory models provide estimates of a particular plume concentrations along a path; whereas Eulerian systems provide simultaneous calculations at every grid sell, essentially capturing the fully coupled calculations of all interacting plumes (actually, air volumes). The formulation of Lagrangian systems restricts realistic treatment of atmospheric mixing, since plumes are subject to degradation through wind shear and other physical phenomena and thus lose individual identity. The Lagrangian treatment of individual plumes requires substantially fewer computational resources, because the solution of fully coupled simultaneous systems (Eulerian) is far more demanding than separate plume-by-plume treatment. Since so many air quality issues are of regional scale, the chances for plume degradation are enhanced and therefore Eulerian systems have become the most common modeling platform.

All modeling systems invoke many approximations both in the description of physical and chemical procesesses as well as in the solution of the system of mathematical equations embodying the physics and chemistry. The spatial extents, or "domains", of model applications often are characterized as being of urban (100-500 km), regional (500-2000 km) or super-regional (> 2000 km) scales. Gridding refers to the horizontal resolution used to delineate simulated air quality concentrations and provide detail on the emissions distribution and meterological variables (e.g., winds and temperatures). Typical grid resolution ranges from 2-5 km and 20- 80 km for urban and regional applications, respectively. Most models produce hourly outputs, which can be aggregated for other averaging times of interest (e.g., 8-hr, 24-hr, seasonal, annual). Models are resolved vertically (typically 5 -15 levels or more) to account for varying meterology and emissions and to approximate vertical mixing phenomena. Some modern systems accomodate nested or variable scale gridding schemes which allow for detailed spatial treatment in urban areas (2- 8 km) and dense less resolution in peripheral/rural portions (20-80 km) of the domain in order to optimize computational resources and apparent precision. A trend toward increasing regional- scale (or mixed regional/urban), Eulerian modeling has been apparent over the last decade, in recognition of the interaction between regional/rural and urban areas and associated "transport" issues.

Models should be viewed as a "system" (Figure 1), including the meteorological and emission preprocessing models and the air quality simulation model (AQSM). Preprocessers process raw data (i.e., emissions inventories and meteorological measurements) into the spatial and temporal fields required by the AQSM. The AQSM calculates concentration fields of air quality species (i.e., compound, element, free radical, or surrogate group), which are determined by the combined interactive effects of source emissions, mixing processes (advection and dispersion), deposition and chemical transformations, mathematically represented by:

ci/ t = (KH ci/ x)/ x + (KH ci/ y)/ y + (KV ci/ z)/ z

- (uci)/ x - (vci)/ y - (wci)/ z + Ri + Si + Li

where:

ci	=	the species concentration

u,v,w	=	horizontal and vertical wind speed components

KH,KH	=	horizontal and vertical turbulent diffusion

Ri	=	net rate of production of species i by chemical reactions

Si	=	emissions rate of species i

Li	=	net rate of removal of species i by surface uptake processes

So-called "transport" into a modeling domain is quantified through boundary conditions which are user-specified concentrations surrounding the established modeled domain (planes on each side and top), which can be brought into the domain through advection and dispersion. The AQSM includes the chemical mechanism which performs the chemical transformation calculations through highly condensed approximations (e.g., 20-50 species; 100 reactions) of the thousands of actual chemical reactions occurring in the atmosphere. Chemical mechanisms have been developed and tested through combinations of fundamental reaction chemistry and smogchambe....).

All gridded modeling systems share the general attributes discussed above. The differentiation among modeling systems is dictated by the required species and averaging times of interest and the desired balance between physical reality and computational resoure demands. For example, so-called ozone models like the UAM and ROM generally are applied for episodic ozone events occurring over a 2 to 14 day period where processes/events such as precipitation, cloud interactions particle formation may not be critically important to determining peak ozone concentartions during the course of the episode. Hence, unlike the Regional Acid Deposition Model (RADM), these ozone models do not include cloud and precipitation processes nor a characterization of sulfur and nitrogen chemistry (gas and aqueous phases) required for precipitation and particle formation processes. Conversely, the REMSAD does not include the detailed oxidant chemistry of the UAM or ROM, which would exact enormous computational burdens on performing "screening" types of analyses over annual time scales.

Nonreactive models: Receptor and local scale techniques Although many current efforts are dedicated to large scale reactive modeling systems, receptor and non-reactive dispersion modeling tools perform important roles. For example, the demand for observational based analyses has and will expand the use of recptor modeling techniques for interpreting ambient data and evaluating emissions. The strong probablility of a fine particle standard requires attention to regional scale reactive modeling systems since the fraction of mass due to secodary formation increases as with decreaing size cutoffs, and the atmospheric residence times for small particles approach those of gases. Nevertheless, primary particles are important components of particle mass regardless of the size cutoff. Furthermore, PM10 standards are not disappearing. Consequently the application of receptor and nonreactive, local scale modeling techniques remains an important component of a comprehensive air quality management effort. The lack of attention in this document given to more classical techniques reflects the current interest in large scale regional issues and the feasibility of market-based trading across pollutants and broad regions.

4.3.2	Model Application Process

Modern model applications utilizing gridded systems are extremely resource and data intensive exercises. A series of sequential steps applicable to regulatory model applications include:

1. Establishing model domain, characterization and selection of modeling episodes,

2. Raw data gathering and processing of model inputs

3. Model testing, including component testing of emissions and meteorological preprocessers

4. Development of emission control stratgies and model application and interpretation of results.

5. Corroboration and evaluation of strategy results.

Guidance documents exist for ozone modeling (EPA, 1991; CARB, ??). Nevertheless, the application process is a mix of art, science and engineering; and the relative attention to any particular step varies strongly among applications. The potential quality of a modeling application often is constrained by available data. The use of national emissions, meteorological and air quality data bases as sole data sources for modeling is a strongly debated topic, and often viewed as more critical than internal differences among the AQSMs. Consequently, most historical modeling applications were associated with special intensive field programs (Sectionn 4.3.2) designed to augment national data bases. Equally perplexing are issues related to "appropriate" use of models. For example, the CAA and EPA guidance require modeled ozone attainment demonstrations; whereas, the scientific community question how a deterministic tool can be applied to a probabilistic standard (will the new standard be probabilistic?) under highly stochastic meteorological and emissions phenomena. All of these problems are compounded with secondary particulate modeling, given the need to characterize additional physical/chemical processes (aqueous and gaseous phases, organic aerosols, size distributions and growth; the shortage of ambient data for model evaluation (especially time-varying speciated data), and related incommensurabilities betweeen models and measurements.

Despite numerous issues related to models and the model application process, the modeling system is an extremely powerful tool for:

1.	Expanding/complementing the time and space constraints of ambient monitoring which often is limited to a small number of surface observations,

2.	Further expanding time by predicting future conditions; the very basis of control strategy analysis, and

3.	Integrating emissions, meterological and air quality data for explaining physical and chemical phenomena that can not be described by data alone (or a single data category).

4.3.3	Modeling Tools

The following summaries of Eulerian AQSMs provides basic information regarding pertinent refernces, applicability (pollutants and spatial scales), and chemical mechanisms. Comments regarding any unintended mischaracterizations or omissions are graetly appreciated. One of the most comprehensive modeling reviews was written by Seigneur (1990) for the Electric Power and Research Institute (EPRI), and along with assistance from numerous model developers provides the basis for the modeling summary. The American Petroleum Institute (API) recently issued a Request for Proposals (RFP) for a review of particle models. PM and ozone modeling are treated together, reflecting the commonality in process treatment among the available Eulerian models. Visibility is a direct function of particle levels, particle size ditributions and particle composition; all properties that particle models strive to simulate. Consequently, visibility is (can be) treated as a postprocessing step in any particle model under discussion.

4.3.1.1	Eulerian AQSMs

UAMIV (applicability: ozone). The Urban Airshed Model (UAM) family of models has been undergoing continued development by Systems Applications International (SAI) from the late 1970s (Roth et al., 1983). Current application versions include the EPA reference model, UAMIV (Morris et al, 1990; Scheffe and Morris, 1992), and the UAMV (discussed below). UAMIV is the current EPA ozone guideline model available through NTIS and the EPA SCRAM Bulletin Board. The model uses a fixed-grid structure with 2-5 km grid resolution and 3-5 vertical layers applied over urban scale domains roughly 100 - 300 km on a side. UAMIV uses the carbon-bond IV chemical mechanism, which includes roughly 35 species and 90 reactions. Most of the important inorganic species and reactions are treated explicitly (i.e., actual species such as NO, ozone, NO2 are simulated). Organic species are gouped into 13 surrogate and explicit classes, largely based on the structure of individual compounds. UAMIV is the most widely applied and tested ozone model, used in nearly all of the recent CAA SIP modeling demonstrations. Boundary conditions for the UAM typically are provided through interpolation of Regional Oxidant Model (ROM) concentration fields in areas influenced strongly by transport (e.g., NE U.S. corridor). UAMIV meterological and emissions inputs typically are derived from the UAM Diagnostic Wind Model (DWM) and the Emissions Preprocessing System (EPS) 2.0. Extensive application guidance and model documentation is available for the UAMIV (EPA, 1992; EPA, 1990 a-e, Scheffe, 1990, Scheffe and Morris, 1992). Descriptions of more recent UAMIV applications for the Northeast U.S. include (Rao et al.,.....).

UAMV (applicability: ozone). Major enhancenments of the most recent UAM model feature variable grid resolution and plume-in-grid (PIG) treatment. Variable grids enable detailed spatial treatment in urban areas (2- 8 km) and dense less resolution in peripheral/rural portions (20-80 km) of the domain in order to optimize computational resources and apparent precision, and avoid the need for running independent regional and urban scale models. Plume-in-grid allows sub-grid scale treatment for a number of large individual sources to better approximate the sequence of mixing (rate of dilution) and chemical reactions. In standard gridded formulation, all emissions from a particular source enter a grid cell and instantly mix throughout the entire cell volume. PIG allows the plume to grow dynamically at sub-grid scales, and plume/cell mixing is delayed until plume dimensions reach those of the cell. UAMV typically is driven with the SAIMM prognostic meteorlogical model (see below). Recent and current UAMV applications include the Lake Michigan Ozone Study (LMOS, reference), Gulf of Mexicao Air Quality Study (GMAQS; SAI, 1995), and the Ozone Transport Assessment Group (OTAG).

ROM (applicability: ozone).	The Regional Oxidant Model (ROM) is a fixed grid model (18.5 km cell dimensions) that has been applied over regional (e.g., Northeast corridor) and super-regional domains (Eastern U.S). ROM incorporates the CBIV chemical mechanism and has been driven by a diagnostic meteorological model and (emissions model). ROM has been applied throughout the late 1980's and into the mid 1990's for providing information for ozone precursor control strategy development in the Eastern U.S. Major EPA-directed efforts include the Regional Ozone (Chu et al., 1992), and continual support from 1990 through 1996 for the Ozone Transport Commission (OTC) and the States' UAM attainment demonstrations.

Modeling for Northeast Transport (ROMNET; EPA/Possiel, 1992?) program, ROM matrix study. Much of the EPA ROM analyses have focused on the exceptionally high ozone eepisode occurring ocer a 2-week period in July, 1988.

RADM (applicability: acid deposition, ozone). The Regional Acid Deposition Model (RADM) was developed by the Natioanl Center for Atmospheric Research (NCAR) an further refined by the State University of New York (Chang et al. 1990) to support the National Acid Precipitation Assessment Program (NAPAP). RADM susbsequently has been applied towards assessing acid deposition standards (EPA, 1995), nitrogen deposition into the Chesapeake Bay watershed (Dennis and ??, 1995) and diagnosing grid cell dimension and vertical mixing effects on ozone (Dennis ....???). The RADM domain includes much of the Eastern and Midwest U.S. (Figure 1). Most RADM applications have been based on 80 km grid cell dimensions and 15 vertical layers, although some applications have used more resolved subdomains and additional vertical layers. The RADM2 gas phase chemical mechanism utilizes a lumped molecule approach (Lurmann et al., 1986) for parameterizing organic species, and also includes inorganic sulfur chemistry required for characterizing sulfate formation (UAM models do not carry sulfur chemistry). RADM also includes an aqueous chemistry submodel for reactions in clouds that account for SO2 oxidation (sulfate formation). Aqueous phase equilibrium is maintained among all the dissolved ions (e.g., H+, NH4+, SO4-2, HSO4-1, etc.). RADM also carries ammonia gas and dissolved ammonium, which affects the precipitation acidity. However, RADM does not treat ammonia-sulfate-nitrate equilibrium which is important in particle formation. The gas phase chemistry in RADM is different from, but equally comprehensive to the CBIV chemistry used in the ozone models. The associated peroxide and intermediate radical species are required for characterizing sulfate and nitrate formation and eventual removal, the objective for deposition assessments. In addition, RADM accounts for cloud processes where much of the sulfate formation occurs, especially in fall through spring periods. Discuss wet and dry deposition

Most RADM applications are based on an aggregate set of meterological cases form 1983-1987 (MM4 prognostic wind model) over different seasons to reflect an annualized protypical year of precipiation. Thus, RADM results are available for annual avearges and hourly values for multiple day episodes. RADM has been evaluated as part of the Eulerian Model Evaluation Field Study (EMEFS, Spicer et al., ?????)

RPM [RADM/RPM](applicability: PM). The Regional Particulate Model (RPM) is a particle formation model which requires available RADM simulations (i.e., RPM is a RADM postprocesser) to produce the necessary radical fields. In it's current form, major extensions to RADM provided by RPM include: (1) nitrate-ammonium-sulfate equilibrium chemistry (modification of MARS; Saxena et al., 1986); (2) treatment of organic aerosol formation ; and (3) bi-modal particle size characterization composed of water (size growth through relative humidity), sulfate, organics, carbon, ammonium, and nitrate. Parameterization of organic aerosol formation utilizing RADM2 organic classes is based on organic aerosol yields from Pandis et al., (1992). Primary particle emissions (check with Frank about elemental-C) currently are not carried in RPM. RPM is under development as a test bed for a particle model to be integrated into MODELS3. Current applications include base-case 1990 emissions; and selected future-year (2010) CAA related control strategies. Developed by EPA (Binkowski and Shankar, 1995).

DAQM (PM,ozone,deposition). The Denver Air Quality Model (DAQM) is an augmentation of RADM with fairly complete treatment of secondary aerosol formation processes. Relative to the RADM/RPM system, DAQM provides an integrated treatment of oxidants and aerosols (i.e., no postprocessing step) and a more comprehensive organic aerosols scheme. DAQM currently is configure a domain covering Colorado with 8 km horizontal grid cell resolution for assessing Rocky Mountain Front Range visibilty and other air quality issues (Middleton et al, 1993; Middleton, 1996).

SAQM (O3, PM?) (acronyms gone berserk) The collaborative San Joaquin Valley Air Quality Study (SJVAQS)/Atmospheric Utility, Signatures, Predictions and Experiments (AUSPEX) research project includes the SJVAQS/AUSPEX Regional Modeling System (SARMAP) which is composed of the SARMAP Air Quality Model (SAQM), an the MM5 meterological model and the EMS-95/GMAP emissions model. SAQM is derived from RADM with several enhancements (Venkatram et al., 1990): variable gridding (12 and 4km nesting); improved numerical methods sscheme; compatibility with MM5; alterbnative chemical mechanisms (CBM-IV) and the Statewide Air Pollution Reserach Center (SAPRC) chemical mechanism (Carter, 1990). SAQM has been evaluated for the August 3-6, 1990 ozone episode (DeMassa et al., 1996). [recent effort to put aerosol chemistry in SAQM for SARMAP??? check again with Lurmann/Ziman]

MODELS3(multipollutant). MODELS3 is EPA's next generation integrated modeling system. Plans for MODELS3 includes a flexible comprehensive infrastructure for preprocessing modules; emissions, meteorological and air quality data bases; and various AQSMs. The initial MODELS3 science model will be derived from RADM-2 and RPM components; MM5 meteorological Model, and the EMS-95 emissions model. Future versions will incorporate capability for various trace metals and SVOCs. MODELS3 is scheduled to undergo testing in 1996/97, community availability expected in 1998. Eventually to include next-generation "morphocule" chemical mechanism (Jeffries, ????). An early prototype version is to be applied (1996-1997) to the Eastern U.S. covering the summer, 1995 as part of a combined southern oxidant Study (SOS) ; Southeast States Air Use Management (SESAUM) ??effort.

REMSAD (PM, deposition). The Regulatory Modeling System for Aerosols and Deposition (REMSAD) is based on the UAMV platform. The major changes include removal of CB-IV chemistry replaced by an empiricized reduced-form oxidant chemistry package (ozone is required input); insertion of inorganic aerosol formation processes (ammonia-nitrate-sulfate system), cloud processes, vertical extension of modeling domain to the upper troposphere, and toxics chemistry module for metals, and semi volatiler organic compounds (SVOC). REMSAD was developed to address Great Waters deposition and secondary PM issues on national, regional and urban scales. A variety of modeling domains have been or are being developed: Contiguous U.S. (60 km); Lake Michigan subnest (20 km); eastern U.S.; NW and SW U.S. Funded by EPA and dveloped by SAI, REMSAD is viewed as a "screening" level tool compatible with current gridded emissions and meterological data bases, and supplementing applications from resource- intensive research-grade models. Current applications based on 1990 MM4 meteorolgy. REMSAD has not been evaluated. References (SAI, 1996; Guthrie et al., 1995; Emery et al., 1995).

CALGRID (ozone). CALGRID was developed for the California Air Resources Board (CARB) during the late 1980's to modernize the existing UAM code and allow for the interchanging of different chemical mechanisms. A set of diagnostic meteorological preprocessers, CALMET, was developed in parallel with the CALGRID AQS. CALGRID has been exercised for regional {Lake Michigan Ozone Study (LMOS) domain; Northeast-Modeling Ozone Cooperative Agreement (MOCA) domain} and urban scale (New England) applications. CALGID has been run with the CB-IV and SAPRC-90 chemical mechanisms. References (Yamartino et al., 1992; Yamartino et al., 1996).

CIT (ozone). The California Institute of Technolgy (CIT) model has been a research tool that was based on the UAM and modified, primarily to adopt different chemical mechanisms allowing more flexible treatment of VOCs. Most CIT applications have focused on Los Angeles. References (Development/technical descriptions:McRae et al., 1982; Russell and Cass, 1984; Harley et al., 1993; Applications: Milford et al., 1991).

URM (ozone, aerosols). The urban regional model (URM) extended the CIT model to multiscale capability. The URM can incorporate various chemical mechanisms, SAPRC-90 is the basic core mechanism. Most applications (NE U.S., Mexico/U.S. border) have addressed ozone; aerosol chemistry has been added. (Wet-deposition??) References (Kumar et al., 1993; Kumar and Russell, 1996).

STEM-II (acid-deposition, oxidants). The Sulfur Transport Eulerian Model-II developed at the Universities of Kentucky and Iowa is a regional scale applied with various grid resolutions (10 to 56 km). STEM-II uses a lumped-molecular chemical mechanism for VOC (Lurmann et al., 1986) and incorporates sulfur-nitrogen-ammonia/ammonium chemistry (gas and aqueous processes) for acid precipitation assessments. Applications include Philadelphia, cntral Japan, Kentucky and the NW U.S. References: (Technical formulation: Carmichael et al., 1986; Applications, Carmicahael et al., 1987)....need more recent info...based only on EPRI 1990 report

ADOM ( acid-deposition, oxidants, PM). The acid deposition and oxidant model (ADOM) developed by ENSR is a regional scale model that has been applied to Eastern North America and Northern Europe. Regioanl scale applications (60 and 120 km grid cell resolution); oxidant chemistry with wet deposition, aerosol and PM capability References (Venkatram et al., 1988).

Table 1: Summary of Eulerian Air Quality Simulation Models
Pollutants/ scales/comments	Chemical Mechanism	Major Applications	Principal references
O3; urban; EPA guideline model; public domain	CB-IV(full gas-phase oxidant chemistry)	State SIPs; 5-City; SCAQS	Roth et al., 1983; EPA, 1990; EPA (1991)
UAMV	O3; variable scale; PIG;proprietary	CB-IV(full gas-phase oxidant chemistry)	LMOS; GMAQS; OTAG	Morris et al (19??
ROM	O3;regional;public domain	CB-IV(full gas-phase oxidant chemistry)	ROMNET; MATRIX study; State UAM SIPs	Lamb (1982); Schere (1986); Possiel et al., 1991??
RADM-II	S/N dep; O3; regional; public/research	Lumped molecular (full gas-phase oxidant chemistry with S); aqueous chemistry; aerosols (sulfate only)	NAPA; acid dep. stnd. feasibility; Ches. Bay N-dep	Chang et al., (1990)
RPM	secondary PM;RADM physical attributes; research (not evaluated)	reads full RADM fields; adds nitrate-sulfate- ammonium; highly parameterized org- aerosol treatment; bi- modal PM	base-1990 and 2010 CAA strategies	Binkowski (1995);
SAQM
DAQM
CIT
URM
STEM-II
CALGRID
ADOM
REMSAD
multipollutant; under development	RADM-II/RPM	SESAUM/SOS prototype	??

[Remaining sections to-be-completed]

4.3.1.2	Meterorological "preprocessing" models

[To-be-completed]

- Diagnostic Models

- DWM

- CALMET

Prognostic Models

- RAMS

- MM4/5

- SAIMM

-.....

4.3.1.3	Emissions "preprocessing" models (Chet)

FREDS

EPS

EMS-95 (GMAP) ???

...........

4.3.1.4	Non-reactive Gaussisan-type models

4.3.1.5	Observational analysis and Observational Based Models

-	ozone correlation/indicator approaches (Noy/NOz; etc.)

-	Ozone OBMs (GIT, SPA)

-	receptor modeling

-- PM source apportionament (major contributions)

-- VOC source apportionament

4.3.2	Major application studies/programs (at least to reference)

4.3.2.1 Major ozone studies, reports, programs

-	SCAQS

-	SARMAP

-	SCCAMP

-	LMOS

-	GMAQS

-	EPA/ROMNET/ROM MATRIX/OTAG

-	SOS

-	NARSTO/NARSTO-NE

-	NAS/185B Report

-	GMAQS/COAST

-	CAAMRAQ

-	EMEFS

4.3.2.2	PM/visibility studies and reports

-GCVTCS

-Denver brown cloud

-Current CD and staff paper on PM

-NAS Vis. Rep

- Phil/Harvard studies