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8.0 ALTERNATIVE MODEL APPLICATIONS 

Recent model inter-comparisons between CALPUFF and full science models such as CAMx 

and CMAQ (Morris et al., 2003) amply demonstrate that CALPUFF substantially overpredicts 

concentrations of visibility-reducing fine particulates under various real-world at the larger 

downwind distances when compared to one-atmosphere models. Perhaps only a few BART eligible 

sources in the CENRAP region will consider seriously the use of alternative models to EPA’s 

CALPUFF model. But for those sources whose anticipated control costs are substantial, the 

application of comprehensive, full-science regional visibility assessment tools will yield more 

realistic BART control requirements than those generated by a puff model. In this chapter, the steps 

required to select and apply a state-of-science regional model and attendant data bases are presented. 

While there are other alternatives to CALPUFF (i.e., PLUVUE-II, SCICHEM), only full-science 

models have the requisite chemistry and physics formulations to surmount the many limitations 

associated with CALPUFF (discussed in Chapter 3). In addition, we briefly summarize the scope and 

availability of the emissions, meteorological, and air quality modeling data bases for the CENRAP 

region that can be used to support alternative BART modeling. 

8.1 Use of One-Atmosphere Models in Refined Assessments 

States or source operators interested in conducting truly refined visibility modeling of BARTeligible sources have several options for using one-atmosphere models supported by current RPO data 

bases. Options available include: 

 > Modeling Systems: CMAQ4.5 or CAMx4.2 

> Sub-Grid-Plume Treatment: CMAQ-PinG, CMAQ-APT-PM, CMAQ-MADRID-APT, 

CAMx PM IRON PiG, and CAMx multi-scale flexi-nesting; 

 > Modeling Years: 2001 (MRPO), 2002 (CENRAP, MRPO, VISTAS, WRAP), 2003 

(MRPO) 

 > Modeling Resolutions: 12 km (VISTAS, WRAP, MPRO), 36 km (CENRAP, MPRO, 

VISTAS, CENRAP, WRAP) 

Figures 8-1 thorough 8-6 present the various grid domains used by the RPOs and the Five States 

Study1

 (Tesche et al., 2005b). Clearly, each CENRAP state is covered by all of the 36 km domains 

and by many of the higher resolution 12 km domains as well. Depending on the source’s location and 

the Class I area(s) of concern, there are likely to be several model/data base configurations available. 

All of the modeling performed on these domains has been rigorously evaluated and publicly 

disseminated. 

 

 

1 The Five States Study is an integrated 8-hr ozone and annual PM2.5 modeling study in the Upper Midwest 

aimed at identifying effective SO2, NOx, and VOC controls to address the residual nonattainment beyond 

imposition of CAIR controls. Using augmented versions of the MRPO 2002 regional haze data sets, a suite of 

nested 36/12/4 km ozone and 36/12 km annual PM2.5 simulations are being carried out for 2002 and 2009 to 

address emissions controls needed for attainment of the 8-hr ozone and annual PM2.5 NAAQS. These data sets 

will be publicly available. 
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 8.1.1 Regional Haze Modeling Data Bases 

The readiness of one-atmosphere regional models for visibility impact assessment has been 

substantially strengthened in the past three years as the result of the RPO efforts to address the 

Regional Haze Rule. CENRAP and the other four RPOs have developed and evaluated 

comprehensive data sets for regional haze model applications (Morris et al., 2004, 2005a,b,d; Baker, 

2005a-c; Brewer and Adlhoch, 2005; Boylan and Russell, 2005; Tesche et al., 2005). Over the 

CENRAP domain, annual fine particulate and visibility modeling data bases now exist for calendar 

years 2001, 2003, and 2003 (Baker, 2005c). Availability of these public data bases significantly 

reduces the resource burden on that subset of states or source operators who may elect to carry out 

alternative BART modeling. 

Emission Inventories. Comprehensive modeling inventories for 2002 have been developed by all of 

the RPOs using common national data sets (2002 NEI) together with local and state-specific emissions 

information. Generally, all of these inventories are consistent with on another although there are 

naturally some differences stemming from availability of local information. The elements of the 

CENRAP modeling inventory well-characterize the work performed by all the RPOs. 

As part of the CENRAP regional haze modeling program, a gridded, hourly emissions 

inventory for 2002 of criteria air pollutants (CAPs) and ammonia (NH3) emissions inventories for 

point, area, and nonroad sources has been developed (Pechan and CEP, 2005; UCR, 2005). The 

original Base_A 2002 inventory files completed during February 2005 were updated to incorporate 

comments provided by the CENRAP State, Local, and Tribal (S/L/T) agencies and the Emissions 

Inventory (EI) and Modeling Workgroups. As a result of the updates, the updated Base_B inventory 

was developed and is now being used in CENRAP’s 2002 and 2018 regional haze modeling. The 

CENRAP modeling inventories and supporting data prepared include the following: 

> Comprehensive, county-level, mass emissions and modeling inventories for point, area, 

and nonroad sources of 2002 emissions for the CAPs and NH3 for the state, local, and 

tribal agencies included in the CENRAP region; 

> Modeling inventory files containing 2018 projection year emissions for EGUs; and 

> A modeling inventory for Ontario fires during 2002. 

The mass emissions inventory files were prepared in the National Emissions Inventory (NEI) Input 

Format Version 3.0 (NIF 3.0) and then converted to the SMOKE/IDA format. These point, area, 

mobile and nonroad inventories were temporally allocated (hourly), chemically speciated (for the CB4 

mechanism) and spatially distributed (on a 36/12 km grid) over the CENRAP domain for the full 

calendar year 2002. The inventories include emissions rates for sulfur dioxide (SO2), oxides of 

nitrogen (NOx), volatile organic compounds (VOC), carbon monoxide (CO), NH3, and particles with 

an aerodynamic diameter less than or equal to a nominal 10 and 2.5 micrometers (i.e., primary PM10

and PM2.5). The inventories included summer day, winter day, and average day emissions. The 

temporal profiles developed by Pechan and CEP (2005) were used by the CENRAP modeling team to 

generate gridded, speciated, hourly emissions of photochemical, particulate, and regional haze 

precursors.

Meteorological Data Bases The MM5 model has been used exclusively by the RPOs in support of the 

regional haze modeling and the modeling files used in the CENRAP and MRPO haze modeling are the 
8-3

same ones that have been processed to yield the CALMET fields discussed in Chapter 5. In particular, 

the following annual MM5 simulations have been used to construct CAMx and CAMQ modeling data 

bases over the U.S. 

> 2001 MM5 data set at 36/12 km resolution developed for EPA by Alpine Geophysics 

(McNally and Tesche, 2002; McNally 2003); 

> 2002 MM5 data set ad 36/12 km resolution developed for VISTAS by Baron 

Advanced Meteorological Services (Olerud and Sims, 2004); 

> 2002 MM5 data set at 36 km resolution developed for CENRAP by Iowa DNR 

(Johnson, 2003a,b); 

> 2003 MM5 data set at 36 km resolution developed for the Midwest RPO (Baker, 

2005; Baker et al., 2004; Kembell-Cook et al., 2005); and 

> 2004 MM5 data set at 36 km resolution developed for the Midwest RPO (Baker, 

2005). 

Each of these studies included a performance evaluation of the MM5 against surface meteorological 

observations and the results of these evaluations are contained in the reports or presentations cited 

above. 

Regional Haze Modeling Data Bases Regional modeling data bases using the emissions and MM5 

fields have been developed and evaluated by VISTAS (CAMx/CMAQ for 2002 at 36/12 km); MRPO 

(CAMx for 2001, 2002, 2003 at 36 km [and some 12 km]); CENRAP (CMAQ/CAMx for 2002 at 36 

km [and some 12 km]). Details of these date base development activities and associated model 

performance evaluations are thoroughly described in the literature references. 

 8.1.2 Obtaining Regional Modeling Data Bases 

 As discussed in Chapter 4, the aggregate file sizes of the regional haze modeling data sets are 

substantial, requiring several 300-500 Gbyte external disk drives to store the information. In addition 

to the CENRAP CMAQ and CAMx modeling data bases, the MRPO and VISTAS have indicated a 

willingness to provide their data sets to CENRAP for use by CENRAP states or source operators. It is 

anticipated that CENRAP will serve as the central repository, collecting one complete distribution of 

the pertinent MRPO and/or VISTAS data bases on a set of high-capacity disk drives and then making 

them available to the states or source operators on an as requested basis for advanced modeling. 

Parallel sources of these data sets are the modeling contractors performing the regional haze modeling 

for CENRAP, VISTAS, and WRAP. 

 8.1.3 Protocol Development 

EPA’s BART guidance clearly indicates the need for a detailed modeling protocol to support 

any application of alternative models for BART analyses. An example of the content of such a oneatmosphere modeling protocol would be the CMAQ/CAMx modeling protocols developed for 

CENRAP (Morris et al., 2004c) and VISTAS (Morris et al, 2004a). In addition, certain components of 

the screening and source-specific protocols developed with CALPUFF (Tables 6-1 and 7-2) would be 

appropriate. The alternative modeling protocol should be submitted to the state, regional EPA office 
8-4

and FLM for review and negotiation. Note that EPA’s role in the development of the protocol is only 

advisory as the “states better understand the BART-eligible source configurations” and factors 

affecting their particular Class I areas (70 FR 39126). 

8.2 Model Applications 

Assessment of BART sources with regional models is a straightforward, albeit computationally 

intensive exercise. Two types of regional model simulations are required. 

 8.2.1 Regional Base Case Simulation 

 

The first step is to re-run one of the existing regional simulations for the 2002 annual basecase to 

establish comparability with the base case run reported by the RPO from whom the CMAQ or CAMx 

modeling files were obtained. The purpose of this annual simulation is two-fold. First, it establishes 

proper operation of the modeling system on the host computer via comparison with the RPO model 

output files. Second, it provides the gridded 24-hourly estimates of (NH4)2SO4, NH4NO3, OC, EC, 

soil, and coarse mass needed to calculate daily extinction coefficients from the IMPROVE extinction 

equation for every grid cell in the domain, including the Class I areas. (Further discussion of natural 

visibility conditions is given in the following section). On a modern Linux cluster of 4-6 nodes, a 36 

km simulation would require approximately two weeks to complete execution. 

 An important question to be addressed in the modeling protocol is whether one annual 

simulation of 2002 would be sufficient to provide the necessary assessment of a BART source’s 

visibility impact. Recent annual CAMx simulations of 2001, 2002, and 2003 by the MRPO (Baker, 

2005c) indicated that for these three years: 

 > CAMx model performance by specie was very consistent from one year to the next; 

> Sulfate predictions exhibited the greatest year to year variability, likely due to the 

interannual variability of atmospheric water content; 

 > CAMx performance was best for sulfate and elemental carbon; and 

> Organic carbon predictions during the summer months were the poorest of the 

secondary particulate species; and 

 > Nitrate performance, a challenge in all regional modeling studies, needs additional 

improvement. 

The performance levels achieved in the MRPO CAMx modeling are generally consistent with that of 

the other RPOs. These results are included in Figure 4-9, presented previously. Because the regional 

models include all of the important known gas-phase, aerosol, and aqueous-phase chemistry producing 

visibility-impairing particulates, a strong case can be made that only one year of modeling would be 

needed instead of the three years of CALPUFF modeling. However, if the reviewing agencies require 

a full three years of modeling data, a valid approach remains nonetheless with the 2001-2003 CAMx 

data sets developed by the MPRO. 

 
8-5

 8.2.2 BART Source Simulation with SGS Technology 

 

 Once the base case is established, the CMAQ or CAMx simulation is repeated with the 

emissions from the BART source are included in the point source emissions file. For this application, 

depending upon the host model chosen, one of several plume-in-grid or nested grid options should be 

used to provide near source resolution of the SO2, NOx, NH3, and VOC emissions of the BART 

source. For most single point BART simulations conducted with a full-science photochemical grid 

model, the plume-in-grid technique is generally recommended over the use of multi-scale grid nesting 

although the latter may still be appropriate in addition to the plume-in-grid approach for other reasons, 

including replication of meteorology and transport phenomena. Prescription of how the sub-grid-scale 

(SGS) technology is used within the regional model is a key point to be addressed in the modeling 

protocol. The use of SGS will allow the simulation of BART plume impacts down to very small scales 

(on the order of 1 km or less) depending upon the need. 

8.2.3 Assessing Modeled BART Source Impacts 

 The direct impacts of the BART sources on 24-hr visibility is quantified by subtracting the 2002 

Base Case simulation from the BART source simulation and plotting the residual aerosol species 

concentrations and HI values in daily tile plots. Figure 8-7 provides an example of such an 

incremental 24-hr average concentration tile plot. From the CAMx PM PinG modeling by Yarwood et 

al, (2005) using the 2002 MPRO data base, incremental 24-hr average SO2, sulfate, nitrate, and NH4

concentration fields on 14 June 2002 are shown. This plume reveals the individual plume signatures 

downwind from their origin. Similar plots of delta-deciviews can be determined using the IMPROVE 

extinction equation with the modeled aerosol fields from both model simulations. Tabular summaries 

of the visibility impacts in all grid cells, but especially the Class I areas are easily produced. 

8.3 Estimation of Natural Visibility Conditions 

8.3.1 Flexibility Suggested by EPA 

Under the Regional Haze Rule, by 2008, the states must develop a strategy for reducing 

regional haze in Class I areas during the following decade. The rate of improvement of visibility is 

intended to be at a rate sufficient to reach “natural conditions” in 2064, and thus depends on 

quantifying what the visibility would be under those conditions. The initial, “default” estimate of 

natural conditions recommended by EPA (2003) – based on pre-1990 research (Trijonis, 1990) does 

not take into account spatial and temporal variability and local geographic and meteorological 

conditions. Consequently, the default natural visibility conditions are out-of-date and do not reflect the 

very substantial theoretical, observational, and modeling data bases now available for estimating 

natural background concentrations (Tombach and Brewer, 2005). Recognizing that the “default” 

concentrations (see Table 6-3) are not appropriate for every Class I area, EPA allows states the option 

to develop "refined" estimates of natural conditions that reflects local conditions. 

States might wish to adopt a refined approach to estimating natural visibility conditions for 

several reasons, including: 

> The default estimates are shown, through more recent Class I area-specific observations 

and/or modeling analyses to be out-dated and/or incorrect; 

> If the default estimates of the natural background conditions are close to the current 
8-6

visibility conditions, small uncertainties can have significant impacts on states’ ability 

to meet SIP goals; 

> In some regions, natural sources are known to exhibit predictable seasonal influences on 

visibility; therefore, states might wish to use refined estimates of natural visibility 

conditions to account for these influences; 

> States which receive significant visibility impacts from biomass smoke might wish to 

distinguish more explicitly between man-made and natural sources. 

EPA identifies several possible refined approaches which can be adopted for developing ore 

realistic estimates of natural visibility conditions and states may identify others that are more 

appropriate for their own situations. Approaches suggested by EPA include: (a) develop refined 

estimates of the constant values of one or more natural species, (b) estimate natural visibility using 

species concentrations that vary (e.g., seasonally, monthly, or climatologically), or (c) adjust the 

estimated constant species to account for infrequent natural events, such as forest fires or wind-blown 

dust, as major influences on visibility. This latter approach would require estimating the frequency 

and magnitude of the natural contribution to particle concentrations during the events. EPA encourages 

flexibility in these approaches so that default and refined annual average, seasonal, monthly, and 

event-specific species concentrations provide the best estimates of natural visibility for each of the 

mandatory Federal Class I areas. States wishing to employ a refined approach should supply 

demonstrations that the refined approach is technically sound and provides regionally representative 

estimates of natural visibility conditions. 

8.3.2 Refined Procedures for One-Atmosphere Modeling 

The whole purpose of estimating natural visibility conditions is to develop a reasonable 

background against which a human observer might be able to discern the added presence of a BART 

source plume. With Lagrangian models such as CALPUFF, this necessity arises because puff models 

cannot calculate the three-dimensional, hourly varying gas phase and particulate concentration fields 

that contain the various visibility-limiting pollutant species. Consequently, some independent estimate 

of natural visibility conditions is required. Historical measurements of pristine conditions measured in 

the 1980s have been used to fill this void. 

One-atmosphere models, by design, eliminate the need to estimate natural background 

conditions against which individual plume contributions are assessed. This is done directly via the 

simulation of the hour-by-hour, day-by-day emissions (including sea salt), meteorology (rain, snow, 

fog), transport, chemical transformation and removal of gaseous and particulate emissions from all 

sources (natural and manmade) in the region of interest for one or more calendar years. Today, the 

CENRAP, WRAP, MRPO, and VISTAS regional haze programs have all developed base year 2002 

fine particulate and visibility estimates for the entire U.S. at grid scales of 36 km and 12 km using the 

CMAQ and/or CAMx models (depending upon RPO). Importantly, the MRPO has even extended the 

CAMx visibility modeling to include the same three year period (2001-2003) being use in the 

screening and source-specific CALPUFF modeling. The 24-hour averaged modeled ground level 

concentration predictions of (NH4)2SO4, NH4NO3, OC, EC, soil, and coarse mass (i.e., all components 

in the IMPROVE extinction equation) from any of these regional simulations provide a direct 

quantification on a day-to-day basis of current background visibility conditions in not only the Class I 

areas, but in every 12 km or 36 km grid cell in the U.S. By simply re-running one or more of these 

regional one-atmosphere model simulations with the BART source emissions included, and then 
8-7

subtracting the outputs of the two simulations, a direct, gridded estimate is produced of the change in 

visibility (measured in terms of ‘delta-deciviews’) compared against realistic, current background 

conditions. As with the screening CALPUFF modeling, if the Δdv value is greater than the 0.5 dv 

threshold, the source is said to contribute to visibility impairment and is thus ‘subject to BART’ 

controls. 

 

 This approach to calculating a realistic natural visibility background condition departs from 

EPA’s suggestions in two respects. First, it is far more rigorous, and thus is more resource intensive to 

carry out. Second, the natural visibility background conditions over the Class I areas simulated by the 

CAMx/CMAQ models in the 2002 base cases will in all likelihood be less ‘pristine’ then those 

estimated by Trijonis and others in the late 1980s. But, we maintain, they will be more representative 

of the actual current conditions in the Class I areas against which the BART plume impacts should 

realistically be compared. 

 Clearly, the acceptability of this refined one-atmosphere modeling procedure for calculating 

natural visibility background conditions in the Class I areas will need to be discussed with state, EPA, 

and FLM staff in order to reach consensus on the merits and acceptability of the approach. The 

fallback is to use the default (EPA, 2003) numbers. 

8.4 Cumulative Impact Assessments 

While the use of regional models incorporating plume-in-grid or multi-scale nesting allow for 

rigorous assessment of 24-hr average point source annual visibility impacts over one or more years, 

where one-atmosphere models are most useful is in cumulative impact assessments. The example 

shown in Figure 8-7 is one such application. The modeling performed by Yarwood et al., (2005) 

treated several dozen point sources with plume-in-grid technology allowing for the opportunity to 

assess the cumulative impact of all of these sources in the context of all anthropogenic and biogenic 

sources in the central U.S. 

 As noted in Chapter 1, each CENRAP state is required to carry out four types of visibility 

modeling under the Regional Haze Rule: (a) single-source modeling to determine which BARTeligible sources are ‘subject to BART’, (b) single-source modeling to determine the degree of visibility 

improvement attributable to proposed BART controls for each source subject to BART, (c) cumulative 

modeling to determine the combined effect of proposed BART controls for sources subject to BART in 

each CENRAP state; and (d) regional-scale modeling to determine if the combined effect of proposed 

BART controls for all CENRAP states ultimately satisfy the RHR visibility improvement goals. The 

third responsibility, cumulative impact assessment of all state sources subject to BART, is best 

addressed through the use of regional models. 

8.5 Concluding Remarks 

Only a few states and/or BART eligible sources are expected to seriously examine the merits 

of using full-science models as an alternative CALPUFF. But for those who do, the review of oneatmosphere modeling capabilities (Chapter 4) and the procedures one would follow in their 

application to BART sources (this chapter) will hopefully provide useful information to inform 

decision-making. Fortuitously, the recent work by the various RPOs and model developers has 

produced a suite of state-of-science models and associated data bases that are ideally suited to 

rigorous BART visibility impact modeling. While the resource requirements of an alternative 

modeling study – using existing models and data bases-- exceed those of a typical CALPUFF 
8-8

analysis, the potential to develop more reasonable control limits on certain BART sources may easily 

eclipse the additional costs of full-science modeling. 

-2000 -1500 -1000 -500 0 500 1000 1500 2000 2500

-2000

-1500

-1000

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0

500

1000

1500

12 km Domain

36 km Domain

Figure 8-1. CENRAP 36/12 km CMAQ/CAMx Domain Used for 2002 Annual OneAtmosphere Regional Haze Simulations. 36 km domain is outer grid; 12 km 

domain is colored red. (Note: Model Simulations on 12 km domain may not be 

performed for full annual cycle). (Source: Morris et al., 2005d). 
8-9

MM5 CAMx 

Figure 8-2. MRPO 36 km CAMx Domain Used for 2001, 2002, 2003 Annual One-Atmosphere 

Regional Haze Simulations (Source: Baker, 2005c). 
8-10

\ 

Figure 8-3. MRPO 36/12 km CAMx Domain Used for 2002 Annual One-Atmosphere Regional 

Haze Simulations. 36 km domain is outer grid; 12 km domain is colored green 

(Source: Koerber, 2005). 
8-11

Figure 8-4. VISTAS 36/12 km CMAQ/CAMx Domain Used for 2002 Annual One-Atmosphere 

Regional Haze Simulations. 36 km domain is outer grid; 12 km domain is colored 

green (Source: Tesche et al., 2005). 
8-12

Figure 8-5. WRAP 36 km CMAQ/CAMx Domain Used for 2002 Annual One-Atmosphere 

Regional Haze Simulations. (Source: Morris et al., 2004b). 
8-13

Figure 8-6. Five States Study 36/12/4 km PM2.5/8-hr Ozone Study Using CAMx for the 2002 

Annual Period. (Source: Tesche, et al., 2005b). 
8-14

24-hr SO2 Concentrations 24-hr SO4 Concentrations

24-hr NO3 Concentrations 24-hr NH4 Concentrations

Figure 8-7. Incremental 24-hr Average SO2, Sulfate, Nitrate, and NH4 Concentrations Fields on 

14 June 2002 from the CAMx PM PiG Simulation Over the Upper Midwest. 

Apparent in this figure are the individual plume footprints downwind from their 

origin. The modeled plumes result from full chemistry interaction between the 

background regional atmosphere and the chemical and physical dynamics of the 

individual plumes. (Adapted from Yarwood et al., 2005).