Air Dispersion Modeling and 

Health Risk Assessment 

Protocol 

Avenal Energy Project 

Kings County, California 

Submitted to:

San Joaquin Valley Air Pollution Control District 

California Energy Commission 

U.S. Environmental Protection Agency, Region 9 

Federal Power Avenal LLC

prepared for:

August 2007

prepared by: 

Sierra Research, Inc. 

1801 J Street 

Sacramento, California 95811 

(916) 444-6666
 

Air Dispersion Modeling and Health Risk Assessment Protocol 

Avenal Energy Project 

Submitted to 

San Joaquin Valley Air Quality Management District 

California Energy Commission 

U.S. Environmental Protection Agency, Region 9 

August 2007 

Sierra Research, Inc. 

1801 J Street 

Sacramento, CA 95811 

(916) 444-6666 
 

Air Dispersion 

Modeling and Health Risk Assessment Protocol 

Avenal Energy Project 

Table of Contents

Section Page

1. Purpose and Background ........................................................................................................... 1 

2. Project Location......................................................................................................................... 3 

3. Proposed Emission Sources ....................................................................................................... 3 

4. Meteorological Data .................................................................................................................. 4 

5. Site Representation – Meteorological Data ............................................................................... 4 

6. Existing Ambient Air Quality Data ........................................................................................... 5 

7. Air Quality Dispersion Models.................................................................................................. 6 

8. Good Engineering Practice (GEP) Stack Height and Downwash ............................................. 8 

9. Receptor Selection ..................................................................................................................... 8 

10. Modeling Scenarios ................................................................................................................ 10 

11. Class I Area Impact Analysis and Class II PSD Significance Thresholds ............................. 11 

12. Ambient Air Quality Impact Analyses .................................................................................. 12 

13. Health Risk Assessment.......................................................................................................... 14 

14. Construction Air Quality Impact Analysis ............................................................................. 16 

15. Cumulative Air Quality Impact Analysis ............................................................................... 17 

16. Final Modeling Submittal ....................................................................................................... 17 

17. References............................................................................................................................... 18 

 

Appendix A: Location Figures 

Appendix B: Information on CTDMPLUS Model 

List of Tables

Page

1. Air Quality Impact Analysis Significance Thresholds..........................................................3 

2. Operating Modes of the Combustion Gas Turbines............................................................11 

3. Significance Levels for Air Quality Impacts in Class II Areas...........................................13 

List of Figures

Page

1. Locations of Project and Surface Meteorological Monitoring Station ...............................20 

2A. Wind Rose for Hanford, CA – Calendar Year 2000 ...........................................................21 

2B. Wind Rose for Hanford, CA – Calendar Year 2001 ...........................................................22 

2C. Wind Rose for Hanford, CA – Calendar Year 2002 ...........................................................23 

2D. Wind Rose for Hanford, CA – Calendar Year 2003 ...........................................................24 

2E. Wind Rose for Hanford, CA – Calendar Year 2004 ...........................................................25 
 -1-

 

Avenal Energy Project 

Modeling Protocol 

July 2007 

1. PURPOSE AND BACKGROUND

On behalf of Federal Power Avenal LLC (Applicant), Sierra Research is submitting this 

modeling protocol to the San Joaquin Valley Air Pollution Control District (SJVAPCD or 

District), California Energy Commission (CEC), and U.S. Environmental Protection Agency (US 

EPA) for approval of the air dispersion and health risk assessment modeling proposed to be 

conducted in support of the Avenal Energy Project (Project) Application for a Determination of 

Compliance (DOC) from the District, Application for Certification (AFC) from the CEC, and 

Prevention of Significant Deterioration (PSD) Permit from the EPA. 

The proposed air-cooled combined-cycle power plant project will consist of constructing and 

operating the following equipment: 

• Two General Electric PG7241 (FA) combustion turbines (or equivalent), rated at 

180 MW (nominal net, at site design conditions); 

• Two heat recovery steam generators (HRSGs) equipped with duct burners; 

• One 300-MW (nominal) condensing steam turbine; 

• One natural gas-fueled auxiliary boiler; 

• Three inlet air chillers with associated air cooling; 

• One closed cooling water air cooler; 

• One fire water pump engine; and 

• One emergency generator engine. 

The project site is located six miles northeast of the residential and business districts of the City 

of Avenal in Kings County, between the California Aqueduct and the Avenal Cutoff Road (see 

location figures in Appendix A). 

Because the Project will generate more than 50 MW of electric power, certification by the CEC 

will be needed to satisfy requirements for public disclosure of potential environmental impacts 

under the California Environmental Quality Act (CEQA). Criteria pollutant emissions of 

nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter with nominal 

aerodynamic diameter less than or equal to 10 microns (PM10) will exceed 50 tons per year, and 

hence, classify the Project as a new major source under District New Source Review regulations, 

Rule 2201. Project emissions of these same pollutants are also expected to exceed the 100 tonper-year major source threshold for the federal Prevention of Significant Deterioration (PSD) 

requirements (40 CFR 52.21) pertaining to federal attainment1 pollutants nitrogen dioxide (NO2), 

 

1

 Sulfur dioxide (SO2) is also an attainment pollutant, while ozone and particulate matter with nominal aerodynamic 

diameter less than or equal to 2.5 microns (PM2.5) are nonattainment pollutants in the project area of the San Joaquin 

Valley Air Basin. 
 -2-

CO, and PM10 emitted from the fossil fuel-fired steam-electric plant category2 listed in the 

federal PSD regulations. Therefore, the permit application will include a demonstration of 

compliance with all applicable portions of the PSD requirements. Emissions of volatile organic 

compounds (VOCs) and sulfur dioxide (SO2) will be below major source levels. 

The Applicant will submit air quality impact analyses to the SJVAPCD, CEC, and US EPA. The 

modeling analyses will include pollutants as required by District Rule 2201, Section 4.3.2;,CEC 

requirements3

 for evaluation of project air quality impacts; and applicable US EPA requirements 

for PSD permits. The purpose of this document is to establish the procedure for meeting 

District, CEC and US EPA air quality modeling requirements for the proposed project. 

This protocol follows modeling guidance provided by the U.S. Environmental Protection Agency 

(USEPA) in its “Guideline on Air Quality Models” (including supplements), the National Park 

Service’s “Permit Application Guidance for New Air Pollution Sources” (Bunyak, 1993), the 

Federal Land Managers’ “Air Quality Related Values Workgroup (FLAG) Draft Phase I Report” 

(October 1999), the “Interagency Workgroup on Air Quality Modeling (IWAQM) Phase II 

Recommendations” (1998), and the SJVAPCD Guidance for Air Dispersion Modeling, Version 

1.2 (August 2006). 

The air quality impact analysis will address the proposed facility operation in terms of the 

following modes: 

• Baseload (100%) operation without duct firing 

• Baseload (100%) operation with duct firing 

• Startup/shutdown operations. 

Impacts from operation of the facility will be compared to the thresholds shown in Table 1: 

 

2

 Fossil fuel-fired steam-electric plant with heat input greater than 250 MMBtu/hour. 

3

 Summarized in CEC Data Adequacy Worksheets, revised March 28, 2007, and available at 

http://www.energy.ca.gov/sitingcases/documents/index.html. 
 -3-

Table 1 

Air Quality Impact Analysis Significance Thresholds

Air Quality Criteria VOC a NO2 PM10 PM2.5 CO SO2

PSD Significant Impact Levels NAb √ √ NA √ NA 

PSD Monitoring Exemption Levels NA √ √ √ NA 

Ambient Air Quality Standards (AAQS) √ √ √ √c √ √

Class I and Class II Visibility √ √ √

Impacts to Soils and Vegetation √ √ √

Class I Area Acid Deposition √ √ √

Notes: 

a. VOC emissions are used as a surrogate for ozone impacts in the PSD review process; no ozone modeling will be 

carried out. 

b. Not applicable. 

c. USEPA guidance (71 FR 6727) provides that compliance with the federal PM2.5 NAAQS should be evaluated 

using the PM10 NAAQS and not modeled directly. At the request of the California Air Resources Board (ARB), 

compliance with both the federal 24-hour average AAQS and the state and federal annual average AAQS for 

PM2.5 will be addressed based on PM2.5 for non-PSD purposes.

2. PROJECT LOCATION

The location of the site is approximately the following: 

• Latitude: 36.088394° North 

• Longitude: 120.061141° West 

or, in Universal Transverse Mercator (UTM) coordinates, 

• Northing: 3,997.742 kilometers (km) and 

• Easting: 764.616 kilometers (North American Datum [NAD] 27, Zone 10). 

The nominal site elevation is 340 feet (104 meters) above mean sea level. 

3. PROPOSED EMISSION SOURCES

The new emission sources for the Project will be the two natural gas-fired combustion turbines 

in combined-cycle mode, one natural gas-fired auxiliary boiler, one natural gas-fueled 

emergency generator engine, and one Diesel-fueled fire water pump engine. The turbines will 

utilize advanced combustion design and emission controls to limit emissions of NOx, CO, and 

VOC. Emissions of PM10 and particulate matter with nominal aerodynamic diameter less than or 

equal to 2.5 microns (PM2.5), and SOx will be kept to a minimum through the exclusive use of 

natural gas in the turbines. The layout of the emitting units is shown in the last three figures in 

Appendix A. 
 -4-

4. METEOROLOGICAL DATA

The Project area has no on-site meteorological monitoring. The District has developed and 

posted on its website five years of meteorological data from its monitoring station at Hanford, 

CA, located 28 miles to the northeast of the project site (see Figure 1), and appropriate for use on 

this project with the model AERMOD4

 (current version 07026). Data from the Hanford station5

have been processed by AERMET software, which formats the data for use in AERMOD. The 

meteorological data from the Hanford monitoring station are representative of conditions at the 

Project because the area is extremely flat both between the two locations and over a much larger 

area to the northwest and southeast along the alignment of the San Joaquin Valley. Air flow in 

the valley can be characterized by up-valley and down-valley winds. The down-valley winds are 

generally a result of airflow into the Valley from the Carquinez Strait and the Altamont Pass that 

then flow south. Strong diurnal wind regimes markedly affect the horizontal transport of air in 

the project area. This results in a pronounced north-northwest component to the wind roses 

shown in Figures 2A through 2E for 2000-2004, respectively. Modeling for each of the five 

years of available data6

 will not vary much because the meteorological data are so constant 

between the years. 

It is expected that the main modeling for the project will be accomplished with AERMOD. If 

needed for supplemental modeling of air dispersion to receptors in terrain above stack-top 

height, Appendix A contains information on the following two additional models and the 

meteorological data needed to run them: Complex Terrain Screening Model (CTSCREEN) and 

the Complex Terrain Dispersion Model PLUS (CTDMPLUS). 

5. SITE REPRESENTATION – METEOROLOGICAL DATA

USEPA defines the term “on-site data” to mean data that would be representative of atmospheric 

dispersion conditions at the source and at locations where the source may have a significant 

impact on air quality. Specifically, the meteorological data requirement originates in the Clean 

Air Act at Section 165(e)(1), which requires an analysis “of the ambient air quality at the 

proposed site and in areas which may be affected by emissions from such facility for each 

pollutant subject to regulation under [the Act] which will be emitted from such facility.” 

This requirement and USEPA’s guidance on the use of on-site monitoring data are also outlined 

in the “On-Site Meteorological Program Guidance for Regulatory Modeling Applications” 

(1987a). The representativeness of the data depends on (a) the proximity of the meteorological 

monitoring site to the area under consideration, (b) the complexity of the topography of the area, 

(c) the exposure of the meteorological sensors, and (d) the period of time during which the data 

 

4

 AERMOD stands for American Meteorological Society/Environmental Protection Agency Regulatory Model, and 

was developed by the American Meteorological Society/Environmental Protection Agency Regulatory Model 

Improvement Committee (AERMIC). 

5

 SJVAPCD. Meteorological data from Hanford during 2000-2004, 

http://www.valleyair.org/busind/pto/Tox_Resources/AirQualityMonitoring.htm.

6

 SJVAPCD. Hanford meteorological data in AERMOD-compatible format, 

http://www.valleyair.org/busind/pto/Tox_Resources/AirQualityMonitoring.htm
 -5-

are collected. The District’s proposed Camp Pendleton meteorological data are representative of 

conditions at the project site. 

Representativeness has also been defined in the “Workshop on the Representativeness of 

Meteorological Observations” (Nappo et. al., 1982) as “the extent to which a set of 

measurements taken in a space-time domain reflects the actual conditions in the same or 

different space-time domain taken on a scale appropriate for a specific application.” 

Representativeness is best evaluated when sites are climatologically similar, as are the project 

site and the Camp Pendleton meteorological monitoring station. Representativeness has 

additionally been defined in the PSD Monitoring Guideline (USEPA 1987b) as data that 

characterize the air quality for the general area in which the proposed project would be 

constructed and operated. Because of the relative proximity of the Hanford meteorological data 

site to the proposed project site, the same large-scale topographic features that influence the 

meteorological data monitoring station also influence the proposed project site in the same 

manner. 

6. EXISTING AMBIENT AIR QUALITY DATA

Background ambient air quality data for the project area during 2004-2006 will be obtained from 

the following monitoring sites that are nearest to the project site by pollutant: 

• Corcoran- Patterson Avenue7: PM10 and PM2.5 

• Visalia-North Church Street: CO 

• Hanford-South Irwin Street: O3 and NO2

• Sacramento-Del Paso Manor: SO2

Modeled concentrations will be added to these representative background concentrations to 

determine compliance with the CAAQS and NAAQS. 

For PM10 and PM2.5, the following data will be used to represent project background: 

• For PM10, the highest 24-hour average and annual average values between 2004 and 2006 

will be used to represent project background. 

• For PM2.5 24-hour impacts, the three-year average of the 98th percentile 24-hour 

monitored levels for the period between 2004 and 2006 will be used to represent project 

background. For annual impacts, the highest monitored annual average between 2004 

and 2006 will be used to represent project background. The maximum modeled 24-hr 

and annual PM2.5 impacts for the project will be added to the above project background 

levels to confirm that the project will not cause or contribute to a violation of an air 

quality standard. The most recent three calendar-year period available, 2004-2006, is 

used to most accurately represent future background levels in the AQIA. 

• If a more refined analysis of potential PM2.5 impacts is needed, then the following 

procedure would be followed to combine background concentrations of 24-hour average 

PM2.5 concentrations during the period 2000 through 2004 (contemporaneous with 

meteorological data) with modeled impact concentrations. Because PM2.5 measurements 

 

7 http://www.arb.ca.gov/adam/cgi-bin/db2www/adamtop4b.d2w/Branch
 -6-

are taken on a once-in-six-day basis, each PM2.5 measurement would be presumed to 

represent the day of measurement and each of the five subsequent days. Missing data 

would be filled in by interpolation using data from the data immediately preceding and 

following the missing data point. These day-specific project background data would be 

combined with contemporaneous modeled project impacts to evaluate compliance with 

the PM2.5 AAQS for non-PSD purposes. The highest three-year average of the combined 

project plus background 98th percentile 24-hour average concentrations would then be 

used to evaluate compliance with the 24-hour average PM2.5 AAQS for non-PSD 

purposes. The highest three-year average of the combined project plus background 

annual average concentrations would be used to evaluate compliance with the annual 

average PM2.5 AAQS for non-PSD purposes. 

7. AIR QUALITY DISPERSION MODELS

Overview 

The following USEPA air dispersion models are proposed for use to quantify pollutant impacts 

on the surrounding environment based on the emission sources’ operating parameters and their 

locations: 

 

• American Meteorological Society/Environmental Protection Agency Regulatory Model 

Improvement Committee (AERMIC) model, also known as AERMOD (Version 07026); 

• Building Profile Input Program – Plume Rise Model Enhancements (BPIP-PRIME, 

current version 04274); and 

• SCREEN3 (Version 96043). 

The following models are not expected to be used, but they are listed in the event that an 

optional specialized modeling analysis is necessary for the project. In addition to the 

information listed below, further information on the use of CTDMPLUS is provided in Appendix 

A. 

• Complex Terrain SCREEN (CTSCREEN, Version 94111) 

• Complex Terrain Dispersion Model (CTDMPLUS, Version 93228) 

• Visibility Screening Model (VISCREEN (Version 88341) 

The first three primary models listed above, and how they are used, are discussed below. 

Simple, Complex, and Intermediate Terrain Impacts 

For modeling project emissions in simple, complex, and intermediate terrain, the USEPArecommended guideline model AERMOD will be used with the AERMET-processed hourly 

meteorological data from the Hanford monitoring station during 2000-2004 provided by the 

District on its website. The AERMOD model requires hourly meteorological data consisting of 

wind vector and speed (with reference height), temperature (with reference height), MoninObukhov length, surface roughness length, heights of the mechanically and convectively 
 -7-

generated boundary layers, surface friction velocity, convective velocity scale, and vertical 

potential temperature gradient in the 500-meter layer above the planetary boundary layer. The 

model assumes that there is no variability in meteorological parameters over a one-hour time 

period, hence the term “steady-state.” The AERMOD model allows input of multiple sources 

and source groupings, eliminating the need for multiple model runs. Complex phenomena such 

as building-induced plume downwash are treated in this model. 

Standard AERMOD control parameters will be used (stack tip downwash, non-screening mode, 

non-flat terrain, sequential meteorological data check employed). Stack-tip downwash, which 

adjusts the effective stack height downward following the methods of Briggs (1972) for cases 

where the stack exit velocity is less than 1.5 times the wind speed at stack top, will be selected 

per USEPA guidance. The rural default option will be used by not invoking the URBANOPT 

option.8

Ambient Ratio Method and Ozone Limiting Method 

Annual NO2 concentrations will be calculated using the Ambient Ratio Method (ARM), adopted 

in Supplement C to the Guideline on Air Quality Models (USEPA, 1995). The Guideline allows 

a nationwide default of 75% for the conversion of nitric oxide (NO) to NO2 on an annual basis 

and the calculation of NO2/NOx ratios. 

If NO2 concentrations need to be examined in more detail, the Plume Volume Molar Ratio Method 

(PVMRM) adaptation of the Ozone Limiting Method (Cole and Summerhays, 1979) will be used. 

AERMOD PVMRM calculates the NO2 concentration using hourly ozone data. Hourly ozone data 

collected at the Hanford monitoring station during the years 2000-2004 will be used in conjunction 

with PVMRM to calculate hourly NO2 concentrations from hourly NOx concentrations. Missing 

hourly ozone data will be substituted prior to use with day-appropriate values (e.g., from the 

previous day, or the next day, for the same hour). Any other missing hourly ozone data will be 

substituted with 40 ppb ozone (typical ozone tropospheric background level). The PVMRM 

involves an initial comparison of the estimated maximum NOx concentration and the ambient O3

concentration left in the plume after reaction of NO with O3 to determine which is the limiting factor 

to NO2 formation. If the remaining O3 concentration is greater than the maximum NOx 

concentration, total conversion is assumed. If the NOx concentration is greater than the remaining 

O3 concentration, the formation of NO2 is limited by the remaining ambient O3 concentration. In 

this case, the NO2 concentration is set equal to the O3 concentration plus a correction factor that 

accounts for in-stack and near-stack thermal conversion. 

Fumigation 

The SCREEN3 model will be used to evaluate inversion breakup fumigation impacts for shortterm averaging periods (24 hours or less), as appropriate. The methodology in USEPA, 1992 

(Screening Procedures for Estimating the Air Quality Impact of Stationary Sources, Revised) 

will be followed for these analyses. Combined impacts for all sources under fumigation 

 

8

 The rural vs. urban option in AERMOD is primarily designed to set the fraction of incident heat flux that is 

transferred into the atmosphere. This fraction becomes important in urban areas having an appreciable “urban heat 

island” effect due to a large presence of land covered by concrete, asphalt, and buildings. This situation does not 

exist for the proposed project site. 
 -8-

conditions will be evaluated, based on USEPA and any applicable SJVAPCD modeling 

guidelines. 

8. GOOD ENGINEERING PRACTICE (GEP) STACK HEIGHT AND DOWNWASH

AERMOD can account for building downwash effects on dispersing plumes. Stack locations 

and heights and building locations and dimensions will be input to BPIP-PRIME. The first part 

of BPIP-PRIME determines and reports on whether a stack is being subjected to wake effects 

from a structure or structures. The second part calculates direction-specific building dimensions 

for each structure that are used by AERMOD to evaluate wake effects. The BPIP-PRIME output 

is formatted for use in AERMOD input files. 

9. RECEPTOR SELECTION

Receptor and source base elevations will be determined from USGS Digital Elevation Model 

(DEM) data using the 71⁄2-minute format (10- to 30-meter spacing between grid nodes). All 

coordinates will be referenced to UTM North American Datum 1927 (NAD27), Zone 10. The 

AERMOD receptor elevations will be interpolated among the DEM nodes according to standard 

AERMAP procedure. For determining concentrations in elevated terrain, the AERMAP terrain 

preprocessor receptor-output (ROU) file option will be chosen; hills will not be imported into 

AERMOD for CTDM-like processing. 

Cartesian coordinate receptor grids will be used to provide adequate spatial coverage 

surrounding the project area for assessing ground-level pollution concentrations, to identify the 

extent of significant impacts, and to identify maximum impact locations. A 250-meter resolution 

coarse receptor grid will be developed and will extend outwards at least 10 km (or more as 

necessary to calculate the significant impact area). 

For the full impact analyses, a nested grid will be developed to fully represent the maximum 

impact area(s). This nested grid will have the following resolutions: 

• 25-meter resolution along the facility fence-line in a single tier of receptors composed of 

four segments extending out to 100 meters from the fenceline 

• 100-meter resolution from 100 meters to 1,000 meters from the fenceline 

• 250-meter resolution from 1 km out to at least 10 km from the site. 

When maximum first-high or maximum second-high impacts occur in the 100- or 250-meter 

spaced areas, an additional refined receptor grid with 25-meter resolution will be placed around 

each maximum coarse grid impact and extended out 1,000 meters in all directions from that 

point of impact. Concentrations within the facility fenceline will not be calculated. 

The following 7.5-minute USGS Digital Elevation Model (DEM) quadrangles in California will 

be employed for modeling the Project: 

• Guijarral Hills, 

• Huron, 

• Westhaven, 

• Avenal, 
 -9-

• La Cima, 

• Kettleman City, 

• Garza Peak, 

• Kettleman Plain; and 

• Los Viejos. 

If more distant impacts are encountered in the AERMOD results, then other quadrangles that 

may be needed include the following (listed from approximately nearest, to farthest): 

• The Dark Hole, 

• Kreyenhaugen Hills, 

• Coalinga, 

• Domengine Ranch, 

• Harris Ranch, 

• Calflax, 

• Vanguard, 

• Lemoore, 

• Stratford, 

• Alcalde Hills, 

• Joaquin Rocks, 

• Lillis Ranch, 

• Tres Picos Farms, 

• Westside, 

• Five Points, 

• Burrel, 

• Riverdale, 

• Ciervo Mountain, 

• Tumey Hills, 

• Monocline Ridge, 

• Levis, 

• Cantua Creek, 

• San Joaquin, 

• Helm, 

• Raisin, 

• Mercey Hot Springs, 

• Chounet Ranch, 

• Chaney Ranch, 

• Coit Ranch, 

• Tranquility, 

• Jamesan, 

• Kerman, 

• Kearney Park, 

• Laguna Seca Ranch, 

• Hammonds Ranch; and 

• Charleston School. 
 -10-

10. MODELING SCENARIOS

Pollutant emissions to the atmosphere from the proposed project will be dominated by the 

products of combustion of natural gas in the combustion turbines. Emission rates will be 

included in the Application for Certification and Application for a Determination of Compliance 

for the project and will be based on vendor data and additional conservative assumptions of 

equipment performance. Turbine emissions and stack parameters, such as flow rate and exit 

temperature, depend on ambient temperature and operating load. To calculate the maximum air 

quality impacts, a screening analysis will be performed to determine which combination of 

operating load and atmospheric temperature produces the maximum ground-level concentration 

(i.e., impact) for each combination of pollutant and averaging time used in an ambient air quality 

standard. 

In the modeling analysis, maximum impacts will be predicted for base load (100%) and reduced 

load (50%) conditions. In addition, design extreme hot, annual average, and extreme cold 

ambient temperatures will be evaluated for each load condition. Each of these conditions has 

unique performance characteristics that affect plume dispersion and thus predicted impacts. The 

results of this screening analysis will be used to select the maximum-impact operational 

scenarios for the modeling analyses. Refined modeling of these maximum-impact scenarios will 

be based on the five years of available meteorological data and will identify the general locations 

of maximum impacts on the coarse grid of receptors, around which fine grids will be used to 

identify the locations of maximum impacts within 25 meters. 

Details of Operating Scenarios 

Maximum emissions during normal operation would occur during periods of maximum fuel 

consumption, which would occur at peak load (with steam power augmentation). Besides 

normal operation, three other modes of operation would occur: commissioning (one initial 

period), startup, and shutdown. In addition to the impacts associated with normal operation, the 

modeling analysis for the project will examine the short-term impacts associated with 

commissioning and gas turbine startup/shutdown activities. Table 2 gives more detail on all four 

operating modes. 
 -11-

Table 2 

Operating Modes of the Combustion Gas Turbines 

Mode Description 

Commissioning 

The process of fine-tuning each of the turbines and their 

associated emission control systems. The facility will follow a 

systematic approach to optimize performance of the turbines 

and the associated control equipment. NOx, CO, and VOC 

emissions are expected to be greater during commissioning 

than during normal operation. This one-time mode affects 

only the initial year of operation. 

Start-up 

Startup NOx and CO emissions are higher because the 

combustors are not fully optimized, and catalyst systems have 

not reached optimal temperatures. 

Normal Operation 

Normal operation begins after the turbines and the control 

equipment are working optimally, at their designated levels. 

Emissions may vary between the extreme hot, annual average, 

and extreme cold ambient conditions. 

Shutdown 

Shutdown occurs at the initiation of the turbine shutdown 

sequence and ends with the cessation of turbine firing. 

Typically, the shutdown process will emit less than the startup process, and thus this mode will be represented by the 

analysis of start-up emissions. 

11. CLASS I AREA IMPACT ANALYSIS AND CLASS II PSD SIGNIFICANCE THRESHOLDS

The project is not located within 100 km of Class I areas, the nearest being the following: 

• Pinnacles National Monument: 113 km 

• Sequoia National Park: 113 km 

• Ventana Wilderness: 127 km 

Therefore, the Project, although a major source, will not need to evaluate impacts to visibility 

and other air quality related values at Class I areas. 
 -12-

12. AMBIENT AIR QUALITY IMPACT ANALYSES

In evaluating the impacts of the proposed project on ambient air quality, we will model the 

ambient impacts of the project, add those impacts to background concentrations, and compare 

the results to the state and federal ambient standards for SO2, NO2, PM10, PM2.5,9 and CO. 

In accordance with USEPA guidelines,10 the highest second-highest modeled concentrations will 

be used to demonstrate compliance with the short-term federal standards and the highest 

modeled concentration will be used to demonstrate compliance with the federal annual and all 

state standards. 

Based on the results of the screening procedure, the operating case that produces the highest 

modeled ambient concentration for each individual pollutant and averaging period will be used 

in the refined air quality impact analysis. The screening analysis will identify the operating 

cases that produce the highest concentration for each pollutant and averaging period, and will 

provide the specific stack parameters (stack height, stack diameter, exhaust temperature and 

exhaust velocity) that will be used to evaluate compliance with applicable PSD increments and 

compliance with the AAQS for all applicable pollutants and averaging periods. 

Compliance with ambient air quality standards will be demonstrated in accordance with the 

three-step process outlined in the October 1990 Draft NSR/PSD Workshop Manual (p. C.51, 

“The Compliance Demonstration”). The first step in the compliance demonstration is to 

determine, for each pollutant and averaging period, whether the proposed new equipment will 

cause a significant ambient impact anywhere. As indicated in the Workshop Manual, “If the 

significant net emissions increase11 from a proposed source would not result in a significant 

ambient impact anywhere, the application is usually not required to go beyond a preliminary 

analysis in order to make the necessary showing of compliance for a particular pollutant.” The 

significance levels for air quality impacts are shown in the Table 3. If the maximum modeled 

impact for any pollutant and averaging period is below the appropriate significance level, no 

further analysis is necessary. 

If the modeled impacts from the Project are above the PSD significance thresholds shown in 

Table 3, the second step of the compliance demonstration is required to show that the proposed 

new source, in conjunction with existing sources, will not cause or contribute to a violation of 

any ambient air quality standard.12

 

9

 As noted above, comparisons to the federal PM2.5 AAQS are for non-PSD purposes. 10 40 CFR part 51, Appendix W, Sections 11.2.3.2 and 11.2.3.3 11 Note that this guidance requires modeling only for pollutants for which there is a significant net increase. 

12 Because the applicable EPA guidance does not define significance levels for PM2.5, PM2.5 impacts will be 

presumed to be above any applicable significance level regardless of the modeled concentration, and the second step 

of the process will be performed. 
 -13-

Table 3 

Significance Levels for Air Quality Impacts in Class II Areasa (μg/m3) 

 Averaging Period 

Pollutant Annual 24-hour 8-hour 3-hour 1-hour 

SO2 1 5 -- 25 -- 

PM10 1 5 -- -- -- 

NOx 1 -- -- -- -- 

CO -- -- 500 -- 2000 

a. From 40 CFR 51.165, shown as Table C-4 in the 1990 draft PSD Workshop Manual. 

If the modeled impacts from the project alone are above the PSD significance thresholds shown 

in Table 3, the second step of the compliance demonstration is required to show that the 

proposed new source, in conjunction with existing sources, will not cause or contribute to a 

violation of any ambient air quality standard.13 As discussed in more detail in Section 6 of this 

protocol, the impacts of existing sources are represented by the existing ambient air quality data. 

In accordance with Section 8.2.1 of Appendix W to 40 CFR Part 51, 

Background concentrations are an essential part of the total air quality concentration to 

be considered in determining source impacts. Background air quality includes pollutant 

concentrations due to: (1) Natural sources; (2) nearby sources other than the one(s) 

currently under consideration; and (3) unidentified sources. Typically, air quality data 

should be used to establish background concentrations in the vicinity of the source(s) 

under consideration. 

Because ambient PM10 and PM2.5 levels result from both secondary pollutant formation as well 

as directly emitted particulate matter, it is appropriate to rely on the locally monitored air quality 

data for PM10 and PM2.5 to represent background concentrations in the project area. 

The impact of the proposed new equipment will be modeled using the maximum allowable 

emission limits as proposed in the application and the design capacities of the turbines, assuming 

continuous operation, in accordance with the guidance in Table 8-2 of Appendix W. If the 

predicted total ground-level concentration obtained by adding the maximum impact of the 

proposed new equipment to the monitored background concentrations is below the state or 

federal ambient air quality standard for each pollutant and averaging period, no further analysis 

is required for that pollutant and averaging period. 

If the proposed new equipment, combined with existing background concentrations, will result in 

predicted total ground-level concentration in excess of an applicable ambient air quality 

standard, then the third step of the compliance demonstration procedure will be undertaken. 

According to the 1990 draft Workshop Manual, 

For a NAAQS violation to which an applicant contributes significantly, a PSD permit 

may be granted only if sufficient emissions reductions are obtained to compensate for the 

 

13 Because the applicable EPA guidance does not define significance levels for PM2.5, PM2.5 impacts will be 

presumed to be above any applicable significance level regardless of the modeled concentration, and the second step 

of the process will be performed. 
 -14-

adverse ambient impacts caused by the proposed source. Emissions reductions are 

considered to compensate for the proposed source’s adverse impact when, at a minimum, 

(1) the modeled net concentration, resulting from the proposed emissions increase and 

the federally enforceable emissions reduction, is less than the applicable significant 

ambient impact level at each affected receptor, and (2) no new violations will occur. 

[emphasis added] 

In accordance with this guidance, only if the modeled impacts from the proposed new 

equipment, combined with existing background concentrations, are predicted to cause a new 

violation of a standard for which the District is currently in attainment will this netting procedure 

be used. The federally enforceable emissions reductions to be used in the netting procedure will 

be provided by emission reduction credits (ERCs) already acquired and certified by the District. 

13. HEALTH RISK ASSESSMENT

A health risk assessment (HRA) will be performed according to the Office of Environmental 

Health Hazard Analysis “Air Toxics Hot Spots Program Guidance Manual for Preparation of 

Health Risk Assessments” (August 2003). The HRA modeling will be prepared using the ARB’s 

Hotspots Analysis and Reporting Program (HARP) computer program (Version 1.3, October 18, 

2005). The HARP model will be used to assess cancer risk as well as non-cancer chronic and 

acute health hazards. The HRA will include the four following pathways: inhalation, dermal 

absorption, and soil and mother’s milk ingestion. 

Because the HARP model incorporates the previously USEPA-approved model ISCST3, a 

special methodology will be employed to be consistent with using AERMOD for the air 

dispersion modeling and retain the health values and risk computations provided by HARP 

Version 1.3. The methodology used to prepare the HRA has been described by the ARB14 and is 

described below. Its use has been approved by the CEC, the SJVAPCD and several other 

Districts15 on previous power plant projects. 

The risk assessment module of the HARP model is run using unit ground level impacts to obtain 

derived cancer risks for each toxic air contaminant (TAC). Cancer risks are obtained for the 

derived (OEHHA) method, the derived (adjusted) method, average point estimate, high-end 

point estimate, and derived (OEHHA) method for worker exposure. The HARP model output is 

cancer risk by TAC and pathway for each type of analysis, based on an exposure of 1.0 μg/m3

. 

Individual cancer risks are expressed in units of risk per μg/m3 of exposure. To calculate the 

weighted risk for each source, the annual average emission rate in grams per second for each 

TAC will be multiplied by the individual cancer risk for that TAC in units of (μg/m3

)

-1. The 

resulting weighted cancer risks for each TAC will then be summed for the source. The same 

approach will be used to determine the non-cancer acute and chronic health hazards associated 

with the Project. 

 

14 Procedure is described in Part B of Topic 8 of the HARP How-To Guides: How to Perform Health Analyses 

Using a Ground Level Concentration. 

15 San Diego APCD, Bay Area AQMD, North Coast Unified AQMD 
 -15-

Health risk from exposure to a carcinogenic TAC is calculated as the product of the exposure 

concentration times a factor representing the risk per unit concentration (i.e., unit risk) for the 

TAC. In the case of cancer risk, the unit risk depends on breathing rate, the cancer potency 

factor of the TAC, dimensional factors, and other terms involving non-inhalation pathways, 

when relevant. In the case of chronic and acute non-cancer impacts, the health hazard per unit 

concentration (i.e., unit impact) is normally calculated as 1 divided by the Reference Exposure 

Level (REL, expressed as a concentration in μg/m3

) for the TAC. 

Exposure concentration is calculated as the product of the actual emission rate (in grams per 

second)16 of the TAC times the concentration per unit of emission (i.e., χ/Q for an emission rate 

of 1 g/s), which is the output from the AERMOD air dispersion modeling calculation. This 

exposure concentration is the “Unit Concentration.” 

HARP automatically passes the “Unit Concentration” for a given source and receptor into its risk 

module where it is multiplied by the actual emission rate (in g/s) for each TAC and the “Unit 

Risk” for the TAC to produce the calculated risk for the TAC. This is done for all the TACs 

emitted by a source and the summed cancer risk, or non-cancer health impact for common toxic 

endpoints in the case of chronic and acute risk, is the total risk or non-cancer health impact at 

that receptor from that source. The total cancer risk or non-cancer health impact at a receptor is 

the sum of the risks or health impacts from all of the sources. 

Because HARP is not designed to pass AERMOD “Unit Concentration” outputs to its Risk 

Module, an alternative procedure will be used. The calculation of cancer risk or non-cancer 

health impact does not require the variables to be multiplied in any particular order. Therefore, 

the final result will be the same if, for a given source, the “Unit Risk” for a TAC is multiplied by 

the actual emissions (g/s) for the TAC, and these products are added together to give a “Source 

Strength”. The “Source Strength” is then used as the source emission rate in AERMOD. 

This methodology thus uses HARP to calculate the “Unit Risks” for all carcinogenic TACs and 

unit chronic and acute health impacts for all non-carcinogenic TACs, including all required 

exposure pathways and toxic end points as well as receptor types, including residents and 

workers. The unit risk or unit health impact for each TAC from a source is multiplied by the 

emission rate of that TAC from the source. These products are summed for all the TACs emitted 

by the source. This is done for each source. Finally, the resulting “Source Strength” for all 

project sources are used as emission rates in AERMOD. The AERMOD output contains the 

resulting cancer risks and non-cancer chronic and acute health impacts. 

 

16 The annual emission rate is the actual long-term emission rate used for analysis of cancer risk and chronic noncancer health impact, and the one-hour emission rate is the actual short-term emission rate used for analysis of acute 

non-cancer health impact. 
 -16-

14. CONSTRUCTION AIR QUALITY IMPACT ANALYSIS

The potential ambient impacts from air pollutant emissions during the construction of the Project 

will be evaluated by air quality modeling that will account for the construction site location and 

the surrounding topography; the sources of emissions during construction, including vehicle and 

equipment exhaust emissions; and fugitive dust. 

Types of Emission Sources − Construction of the Project will require 27 months, and can be 

viewed as three main sequential phases: site preparation; construction of foundations; 

and installation and assembly of the Project turbines and associated equipment. The construction 

impacts analysis will include a schedule for construction operation activities. Site preparation 

includes site excavation, excavation of footings and foundations, and backfilling operations. 

Fugitive dust emissions from the construction of the project result from the following activities: 

• Excavation and grading at the construction site; 

• Onsite travel on paved and unpaved roads and across the unpaved construction site; 

• Aggregate and soil loading and unloading operations; 

• Raw material transfer to and from material stockpiles; and 

• Wind erosion of areas disturbed during construction activities. 

Engine exhaust will be emitted from the following sources: 

• Heavy equipment used for excavation, grading, and construction of onsite structures; 

• Water truck used to control construction dust emissions; 

• Diesel-fueled welding machines, gasoline-powered generators, air compressors, and 

water pumps; 

• Gasoline-fueled pickup trucks and Diesel-fueled flatbed trucks used onsite to transport 

workers and materials around the construction site; 

• Transport of mechanical and electrical equipment to the project site; 

• Transport of rubble and debris from the site to an appropriate landfill; and 

• Transport of raw materials to and from stockpiles. 

Emissions from a peak activity day will be modeled. Annual average emissions over the 27-

month construction period will also be calculated and modeled for comparison with annual 

standards. 

Existing Ambient Levels – The background data discussed earlier will be used to represent 

existing ambient levels for the construction analysis as well as the analysis of the impacts of 

project operations. 

Model Type − The USEPA model, ISCST3, has been approved by the CEC for use in 

construction modeling, and will be used to estimate ambient impacts from construction 

emissions. The modeling options and meteorological data described above will be used for the 

modeling analysis. 
 -17-

The construction site will be represented as both a set of volume sources and a separate set of 

area sources in the modeling analysis. Emissions will be divided into two categories: exhaust 

emissions and dust emissions. For exhaust emissions, a plume height of 4.6 meters (15 feet) will 

be used. Plume height refers to the distance measured from ground level to the centerline of the 

emissions plume. For dust emissions, a release height of two meters will be used due to the 

ambient plume temperatures and negligible plume velocities. 

For the construction modeling analysis, the receptor grid will begin at the property boundary and 

will extend approximately one kilometer in all directions. Receptor spacing will be 60 meters, 

except for three tiers of receptors spaced along the project boundary at 25-meter spacing. 

15. CUMULATIVE AIR QUALITY IMPACT ANALYSIS

The CEC has defined this analysis as “a cumulative air quality modeling impacts analysis of the 

project’s typical operating mode in combination with other stationary source emissions sources 

within a six-mile radius which have received construction permits but are not yet operating, or 

are in the permitting process.” For each criteria pollutant the CEC staff considers facilities 

having an emission increase of less than five tons per year to be de minimis, so such facilities 

may be excluded. The District has already been requested to provide information on any sources 

that might be appropriate for a cumulative air quality impact analysis, as defined above. 

Upon receipt of sufficient information from the District to allow air dispersion modeling of the 

non-project sources to be included in the cumulative air quality impact analysis, AERMOD will 

be used in a procedure similar to that described earlier in this protocol. 

16. FINAL MODELING SUBMITTAL

The final modeling analyses will include the following materials: 

• Summaries of maximum modeled impacts for each air quality scenario showing 

meteorological conditions and receptor location and elevation; 

• All modeling inputs and outputs (including BPIP-PRIME and meteorological files) in 

electronic format, together with a description of all filenames; 

• Plot plan showing emission points, nearby buildings (including dimensions), crosssection lines, property lines, fencelines, roads, and UTM coordinates; and 

• A table showing building heights used in the modeling analysis. 

The HRA results will include AERMOD output plot files, spreadsheets that relate the computed 

risks with UTM coordinates, and textual discussion that explains the methodology to the lay 

public. 
 -18-

17. REFERENCES

Briggs, G.A. (1972). Discussion on Chimney Plumes in Neutral and Stable Surroundings. 

Atmos. Environ. 6:507-510. 

Bunyak, John, 1993. Permit Application Guidance for New Air Pollution Sources. Natural 

Resources Report NPS/NRAQD/NRR-93/09, US Dept. of the Interior, National Park Service. 

Cole, H.S. and J.E. Summerhays (1979). A review of techniques available for estimating 

short-term NO2 concentrations. Journal of the Air Pollution Control Association 29(8) 790-888.

Irwin, J.S. (1978). Proposed Criteria for Selection of Urban Versus Rural Dispersion 

Coefficients. Staff Report. Meteorology and Assessment Division, U.S. Environmental 

Protection Agency, Research Triangle Park, NC. (Air Docket Reference No. II-B-8 for the 

Fourth Conference on Air Quality Modeling). 

Nappo, C. J. et al. (1982). The Workshop on the Representativeness of Meteorological 

Observations, June 1981, Boulder, Co. Bull. Amer. Meteor. Soc., Vol. 63, No. 7, pp. 761-764. 

American Meteorological Society, Boston, MA. 

San Diego APCD (2005). Supplemental Guidelines for Submission of Air Toxics “Hot Spots” 

Program Health Risk Assessments (HRAs), Version 1.0, March 1, 2005. 

USEPA, 1985. Guideline for Determination of Good Engineering Practice Stack Height

(Technical Support Document for the Stack Height Regulations) - Revised. EPA-450/4-80-

023R. 

USEPA (1986). Guideline on Air Quality Models (Revised). U.S. EPA-45/2-78-027R, Office of 

Air Quality Planning and Standards, Research Triangle Park, NC. 

USEPA (1987a). Supplement A to the Guideline on Air Quality Models (Revised). Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. 

USEPA (1987b). Ambient Monitoring Guidelines for Prevention of Significant Deterioration 

(PSD). Office of Air Quality Planning and Standards, and Office of Research and Development, 

Research Triangle Park, NC. 

USEPA (1988). Workbook for Plume Visual Impact Screening and Analysis. Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. EPA-450/4-88-015 (Including 

October 1992 Revisions). 

USEPA, 1990. Supplement B to the Guideline on Air Quality Models (Revised). Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. 

USEPA, 1990. New Source Review Workshop Manual − Draft. Office of Air Quality Planning 

and Standards, Research Triangle Park, NC. 
 -19-

USEPA, 1992. Procedures for Substituting Values for Missing NWS Meteorological Data for 

Use in Regulatory Air Quality Models. Dennis Atkinson and Russell F. Lee, Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. July 7, 1992. 

USEPA (1992). Screening Procedures for Estimating the Air Quality Impact of Stationary 

Sources, (Revised). Office of Air Quality Planning and Standards, Research Triangle Park, NC. 

USEPA (1995). Supplement C to the Guideline on Air Quality Models (revised). Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. 

USEPA (1998). Interagency Workgroup on Air Quality Modeling (IWAQM) Phase 2 Summary 

Report and Recommendations for Modeling Long-Range Transport Impacts. Air Quality 

Modeling Group (MD-14), Research Triangle Park. National Park Service - Air Resource 

Division, Denver, Colorado. USDA Forest Service - Air Quality Program, Fort Collins, 

Colorado. US Fish and Wildlife Service Air Quality Branch, Denver, Colorado.

USEPA (2000). Meteorological Monitoring Guidance for Regulatory Modeling Applications. 

Publication No. EPA-454/R-99-005. Office of Air Quality Planning & Standards, Research 

Triangle Park, NC. (PB 2001-103606) (Available at www.epa.gov/scram001/) 

USEPA (2003). 40 CFR Part 51, Appendix W. Guideline on Air Quality Models. Last update 

5/13/2003. 

USDA Forest Service Air Quality Program, National Park Service Air Resource Division, US 

Fish and Wildlife Service - Air Quality Branch (2000). Federal Land Managers’ Air Quality 

Related Values Workgroup (FLAG) Phase I Report. December 2000. 
 -20-

Figure 1 

Locations of Project and Surface Meteorological Monitoring Station 

Avenal Energy Project 
 -21-

NORTH

SOUTH

WEST EAST

5%

10%

15%

20%

25%

WIND SPEED

(Knots)

>= 22

17 - 21

11 - 17

7 - 11

4 - 7

1 - 4

Calms: 25.88%

Figure 2A: Wind Rose for Hanford, CA – Calendar Year 2000 
 -22-

NORTH

SOUTH

WEST EAST

6%

12%

18%

24%

30%

WIND SPEED

(Knots)

>= 22

17 - 21

11 - 17

7 - 11

4 - 7

1 - 4

Calms: 12.69%

Figure 2B: Wind Rose for Hanford, CA – Calendar Year 2001 
 -23-

NORTH

SOUTH

WEST EAST

6%

12%

18%

24%

30%

WIND SPEED

(Knots)

>= 22

17 - 21

11 - 17

7 - 11

4 - 7

1 - 4

Calms: 23.39%

Figure 2C: Wind Rose for Hanford, CA – Calendar Year 2002 
 -24-

NORTH

SOUTH

WEST EAST

5%

10%

15%

20%

25%

WIND SPEED

(Knots)

>= 22

17 - 21

11 - 17

7 - 11

4 - 7

1 - 4

Calms: 21.32%

Figure 2D: Wind Rose for Hanford, CA – Calendar Year 2003 
 -25-

NORTH

SOUTH

WEST EAST

5%

10%

15%

20%

25%

WIND SPEED

(Knots)

>= 22

17 - 21

11 - 17

7 - 11

4 - 7

1 - 4

Calms: 21.90%

Figure 2E: Wind Rose for Hanford, CA – Calendar Year 2004
 

Appendix A 

Location Figures 





SITE

BOUNDARY

5

AVENAL CUTOFF ROAD

GAS PIPELINE

G

G

G

G

E

E E

G

ALTERNATEGAS PIPELINEROUTE

EXISTING PG&E 230 KTRANSMISSION LINE

EXISTING PG&E 500 KVTRANSMISSION LINES

E

E

0 2,000 4,000 FEET

 SCALE

REFERENCE: 

U.S.G.S 7.5 MINUTE TOPOGRAPHIC SERIES MAPOF LA CIMA, CALIFORNIA, DATED 1978. 

NATURAL GA

INTERCONN

FEDERAL PO

AVENAL ENERGY
 

Appendix B 

Information on CTDMPLUS Model 

The CTDMPLUS and CTSCREEN Models 

Complex terrain impacts may need to be modeled with more accuracy than that provided by 

AERMOD. The use of more refined modeling techniques is specifically addressed in EPA’s 

Appendix W1 modeling guidance, as follows: 

Since AERMOD treats dispersion in complex terrain, we have merged sections 4 and 5 of 

appendix W, as proposed in the April 2000 NPR [Notice of Proposed Rulemaking]. And 

while AERMOD produces acceptable regulatory design concentrations in complex 

terrain, it does not replace CTDMPLUS for detailed or receptor-oriented complex 

terrain analysis, as we have made clear in Guideline section 4.2.2. CTDMPLUS remains 

available for use in complex terrain. [p. 68225] 

4.2.2 Refined Analytical Techniques 

d. If the modeling application involves a well defined hill or ridge and a detailed 

dispersion analysis of the spatial pattern of plume impacts is of interest, CTDMPLUS, 

listed in Appendix A, is available. CTDMPLUS provides greater resolution of 

concentrations about the contour of the hill feature than does AERMOD through a 

different plume-terrain interaction algorithm. [p. 68233] 

CTSCREEN is the same basic model as CTDMPLUS, except that meteorological data are 

handled internally in a simplified manner. As discussed in the CTSCREEN users guide,2

Since [CTDMPLUS] accounts for the three-dimensional nature of plume and terrain 

interaction, it requires detailed terrain and meteorological data that are representative 

of the modeling domain. Although the terrain data may be readily obtained from 

topographic maps and digitized for use in the CTDMPLUS, the required meteorological 

data may not be as readily available. 

Since the meteorological input requirements of the CTDMPLUS can limit its application, 

the EPA's Complex-Terrain-Modeling, Technology-Transfer Workgroup developed a 

methodology to use the advanced techniques of CTDMPLUS in situations where on-site 

meteorological measurements are limited or unavailable. This approach uses 

CTDMPLUS in a "screening" mode--actual source and terrain characteristics are 

modeled with an extensive array of predetermined meteorological conditions. 

 

1 40 CFR 51 Subpart W, as amended November 9, 2005 at 70 FR 68218, “Revision to the Guideline on Air Quality 

Models: Adoption of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other 

Revisions.” 

2

 USEPA, EPA-600/8-90-087, “User’s Guide to CTDMPLUS: Volume 2. The Screening Mode (CTSCREEN),” 

October 1990. 
 

This CTDMPLUS screening mode (CTSCREEN) serves several purposes in regulatory 

applications. When meteorological data are unavailable, CTSCREEN can be used to 

obtain conservative (safely above those of refined models), yet realistic, impact estimates 

for particular sources. 

Therefore, the use of the CTSCREEN version of CTDMPLUS is consistent with EPA guidance. 

Meteorological Data for CTDMPLUS 

The discussion in Section 6 of the text focused on meteorological data needed to run AERMOD. 

As discussed in a later section, an additional model, Complex Terrain Dispersion Model PLUS 

(CTDMPLUS), may be used in lieu of the model Complex Terrain Screening Model 

(CTSCREEN) for receptors in the terrain above stack-top height. CTDMPLUS is a USEPAapproved air dispersion model, and is fully supported with user guidance documentation.3

CTDMPLUS requires an extensive suite of meteorological data composed of not only wind 

speed, direction, and temperature, but also horizontal and vertical wind direction standard 

deviations (sigma theta and sigma phi, respectively), and vertical wind speed standard deviation 

(sigma w). The AERMOD-compatible meteorological data set provided by the SJVAPCD, 

derived from measurements taken at Hanford, CA, does not include these non-standard 

measurements. 

It is possible to develop conservative values for the standard deviation parameters sigma theta, 

sigma phi, and sigma w that are consistent with the available meteorological data, and use them 

to prepare a meteorological data set that is usable in CTDMPLUS and yields conservative (i.e., 

high) ground-level concentrations. 

If modeling with CTDMPLUS is required, the District will be requested to develop ISCST3-

compatible meteorological data sets for the same three years of 2003-2005. The ISCST3 

meteorological data set would be used to create the CTDMPLUS-compatible meteorological data 

set. Because all three of these Gaussian dispersion models—ISCST3, AERMOD, and 

CTDMPLUS—require upper air data as well as surface data, the upper air data from the Oakland 

Metropolitan Airport (ID No. 23230) would be used because it is the same station used by the 

SJVAPCD to develop the AERMOD-compatible meteorological dataset. 

The following meteorological parameters are needed for CTDMPLUS and would be taken 

directly from the AERMET files: 

• Observed mixing height, provided as the height of the convective or planetary boundary 

layer (PBL); 

 

3

 USEPA. Technology Transfer Network, Support Center for Regulatory Atmospheric Modeling, 

http://www.epa.gov/scram001/dispersion_prefrec.htm#ctdmplus
 

• Calculated mixing height, provided as the height of the mechanical, or surface, boundary 

layer (SBL); 

• Friction velocity (USTAR); 

• Monin-Obukhov length (L); and 

• Roughness length (ZO). 

The remaining standard deviations (sigma values) are not available from AERMOD and must be 

obtained from ISCST3-compatible files that would be developed from the Hanford, CA 

meteorological data. Stability classes determined by MPRM4 or PCRAMMET5

 from the 

measured Hanford meteorological data would be used to select the most conservative values 

from the following ranges recommended in USEPA’s Meteorological Monitoring Guidance 

document:6

 Sigma Phi (σΦ)/ Sigma Theta (σθ)/ 

Stability Regulatory Range Regulatory Range 

Category (degrees) (degrees)

A 11.5 22.5 

B 10.0 – 11.5 17.5 – 22.5 

C 7.8 – 10.0 12.5 – 17.5 

D 5.0 – 7.8 7.5 – 12.5 

E 2.4 – 5.0 3.8 – 7.5 

F < 2.4 < 3.8 

The most conservative values (that is, the values that produce the highest modeled impacts) for 

sigma theta and sigma phi within each range would be determined by conducting a sensitivity 

analysis for all combinations of stack conditions to be modeled using CTDMPLUS and receptor 

locations for which CTDMPLUS would be used (that is, receptors above stack height). The 

sensitivity analysis would use the upper and lower values of each range for each stability 

category. For example, for stability category D, four combinations would be evaluated as 

follows: 

σΦ σθ

5.0 7.5 

5.0 12.5 

7.8 7.5 

7.8 12.5 

 

4

 The Meteorological Processor for Regulatory Models 

5

 EPA meteorological preprocessor 

6 Tables 6-8a and 6-9a in Meteorological Monitoring Guidance for Regulatory Modeling Applications, EPA-454/R99-005, US EPA Office of Air and Radiation, Office of Air Quality Planning and Standards, February 2000. 
 

For stability category A, maximum values for σΦ and σθ of 15.0 and 27.0, respectively, would be 

evaluated. For stability category F, minimum values for σΦ and σθ of 1.0 and 2.0, respectively, 

would be evaluated.

Sigma-w would be estimated by multiplying sigma-phi (after conversion from degrees to 

radians) by the horizontal wind speed.