Modeling for North 

American Background 

Concentrations 

Final Report 

Contract No. EP-C-09-009 

Work Assignment 2-55

October 28, 2011 

Prepared for 

U.S. EPA 

National Center for Environmental Assessment 

Research Triangle Park, NC 27711 

Prepared by 

ICF International 

101 Lucas Valley Road, Suite 260 

San Rafael, CA 94903 

415.507.7164 

11-035 
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Table of Contents 

1. Introduction ........................................................................................................................... 1

2. Description of GEOS-Chem ................................................................................................. 3

3. GEOS-Chem Model Application Methodology ................................................................... 5

3.1. Modeling Domain .......................................................................................................... 5

3.2. Simulation Periods ........................................................................................................ 7

3.3. Input Acquisition and Preparation ................................................................................. 7

Meteorological Inputs .......................................................................................................................................... 7

Emission Inputs ..................................................................................................................................................... 8

Terrain, Land-Use, Chemistry and Other Inputs ........................................................................................ 8

3.4. Model Options and Parameter Specifications .............................................................. 9

3.5. Scenarios ...................................................................................................................... 9

4. Model Performance Evaluation Procedures & Results ................................................... 11

4.1. Comparison with Observed Air Quality Data .............................................................. 11

Summary Model Performance Results for 2006 ...................................................................................... 12

Summary Model Performance Results for 2007 ...................................................................................... 49

Summary Model Performance Results for 2008 ...................................................................................... 88

Year-to-Year Variations in Model Performance ...................................................................................... 127

4.2. Case Studies ............................................................................................................ 127

Case Study #1: 21 April 2006 ....................................................................................................................... 128

Case Study #2: 14 July 2006 ........................................................................................................................ 129

Case Study #3: 17-20 August 2006 ............................................................................................................ 131

Additional Case Studies ................................................................................................................................. 134

4.3. Summary of Model Performance .............................................................................. 134

5. Estimation of North American Background Ozone (NAB Scenario) ........................... 137

6. Additional Scenarios and Comparisons ......................................................................... 155

6.1. U.S. and Natural Background Scenarios .................................................................. 155

6.2. Comparison of North American and Natural Background Ozone Concentrations .... 175

7. Attributes and Limitations ................................................................................................ 183

8. References ......................................................................................................................... 185

List of Figures 

Figure 3-1. GEOS-Chem Nested-Grid Modeling Domain for North America. ................................................................. 5

Figure 4-1. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Each Season and the Entire Year, Averaged Over All Sites in Entire Continental U.S.: 2006. ............... 23

Figure 4-2. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the West Coast Subregion: 2006. ............................................................... 24

Figure 4-3. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Intermountain West Subregion: 2006. ................................................... 26

Figure 4-4. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in Central States Subregion: 2006. ................................................................. 27

Figure 4-5. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Great Lakes Subregion: 2006. .............................................................. 29

Figure 4-6. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Southeast Subregion: 2006. .................................................................. 31

Figure 4-7. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Northeast Subregion: 2006. .................................................................. 32

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Figure 4-8. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Each Season and the Entire Year, Averaged Over All Sites in Entire Continental U.S.: 2007. ............... 59

Figure 4-9. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the West Coast Subregion: 2007. ............................................................... 60

Figure 4-10. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Intermountain West Subregion: 2007. ................................................... 61

Figure 4-11. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in Central States Subregion: 2007. ................................................................. 63

Figure 4-12. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Great Lakes Subregion: 2007. .............................................................. 65

Figure 4-13. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Southeast Subregion: 2007. .................................................................. 66

Figure 4-14. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Northeast Subregion: 2007. .................................................................. 68

Figure 4-15. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Each Season and the Entire Year, Averaged Over All Sites in Entire Continental U.S.: 2008. ............... 98

Figure 4-16. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the West Coast Subregion: 2008. ............................................................... 99

Figure 4-17. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Intermountain West Subregion: 2008. ................................................. 100

Figure 4-18. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in Central States Subregion: 2008. ............................................................... 102

Figure 4-19. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Great Lakes Subregion: 2008. ............................................................ 104

Figure 4-20. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Southeast Subregion: 2008. ................................................................ 105

Figure 4-21. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for 

Selected CASTNet Sites in the Northeast Subregion: 2008. ................................................................ 107

Figure 4-22. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for the 

Lassen NP CASTNet Site for 11-29 April 2006. ................................................................................... 128

Figure 4-23. Vertical Profiles of Simulated and Observed Ozone Concentrations (ppb) for Trinidad Head for 21 

April 2006. Observed Profile is Based on Ozonesonde Data from the INTEX-B Program. ................... 129

Figure 4-24. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for the 

Rocky Mountain NP CASTNet Site for 8-20 July 2006. ........................................................................ 130

Figure 4-25. Vertical Profiles of Simulated and Observed Ozone Concentrations (ppb) for Boulder for 14 July 

2006. Observed Profile is Based on Ozonesonde Data from the INTEX-B Program. ........................... 131

Figure 4-26. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for the 

Lassen Volcanic NP and Pinnacles NM CASTNet Sites for 12-25 August 2006. ................................. 132

Figure 4-27. Vertical Profiles of Simulated and Observed Ozone Concentrations (ppb) for Trinidad Head for 17-

20 August 2006. Observed Profile is Based on Ozonesonde Data from the INTEX-B Program. .......... 133

Figure 5-1. Mean MDA8 Ozone Concentrations (ppb) for the Base (Current Atmosphere) and North American 

Background Scenarios, Based on the 2006, 2007, and 2008 Simulation Period. ................................. 137

Figure 5-2. Mean MDA8 Ozone Concentrations (ppb) for the North American Background Scenario, Based on 

the 2006, 2007, and 2008 Simulation Period. ....................................................................................... 138

Figure 5-3. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Lassen Volcanic NP, CA. ............................................................................ 147

Figure 5-4. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Pinedale, WY. ............................................................................................. 148

Figure 5-5. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Big Bend NP, TX. ....................................................................................... 149

Figure 5-6. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Perkinstown, WI. ......................................................................................... 150

Figure 5-7. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Great Smoky Mountains NP, TN. ............................................................... 151

Figure 5-8. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Blackwater NWR, MD. ................................................................................ 152

Figure 6-1. Mean MDA8 Ozone Concentrations (ppb) for the Base (Current Atmosphere), NAB, USB and NB 

Scenarios, Based on the 2006, 2007, and 2008 Simulation Period. ..................................................... 155

Figure 6-2. Mean MDA8 Ozone Concentrations (ppb) for the U.S. Background Scenario, Based on the 2006, 

2007, and 2008 Simulation Period. ....................................................................................................... 157

Figure 6-3. Mean MDA8 Ozone Concentrations (ppb) for the Natural Background Scenario, Based on the 2006, 

2007, and 2008 Simulation Period. ....................................................................................................... 158

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Figure 6-4. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Lassen Volcanic NP, CA. ............................................................................ 167

Figure 6-5. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Pinedale, WY. ............................................................................................. 168

Figure 6-6, Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Big Bend NP, TX. ....................................................................................... 169

Figure 6-7. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Perkinstown, WI. ......................................................................................... 170

Figure 6-8. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Great Smoky Mountains NP, TN. ............................................................... 171

Figure 6-9. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American 

Background Scenarios for Blackwater NWR, MD. ................................................................................ 172

Figure 6-10. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Intermountain West CASTNet 

sites. The Box-and-Whisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize 

NAB and NB for 10-ppb Bins of Observed Ozone Concentration: 2006-2008. ..................................... 176

Figure 6-11. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Great Lakes CASTNet sites. 

The Box-and-Whisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB 

and NB for 10-ppb Bins of Observed Ozone Concentration: 2006-2008. ............................................. 177

Figure 6-12. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Southeast CASTNet sites. The 

Box-and-Whisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB and 

NB for 10-ppb Bins of Observed Ozone Concentration: 2006-2008. .................................................... 178

Figure 6-13. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Northeast CASTNet sites. The 

Box-and-Whisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB and 

NB for 10-ppb Bins of Observed Ozone Concentration: 2006-2008. .................................................... 179

Figure 6-14. Frequency Distributions of MDA8 Ozone Concentrations (ppb) for March – August 2006, 2007 & 

2008 for Low-Elevation (<1.5 km) and High-Elevation (>1.5 km) CASTNet Sites. Observed Values 

are in Black; Model Results are in Red, NAB is in Blue, and NB is in Green (Dashed). ....................... 180

List of Tables 

Table 3-1. GEOS-Chem Reduced Vertical Grid Interface Levels Based on 47 Sigma-Pressure Levels. ....................... 6

Table 3-2. Background Methane Concentrations (ppb) in GEOS-Chem Simulations. ................................................... 9

Table 4-1. Definition and Description of Measures/Metrics for GEOS-Chem Model Performance Evaluation. ............ 12

Table 4-2. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone 

Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using CASTNet Data: 

2006. ....................................................................................................................................................... 14

Table 4-3. Seasonal and Annual GEOS-CHEM Model Performance Statistics for Maximum Daily 8-hour Average 

(MDA8) Ozone Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using 

CASTNet Data: 2006. ............................................................................................................................. 19

Table 4-4. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone 

Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using AQS Data: 2006. .......... 34

Table 4-5. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average NO2

Concentration (ppb) for the Base Case (Current Atmosphere Scenario): 2006. ..................................... 37

Table 4-6. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average SO2

Concentration (ppb) for the Base Case (Current Atmosphere Scenario): 2006. ..................................... 40

Table 4-7. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average CO 

Concentration (ppm) for the Base Case (Current Atmosphere Scenario): 2006. .................................... 43

Table 4-8. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average PM2.5

Concentration (μgm-3) for the Base Case (Current Atmosphere Scenario): 2006. .................................. 46

Table 4-9. Annual GEOS-CHEM Model Performance Statistics for Dry Deposition (kg/ha) for the Base Case 

(Current Atmosphere Scenario) for the Continental U.S.: 2006. ............................................................. 49

Table 4-10. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone 

Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using CASTNet Data: 2007. ..... 50

Table 4-11. Seasonal and Annual GEOS-CHEM Model Performance Statistics for Maximum Daily 8-hour 

Average (MDA8) Ozone Concentration (ppb) for the Base Case (Current Atmosphere Scenario) 

Using CASTNet Data: 2007. ................................................................................................................... 55

Table 4-12. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone 

Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using AQS Data: 2007. .......... 70

Table 4-13. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average NO2

Concentration (ppb) for the Base Case (Current Atmosphere Scenario): 2007. ..................................... 73

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Table 4-14. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average SO2

Concentration (ppb) for the Base Case (Current Atmosphere Scenario): 2007. ..................................... 77

Table 4-15. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average CO 

Concentration (ppm) for the Base Case (Current Atmosphere Scenario): 2007. .................................... 80

Table 4-16. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average PM2.5

Concentration (μgm-3) for the Base Case (Current Atmosphere Scenario): 2007. .................................. 84

Table 4-17. Annual GEOS-CHEM Model Performance Statistics for Dry Deposition (kg/ha) for the Base Case 

(Current Atmosphere Scenario) for the Continental U.S.: 2007. ............................................................. 87

Table 4-18. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone 

Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using CASTNet Data: 2008. ..... 89

Table 4-19. Seasonal and Annual GEOS-CHEM Model Performance Statistics for Maximum Daily 8-hour 

Average (MDA8) Ozone Concentration (ppb) for the Base Case (Current Atmosphere Scenario) 

Using CASTNet Data: 2008. ................................................................................................................... 94

Table 4-20. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone 

Concentration (ppb) for the Base Case (Current Atmosphere Scenario) Using AQS Data: 2008. ........ 109

Table 4-21. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average NO2

Concentration (ppb) for the Base Case (Current Atmosphere Scenario): 2008. ................................... 112

Table 4-22. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average SO2

Concentration (ppb) for the Base Case (Current Atmosphere Scenario): 2008. ................................... 116

Table 4-23. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average CO 

Concentration (ppm) for the Base Case (Current Atmosphere Scenario): 2008. .................................. 119

Table 4-24. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average PM2.5

Concentration (μgm-3) for the Base Case (Current Atmosphere Scenario): 2008. ................................ 123

Table 4-25. Annual GEOS-CHEM Model Performance Statistics for Dry Deposition (kg/ha) for the Base Case 

(Current Atmosphere Scenario) for the Continental U.S.: 2008. ........................................................... 126

Table 5-1. GEOS-Chem-Derived Seasonal and Annual Mean 24-Hour Average Observed, Current Atmosphere, 

and NAB Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. .......... 139

Table 5-2. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, and NAB 

Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. ......................... 143

Table 5-3. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, and NAB 

Ozone Concentrations (ppb) for Selected CASTNet Sites, Based on the 2006, 2007 and 2008 

Simulation Period.................................................................................................................................. 153

Table 6-1. GEOS-Chem-Derived Seasonal and Annual Mean 24-Hour Average Observed, Current Atmosphere, 

NAB, USB, and NB Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation 

Period. .................................................................................................................................................. 159

Table 6-2. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, NAB, USB, 

and NB Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. ............ 163

Table 6-3. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, NAB, USB, 

and NB Ozone Concentrations (ppb) for the Spring, Summer, and Annual Periods for Selected 

CASTNet Sites, Based on the 2006, 2007 and 2008 Simulation Period. .............................................. 173
1. Introduction 

Sections 108 and 109 of the Clean Air Act require the U.S. Environmental Protection Agency 

(EPA) to conduct periodic reviews of the National Ambient Air Quality Standards (NAAQS) for 

criteria pollutants. These periodic reviews are part of an Integrated Science Assessment (ISA). 

Currently, the National Center for Environmental Assessment (NCEA), which is part of EPA’s 

Office of Research and Development (ORD), is reviewing the NAAQS for ozone (O3) and other 

oxidants. This document summarizes modeling work conducted by ICF International, in 

collaboration with Harvard University, to support this assessment. 

An important aspect of the O3 ISA is the estimation of policy relevant background (PRB) ozone 

concentrations. The concentrations are defined as those that would occur in the absence of all 

anthropogenic sources in continental North America, hereafter referred to as North American 

background (or NAB) concentrations. For this assessment, NAB O3 was estimated using the 

GEOS-Chem model and includes contributions from 1) all natural sources (throughout the 

world) and 2) all anthropogenic sources outside of North America. 

GEOS-Chem is a global chemical transport model (CTM) that simulates reactions of 

photochemical precursor emissions of oxides of nitrogen (NOx), volatile organic compounds 

(VOCs), and carbon monoxide (CO). The GEOS-Chem model is driven by assimilated 

meteorological observations from the Goddard Earth Observing System (GEOS) and is able to 

represent long-range transport (for example, from outside CNA) as well as stratospherictropospheric exchange processes. For this assessment, GEOS-Chem was applied to estimate 

NAB O3 and its variability as well as the relative contribution of natural versus anthropogenic 

emissions to NAB O3. 

This study represents the latest in a series of assessments of PRB ozone concentrations. 

Previous studies have used measurements and transport models to estimate PRB ozone 

concentrations. EPA (2006) noted that the use of models has the advantage of being able to 

explicitly account for long-range transport and to apportion the contributions to PRB ozone 

between natural and anthropogenic sources. 

The most recent PRB O3 assessment was completed in 2008. For this assessment, estimates of 

NAB ozone concentrations were based on output from the GEOS-Chem model using a 2001 

simulation period with horizontal resolution of 2°×2.5° (degrees of longitude and latitude) (Fiore 

et al., 2003). NAB O3 concentrations for the eastern U.S. were estimated to be generally within 

the range of 15 to 35 parts per billion (ppb) during the summer months and higher in the 

springtime due to enhanced contributions from pollution sources outside North America and 

stratospheric ozone intrusion. 

Following this, Wang et al. (2009) also modeled NAB O3 concentrations using GEOS-Chem. This 

later modeling was done for 2001 at higher spatial resolution (1°×1°) using the GEOS-3 

meteorological data. The resulting NAB O3 concentrations are estimated to be generally within the 

range of 18 to 35 ppb for summer, and are consistent with those reported by Fiore et al. (2003). 

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Introduction 

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For the current analysis, GEOS-Chem was applied using nested grids with 2°×2.5° horizontal 

resolution at the global scale and 0.5°×0.667° horizontal resolution over North America using 

the most recent version of the GEOS-5 meteorological data. The model was applied for the 

years 2006, 2007 and 2008. Zhang et al. (2011) recently completed a similar study for North 

America, using the same grid configuration but a different version of the GEOS-Chem model. 

This report summarizes the GEOS-Chem application and results and includes a description of 

the GEOS-Chem model, an overview of the model application and evaluation methods, an 

assessment of model performance for the base case simulations, modeling results for NAB O3

and other simulations, and a discussion of the attributes and limitations of the modeling 

methods and results. 
2. Description of GEOS-Chem 

The GEOS–Chem model is a three-dimensional global-scale chemical transport model (CTM). It 

was developed and continues to be refined by researchers at Harvard University, as well as 

numerous other institutions. The GEOS-Chem model simulates the emission, chemical 

transformation, transport, and deposition of air pollutants throughout and above the 

troposphere. The model incorporates detailed emission inventories of anthropogenic and 

biogenic emissions that account for fossil fuel combustion and usage, biomass burning, biofuel 

burning, biogenic, and aerosol emissions. 

The GEOS-Chem chemical mechanism is designed to simulate tropospheric chemistry and 

includes reactions that represent the photochemical production of ozone as well as a detailed 

representation of aerosol formation. The chemical mechanism is described in some detail by 

Evans et al. (2003) and Mao et al. (2010). A list of species included in the GEOS-Chem model 

is provided in the on-line user’s guide (Yantosca and Carouge, 2011). GEOS-Chem includes a 

state-of-the-science photolysis routine (FAST–J) and the SMVGEAR II chemistry solver 

package. The ozone in the stratosphere is represented by a linearized ozone (Linoz) 

parameterization (McLinden et al., 2000). 

Advection and transport within the model are driven by assimilated meteorological observations 

from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling Assimilation 

Office. GEOS–Chem includes the state-of-the-art transport (TPCORE) routine. 

As summarized by Bey et al. (2001), dry deposition is treated with an enhanced resistance in 

series method (Wesley, 1989; Wang et al., 1998). Deposition velocities are calculated based on 

local surface types and meteorological conditions. Wet deposition is based on the algorithm 

developed by Liu et al. (2001). The algorithm accounts for scavenging by convective clouds and 

large-scale precipitation. 

Version 9-01-01 of the GEOS-Chem model was applied for this study. Compared to the prior 

version of the model, version 9-01-01 includes several updates. Two of the updates that are 

likely to affect the simulated ozone concentrations are 1) a correction for the yield of isoprene 

nitrates that increases the lifetime of NOx and may result in greater ozone production, and 2) a 

correction for lightning NOx emissions (which may also increase ozone). A full list of updates is 

provided in the release notes for this version of the model 

(http://acmg.seas.harvard.edu/geos/doc/man/appendix_9.html). 

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Description of GEOS-Chem 

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3. GEOS-Chem Model Application Methodology 

3.1. Modeling Domain 

For the current analysis, Version 9-01-01 of GEOS-Chem was applied using nested grids with 

2°×2.5° horizontal resolution at the global scale and 0.5°×0.667° horizontal resolution over 

North America (140°-40°W, 10°-70°N). In addition, a coarser resolution grid (4°×5°) is also 

being used for the start-up simulation period—an annual simulation period run to spin up the 

model and to ensure a reasonable representation of long-range (global) transport. The global 

domain is illustrated by Bey et al. (2001). The nested-grid modeling grid domain for North 

America is illustrated in Figure 3-1 and the grid spacing at different grid resolution is shown. 

Figure 3-1. GEOS-Chem Nested-Grid Modeling Domain for North America. 

(a) 4°×5° Global-Scale Resolution (Spin-Up Period Only). 

(b) 2°×2.5° Global-Scale Resolution. 

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GEOS-Chem Model Application Methodology 

(c) 0.5°×0.667° Continental-Scale Resolution. 

Source (Zhang, 2011, pers. comm.) 

The modeling domain includes 47 vertical layers that increase in thickness with height above 

ground. The top of the modeling domain is at approximately 80 kilometers (km) with layers 

defined as shown in Table 3-1. 

Table 3-1. GEOS-Chem Reduced Vertical Grid Interface Levels Based on 47 Sigma-Pressure Levels. 

Level Altitude (km) Pressure (hPa) 

47 80.581 0.01 

46 68.392 0.066 

45 59.924 0.211 

44 51.788 0.617 

43 44.286 1.651 

42 37.574 4.077 

41 31.716 9.293 

40 26.596 19.792 

39 24.24 28.368 

38 22.004 40.175 

37 19.855 56.388 

36 17.773 78.512 

35 16.753 92.366 

34 15.731 108.663 

33 14.706 127.837 

32 13.674 150.393 

31 12.633 176.93 

30 11.578 208.244 

29 10.504 245.246 

28 9.409 288.927 

27 8.32 339.005 

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GEOS-Chem Model Application Methodology 

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Level Altitude (km) Pressure (hPa) 

26 7.582 377.07 

25 6.905 415.155 

24 6.277 453.269 

23 5.692 491.401 

22 5.142 529.55 

21 4.623 567.706 

20 4.132 605.88 

19 3.665 644.054 

18 3.219 682.239 

17 2.932 707.699 

16 2.654 733.16 

15 2.382 758.621 

14 2.118 784.088 

13 1.86 809.556 

12 1.659 829.929 

11 1.51 845.211 

10 1.363 860.493 

9 1.218 875.776 

8 1.075 891.059 

7 0.934 906.342 

6 0.795 921.626 

5 0.657 936.911 

4 0.521 952.195 

3 0.387 967.48 

2 0.254 982.765 

1 0.123 998.051 

0 -0.006 1013.25 

3.2. Simulation Periods 

The GEOS-Chem model was applied for the years 2006, 2007 and 2008. In addition, the 

modeling was run for a one-year spin-up period (2005). 

3.3. Input Acquisition and Preparation 

Meteorological Inputs 

As noted earlier, the GEOS-Chem model is driven by assimilated meteorological observations 

from the Goddard Earth Observing System (GEOS) and is able to represent long-range 

transport as well as stratospheric-tropospheric exchange processes. For this analysis, 

meteorological inputs for the four simulation years were provided by a researcher from the 

Harvard University Atmospheric Chemistry Modeling (ACM) Group (Zhang, 2011, pers. comm.). 

The version of the model used for this study utilizes the GEOS-5 data product from NASA’s 
Modeling for North American Background Concentrations 

GEOS-Chem Model Application Methodology 

Global Modeling and Assimilation Office (GMAO). Data used by GEOS-Chem include surface 

albedo, parameters defining properties at the surface including moisture content and land type, 

various precipitation measures, cloud fraction, heat and radiation fluxes, PBL thickness, air 

temperature, tropopause pressure, ground (skin) temperature, U and V wind components, 

friction velocity, specific humidity and others. More information can be found at 

http://wiki.seas.harvard.edu/geos-chem/index.php/Overview_of_GMAO_met_data_products. 

Emission Inputs 

Baseline emission inputs were also provided by Zhang (2011, pers. comm.). The emission 

inventories include both anthropogenic and biogenic emissions, and account for fossil fuel 

combustion and usage, biomass burning, biofuel burning, and natural aerosol emissions. 

Examples of categories of emissions included are aircraft emissions, shipping emissions, and 

soil and fertilizer NOx emissions. The emissions also include estimates for NOx generated by 

lightning. Various sources of data provide global coverage with the more reliable and highly 

resolved emissions data sources taking precedence. For example, the 2005 EPA National 

Emission Inventory (NEI) inventory is used in order to enhance the emissions estimates over 

the United States. Temporal resolution for emissions varies depending on data source from 

annual to seasonal. Within the model, emissions are introduced at hourly intervals. More 

information on the emissions can be found at http://wiki.seas.harvard.edu/geoschem/index.php/Anthropogenic_emissions. 

Anthropogenic emissions were projected to each simulated year using scale factors. Scale 

factors were developed for each region of the world based on available information. More 

detailed information and links to additional documents can be found at 

http://wiki.seas.harvard.edu/geos-chem/index.php/Scale_factors_for_anthropogenic_emissions. 

Terrain, Land-Use, Chemistry and Other Inputs 

All other inputs for the application of GEOS-Chem were provided by Zhang (2011, pers. 

comm.). These include Total Ozone Mapping Spectrometer (TOMS) data, surface UV albedo, 

dry deposition coefficients, and land use codes. Note that roughness lengths and terrain heights 

are included in the GEOS-5 data. Other files define the chemical mechanism and provide data 

for calculating photolysis rates. 

The leaf area index used for ozone dry deposition was based on data from the Moderate 

Resolution Imaging Spectroradiometer (MODIS). Use of MODIS data is expected to result in 

less ozone dry deposition and higher ozone concentrations compared to the use of Advanced 

Very High Resolution Radiometer (AVHRR) derived values. 

ICF International 8 October 28, 2011 11-035
Modeling for North American Background Concentrations 

GEOS-Chem Model Application Methodology 

3.4. Model Options and Parameter Specifications 

For this application, version 9-01-01 of the GEOS-Chem model was applied. Some of the key 

options are listed below: 

 Linoz stratospheric ozone chemistry was utilized. 

 Sulfate, carbon, dust, and sea salt aerosols were included. 

 Both wet and dry depositions were simulated. 

 Tropospheric chemistry module was enabled. 

 Secondary organic aerosols were not included. 

3.5. Scenarios 

This analysis considered the following ozone air quality/emission scenarios for 2006, 2007, and 

2008: 

1. Current atmosphere—all anthropogenic and natural emissions for sources throughout the 

modeling domain. 

2. North American background—current atmosphere emissions minus anthropogenic 

emissions for sources in the U.S., Canada, and Mexico. 

3. U.S. background—current atmosphere emissions minus anthropogenic emissions for 

sources in the U.S. 

4. Natural background—all natural emissions for sources throughout the modeling domain. 

Background methane concentrations vary stepwise by latitude in GEOS-Chem. For the base 

case simulations, GEOS-Chem has values built-in for the years 2006 and 2007. For 2008 and 

for each of the other 3 scenarios, new background methane concentrations were supplied by 

EPA (Dutton, 2011, pers. comm.). The specific values used for background methane 

concentration are presented in Table 3-2. NAB O3 corresponds to the North American 

background scenario. 

Table 3-2. Background Methane Concentrations (ppb) in GEOS-Chem Simulations. 

(a) 2006 

Latitude Current 

Atmosphere 

U.S. 

 Background 

North American 

Background 

Natural 

Background 

90 S – 30 S 1726.53 1623.49 1582.60 700 

30 S – 0 S 1735.17 1631.62 1590.51 700 

0 N – 30 N 1796.30 1689.10 1646.55 700 

30 N – 90 N 1848.20 1737.90 1694.12 700 

Global Average 1776.55 1670.53 1628.45 700 

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GEOS-Chem Model Application Methodology 

ICF International 10 October 28, 2011 11-035

(b) 2007 

Latitude Current 

Atmosphere 

U.S. 

 Background 

North American 

Background 

Natural 

Background 

90 S – 30 S 1732.52 1629.13 1588.09 700 

30 S – 0 S 1741.68 1637.74 1596.48 700 

0 N – 30 N 1801.38 1693.88 1651.21 700 

30 N – 90 N 1855.55 1744.81 1700.86 700 

Global Average 1782.78 1676.39 1634.16 700 

(c) 2008 

Latitude Current 

Atmosphere 

U.S. 

 Background 

North American 

Background 

Natural 

Background 

90 S – 30 S 1740.56 1636.68 1595.45 700 

30 S – 0 S 1747.71 1643.41 1602.01 700 

0 N – 30 N 1804.09 1696.42 1653.69 700 

30 N – 90 N 1857.41 1746.57 1702.57 700 

Global Average 1787.44 1680.77 1638.43 700 
4. Model Performance Evaluation Procedures & Results 

The model performance evaluation of the “Current Atmosphere” scenario for the three 

simulation years is presented in this section of the report. This evaluation focuses primarily on 

the ability of the GEOS-Chem model to replicate observed ozone and other pollutant 

concentrations. The evaluation also includes a qualitative assessment of how well the 

stratospheric-tropospheric exchange is being simulated and an examination of the effects of 

interannual variability in meteorology on the simulation results. 

4.1. Comparison with Observed Air Quality Data 

There are several monitoring networks with available data for the U.S. corresponding to the time 

period and species of interest. The networks generally focus on one or more specific pollutants, 

either air concentrations or deposition, and specific geographic/demographic regions (i.e., 

urban, rural, protected environments, etc.). In addition, they may utilize different monitoring 

techniques, measurement periods, and quality assurance/reporting procedures. 

For this analysis, we rely primarily on data from the Clean Air Status and Trends Network 

(CASTNet) and the EPA’s Air Quality System (AQS) databases. The CASTNet database 

includes air concentration data for ozone, nitrogen dioxide (NO2), and sulfur dioxide (SO2) as 

well as deposition data (fluxes) for nitrogen and sulfur. The network consists of more than 120 

sites in the U.S. and is primarily intended to monitor air quality in rural areas and areas where 

urban influences are minimal. For this study, these data are used primarily to examine how well 

the GEOS-Chem model is able to represent non-urban ozone and other pollutant concentrations 

for selected regions and sites throughout the U.S. The CASTNet data are also used to examine 

the reasonableness of the modeled dry deposition amounts for sulfur and nitrogen. 

The AQS database includes air concentration data for ozone, NO2, SO2, carbon monoxide (CO), 

and particulate matter (PM10 and PM2.5). The network is primarily intended to monitor air quality 

in areas where people live, and, thus, a large proportion of the sites are located in urban and 

suburban areas. For this study, these data are used primarily to examine how well the GEOSChem model is able to represent the range of ozone and other pollutant concentrations that 

occur throughout the U.S. during the three-year simulation period, as well as the spatial 

variability of the ozone concentrations. 

Statistical and graphical analyses are used to compare the simulated and observed 

concentrations. Comparisons are presented for the entire U.S., selected subregions, and 

individual sites representing key areas in the U.S. that have been identified as important in 

characterizing background ozone (e.g., Zhang and Jacob, 2011). The subregions include the 

West Coast (WA, OR, CA), the Intermountain West (ID, MT, WY, NV, UT, CO, AZ, NM), the 

Central States (ND, SD, MN, NE, IA, KS, MO, OK, AR, TX, LA), the Great Lakes region (WI, IL, 

IN, OH, MI), the Southeast (KY, WV, VA, TN, NC, SC, MS, AL, GA, FL) and the Northeast (ME, 

NH, VT, MA, RI, CT, NY, PA, NJ, DE, MD, DC). A variety of graphical and statistical measures 

and a range of averaging times (daily, monthly, seasonal, etc.) are considered. 

The statistical measures are listed and defined in Table 4-1. 

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Table 4-1. Definition and Description of Measures/Metrics for GEOS-Chem Model Performance Evaluation. 

Metric Definition 

# of data pairs The number of observation/simulation data pairs 

Mean observation value The average observed concentration 

Mean simulation value The average simulated concentration 

Mean residual 

⎟∑ − ⎞ ⎜

⎛ N

OS )( 1

⎠⎝ l=

ll N 1

 

where N is the number of data pairs, and Sl and Ol are the simulated and observed values 

at site l, respectively, over a given time interval. 

Mean unsigned error ∑ ⎝ ⎠l=

ll N 1

⎟ − ⎞ ⎜

⎛ N

OS 1

Normalized bias %100)( 1

⎠⎝ l=1

lll N ⎟ ⋅− ⎞ ⎜

⎛ ∑

N

OOS 

Normalized gross error %100 1

⎠⎝ l=1

lll N ⎟ ⋅− ⎞ ⎜

⎛ ∑

N

OOS

Fractional bias %100)(5.0)( 1

⎠⎝ l=1

ll ll N ⎟ − ⋅+ ⎞ ⎜

⎛ ∑

N

OSOS

Fractional error %100)(5.0 1 ⎟ − ⋅+ ⎞ ⎜

⎛ ∑

N

OSOS ⎠⎝ l=1

ll ll N

Coefficient of Determination (R2) R2 is the square of the correlation (R), which is defined as the sample covariance between 

two datasets divided by the product of the standard deviations for each dataset 

In calculating the statistical measures, the GEOS-Chem model output was paired with the 

observed data for the appropriate locations (grid cells) and time intervals. 

Summary Model Performance Results for 2006 

In this section, the GEOS-Chem results for 2006 are compared to observed data for ozone, 

other gaseous species, fine particulate matter, and dry deposition. 

Ozone 

Two metrics were used for the calculation of statistical measures for ozone: 24-hour average 

ozone and maximum daily 8-hour average (MDA8) ozone. MDA8 is also used for the National 

Ambient Air Quality Standard (NAAQS) for 8-hour ozone. 

Among the datasets available for this study, the CASTNet dataset is best suited to the 

evaluation of model performance for ozone since the network is designed to monitor rural or 

background ozone concentrations that are not driven by local urban-scale sources. The 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 13 October 28, 2011 11-035

CASTNet data are expected to be reasonably consistent with the spatial and temporal scales 

simulated by the GEOS-Chem model. 

Table 4-2 lists the seasonal and annual performance metrics for 24-hour average ozone for the 

entire U.S. and each of the six model performance subregions for 2006, calculated using the 

CASTNet data. Units are ppb. 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-2. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone Concentration (ppb) for 

(Current Atmosphere Scenario) Using CASTNet Data: 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 6,900 29.34 30.78 1.45 6.25 13.20 29.60 4.60 Spring (Mar-May) 7,217 41.93 45.35 3.41 7.82 13.80 22.70 9.20 Summer (Jun-Aug) 6,982 41.05 51.98 10.93 13.05 36.80 40.60 25.80 Fall (Sep-Nov) 7,000 29.62 39.28 9.66 11.31 48.70 53.40 30.10 Annual 28,099 35.55 41.91 6.35 9.60 28.00 36.50 17.40 

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 (b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 788 32.62 36.92 4.30 9.09 32.70 44.10 17.80 Spring (Mar-May) 814 42.37 44.31 1.94 9.31 14.80 29.00 8.00 Summer (Jun-Aug) 800 48.89 53.19 4.30 10.11 17.20 27.50 10.90 Fall (Sep-Nov) 781 36.54 43.31 6.77 10.25 46.90 54.10 24.10 Annual 3,183 40.16 44.47 4.30 9.69 27.70 38.50 15.10 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,117 40.45 39.86 -0.59 5.30 6.00 19.50 0.40 Spring (Mar-May) 1,186 50.06 47.53 -2.53 6.25 -1.80 14.50 -4.00 Summer (Jun-Aug) 1,121 48.94 52.59 3.65 7.22 11.70 18.00 8.70 Fall (Sep-Nov) 1,166 38.85 42.89 4.04 6.43 17.70 23.20 11.70 Annual 4,590 44.60 45.72 1.12 6.30 8.40 18.80 4.10 

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(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 733 30.27 31.16 0.89 6.17 10.00 26.70 2.90 Spring (Mar-May) 728 40.30 42.61 2.31 7.23 13.70 23.90 8.00 Summer (Jun-Aug) 738 40.61 48.92 8.32 11.11 34.10 39.50 21.90 Fall (Sep-Nov) 783 30.38 36.51 6.13 8.55 31.10 38.10 19.90 Annual 2,982 35.31 39.76 4.45 8.28 22.40 32.20 13.30 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,116 22.15 21.10 -1.05 4.81 -2.50 26.20 -9.30 Spring (Mar-May) 1,144 37.08 42.38 5.30 7.30 17.40 22.50 13.20 Summer (Jun-Aug) 1,083 37.61 54.70 17.08 17.36 49.40 50.00 36.50 Fall (Sep-Nov) 1,095 22.78 31.83 9.06 10.56 44.40 50.90 29.30 Annual 4,438 29.93 37.43 7.51 9.93 26.90 37.20 17.20 

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(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,864 28.34 31.85 3.51 6.96 21.50 31.80 13.00 Spring (Mar-May) 1,996 42.12 48.12 6.00 8.77 19.80 25.20 14.50 Summer (Jun-Aug) 1,976 39.10 52.45 13.35 14.98 47.30 50.10 32.20 Fall (Sep-Nov) 1,871 29.13 42.93 13.80 14.46 63.60 65.50 40.10 Annual 7,707 34.86 44.04 9.18 11.31 37.90 43.00 24.90 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,282 24.81 25.74 0.94 5.60 10.90 31.00 1.10 Spring (Mar-May) 1,349 39.25 43.93 4.68 7.67 14.90 22.00 10.80 Summer (Jun-Aug) 1,264 35.35 49.41 14.06 14.47 45.70 46.60 33.50 Fall (Sep-Nov) 1,304 23.23 36.33 13.10 14.06 70.40 74.10 42.60 Annual 5,199 30.72 38.87 8.15 10.41 35.30 43.30 21.90 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Based on the 24-hour average ozone metric, ozone is overestimated for most regions and time 

periods. The greatest amount of overestimation tends to occur during the summer months 

(June–August) for the eastern portion of the U.S. and during the autumn months (September–

November) for the western portion of the U.S. Considering all periods and regions, the best 

agreement with the observations tends to occur during the spring months (March–May). Among 

the subregions, the best performance for 24-hour ozone is achieved for the Intermountain West 

subregion for which the normalized bias ranges from approximately -2 to 18 percent. 

Table 4-3 lists the seasonal and annual performance metrics for maximum daily 8-hour average 

(MDA8) ozone for the entire U.S. and each of the six model performance subregions for 2006, 

calculated using the CASTNet data. Units are ppb. 

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Model Performance Evaluation Procedures & Results 

Table 4-3. Seasonal and Annual GEOS-CHEM Model Performance Statistics for Maximum Daily 8-hour Average (MDA8) Ozone Concentrati

(Current Atmosphere Scenario) Using CASTNet Data: 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 6,920 35.28 37.87 2.59 5.65 12.10 20.80 7.50 Spring (Mar-May) 7,282 51.00 54.97 3.97 7.36 10.40 16.40 7.90 Summer (Jun-Aug) 7,058 53.36 64.66 11.30 13.63 26.50 30.10 19.80 Fall (Sep-Nov) 7,084 38.25 48.84 10.60 11.74 35.30 37.90 25.30 Annual 28,344 44.56 51.68 7.11 9.60 21.10 26.20 15.10 

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(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 796 38.35 43.27 4.93 6.98 21.70 26.20 14.70 Spring (Mar-May) 821 51.87 53.40 1.53 7.39 6.40 16.20 4.00 Summer (Jun-Aug) 812 62.91 66.61 3.71 11.51 8.90 20.10 5.60 Fall (Sep-Nov) 790 45.98 52.57 6.58 9.86 24.80 30.10 16.80 Annual 3,219 49.86 54.02 4.16 8.93 15.30 23.10 10.20 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,121 44.19 44.57 0.38 4.30 3.70 12.00 1.80 Spring (Mar-May) 1,192 56.15 54.62 -1.52 5.25 -1.50 9.70 -2.40 Summer (Jun-Aug) 1,131 57.15 60.77 3.61 7.04 7.60 13.00 6.20 Fall (Sep-Nov) 1,174 44.46 49.57 5.10 6.39 13.70 16.40 11.40 Annual 4,618 50.52 52.40 1.88 5.75 5.90 12.80 4.20 

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(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 730 36.24 37.55 1.31 5.18 6.70 17.30 3.40 Spring (Mar-May) 734 49.25 50.87 1.62 6.54 6.90 15.80 4.40 Summer (Jun-Aug) 743 51.88 60.07 8.19 11.14 23.30 28.20 16.20 Fall (Sep-Nov) 794 39.20 44.64 5.44 7.38 19.00 23.60 13.80 Annual 3,001 44.08 48.26 4.18 7.57 14.10 21.30 9.60 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,115 28.08 29.28 1.21 4.98 7.20 21.70 2.60 Spring (Mar-May) 1,159 46.99 54.01 7.02 8.32 17.10 19.70 13.70 Summer (Jun-Aug) 1,091 52.12 70.37 18.25 18.72 38.60 39.50 29.30 Fall (Sep-Nov) 1,110 31.97 43.48 11.50 12.36 39.90 42.80 29.00 Annual 4,475 39.80 49.22 9.42 11.03 25.50 30.80 18.60 

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(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,873 35.88 39.92 4.04 6.30 16.80 22.30 11.80 Spring (Mar-May) 2,017 52.63 58.39 5.76 7.65 13.90 17.10 11.10 Summer (Jun-Aug) 2,002 51.70 64.56 12.85 14.23 31.60 33.60 23.80 Fall (Sep-Nov) 1,890 39.23 52.93 13.70 14.01 42.00 42.70 31.00 Annual 7,782 45.11 54.21 9.10 10.56 26.00 28.80 19.40 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,285 30.43 33.30 2.87 5.88 14.20 24.20 8.30 Spring (Mar-May) 1,359 47.92 54.19 6.27 8.35 14.50 18.60 11.40 Summer (Jun-Aug) 1,279 48.47 64.82 16.35 16.94 37.90 39.10 28.40 Fall (Sep-Nov) 1,326 31.41 47.16 15.76 16.43 57.00 59.00 38.60 Annual 5,249 39.60 49.89 10.29 11.88 30.90 35.20 21.70 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

For 2006, MDA8 ozone is overestimated for nearly all regions and time periods but the bias and 

error statistics suggest reasonably good performance. Model performance varies by season and 

by subregion. Overestimation of ozone during the summer and autumn months is prevalent in 

the eastern U.S. (Central States, Great Lakes, Southeast, and Northeast subregions). For the 

western subregions, there is less overestimation of ozone and overall good agreement with the 

observations. 

The best agreement with the observations is achieved for the Intermountain West subregion. 

The normalized bias for this subregion for the annual period is 5.9 percent, while the 

corresponding gross error is 12.8 percent. The fractional bias is 4.2 percent and the 

corresponding fractional error is 11.7 percent. 

For the entire U.S., the mean residual or difference between the simulated and observed values 

ranges from approximately 2.6 to 11.3 ppb for the four seasonal periods and is 7.1 for the 

annual period. The normalized bias for the annual period is 21.1 percent, while the 

corresponding gross error is 26.2 percent. The fractional bias is 15.1 percent and the 

corresponding fractional error is 20.9 percent. Figure 4-1 displays the agreement between the 

mean simulated and observed MDA8 concentrations for the seasonal and annual periods for the 

entire U.S. 

Figure 4-1. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Each Season 

and the Entire Year, Averaged Over All Sites in Entire Continental U.S.: 2006. 

Seasonal & Annual Mean Ozone Concentration (ppb)

25

35

45

55

65

75

25 35 45 55 65 75

Observed

GEOS-Chem

Spring Summer Fall Winter Annual

A subset of the CASTNet sites was selected for a more detailed comparison of the simulated 

and observed MDA8 ozone concentrations. For each subregion, four sites were selected based 

on data availability, location within the subregion, and elevation. The criteria used to select the 

sites included good data coverage for all three modeled years and representation of different 

portions of each subregion and a range of elevations, as appropriate. 

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ICF International 24 October 28, 2011 11-035

Time-series and scatter plots illustrating site-specific model performance for MDA8 ozone 

concentration for the West Coast subregion are provided in Figure 4-2. The West Coast sites 

(listed in order of elevation) are: Pinnacles National Monument (NM), CA (elevation 335 meters 

(m)) Mount Rainier National Park (NP), WA (elevation 415 m), Joshua Tree NM, CA (elevation 

1244 m), and Lassen Volcanic NP, CA (elevation 1756 m). In the time series plots, the observed 

values are in blue and the simulated values are in green. The range scale used for MDA8 ozone 

is 0 to 100 ppb, with a few exceptions. 

Figure 4-2. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the West Coast Subregion: 2006. 

(a) Pinnacles NM 

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: 2006

R2 = 0.5112

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Simulated

Observed

(b) Mount Rainier NP 

CASTNet Site Mount Rainier NP, WA (MOR409) MDA8: 2006

R2 = 0.5498

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

CASTNet Site Mount Rainier NP, WA (MOR409) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

 (c) Joshua Tree NM 

CASTNet Site Joshua Tree NM, CA (JOT403) MDA8: 2006

0

20

40

60

80

100

120

140

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Joshua Tree NM, CA (JOT403) MDA8: 2006

R2 = 0.7288

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Observed

Simulated

(d) Lassen Volcanic NP 

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2007

R2 = 0.4956

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110

Observed

Simulated

Time-series and scatter plots for the Intermountain West subregion are provided in Figure 4-3. 

The Intermountain West sites are: Chiricahua NM, AZ (elevation 1570 m), Great Basin NP, NV 

(elevation 2060 m), Pinedale, WY (elevation 2388 m), and Rocky Mountain NP, CO (elevation 

2743 m). 

ICF International 25 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Figure 4-3. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Intermountain West Subregion: 2006. 

(a) Chiricahua NM 

CASTNet Site Chiricahua NM, AZ (CHA467) MDA8: 2006

R2 = 0.4144

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

CASTNet Site Chiricahua NM, AZ (CHA467) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

(b) Great Basin NP 

CASTNet Site Great Basin NP, NV (GRB411) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Great Basin NP, NV (GRB411) MDA8: 2006

R2 = 0.5271

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Pinedale 

CASTNet Site Pinedale, WY (PND165) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Pinedale, WY (PND165) MDA8: 2006

R2 = 0.5716

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 26 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Rocky Mountain NP 

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: 2006

R2 = 0.4177

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Central States subregion are provided in Figure 4-4. The 

Central States sites are: Caddo Valley, AR (elevation 71 m), Konza Prairie, KS (elevation 348 

m), Theodore Roosevelt NP, ND (elevation 850 m), and Big Bend NP, TX (elevation 1052 m). 

Figure 4-4. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in Central States Subregion: 2006. 

(a) Caddo Valley 

CASTNet Site Caddo Valley, AR (CAD150) MDA8: 2006

R2 = 0.5406

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

CASTNet Site Caddo Valley, AR (CAD150) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

ICF International 27 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) Konza Prairie 

CASTNet Site Konza Prairie, KS (KNZ184) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Konza Prairie, KS (KNZ184) MDA8: 2006

R2 = 0.7493

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Theodore Roosevelt NP 

CASTNet Site Theodore Roosevelt NP, ND (THR422) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Theodore Roosevelt NP, ND (THR422) MDA8: 2006

R2 = 0.6613

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Big Bend NP 

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2006

R2 = 0.3867

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 28 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Time-series and scatter plots for the Great Lakes subregion are provided in Figure 4-5. The 

Great Lakes sites are: Salamonie Reservoir, IN (elevation 250 m), Ann Arbor, MI (elevation 267 

m), Oxford, OH (elevation 284 m), and Perkinstown, WI (elevation 472 m). 

Figure 4-5. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Great Lakes Subregion: 2006. 

(a) Salamonie Reservoir 

CASTNet Site Salamonie Reservoir, IN (SAL133) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Salamonie Reservoir, IN (SAL133) MDA8: 2006

R2 = 0.6938

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(b) Ann Arbor 

CASTNet Site Unionville, MI (ANA115) MDA8: 2006

R2 = 0.7056

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

CASTNet Site Unionville, MI (ANA115) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

ICF International 29 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Oxford 

CASTNet Site Oxford, OH (OXF122) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Oxford, OH (OXF122) MDA8: 2006

R2 = 0.6417

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120

Observed

Simulated

(d) Perkinstown 

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2006

R2 = 0.6848

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Southeast subregion are provided in Figure 4-6. The 

Southeast sites are: Sumatra, FL (elevation 14 m), Cadiz, KY (elevation 189 m), Sand 

Mountain, AL (elevation 352 m), and Great Smoky Mountain NP, TN (elevation 793 m). 

ICF International 30 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Figure 4-6. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Southeast Subregion: 2006. 

(a) Sumatra 

CASTNet Site Sumatra, FL (SUM156) MDA8: 2006

R2 = 0.387

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

CASTNet Site Sumatra, FL (SUM156) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

(b) Cadiz 

CASTNet Site Cadiz, KY (CDZ171) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Cadiz, KY (CDZ171) MDA8: 2006

R2 = 0.5638

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Sand Mountain 

CASTNet Site Sand Mountain, AL (SND152) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Sand Mountain, AL (SND152) MDA8: 2006

R2 = 0.7116

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 31 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Great Smoky Mountain NP 

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2006

R2 = 0.6712

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Northeast subregion are provided in Figure 4-7. The 

Northeast sites are Blackwater National Wildlife Refuge (NWR), MD (elevation 4 m), Acadia NP, 

ME (elevation 158 m), Penn State, PA (elevation 378 m), and Huntington Wildlife Forest, NY 

(elevation 502 m). 

Figure 4-7. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Northeast Subregion: 2006. 

(a) Blackwater NWR 

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2006

R2 = 0.7252

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120

Observed

Simulated

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

ICF International 32 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) Acadia NP 

CASTNet Site Acadia NP, ME (ACA416) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Acadia NP, ME (ACA416) MDA8: 2006

R2 = 0.4467

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Observed

Simulated

(c) Penn State 

CASTNet Site Penn State, PA (PSU106) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Penn State, PA (PSU106) MDA8: 2006

R2 = 0.6058

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Huntington Wildlife Forest 

CASTNet Site Huntington Wildlife Forest, NY (HWF187) MDA8: 2006

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Huntington Wildlife Forest, NY (HWF187) MDA8: 2006

R2 = 0.4047

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 33 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

For the more urban sites included in the AQS network, only 24-hour average ozone was 

considered. Due to the relatively coarse grid resolution used for both the emissions and the 

modeling, some of the diurnal features of urban-scale ozone concentrations (including high 

daytime peaks and near zero concentrations at night) are not well simulated and model 

performance is worse than for the CASTNet sites. The 24-hour ozone concentrations are 

overestimated for all subregions and periods. 

Table 4-4 lists the seasonal and annual performance metrics for 24-hour average ozone for the 

entire U.S. and each of the six model performance subregions for 2006, calculated using the 

AQS data. Normalized bias and error statistics are not included in this table since they are not 

applicable for very low concentrations, such as those that sometimes characterize urban-scale 

ozone. Units are ppb. 

Table 4-4. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario) Using AQS Data: 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 50,917 22.11 31.12 9.02 10.62 38.30 45.40 0.258 

Spring 

(Mar-May) 89,599 35.34 44.46 9.12 10.85 24.20 28.50 0.215 

Summer 

(Jun-Aug) 106,135 35.68 51.49 15.81 16.83 37.60 40.10 0.251 

Fall 

(Sep-Nov) 82,734 25.19 40.84 15.65 16.18 49.90 51.50 0.271 

Annual 329,385 30.85 43.75 12.90 14.08 37.10 40.60 0.345 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 14,320 20.05 32.33 12.27 13.47 55.50 59.30 0.194 

Spring 

(Mar-May) 15,774 33.65 40.43 6.78 9.93 18.90 27.60 0.234 

Summer 

(Jun-Aug) 17,753 36.98 47.45 10.47 13.77 22.30 31.20 0.311 

Fall 

(Sep-Nov) 16,442 27.24 41.14 13.90 15.05 44.80 47.70 0.292 

Annual 64,289 29.90 40.74 10.84 13.09 34.60 40.80 0.334 

ICF International 34 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 8,813 24.85 35.96 11.11 12.62 44.70 48.90 0.143 

Spring 

(Mar-May) 10,809 39.23 45.44 6.21 9.23 16.80 23.50 0.162 

Summer 

(Jun-Aug) 12,560 42.57 52.74 10.17 11.59 22.40 25.30 0.154 

Fall 

(Sep-Nov) 10,876 27.98 42.64 14.66 15.25 45.30 46.80 0.112 

Annual 43,058 34.42 44.92 10.50 12.13 31.30 35.10 0.316 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 12,193 23.20 29.08 5.88 7.66 25.20 32.80 0.295 

Spring 

(Mar-May) 16,192 34.40 41.86 7.47 8.88 21.60 25.00 0.333 

Summer 

(Jun-Aug) 17,933 35.28 50.33 15.05 15.67 39.00 40.30 0.436 

Fall 

(Sep-Nov) 15,546 26.52 37.97 11.45 12.00 37.50 39.30 0.429 

Annual 61,864 30.47 40.82 10.35 11.39 31.30 34.60 0.469 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 2,991 17.03 19.22 2.20 4.70 10.90 32.20 0.697 

Spring 

(Mar-May) 12,164 33.28 44.26 10.98 11.81 29.60 31.70 0.207 

Summer 

(Jun-Aug) 17,151 34.37 55.03 20.66 20.78 46.40 46.60 0.187 

Fall 

(Sep-Nov) 9,998 20.59 36.75 16.16 16.39 55.80 57.10 0.366 

Annual 42,304 29.57 45.08 15.51 16.02 41.30 43.80 0.416 

ICF International 35 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 7,287 25.04 37.01 11.98 12.78 41.10 43.80 0.194 

Spring 

(Mar-May) 21,817 36.63 48.84 12.21 13.05 30.30 32.20 0.182 

Summer 

(Jun-Aug) 24,048 33.69 52.53 18.84 19.27 46.60 47.40 0.199 

Fall 

(Sep-Nov) 18,264 25.85 46.20 20.35 20.49 58.80 59.20 0.220 

Annual 71,416 31.70 48.20 16.50 17.02 44.20 45.40 0.238 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 5,313 19.58 23.06 3.48 6.59 21.70 39.70 0.521 

Spring 

(Mar-May) 12,843 35.27 44.71 9.44 11.12 24.20 28.50 0.183 

Summer 

(Jun-Aug) 16,690 33.90 51.09 17.19 17.68 41.60 42.70 0.378 

Fall 

(Sep-Nov) 11,608 20.95 37.77 16.82 17.25 58.00 60.10 0.251 

Annual 46,454 29.40 42.79 13.39 14.49 38.60 42.80 0.415 

NO2, SO2, CO, and PM2.5 

NO2, SO2, CO and PM2.5 data from the AQS network were also used to examine model 

performance for these species. NO2, SO2, CO concentrations at the urban AQS sites are 

typically characterized by diurnal profiles that reflect subgrid-scale traffic and activity patterns 

within the urban area. Concentrations can vary considerably in time and space. Due in part to 

the relatively coarse grid resolution used for both the emissions and the modeling, the spatial 

and temporal variability of these species are not captured. Nevertheless, simulated and 

observed 24-hour average concentrations for these species are examined and compared. PM2.5

is a secondary pollutant and is characterized by concentration patterns that are less variable in 

time and space. A key regulatory metric for PM2.5 is 24-hour average PM2.5, thus this metric is 

also used for the statistical calculations for PM2.5. 

ICF International 36 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-5 lists the seasonal and annual performance metrics for 24-hour average NO2 for the 

entire U.S. and each of the six model performance subregions for 2006, calculated using the 

AQS data. Normalized bias and error statistics are not included in this table since they are not 

applicable for very low concentrations, such as those found in the NO2, SO2, and CO data. Units 

are ppb. 

Table 4-5. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average NO2 Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario): 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 33,487 14.84 7.56 -7.28 8.29 -54.80 73.90 0.283 

Spring 

(Mar-May) 35,436 10.54 5.15 -5.39 6.19 -57.40 77.20 0.307 

Summer 

(Jun-Aug) 36,115 9.35 5.00 -4.35 5.41 -50.10 74.30 0.263 

Fall 

(Sep-Nov) 34,446 12.66 6.16 -6.50 7.35 -58.80 76.70 0.296 

Annual 139,484 11.79 5.94 -5.85 6.78 -55.20 75.50 0.312 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 8,877 17.38 6.99 -10.39 10.86 -80.90 90.60 0.317 

Spring 

(Mar-May) 9,037 10.72 5.21 -5.51 6.16 -69.80 82.00 0.340 

Summer 

(Jun-Aug) 9,004 11.07 5.43 -5.64 6.28 -67.40 78.70 0.416 

Fall 

(Sep-Nov) 8,762 16.25 6.13 -10.12 10.42 -86.80 92.70 0.354 

Annual 35,680 13.82 5.93 -7.89 8.41 -76.10 86.00 0.359 

ICF International 37 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 4,105 16.55 4.42 -12.14 12.38 -93.00 113.60 0.496 

Spring 

(Mar-May) 4,452 10.17 3.15 -7.03 7.30 -80.50 106.20 0.487 

Summer 

(Jun-Aug) 4,641 8.36 3.13 -5.23 5.54 -74.80 97.10 0.372 

Fall 

(Sep-Nov) 4,404 12.05 3.66 -8.39 8.66 -82.70 105.10 0.458 

Annual 17,602 11.65 3.57 -8.09 8.36 -82.50 105.30 0.469 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 8,078 10.50 6.68 -3.82 5.31 -29.50 59.70 0.256 

Spring 

(Mar-May) 8,368 7.59 4.80 -2.79 4.09 -30.40 61.60 0.203 

Summer 

(Jun-Aug) 8,385 6.88 5.79 -1.09 3.83 -11.90 59.50 0.118 

Fall 

(Sep-Nov) 8,326 9.29 6.13 -3.17 4.74 -28.20 59.30 0.242 

Annual 33,157 8.55 5.84 -2.71 4.48 -24.90 60.00 0.215 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 1,751 16.86 11.66 -5.20 6.17 -29.30 41.80 0.449 

Spring 

(Mar-May) 1,952 15.58 7.56 -8.02 8.34 -59.30 66.10 0.460 

Summer 

(Jun-Aug) 2,081 13.08 6.21 -6.87 7.27 -60.00 68.80 0.346 

Fall 

(Sep-Nov) 1,866 15.47 9.40 -6.07 6.55 -43.50 51.60 0.489 

Annual 7,650 15.17 8.58 -6.59 7.11 -48.80 57.70 0.408 

ICF International 38 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 4,631 11.47 7.25 -4.22 5.56 -33.50 58.80 0.320 

Spring 

(Mar-May) 4,967 9.36 4.62 -4.74 5.46 -53.40 72.80 0.212 

Summer 

(Jun-Aug) 4,996 7.75 4.01 -3.73 4.49 -42.40 71.40 0.144 

Fall 

(Sep-Nov) 4,847 10.01 5.41 -4.60 5.46 -49.00 69.00 0.276 

Annual 19,441 9.61 5.29 -4.32 5.23 -44.70 68.10 0.277 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 6,045 17.84 10.84 -6.99 8.41 -46.40 60.90 0.299 

Spring 

(Mar-May) 6,660 13.65 6.62 -7.02 8.01 -59.70 76.20 0.255 

Summer 

(Jun-Aug) 7,008 10.81 5.14 -5.66 6.19 -57.70 73.10 0.349 

Fall 

(Sep-Nov) 6,241 13.83 7.70 -6.13 7.29 -54.00 69.30 0.300 

Annual 25,954 13.90 7.47 -6.43 7.44 -54.70 70.10 0.333 

For 2006, the 24-hour average NO2 concentrations are underestimated for all subregions and 

time periods. Model performance is worst for the West Coast and Intermountain West 

subregions. Elsewhere, the fractional bias indicates that the modeled values are within a factor 

of two (67 percent) of the observed values. 

Table 4-6 lists the seasonal and annual performance metrics for 24-hour average SO2 for the 

entire U.S. and each of the six model performance subregions for 2006, calculated using the 

AQS data. Units are ppb. 

ICF International 39 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-6. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average SO2 Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario): 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 42,217 4.13 6.12 1.99 3.97 49.40 88.80 0.178 

Spring 

(Mar-May) 43,555 3.15 4.41 1.27 3.06 51.90 93.80 0.127 

Summer 

(Jun-Aug) 42,983 2.69 4.45 1.76 3.06 63.50 97.80 0.131 

Fall 

(Sep-Nov) 42,583 3.07 5.21 2.14 3.59 65.00 97.50 0.150 

Annual 171,338 3.25 5.04 1.79 3.41 57.50 94.50 0.157 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 3,142 1.46 1.60 0.14 1.06 33.70 86.20 0.152 

Spring 

(Mar-May) 3,302 1.07 1.28 0.21 0.99 60.30 105.10 0.066 

Summer 

(Jun-Aug) 3,306 1.36 1.54 0.18 0.99 51.20 92.00 0.144 

Fall 

(Sep-Nov) 3,270 1.40 1.62 0.22 1.00 40.20 87.50 0.208 

Annual 13,020 1.32 1.51 0.19 1.01 46.50 92.80 0.144 

ICF International 40 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 2,856 2.64 0.94 -1.70 2.17 -30.40 111.20 0.009 

Spring 

(Mar-May) 2,926 1.85 0.77 -1.07 1.60 -4.00 120.40 0.032 

Summer 

(Jun-Aug) 2,938 1.82 0.80 -1.02 1.52 -9.20 113.70 0.015 

Fall 

(Sep-Nov) 2,834 2.05 0.87 -1.18 1.63 -13.40 108.10 0.052 

Annual 11,554 2.09 0.84 -1.24 1.73 -14.10 113.40 0.026 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 8,766 2.24 3.23 0.98 2.67 72.40 106.30 0.033 

Spring 

(Mar-May) 9,075 2.04 2.52 0.48 2.22 70.20 110.60 0.032 

Summer 

(Jun-Aug) 8,931 1.66 2.93 1.27 2.36 87.40 115.40 0.053 

Fall 

(Sep-Nov) 8,813 1.98 3.06 1.07 2.58 82.10 113.50 0.058 

Annual 35,585 1.98 2.93 0.95 2.45 78.00 111.40 0.041 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 8,075 4.83 8.69 3.86 5.42 71.50 87.60 0.064 

Spring 

(Mar-May) 8,327 4.18 6.51 2.33 4.18 62.80 86.20 0.064 

Summer 

(Jun-Aug) 8,180 3.46 6.52 3.06 4.37 75.90 95.10 0.025 

Fall 

(Sep-Nov) 8,205 3.83 7.86 4.04 5.15 83.70 98.50 0.094 

Annual 32,787 4.07 7.39 3.31 4.77 73.50 91.80 0.067 

ICF International 41 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 9,629 4.28 7.11 2.83 4.71 70.20 89.40 0.084 

Spring 

(Mar-May) 9,891 3.58 5.55 1.97 3.73 65.20 89.30 0.088 

Summer 

(Jun-Aug) 9,636 3.15 5.48 2.33 3.70 72.60 94.60 0.091 

Fall 

(Sep-Nov) 9,494 3.66 6.18 2.53 4.21 73.80 94.90 0.119 

Annual 38,650 3.67 6.08 2.41 4.09 70.40 92.00 0.099 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 9,749 6.57 9.15 2.58 4.85 30.30 64.00 0.197 

Spring 

(Mar-May) 10,034 4.01 5.73 1.72 3.44 37.50 73.90 0.149 

Summer 

(Jun-Aug) 9,992 3.29 5.52 2.23 3.25 57.80 81.90 0.224 

Fall 

(Sep-Nov) 9,967 3.89 6.90 3.01 4.27 61.50 83.10 0.147 

Annual 39,742 4.43 6.81 2.38 3.95 46.80 75.80 0.208 

For 2006, model performance for 24-hour average SO2 concentrations varies by subregion. SO2

is underestimated in the Intermountain West, where observed values are low and overestimated 

elsewhere, including in the East where observed values are higher. For three of the subregions 

(West Coast, Intermountain West, and Northeast), the fractional bias values indicate that the 

modeled values are within a factor of two (67 percent) of the observed values. For the remaining 

three subregions, the bias values are generally somewhat higher than this threshold. The R2

values for SO2 are very low. 

Table 4-7 lists the seasonal and annual performance metrics for 24-hour average CO for the 

entire U.S. and each of the six model performance subregions for 2006, calculated using the 

AQS data. Units are parts per million (ppm). 

ICF International 42 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-7. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average CO Concentration 

(ppm) for the Base Case (Current Atmosphere Scenario): 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 32,771 0.58 0.27 -0.31 0.37 -51.50 78.50 0.015 

Spring 

(Mar-May) 32,816 0.42 0.23 -0.19 0.24 -39.70 69.80 0.026 

Summer 

(Jun-Aug) 31,289 0.38 0.23 -0.15 0.21 -27.90 69.50 0.017 

Fall 

(Sep-Nov) 31,633 0.48 0.24 -0.24 0.29 -43.00 76.60 0.043 

Annual 128,509 0.47 0.24 -0.22 0.28 -40.60 73.60 0.032 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 7,459 0.69 0.27 -0.42 0.44 -68.50 83.00 0.107 

Spring 

(Mar-May) 7,613 0.43 0.24 -0.18 0.23 -39.30 64.00 0.052 

Summer 

(Jun-Aug) 6,994 0.35 0.24 -0.11 0.20 -23.10 70.00 0.032 

Fall 

(Sep-Nov) 6,677 0.51 0.26 -0.25 0.30 -46.70 74.80 0.073 

Annual 28,743 0.50 0.25 -0.24 0.29 -44.70 72.90 0.070 

ICF International 43 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 6,475 0.74 0.19 -0.55 0.56 -98.80 106.10 0.124 

Spring 

(Mar-May) 5,824 0.42 0.17 -0.26 0.27 -64.90 84.80 0.099 

Summer 

(Jun-Aug) 5,167 0.34 0.17 -0.17 0.21 -41.70 77.70 0.007 

Fall 

(Sep-Nov) 6,055 0.53 0.17 -0.36 0.38 -75.10 94.20 0.084 

Annual 23,521 0.52 0.17 -0.34 0.36 -71.80 91.50 0.101 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 4,469 0.40 0.22 -0.18 0.26 -28.30 79.90 0.013 

Spring 

(Mar-May) 4,691 0.34 0.19 -0.15 0.21 -29.00 77.50 0.002 

Summer 

(Jun-Aug) 4,436 0.31 0.18 -0.12 0.19 -21.60 79.90 0.003 

Fall 

(Sep-Nov) 4,482 0.35 0.19 -0.16 0.23 -25.20 82.50 0.003 

Annual 18,078 0.35 0.19 -0.15 0.22 -26.10 80.00 0.000 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 3,298 0.47 0.36 -0.11 0.24 -19.20 56.00 0.004 

Spring 

(Mar-May) 3,383 0.45 0.27 -0.19 0.25 -35.70 62.60 0.000 

Summer 

(Jun-Aug) 3,330 0.41 0.26 -0.14 0.21 -26.60 60.10 0.000 

Fall 

(Sep-Nov) 3,288 0.47 0.29 -0.18 0.25 -32.60 63.20 0.022 

Annual 13,299 0.45 0.29 -0.16 0.24 -28.60 60.50 0.006 

ICF International 44 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 5,241 0.56 0.27 -0.29 0.33 -51.30 70.10 0.025 

Spring 

(Mar-May) 5,205 0.47 0.23 -0.24 0.28 -46.50 69.20 0.000 

Summer 

(Jun-Aug) 5,155 0.47 0.24 -0.23 0.26 -52.90 66.60 0.001 

Fall 

(Sep-Nov) 5,067 0.54 0.24 -0.30 0.32 -56.60 73.20 0.018 

Annual 20,668 0.51 0.24 -0.27 0.30 -51.80 69.80 0.012 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 5,829 0.49 0.37 -0.12 0.24 -10.20 59.00 0.194 

Spring 

(Mar-May) 6,100 0.41 0.28 -0.13 0.20 -16.90 58.40 0.176 

Summer 

(Jun-Aug) 6,207 0.38 0.29 -0.08 0.19 -2.50 59.60 0.110 

Fall 

(Sep-Nov) 6,064 0.43 0.31 -0.13 0.22 -10.60 64.30 0.213 

Annual 24,200 0.43 0.31 -0.11 0.21 -10.00 60.30 0.190 

For 2006, CO concentrations are underestimated for all regions and time periods. The modeled 

values exhibit less seasonal variation than the observed values and this results in very low 

correlation (R2

). The best performance for CO is for the Northeast subregion. The worst 

performance is for the West and Intermountain West subregions for the wintertime period. 

Table 4-8 lists the seasonal and annual performance metrics for 24-hour average PM2.5 for the 

entire U.S. and each of the six model performance subregions for 2006, calculated using the 

AQS data. Units are micrograms per cubic meter (μgm-3). 

ICF International 45 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-8. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average PM2.5 Concentration 

(μgm-3) for the Base Case (Current Atmosphere Scenario): 2006. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 35,399 11.21 9.88 -1.34 5.24 -13.30 49.00 0.116 

Spring 

(Mar-May) 34,570 10.54 11.35 0.81 4.68 10.50 43.50 0.179 

Summer 

(Jun-Aug) 33,872 14.13 9.62 -4.51 7.43 -42.00 61.00 0.080 

Fall 

(Sep-Nov) 33,477 10.79 9.93 -0.86 4.83 -10.50 47.60 0.223 

Annual 137,318 11.66 10.20 -1.46 5.54 -13.70 50.20 0.117 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 3,658 16.03 6.61 -9.41 10.88 -66.70 81.30 0.018 

Spring 

(Mar-May) 3,305 8.72 9.91 1.19 4.09 12.10 44.30 0.410 

Summer 

(Jun-Aug) 3,039 11.01 7.16 -3.85 4.82 -41.20 53.20 0.181 

Fall 

(Sep-Nov) 3,352 12.96 8.25 -4.71 6.89 -41.70 60.10 0.047 

Annual 13,354 12.31 7.96 -4.34 6.82 -35.10 60.40 0.047 

ICF International 46 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 3,693 10.52 4.76 -5.76 7.34 -61.60 80.00 0.010 

Spring 

(Mar-May) 3,628 6.55 11.31 4.76 6.40 44.10 63.50 0.025 

Summer 

(Jun-Aug) 3,648 8.10 5.06 -3.04 4.39 -45.50 62.40 0.000 

Fall 

(Sep-Nov) 3,611 7.81 4.62 -3.19 4.40 -44.00 62.30 0.013 

Annual 14,580 8.26 6.43 -1.83 5.64 -26.90 67.10 0.000 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 6,278 9.23 9.75 0.52 4.05 4.80 43.00 0.156 

Spring 

(Mar-May) 6,304 10.70 9.96 -0.74 4.28 -3.10 41.40 0.089 

Summer 

(Jun-Aug) 6,162 13.03 7.97 -5.05 7.14 -55.40 68.50 0.109 

Fall 

(Sep-Nov) 5,847 10.17 9.11 -1.07 4.21 -9.40 43.90 0.218 

Annual 24,591 10.78 9.21 -1.58 4.92 -15.70 49.20 0.105 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 5,145 12.33 14.12 1.78 4.89 12.40 37.40 0.289 

Spring 

(Mar-May) 4,884 11.64 12.55 0.91 4.77 13.80 39.80 0.242 

Summer 

(Jun-Aug) 4,795 15.07 10.09 -4.98 7.02 -41.40 56.70 0.160 

Fall 

(Sep-Nov) 4,725 11.70 12.82 1.13 4.24 12.20 36.20 0.426 

Annual 19,549 12.68 12.43 -0.25 5.22 -0.50 42.40 0.204 

ICF International 47 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

 (f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 10,073 10.17 9.01 -1.17 3.73 -13.00 39.80 0.258 

Spring 

(Mar-May) 9,916 12.69 11.35 -1.34 4.40 -8.60 35.80 0.092 

Summer 

(Jun-Aug) 9,791 17.18 10.49 -6.69 9.47 -52.50 66.20 0.027 

Fall 

(Sep-Nov) 9,476 11.71 9.34 -2.37 4.49 -21.60 43.00 0.246 

Annual 39,256 12.93 10.05 -2.88 5.51 -23.80 46.20 0.093 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias 

(%) 

Fractional 

Error 

(%) 

R2

Winter 

(Dec-Feb) 6,552 11.93 13.28 1.35 4.77 11.40 39.20 0.376 

Spring 

(Mar-May) 6,533 9.69 12.97 3.28 4.76 35.10 45.70 0.473 

Summer 

(Jun-Aug) 6,437 15.11 11.84 -3.26 6.80 -19.40 50.30 0.228 

Fall 

(Sep-Nov) 6,466 10.15 13.50 3.35 5.34 28.30 46.70 0.449 

Annual 25,988 11.71 12.90 1.19 5.41 14.00 45.50 0.301 

For 2006, model performance for 24-hour average PM2.5 is mixed. For many of the regions and 

time periods, the bias and error statistics indicate good model performance. Most troubling is 

that the seasonal concentration pattern that is characterized by higher concentrations during the 

summer months is not replicated by the model. The modeled values tend to be lowest during 

the summer months and consistently underestimate the observed values. This cyclical 

mismatch could be due to the fact that different meteorological processes affect PM2.5 in the 

winter and the summer months and that GEOS-Chem is less able to simulate the summertime 

conditions. Local stability and low wind speeds generally contribute to high PM2.5 concentrations 

throughout the year but are especially important during the winter months, while regional-scale 

transport can be important during the summer months. 

ICF International 48 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 49 October 28, 2011 11-035

Dry Deposition 

Modeled dry deposition amounts are compared with data from the CASTNet sites in Table 4-9. 

Dry deposition data are available for approximately 80 sites within the U.S. for the following 

species: nitric acid (HNO3), ammonium (NH4), particulate nitrate (NO3), ozone, SO2, and sulfate 

(SO4). Table 4-8 compares the simulated and observed annual deposition amounts and 

provides summary annual performance metrics for each of these deposition species for the 

entire U.S. Units are kilograms per hectare (kg/ha). 

Table 4-9. Annual GEOS-CHEM Model Performance Statistics for Dry Deposition (kg/ha) for the Base Case (Current 

Atmosphere Scenario) for the Continental U.S.: 2006. 

Time Period # 

Data Pairs 

Mean 

Observed 

(kg/ha) 

Mean 

Simulated 

(kg/ha) 

Mean 

Residual 

(kg/ha) 

Mean 

Unsigned 

Error 

(kg/ha) 

Fractional 

Bias 

(%) 

Fractional 

Error 

 (%) 

 R2 

Nitric acid (HNO3) 82 4.88 6.44 1.57 2.73 23.50 52.70 0.275 

Ammonium (NH4) 82 0.32 0.23 -0.09 0.13 -33.40 53.60 0.516 

Particulate nitrate (NO3) 82 0.27 0.30 0.03 0.18 6.00 80.20 0.205 

Ozone (O3) 81 40.24 35.60 -4.64 12.81 -10.00 34.50 0.064 

Sulfur dioxide (SO2) 82 3.36 4.33 0.97 1.58 40.10 51.80 0.536 

Sulfate (SO4) 82 0.96 0.45 -0.51 0.54 -70.20 77.10 0.468 

Overall, model performance for dry deposition for 2006 is quite good. Dry deposition of ozone is 

underestimated by the model, which could contribute to overestimation of the ozone 

concentrations. Ammonium and sulfate dry deposition are also underestimated. 

Summary Model Performance Results for 2007 

In this section, the GEOS-Chem results for 2007 are compared to observed data for ozone, 

other gaseous species, fine particulate matter, and dry deposition. 

Ozone 

The two metrics used for the calculation of statistical measures for ozone are: 24-hour average 

ozone and MDA8 ozone. 

Table 4-10 lists the seasonal and annual performance metrics for 24-hour average ozone for the 

entire U.S. and each of the six model performance subregions for 2007, calculated using the 

CASTNet data. Units are ppb. 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-10. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone Concentration (ppb) fo

(Current Atmosphere Scenario) Using CASTNet Data: 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 6,910 28.48 30.17 1.69 6.17 14.70 30.60 5.50 Spring (Mar-May) 7,156 42.24 45.70 3.46 7.35 13.40 21.30 9.30 Summer (Jun-Aug) 7,123 40.31 51.84 11.53 13.30 39.50 42.80 27.50 Fall (Sep-Nov) 7,112 32.16 41.25 9.09 10.65 43.80 47.80 27.30 Annual 28,301 35.86 42.33 6.47 9.39 27.90 35.70 17.50 ICF International 50 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 765 32.11 35.57 3.46 8.44 31.60 43.60 15.80 Spring (Mar-May) 816 44.90 46.44 1.54 8.12 13.90 26.00 7.30 Summer (Jun-Aug) 801 47.79 51.29 3.50 10.02 20.10 32.20 11.10 Fall (Sep-Nov) 754 36.21 43.34 7.13 10.46 61.70 68.40 26.80 Annual 3,136 40.43 44.28 3.85 9.25 31.30 42.10 15.10 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,101 39.06 38.95 -0.11 4.63 4.30 15.60 1.00 Spring (Mar-May) 1,140 48.31 47.91 -0.40 5.36 2.30 13.00 0.40 Summer (Jun-Aug) 1,165 47.65 52.82 5.17 8.00 14.90 20.10 11.50 Fall (Sep-Nov) 1,165 39.81 43.68 3.87 6.43 18.00 23.60 11.30 Annual 4,571 43.75 45.92 2.18 6.13 10.00 18.20 6.10 ICF International 51 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 736 28.17 31.09 2.91 7.05 28.70 41.50 13.00 

Spring 

(Mar-May) 760 38.49 42.47 3.98 7.49 17.60 25.10 11.90 

Summer (Jun-Aug) 722 36.16 46.88 10.72 11.54 39.50 41.60 27.50 Fall (Sep-Nov) 811 30.04 38.14 8.10 9.68 46.10 50.40 26.60 Annual 3,029 33.16 39.60 6.43 8.94 33.10 39.80 19.80 

(e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,134 23.46 21.43 -2.04 5.05 -9.10 24.40 -15.20 Spring (Mar-May) 1,136 38.49 42.75 4.27 7.07 13.50 20.80 9.70 Summer (Jun-Aug) 1,169 38.16 55.07 16.91 17.04 48.50 48.80 36.10 Fall (Sep-Nov) 1,135 28.40 38.33 9.93 10.99 38.90 43.30 27.50 Annual 4,574 32.18 39.52 7.34 10.09 23.10 34.40 14.70 ICF International 52 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 53 11-035

(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,876 26.59 30.90 4.31 7.09 26.20 34.90 16.30 Spring (Mar-May) 1,956 44.10 48.69 4.59 8.14 15.30 21.90 11.30 Summer (Jun-Aug) 1,935 40.26 53.37 13.10 14.65 46.80 49.40 31.40 Fall (Sep-Nov) 1,965 32.66 43.54 10.87 12.05 45.50 48.10 31.10 Annual 7,732 35.98 44.23 8.25 10.50 33.50 38.60 22.60 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,298 24.63 25.57 0.94 5.27 9.70 28.90 1.30 Spring (Mar-May) 1,348 38.06 43.33 5.27 7.60 17.40 23.00 13.10 Summer (Jun-Aug) 1,331 33.60 48.94 15.34 15.61 54.40 55.00 38.00 Fall (Sep-Nov) 1,282 26.74 38.88 12.14 12.77 56.90 59.60 36.70 Annual 5,259 30.86 39.28 8.42 10.31 34.50 41.50 22.20 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Model performance is similar to that for 2006 and 24-hour average ozone is overestimated for 

most regions and time periods. The greatest amount of overestimation tends to occur during the 

summer and autumn months, and the best performance for 24-hour ozone is achieved for the 

western subregions. 

Table 4-11 lists the seasonal and annual performance metrics for maximum daily 8-hour 

average (MDA8) ozone for the entire U.S. and each of the six model performance subregions 

for 2007, calculated using the CASTNet data. Units are ppb. 

ICF International 54 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-11. Seasonal and Annual GEOS-CHEM Model Performance Statistics for Maximum Daily 8-hour Average (MDA8) Ozone Concentrat

(Current Atmosphere Scenario) Using CASTNet Data: 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 6,928 33.94 36.68 2.74 5.65 13.40 22.00 8.10 Spring (Mar-May) 7,230 51.44 55.57 4.13 7.10 10.50 15.60 8.20 Summer (Jun-Aug) 7,226 52.54 64.43 11.89 13.82 28.60 31.60 21.20 Fall (Sep-Nov) 7,160 41.44 51.47 10.03 11.02 31.50 33.50 23.00 Annual 28,544 44.96 52.20 7.24 9.43 21.00 25.70 15.20 ICF International 55 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 768 37.51 41.66 4.15 6.59 21.00 26.40 13.60 Spring (Mar-May) 823 54.48 56.47 1.99 7.74 6.50 15.80 4.40 Summer (Jun-Aug) 815 61.40 65.20 3.80 12.06 10.50 22.90 6.30 Fall (Sep-Nov) 757 44.73 52.88 8.14 10.62 34.70 38.70 21.30 Annual 3,163 49.81 54.26 4.45 9.26 17.80 25.70 11.10 

(c) Intermountain West 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,124 42.75 43.53 0.79 3.80 3.90 10.50 2.50 Spring (Mar-May) 1,167 54.35 54.86 0.51 4.67 2.00 9.20 1.30 Summer (Jun-Aug) 1,184 55.52 61.24 5.72 8.37 12.00 16.30 9.80 Fall (Sep-Nov) 1,172 45.57 50.83 5.26 6.68 14.70 17.40 11.60 Annual 4,647 49.63 52.73 3.10 5.91 8.20 13.40 6.40 ICF International 56 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 738 33.81 36.84 3.03 6.25 18.50 27.30 10.20 Spring (Mar-May) 763 47.77 50.74 2.98 6.26 9.80 15.80 7.10 Summer (Jun-Aug) 733 46.94 57.55 10.61 11.63 26.30 28.40 20.80 Fall (Sep-Nov) 813 39.11 47.13 8.02 9.12 27.90 30.40 19.90 Annual 3,047 41.88 48.05 6.17 8.31 20.70 25.50 14.60 

 (e) Great Lakes States 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,130 28.90 28.64 -0.27 4.54 -0.40 17.60 -3.40 Spring (Mar-May) 1,146 49.13 55.24 6.11 7.92 14.10 17.80 11.30 Summer (Jun-Aug) 1,176 52.57 70.56 17.99 18.35 37.90 38.60 29.40 Fall (Sep-Nov) 1,150 39.14 51.21 12.06 12.49 34.80 36.20 26.60 Annual 4,602 42.55 51.61 9.06 10.90 21.80 27.70 16.10 ICF International 57 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 58 11-035

(f) Southeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,878 33.14 38.46 5.32 7.13 21.90 26.50 16.00 Spring (Mar-May) 1,970 54.66 58.88 4.22 6.84 9.90 14.00 8.10 Summer (Jun-Aug) 1,971 53.13 65.54 12.41 13.64 32.00 33.70 23.00 Fall (Sep-Nov) 1,970 43.09 53.70 10.60 11.51 31.10 32.60 23.90 Annual 7,789 46.16 54.33 8.17 9.81 23.70 26.70 17.80 

(g) Northeast 

Time 

Period 

# 

Data Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias 

(%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias 

(%) 

Winter (Dec-Feb) 1,290 29.79 32.13 2.34 5.17 13.80 23.80 7.00 Spring (Mar-May) 1,361 46.46 53.81 7.35 8.93 18.30 21.40 14.40 Summer (Jun-Aug) 1,347 46.71 63.54 16.83 17.20 42.20 42.90 30.70 Fall (Sep-Nov) 1,298 36.77 50.78 14.01 14.33 45.00 46.10 31.90 Annual 5,296 40.09 50.26 10.17 11.44 29.80 33.50 21.00 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Model performance for MDA8 for 2007 is similar to that for 2006. Again, the 8-hour average 

ozone concentrations are overestimated for most regions and time periods but the bias and 

error statistics suggest reasonably good performance. Model performance varies by season and 

by subregion. Overestimation of ozone during the summer and autumn months is prevalent in 

the eastern U.S. (Central States, Great Lakes, Southeast, and Northeast subregions). For the 

western subregions, there is a mix of over- and underestimation of ozone and overall very good 

agreement with the observations. 

The best agreement with the observations is achieved for the Intermountain West subregion. 

The normalized bias for this subregion for the annual period is 8.2 percent, while the 

corresponding gross error is 13.4 percent. The fractional bias is 6.4 percent and the 

corresponding fractional error is 11.9 percent. 

For the entire U.S., the mean residual or difference between the simulated and observed values 

ranges from approximately 2.7 to 11.9 ppb for the four seasonal periods and is 7.2 ppb for the 

annual period. The normalized bias for the annual period is 21 percent, while the corresponding 

gross error is 25.7 percent. The fractional bias is 15.2 percent and the corresponding fractional 

error is 20.4 percent. Figure 4-8 summarizes the agreement between the mean simulated and 

observed MDA8 concentrations for the seasonal and annual periods for the entire U.S. 

Figure 4-8. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Each Season 

and the Entire Year, Averaged Over All Sites in Entire Continental U.S.: 2007. 

Seasonal & Annual Mean Ozone Concentration (ppb)

25

35

45

55

65

75

25 35 45 55 65 75

Observed

GEOS-Chem

Spring Summer Fall Winter Annual

Time-series and scatter plots illustrating site-specific model performance for MDA8 ozone 

concentrations for the West Coast subregion are provided in Figure 4-9. The West Coast sites 

(listed in order of elevation) are: Pinnacles NM, CA (elevation 335 meters (m)) Mount Rainier 

NP, WA (elevation 415 m), Joshua Tree NM, CA (elevation 1244 m), and Lassen Volcanic NP, 

CA (elevation 1756 m). In the time series plots, the observed values are in blue and the 

simulated values are in green. Note that the scale varies by site, based on concentration. 

ICF International 59 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 60 October 28, 2011 11-035

Figure 4-9. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the West Coast Subregion: 2007. 

(a) Pinnacles NM 

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: 2007

R2 = 0.4348

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Simulated

Observed

 

(b) Mount Rainier NP 

CASTNet Site Mount Rainier NP, WA (MOR409) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Mount Rainier NP, WA (MOR409) MDA8: 2007

R2 = 0.4205

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

 (c) Joshua Tree NM 

CASTNet Site Joshua Tree NM, CA (JOT403) MDA8: 2007

0

20

40

60

80

100

120

140

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Joshua Tree NM, CA (JOT403) MDA8: 2007

R2 = 0.789

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Observed

Simulated
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Lassen Volcanic NP 

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2007

R2 = 0.4956

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110

Observed

Simulated

Time-series and scatter plots for the Intermountain West subregion are provided in Figure 4-10. 

The Intermountain West sites are: Chiricahua NM, AZ (elevation 1570 m), Great Basin NP, NV 

(elevation 2060 m), Pinedale, WY (elevation 2388 m), and Rocky Mountain NP, CO (elevation 

2743 m). 

Figure 4-10. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Intermountain West Subregion: 2007. 

(a) Chiricahua NM 

CASTNet Site Chiricahua NM, AZ (CHA467) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Chiricahua NM, AZ (CHA467) MDA8: 2007

R2 = 0.4573

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 61 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) Great Basin NP 

CASTNet Site Great Basin NP, NV (GRB411) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Great Basin NP, NV (GRB411) MDA8: 2007

R2 = 0.7021

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Pinedale 

CASTNet Site Pinedale, WY (PND165) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Pinedale, WY (PND165) MDA8: 2007

R2 = 0.5893

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Rocky Mountain NP 

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: 2007

R2 = 0.4325

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 62 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Time-series and scatter plots for the Central States subregion are provided in Figure 4-11. The 

Central States sites are: Caddo Valley, AR (elevation 71 m), Konza Prairie, KS (elevation 348 

m), Theodore Roosevelt NP, ND (elevation 850 m), and Big Bend NP, TX (elevation 1052 m). 

Figure 4-11. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in Central States Subregion: 2007. 

(a) Caddo Valley 

CASTNet Site Caddo Valley, AR (CAD150) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Caddo Valley, AR (CAD150) MDA8: 2007

R2 = 0.5407

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(b) Konza Prairie 

CASTNet Site Konza Prairie, KS (KNZ184) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Konza Prairie, KS (KNZ184) MDA8: 2007

R2 = 0.5434

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 63 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Theodore Roosevelt NP 

CASTNet Site Theodore Roosevelt NP, ND (THR422) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Theodore Roosevelt NP, ND (THR422) MDA8: 2007

R2 = 0.5871

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Observed

Simulated

(d) Big Bend NP 

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Big Bend NP, TX, (BBE401) MDA8: 2007

R2 = 0.3571

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Great Lakes subregion are provided in Figure 4-12. The 

Great Lakes sites are: Salamonie Reservoir, IN (elevation 250 m), Ann Arbor, MI (elevation 267 

m), Oxford, OH (elevation 284 m), and Perkinstown, WI (elevation 472 m). 

ICF International 64 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Figure 4-12. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Great Lakes Subregion: 2007. 

(a) Salamonie Reservoir 

CASTNet Site Salamonie Reservoir, IN (SAL133) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Salamonie Reservoir, IN (SAL133) MDA8: 2007

R2 = 0.7525

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(b) Ann Arbor 

CASTNet Site Unionville, MI (ANA115) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Unionville, MI (ANA115) MDA8: 2007

R2 = 0.5991

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Oxford 

CASTNet Site Oxford, OH (OXF122) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Oxford, OH (OXF122) MDA8: 2007

R2 = 0.7834

0

10

20

30

40

50

60

70

80

90

100

110

120

0 20 40 60 80 100 120

Observed

Simulated

ICF International 65 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Perkinstown 

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2008

R2 = 0.4394

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Southeast subregion are provided in Figure 4-13. The 

Southeast sites are: Sumatra, FL (elevation 14 m), Cadiz, KY (elevation 189 m), Sand 

Mountain, AL (elevation 352 m), and Great Smoky Mountain NP, TN (elevation 793 m). 

Figure 4-13. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Southeast Subregion: 2007. 

(a) Sumatra 

CASTNet Site Sumatra, FL (SUM156) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Sumatra, FL (SUM156) MDA8: 2007

R2 = 0.3899

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 66 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) Cadiz 

CASTNet Site Cadiz, KY (CDZ171) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Cadiz, KY (CDZ171) MDA8: 2007

R2 = 0.7331

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Sand Mountain 

CASTNet Site Sand Mountain, AL (SND152) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Sand Mountain, AL (SND152) MDA8: 2007

R2 = 0.6845

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Great Smoky Mountain NP 

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2007

R2 = 0.6863

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 67 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Time-series and scatter plots for the Northeast subregion are provided in Figure 4-14. The 

Northeast sites are Blackwater NWR, MD (elevation 4 m), Acadia NP, ME (elevation 158 m), 

Penn State, PA (elevation 378 m), and Huntington Wildlife Forest, NY (elevation 502 m). 

Figure 4-14. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Northeast Subregion: 2007. 

(a) Blackwater NWR 

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2007

R2 = 0.6889

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120

Observed

Simulated

(b) Acadia NP 

CASTNet Site Acadia NP, ME (ACA416) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Acadia NP, ME (ACA416) MDA8: 2007

R2 = 0.4028

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Observed

Simulated

ICF International 68 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Penn State 

CASTNet Site Penn State, PA (PSU106) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Penn State, PA (PSU106) MDA8: 2007

R2 = 0.7081

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Huntington Wildlife Forest 

CASTNet Site Huntington Wildlife Forest, NY (HWF187) MDA8: 2007

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Huntington Wildlife Forest, NY (HWF187) MDA8: 2007

R2 = 0.5308

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

For the more urban sites included in the AQS network, only 24-hour average ozone was 

considered. The results are similar to those for 2006 and the 24-hour ozone concentrations are 

overestimated for all subregions and periods. 

Table 4-12 lists the seasonal and annual performance metrics for 24-hour average ozone for the 

entire U.S. and each of the six model performance subregions for 2007, calculated using the 

AQS data. Normalized bias and error statistics are not included in this table. Units are ppb. 

ICF International 69 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-12. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario) Using AQS Data: 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2 

Winter 

(Dec-Feb) 51,714 22.02 30.28 8.26 9.97 35.70 43.20 0.247 

Spring 

(MarMay) 91,020 36.47 44.92 8.45 10.20 22.10 26.30 0.263 

Summer 

(Jun-Aug) 107,622 34.35 50.56 16.21 17.12 38.90 41.30 0.280 

Fall (SepNov) 84,132 26.90 42.13 15.23 15.74 47.30 48.70 0.316 

Annual 334,515 31.14 43.77 12.62 13.78 35.90 39.40 0.352 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2 

Winter 

(Dec-Feb) 14,826 20.99 31.13 10.15 11.53 46.70 51.10 0.196 

Spring 

(Mar-May) 16,454 35.24 42.31 7.07 10.13 18.90 26.80 0.265 

Summer 

(Jun-Aug) 18,184 34.12 43.79 9.67 12.96 20.90 30.70 0.345 

Fall (SepNov) 16,740 26.07 40.36 14.29 15.34 46.80 49.70 0.239 

Annual 66,204 29.42 39.72 10.30 12.54 32.70 39.10 0.323 

ICF International 70 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2 

Winter 

(Dec-Feb) 9,333 24.63 35.24 10.61 11.84 43.10 46.60 0.143 

Spring 

(Mar-May) 11,612 39.38 46.37 6.99 9.33 18.70 23.70 0.165 

Summer 

(Jun-Aug) 13,576 40.97 52.52 11.56 12.68 25.50 27.90 0.138 

Fall (SepNov) 11,700 29.61 42.63 13.02 13.77 40.40 42.30 0.176 

Annual 46,221 34.40 44.98 10.59 11.95 31.10 34.30 0.325 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 12,495 21.49 27.45 5.97 7.74 28.70 36.20 0.332 

Spring 

(Mar-May) 16,455 34.31 42.10 7.78 9.11 21.60 25.00 0.277 

Summer 

(Jun-Aug) 17,996 29.05 46.85 17.80 17.95 49.70 50.10 0.424 

Fall (SepNov) 15,878 26.10 39.63 13.53 13.87 43.80 44.90 0.378 

Annual 62,824 28.18 39.92 11.74 12.57 36.70 39.40 0.388 

ICF International 71 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 2,950 18.81 19.93 1.12 5.07 3.20 30.90 0.549 

Spring 

(Mar-May) 12,226 35.92 45.12 9.20 10.42 23.80 26.80 0.282 

Summer 

(Jun-Aug) 17,249 34.87 55.45 20.58 20.63 45.80 45.90 0.282 

Fall (SepNov) 10,062 26.81 42.95 16.14 16.33 47.90 48.80 0.516 

Annual 42,487 32.15 47.05 14.90 15.60 37.00 40.10 0.410 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,702 24.13 36.93 12.81 13.56 44.00 46.60 0.150 

Spring 

(Mar-May) 21,524 38.43 48.78 10.35 11.42 25.30 27.60 0.200 

Summer 

(Jun-Aug) 23,822 34.97 53.53 18.55 19.05 44.80 45.70 0.172 

Fall (SepNov) 18,079 27.91 45.87 17.96 18.20 52.10 52.60 0.317 

Annual 70,127 33.18 48.51 15.33 15.96 40.60 42.00 0.264 

ICF International 72 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,435 20.90 23.32 2.41 5.65 15.80 33.80 0.578 

Spring 

(Mar-May) 12,749 35.54 43.99 8.45 10.14 22.30 26.60 0.305 

Summer 

(Jun-Aug) 16,795 33.64 51.19 17.55 17.97 41.70 42.70 0.341 

Fall (SepNov) 11,673 25.14 41.22 16.08 16.48 50.90 52.40 0.368 

Annual 46,652 30.55 43.48 12.93 14.02 35.70 39.70 0.406 

NO2, SO2, CO, and PM2.5 

Table 4-13 lists the seasonal and annual performance metrics for 24-hour average NO2 for the 

entire U.S. and each of the six model performance subregions for 2007, calculated using the 

AQS data. Normalized bias and error statistics are not included in this table. Units are ppb. 

Table 4-13. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average NO2 Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario): 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 33,420 14.31 7.48 -6.83 7.85 -52.20 72.10 0.280 

Spring 

(Mar-May) 34,583 10.62 5.36 -5.26 6.09 -53.20 75.60 0.347 

Summer 

(Jun-Aug) 34,685 8.79 5.10 -3.68 4.93 -41.80 70.70 0.260 

Fall (SepNov) 33,899 12.03 6.08 -5.95 6.97 -52.60 75.60 0.292 

Annual 136,587 11.41 5.99 -5.41 6.45 -49.90 73.50 0.315 

ICF International 73 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 8,826 16.52 6.66 -9.86 10.25 -80.00 89.50 0.364 

Spring 

(Mar-May) 9,120 11.58 5.57 -6.01 6.72 -68.70 81.10 0.378 

Summer 

(Jun-Aug) 9,133 10.12 5.53 -4.59 5.39 -57.70 71.70 0.410 

Fall (SepNov) 9,043 14.99 6.23 -8.76 9.19 -80.20 88.80 0.392 

Annual 36,122 13.27 5.99 -7.28 7.87 -71.60 82.70 0.381 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,084 15.83 4.49 -11.34 11.61 -88.00 107.70 0.472 

Spring 

(Mar-May) 4,170 10.15 3.19 -6.95 7.18 -77.80 106.60 0.498 

Summer 

(Jun-Aug) 3,820 8.50 3.32 -5.18 5.51 -77.40 93.70 0.398 

Fall (SepNov) 3,882 12.90 3.98 -8.92 9.17 -83.70 103.20 0.498 

Annual 15,956 11.88 3.75 -8.13 8.40 -81.70 103.00 0.476 

ICF International 74 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 8,112 10.35 7.04 -3.32 4.95 -24.00 57.50 0.261 

Spring 

(Mar-May) 8,422 7.78 5.20 -2.58 4.03 -22.40 62.10 0.260 

Summer 

(Jun-Aug) 8,372 6.47 6.11 -0.36 3.51 4.30 59.00 0.190 

Fall (SepNov) 8,240 8.88 6.17 -2.71 4.52 -21.00 58.90 0.232 

Annual 33,146 8.35 6.12 -2.23 4.25 -15.70 59.40 0.237 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 1,706 17.76 11.98 -5.78 6.96 -30.00 43.80 0.402 

Spring 

(Mar-May) 1,888 14.90 7.77 -7.13 7.76 -51.40 62.50 0.430 

Summer 

(Jun-Aug) 1,848 12.74 6.15 -6.59 7.03 -54.50 65.30 0.358 

Fall (SepNov) 1,472 15.70 9.04 -6.66 7.28 -45.30 54.90 0.350 

Annual 6,914 15.20 8.64 -6.55 7.26 -45.60 57.00 0.389 

ICF International 75 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,624 11.06 7.06 -4.00 5.37 -34.80 59.60 0.274 

Spring 

(Mar-May) 4,756 8.79 4.38 -4.41 5.06 -50.90 71.60 0.321 

Summer 

(Jun-Aug) 4,784 6.96 3.91 -3.05 4.00 -37.20 68.40 0.093 

Fall (SepNov) 4,769 8.61 5.03 -3.58 5.02 -32.20 70.30 0.155 

Annual 18,933 8.84 5.08 -3.76 4.86 -38.80 67.50 0.252 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,068 16.92 10.38 -6.54 7.88 -43.60 58.80 0.278 

Spring 

(Mar-May) 6,227 13.49 6.81 -6.68 7.50 -56.40 71.10 0.299 

Summer 

(Jun-Aug) 6,728 10.23 4.84 -5.39 5.84 -57.30 73.70 0.376 

Fall (SepNov) 6,493 13.08 7.12 -5.96 7.01 -52.40 70.60 0.250 

Annual 25,516 13.34 7.22 -6.12 7.03 -52.60 68.70 0.333 

For 2007, the 24-hour average NO2 concentrations are underestimated for all subregions and 

time periods. As for 2006, model performance is worst for the West Coast and Intermountain 

West subregions. Elsewhere, the fractional bias indicates that the modeled values are well 

within a factor of two (67 percent) of the observed values. 

Table 4-14 lists the seasonal and annual performance metrics for 24-hour average SO2 for the 

entire U.S. and each of the six model performance subregions for 2007, calculated using the 

AQS data. Units are ppb. 

ICF International 76 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-14. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average SO2 Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario): 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 41,856 3.78 6.15 2.37 4.14 55.70 92.70 0.125 

Spring 

(Mar-May) 43,300 3.00 4.49 1.49 3.09 54.30 93.80 0.128 

Summer 

(Jun-Aug) 42,637 2.63 4.44 1.81 3.09 64.00 98.70 0.125 

Fall (SepNov) 42,319 3.03 4.98 1.95 3.45 61.80 96.00 0.125 

Annual 170,112 3.11 5.01 1.90 3.44 59.00 95.30 0.131 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 3,310 1.36 1.48 0.12 1.09 33.70 94.10 0.132 

Spring 

(Mar-May) 3,605 1.07 1.24 0.17 0.86 40.70 97.80 0.154 

Summer 

(Jun-Aug) 3,583 1.07 1.43 0.36 0.89 55.20 95.30 0.158 

Fall (SepNov) 3,604 1.17 1.59 0.42 0.92 45.80 87.00 0.244 

Annual 14,102 1.17 1.44 0.27 0.94 44.00 93.50 0.166 

ICF International 77 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 2,772 2.19 0.94 -1.26 1.78 -9.60 107.00 0.011 

Spring 

(Mar-May) 2,863 1.93 0.70 -1.23 1.62 -8.80 115.00 0.050 

Summer 

(Jun-Aug) 2,737 2.01 0.74 -1.26 1.63 -17.00 113.30 0.066 

Fall (SepNov) 2,720 2.09 0.86 -1.23 1.73 3.00 111.20 0.063 

Annual 11,092 2.05 0.81 -1.24 1.69 -8.20 111.70 0.039 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 8,700 2.17 3.50 1.32 2.87 79.80 109.90 0.020 

Spring 

(Mar-May) 9,130 1.96 2.60 0.64 2.29 72.10 113.10 0.030 

Summer 

(Jun-Aug) 9,105 1.62 3.06 1.43 2.51 88.90 117.50 0.020 

Fall (SepNov) 9,131 2.10 3.02 0.92 2.62 77.10 112.40 0.029 

Annual 36,066 1.96 3.04 1.08 2.57 79.50 113.30 0.024 

ICF International 78 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 7,804 4.24 9.05 4.81 5.94 83.10 95.20 0.038 

Spring 

(Mar-May) 7,766 3.87 6.97 3.10 4.51 74.80 91.40 0.052 

Summer 

(Jun-Aug) 7,444 3.39 6.77 3.38 4.64 81.80 99.00 0.025 

Fall (SepNov) 7,306 4.10 7.77 3.67 5.33 79.70 96.40 0.017 

Annual 30,320 3.91 7.65 3.75 5.11 79.80 95.40 0.035 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 9,252 4.12 7.28 3.16 4.83 72.20 90.90 0.067 

Spring 

(Mar-May) 9,813 3.51 5.36 1.85 3.54 63.80 88.60 0.123 

Summer 

(Jun-Aug) 9,652 3.15 5.54 2.39 3.74 77.70 96.60 0.106 

Fall (SepNov) 9,416 3.44 6.01 2.58 3.95 76.90 94.80 0.131 

Annual 38,133 3.55 6.03 2.48 4.00 72.60 92.70 0.106 

ICF International 79 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 10,018 5.96 8.73 2.77 5.14 34.30 69.70 0.106 

Spring 

(Mar-May) 10,123 3.90 6.06 2.15 3.67 44.20 74.80 0.124 

Summer 

(Jun-Aug) 10,116 3.28 5.42 2.14 3.18 54.80 78.10 0.213 

Fall (SepNov) 10,142 3.81 6.61 2.80 3.98 55.60 78.40 0.181 

Annual 40,399 4.23 6.70 2.47 3.99 47.30 75.20 0.169 

For 2007, model performance for 24-hour average SO2 concentrations varies by subregion. SO2

is underestimated in the Intermountain West, where observed values are low and overestimated 

elsewhere, including in the East where observed values are higher. Fractional error values are 

high and R2

 values are very low. Overall the best agreement with the observed values is for the 

Northeast. 

Table 4-15 lists the seasonal and annual performance metrics for 24-hour average CO for the 

entire U.S. and each of the six model performance subregions for 2007, calculated using the 

AQS data. Units are ppm. 

Table 4-15. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average CO Concentration 

(ppm) for the Base Case (Current Atmosphere Scenario): 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 31,070 0.51 0.27 -0.24 0.31 -39.80 71.70 0.016 

Spring 

(Mar-May) 31,016 0.40 0.24 -0.16 0.22 -30.80 65.80 0.034 

Summer 

(Jun-Aug) 30,176 0.34 0.23 -0.11 0.19 -19.60 67.30 0.012 

Fall (SepNov) 31,088 0.43 0.23 -0.20 0.25 -38.60 71.30 0.051 

Annual 123,350 0.42 0.24 -0.18 0.24 -32.30 69.10 0.032 

ICF International 80 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,828 0.60 0.27 -0.33 0.36 -56.80 75.00 0.112 

Spring 

(Mar-May) 6,991 0.39 0.26 -0.13 0.19 -25.10 60.80 0.121 

Summer 

(Jun-Aug) 6,907 0.31 0.24 -0.07 0.17 -18.00 66.10 0.018 

Fall (SepNov) 7,013 0.49 0.26 -0.23 0.27 -49.90 68.40 0.127 

Annual 27,739 0.45 0.26 -0.19 0.25 -37.40 67.50 0.088 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,891 0.65 0.19 -0.46 0.47 -85.50 96.60 0.127 

Spring 

(Mar-May) 5,175 0.40 0.17 -0.23 0.25 -54.10 79.90 0.094 

Summer 

(Jun-Aug) 4,468 0.30 0.17 -0.14 0.19 -26.50 77.50 0.009 

Fall (SepNov) 5,418 0.47 0.17 -0.29 0.32 -62.00 88.60 0.099 

Annual 20,952 0.47 0.18 -0.29 0.32 -59.10 86.30 0.107 

ICF International 81 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,266 0.37 0.23 -0.14 0.23 -20.10 74.50 0.001 

Spring 

(Mar-May) 4,398 0.33 0.19 -0.13 0.20 -20.60 75.40 0.001 

Summer 

(Jun-Aug) 4,498 0.29 0.18 -0.11 0.16 -21.60 69.40 0.031 

Fall (SepNov) 4,465 0.34 0.19 -0.15 0.20 -30.10 71.00 0.018 

Annual 17,627 0.33 0.20 -0.13 0.20 -23.10 72.50 0.007 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 2,950 0.44 0.36 -0.08 0.21 -14.40 51.30 0.016 

Spring 

(Mar-May) 3,088 0.41 0.27 -0.14 0.19 -30.70 50.70 0.035 

Summer 

(Jun-Aug) 2,919 0.38 0.25 -0.14 0.19 -24.40 55.00 0.009 

Fall (SepNov) 2,762 0.44 0.26 -0.18 0.24 -35.00 65.20 0.008 

Annual 11,719 0.42 0.29 -0.14 0.21 -26.00 55.30 0.016 

ICF International 82 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,090 0.48 0.27 -0.21 0.27 -38.40 63.40 0.015 

Spring 

(Mar-May) 5,122 0.44 0.23 -0.21 0.24 -44.90 63.90 0.012 

Summer 

(Jun-Aug) 5,087 0.41 0.23 -0.18 0.23 -34.60 67.10 0.007 

Fall (SepNov) 5,197 0.44 0.22 -0.22 0.26 -47.10 68.90 0.012 

Annual 20,496 0.44 0.24 -0.21 0.25 -41.30 65.90 0.007 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,045 0.42 0.36 -0.06 0.21 -0.70 56.80 0.122 

Spring 

(Mar-May) 6,242 0.39 0.29 -0.10 0.20 -9.80 59.50 0.072 

Summer 

(Jun-Aug) 6,297 0.34 0.28 -0.06 0.19 3.40 63.10 0.059 

Fall (SepNov) 6,233 0.37 0.28 -0.09 0.19 -3.30 62.50 0.119 

Annual 24,817 0.38 0.30 -0.08 0.19 -2.60 60.50 0.096 

For 2007, CO concentrations are underestimated for all regions and time periods. Again, the 

modeled values exhibit less seasonal variation than the observed values. The best performance 

for CO is for the Northeast subregion. 

Table 4-16 lists the seasonal and annual performance metrics for 24-hour average PM2.5 for the 

entire U.S. and each of the six model performance subregions for 2007, calculated using the 

AQS data. Units are μgm-3. 

ICF International 83 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-16. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average PM2.5 Concentration 

(μgm-3) for the Base Case (Current Atmosphere Scenario): 2007. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 34,581 11.87 10.18 -1.69 5.16 -14.80 46.20 0.166 

Spring 

(Mar-May) 37,027 11.26 11.43 0.17 4.98 6.80 42.70 0.141 

Summer 

(Jun-Aug) 36,252 13.93 10.74 -3.19 7.01 -29.20 55.90 0.102 

Fall (SepNov) 36,120 11.42 9.55 -1.87 5.04 -16.40 47.70 0.186 

Annual 143,980 12.12 10.48 -1.64 5.55 -13.30 48.10 0.138 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 3,619 15.21 6.37 -8.84 10.57 -58.50 77.80 0.009 

Spring 

(Mar-May) 3,731 9.36 9.65 0.29 4.47 8.50 44.70 0.147 

Summer 

(Jun-Aug) 3,539 9.22 8.49 -0.73 4.16 -10.40 46.60 0.162 

Fall (SepNov) 3,784 13.81 8.52 -5.29 7.89 -35.20 58.10 0.048 

Annual 14,673 11.92 8.27 -3.65 6.78 -23.90 56.80 0.030 

ICF International 84 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 3,251 11.72 4.62 -7.10 8.44 -65.70 83.00 0.023 

Spring 

(Mar-May) 3,654 6.32 10.34 4.02 5.96 31.00 58.30 0.022 

Summer 

(Jun-Aug) 3,455 9.30 8.66 -0.64 6.27 -18.90 62.20 0.053 

Fall (SepNov) 3,315 8.46 5.56 -2.91 5.07 -39.80 63.50 0.020 

Annual 13,675 8.88 7.40 -1.48 6.41 -21.80 66.40 0.005 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,694 11.11 11.46 0.35 3.84 4.10 36.60 0.462 

Spring 

(Mar-May) 6,065 10.87 10.56 -0.32 4.20 1.70 39.00 0.181 

Summer 

(Jun-Aug) 6,119 12.76 9.59 -3.17 6.65 -35.40 60.90 0.077 

Fall (SepNov) 5,914 10.43 8.71 -1.71 4.63 -15.40 48.70 0.176 

Annual 23,792 11.30 10.07 -1.24 4.85 -11.50 46.50 0.187 

ICF International 85 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,361 14.11 15.03 0.92 4.52 4.70 32.20 0.446 

Spring 

(Mar-May) 6,000 12.91 13.75 0.84 5.05 13.10 39.40 0.342 

Summer 

(Jun-Aug) 5,938 14.52 11.34 -3.18 6.13 -27.60 49.10 0.220 

Fall (SepNov) 5,807 13.23 12.22 -1.01 5.15 -3.80 41.00 0.275 

Annual 23,106 13.68 13.04 -0.64 5.23 -3.50 40.60 0.295 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 9,925 10.01 8.77 -1.24 3.46 -12.80 37.40 0.283 

Spring 

(Mar-May) 10,522 13.95 10.94 -3.01 5.41 -15.80 39.00 0.138 

Summer 

(Jun-Aug) 10,156 16.56 9.82 -6.74 8.68 -50.60 63.90 0.053 

Fall (SepNov) 10,123 11.36 8.54 -2.82 4.59 -27.20 46.60 0.265 

Annual 40,726 13.00 9.54 -3.46 5.55 -26.60 46.70 0.144 

ICF International 86 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,731 12.04 12.47 0.43 4.83 3.10 40.30 0.342 

Spring 

(Mar-May) 7,055 9.93 12.50 2.57 4.57 28.20 43.50 0.411 

Summer 

(Jun-Aug) 7,045 15.68 13.41 -2.27 6.37 -15.60 44.80 0.323 

Fall (SepNov) 7,177 11.27 11.96 0.69 4.30 10.10 38.90 0.407 

Annual 28,008 12.23 12.58 0.36 5.02 6.50 41.90 0.348 

For 2007, model performance for 24-hour average PM2.5 is a mix of over and underestimation of 

the observed values. For many of the regions and time periods, the bias and error statistics 

indicate good model performance. However, as for 2006, the annual cycle of PM2.5

concentrations indicated by the observations is not replicated by the model. 

Dry Deposition 

Table 4-17 compares the simulated and observed annual deposition amounts and provides 

summary annual performance metrics for dry deposition species for the entire U.S. for 2007. 

Table 4-17. Annual GEOS-CHEM Model Performance Statistics for Dry Deposition (kg/ha) for the Base Case (Current 

Atmosphere Scenario) for the Continental U.S.: 2007. 

Time Period 

# Data 

Pairs 

Mean 

Observed 

(kg/ha) 

Mean 

Simulated 

(kg/ha) 

Mean 

Residual 

(kg/ha) 

Mean 

Unsigned 

Error 

(kg/ha) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Nitric acid 

(HNO3) 82 5.12 6.56 1.43 2.71 24.40 51.30 0.284 

Ammonium 

(NH4) 82 0.35 0.23 -0.13 0.16 -40.60 60.90 0.479 

Particulate 

nitrate (NO3) 82 0.29 0.31 0.02 0.19 -0.80 80.30 0.171 

Ozone (O3) 80 37.78 35.77 -2.01 11.32 -4.60 32.10 0.100 

Sulfur dioxide 

(SO2) 82 3.03 4.33 1.30 1.75 44.30 54.40 0.456 

Sulfate (SO4) 82 1.02 0.44 -0.58 0.61 -73.60 82.40 0.453 

ICF International 87 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 88 October 28, 2011 11-035

Model performance for dry deposition for 2007 is similar to that for 2006, but is characterized by 

less underestimation of ozone dry deposition. The modeled results are about the same for both 

years; the observed values are different. 

Summary Model Performance Results for 2008 

In this section, the GEOS-Chem results for 2008 are compared to observed data for ozone, 

other gaseous species, fine particulate matter, and dry deposition. 

Ozone 

The two metrics used for the calculation of statistical measures for ozone are: 24-hour average 

ozone and MDA8 ozone. Table 4-18 lists the seasonal and annual performance metrics for 24-

hour average ozone for the entire U.S. and each of the six model performance subregions for 

2008, calculated using the CASTNet data. Units are ppb. 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-18. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone Concentration (ppb) fo

(Current Atmosphere Scenario) Using CASTNet Data: 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 6,993 29.27 30.42 1.15 6.04 11.60 27.50 4.30 Spring (Mar-May) 7,097 41.96 44.56 2.59 7.65 11.70 22.10 7.30 Summer (Jun-Aug) 6,907 38.75 50.74 12.00 13.68 42.30 45.30 29.40 Fall (SepNov) 6,960 30.31 39.99 9.68 10.97 46.60 50.50 29.20 Annual 27,957 35.09 41.41 6.32 9.56 27.90 36.20 17.50 ICF International 89 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 627 35.25 35.31 0.06 7.84 13.30 31.50 4.40 Spring (Mar-May) 704 46.83 45.45 -1.38 7.85 3.10 20.00 -0.60 Summer (Jun-Aug) 589 52.08 54.77 2.69 10.22 12.10 24.60 6.70 Fall (SepNov) 634 38.54 45.62 7.08 9.74 47.50 52.80 23.10 Annual 2,554 43.14 45.15 2.01 8.86 18.70 32.00 8.20 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,143 40.41 39.04 -1.36 4.85 0.90 15.10 -2.30 Spring (Mar-May) 1,133 50.49 47.81 -2.67 6.64 -1.80 15.10 -4.20 Summer (Jun-Aug) 1,164 46.08 50.88 4.80 7.88 17.10 22.80 11.80 Fall (SepNov) 1,146 38.21 43.47 5.25 6.92 28.60 32.60 15.20 Annual 4,586 43.79 45.32 1.53 6.58 11.20 21.40 5.20 ICF International 90 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 866 28.97 31.75 2.79 6.63 19.60 31.10 11.00 Spring (Mar-May) 893 38.74 41.54 2.80 7.03 14.80 23.60 9.60 Summer (Jun-Aug) 878 35.12 45.38 10.26 11.56 41.70 44.60 27.40 Fall (SepNov) 889 29.79 36.89 7.10 8.50 35.60 39.80 22.40 Annual 3,526 33.18 38.92 5.74 8.43 27.90 34.80 17.60 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,169 23.78 22.75 -1.03 5.26 -0.50 25.60 -6.60 Spring (Mar-May) 1,183 38.46 40.75 2.30 6.79 7.30 19.20 4.20 Summer (Jun-Aug) 1,127 35.57 52.26 16.69 16.81 50.20 50.50 37.60 Fall (SepNov) 1,107 25.66 35.18 9.52 10.78 39.60 45.20 28.10 Annual 4,586 30.92 37.65 6.73 9.83 23.60 34.80 15.40 ICF International 91 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 92 11-035

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,869 27.03 30.79 3.76 6.84 21.70 31.40 13.90 Spring (Mar-May) 1,872 40.65 47.03 6.38 9.31 23.30 29.00 16.60 Summer (Jun-Aug) 1,787 37.15 51.57 14.43 15.62 53.90 56.00 36.10 Fall (SepNov) 1,879 30.19 43.26 13.07 13.92 57.60 60.00 37.40 Annual 7,407 33.71 43.07 9.36 11.38 39.00 44.00 25.90 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,319 25.02 26.04 1.02 5.36 11.40 30.00 1.50 Spring (Mar-May) 1,312 39.23 43.22 3.99 7.24 13.00 20.90 9.30 Summer (Jun-Aug) 1,362 33.79 49.99 16.20 16.34 55.60 55.90 40.00 Fall (SepNov) 1,305 23.85 35.69 11.84 12.71 59.40 63.10 38.30 Annual 5,298 30.51 38.83 8.32 10.46 35.00 42.60 22.40 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Model performance is similar to that for 2006 and 2007. Ozone is overestimated for most 

regions and time periods and the greatest amount of overestimation tends to occur during the 

summer months and autumn months. 

Table 4-19 lists the seasonal and annual performance metrics for maximum daily 8-hour 

average (MDA8) ozone for the entire U.S. and each of the six model performance subregions 

for 2008, calculated using the CASTNet data. Units are ppb. 

ICF International 93 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-19. Seasonal and Annual GEOS-CHEM Model Performance Statistics for Maximum Daily 8-hour Average (MDA8) Ozone Concentrat

(Current Atmosphere Scenario) Using CASTNet Data: 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 6,981 34.75 37.07 2.33 5.64 12.40 21.20 7.30 Spring (Mar-May) 7,182 50.35 53.96 3.60 7.56 10.30 17.40 7.40 Summer (Jun-Aug) 6,995 50.19 63.23 13.04 14.86 31.90 34.60 24.00 Fall (SepNov) 7,020 38.82 49.70 10.87 11.81 34.60 36.70 25.50 Annual 28,178 43.57 51.01 7.44 9.95 22.20 27.40 16.00 ICF International 94 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

 (b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 630 40.75 41.68 0.93 5.64 7.50 17.60 4.10 Spring (Mar-May) 709 55.95 55.19 -0.76 7.16 0.80 13.60 -1.00 Summer (Jun-Aug) 597 68.13 70.68 2.55 13.56 7.00 21.20 3.60 Fall (SepNov) 637 48.20 55.42 7.22 10.55 26.80 31.90 17.20 Annual 2,573 53.14 55.53 2.40 9.11 10.30 20.90 5.80 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,143 44.14 43.60 -0.54 3.94 0.60 9.90 -0.60 Spring (Mar-May) 1,150 56.29 54.60 -1.70 5.51 -1.30 10.70 -2.60 Summer (Jun-Aug) 1,173 53.77 59.03 5.26 8.14 12.80 17.70 9.70 Fall (SepNov) 1,162 43.82 50.50 6.68 7.52 18.90 20.80 14.70 Annual 4,628 49.52 51.97 2.45 6.30 7.80 14.80 5.40 ICF International 95 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 867 34.49 37.68 3.19 6.05 19.70 27.20 10.10 Spring (Mar-May) 909 47.46 49.59 2.13 6.51 8.50 16.50 5.80 Summer (Jun-Aug) 891 44.83 56.12 11.29 12.45 31.20 33.50 22.90 Fall (SepNov) 898 38.34 45.55 7.21 8.41 23.90 26.70 17.80 Annual 3,565 41.35 47.31 5.96 8.36 20.80 25.90 14.10 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,163 29.09 30.10 1.01 5.26 7.10 20.50 3.40 Spring (Mar-May) 1,189 47.28 52.09 4.81 7.68 11.20 17.50 8.50 Summer (Jun-Aug) 1,140 48.04 67.60 19.56 19.74 42.90 43.30 33.40 Fall (SepNov) 1,114 35.18 47.49 12.30 13.04 37.50 40.00 28.60 Annual 4,606 39.95 49.26 9.31 11.35 24.40 30.10 18.20 ICF International 96 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

ICF International 97 11-035

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,866 33.75 38.39 4.64 6.73 18.60 24.10 13.80 Spring (Mar-May) 1,896 50.02 56.65 6.62 8.73 18.10 21.60 13.90 Summer (Jun-Aug) 1,819 48.83 63.28 14.45 15.12 37.40 38.40 28.10 Fall (SepNov) 1,891 39.57 52.98 13.41 13.88 40.70 41.80 30.40 Annual 7,472 43.02 52.77 9.75 11.09 28.70 31.40 21.50 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error (ppb) 

Normalized 

Bias (%) 

Normalized 

Gross Error 

(%) 

Fractional 

Bias (%) 

Winter (Dec-Feb) 1,312 30.30 33.09 2.79 5.62 15.90 25.40 8.00 Spring (Mar-May) 1,329 47.43 53.57 6.14 8.49 14.80 19.50 11.30 Summer (Jun-Aug) 1,375 46.39 64.47 18.08 18.32 42.90 43.30 32.80 Fall (SepNov) 1,318 32.23 46.21 13.98 14.51 48.40 49.90 34.40 Annual 5,334 39.19 49.52 10.33 11.81 30.60 34.60 21.70 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Model performance for MDA8 for 2008 is about the same as that for 2006 and 2007, only very 

slightly worse overall. Again, the 8-hour average ozone concentrations are overestimated for nearly 

all regions and time periods but the bias and error statistics suggest reasonably good performance. 

Similar to the other years, model performance varies by season and by subregion and the best 

agreement with the observations is achieved for the Intermountain West subregion. The 

normalized bias for this subregion for the annual period is 7.8 percent, while the corresponding 

gross error is 14.8 percent. The fractional bias is 5.4 percent and the corresponding fractional 

error is 12.9 percent. 

For the entire U.S., the mean residual or difference between the simulated and observed values 

ranges from approximately 2.3 to 13 ppb for the four seasonal periods and is 7.4 for the annual 

period. The normalized bias for the annual period is 22.2 percent, while the corresponding gross 

error is 27.4 percent. The fractional bias is 16 percent and the corresponding fractional error is 

21.8 percent. Figure 4-15 displays the agreement between the mean simulated and observed 

MDA8 concentrations for the seasonal and annual periods for the entire U.S. 

Figure 4-15. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Each 

Season and the Entire Year, Averaged Over All Sites in Entire Continental U.S.: 2008. 

Seasonal & Annual Mean Ozone Concentration (ppb)

25

35

45

55

65

75

25 35 45 55 65 75

Observed

GEOS-Chem

Spring Summer Fall Winter Annual

Time-series and scatter plots illustrating site-specific model performance for MDA8 ozone 

concentration for the West Coast subregion are provided in Figure 4-16. The West Coast sites 

(listed in order of elevation) are: Pinnacles NM, CA (elevation 335 meters (m)) Mount Rainier 

NP, WA (elevation 415 m), Joshua Tree NM, CA (elevation 1244 m), and Lassen Volcanic NP, 

CA (elevation 1756 m). In the time series plots, the observed values are in blue and the 

simulated values are in green. Note that the scale varies by site, based on concentration. 

ICF International 98 October 28, 2011 11-035
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Model Performance Evaluation Procedures & Results 

ICF International 99 October 28, 2011 11-035

Figure 4-16. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the West Coast Subregion: 2008. 

(a) Pinnacles NM 

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: 2008

R2 = 0.6047

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Simulated

Observed

(b) Mount Rainier NP 

CASTNet Site Mount Rainier NP, WA (MOR409) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Mount Rainier NP, WA (MOR409) MDA8: 2008

R2 = 0.3282

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

 (c) Joshua Tree NM 

CASTNet Site Joshua Tree NM, CA (JOT403) MDA8: 2008

0

20

40

60

80

100

120

140

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Joshua Tree NM, CA (JOT403) MDA8: 2008

R2 = 0.6758

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Observed

Simulated
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Lassen Volcanic NP 

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2008

R2 = 0.502

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110

Observed

Simulated

Time-series and scatter plots for the Intermountain West subregion are provided in Figure 4-17. 

The Intermountain West sites are: Chiricahua NM, AZ (elevation 1570 m), Great Basin NP, NV 

(elevation 2060 m), Pinedale, WY (elevation 2388 m), and Rocky Mountain NP, CO (elevation 

2743 m). 

Figure 4-17. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Intermountain West Subregion: 2008. 

(a) Chiricahua NM 

CASTNet Site Chiricahua NM, AZ (CHA467) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Chiricahua NM, AZ (CHA467) MDA8: 2008

R2 = 0.5026

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 100 October 28, 2011 11-035
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Model Performance Evaluation Procedures & Results 

(b) Great Basin NP 

CASTNet Site Great Basin NP, NV (GRB411) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Great Basin NP, NV (GRB411) MDA8: 2008

R2 = 0.512

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Pinedale 

CASTNet Site Pinedale, WY (PND165) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Pinedale, WY (PND165) MDA8: 2008

R2 = 0.3767

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Rocky Mountain NP 

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: 2008

R2 = 0.3914

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 101 October 28, 2011 11-035
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Model Performance Evaluation Procedures & Results 

Time-series and scatter plots for the Central States subregion are provided in Figure 4-18. The 

Central States sites are: Caddo Valley, AR (elevation 71 m), Konza Prairie, KS (elevation 348 

m), Theodore Roosevelt NP, ND (elevation 850 m), and Big Bend NP, TX (elevation 1052 m). 

Figure 4-18. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in Central States Subregion: 2008. 

(a) Caddo Valley 

CASTNet Site Caddo Valley, AR (CAD150) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Caddo Valley, AR (CAD150) MDA8: 2008

R2 = 0.3452

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(b) Konza Prairie 

CASTNet Site Konza Prairie, KS (KNZ184) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Konza Prairie, KS (KNZ184) MDA8: 2008

R2 = 0.5428

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 102 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Theodore Roosevelt NP 

CASTNet Site Theodore Roosevelt NP, ND (THR422) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Theodore Roosevelt NP, ND (THR422) MDA8: 2008

R2 = 0.5645

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Observed

Simulated

(d) Big Bend NP 

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2008

R2 = 0.4607

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Great Lakes subregion are provided in Figure 4-19. The 

Great Lakes sites are: Salamonie Reservoir, IN (elevation 250 m), Ann Arbor, MI (elevation 267 

m), Oxford, OH (elevation 284 m), and Perkinstown, WI (elevation 472 m). 

ICF International 103 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Figure 4-19. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Great Lakes Subregion: 2008. 

(a) Salamonie Reservoir 

CASTNet Site Salamonie Reservoir, IN (SAL133) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Salamonie Reservoir, IN (SAL133) MDA8: 2008

R2 = 0.7053

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(b) Ann Arbor 

CASTNet Site Unionville, MI (ANA115) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Unionville, MI (ANA115) MDA8: 2008

R2 = 0.596

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Oxford 

CASTNet Site Oxford, OH (OXF122) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Oxford, OH (OXF122) MDA8: 2008

R2 = 0.6467

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120

Observed

Simulated

ICF International 104 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Perkinstown 

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2008

R2 = 0.4394

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

Time-series and scatter plots for the Southeast subregion are provided in Figure 4-20. The 

Southeast sites are: Sumatra, FL (elevation 14 m), Cadiz, KY (elevation 189 m), Sand 

Mountain, AL (elevation 352 m), and Great Smoky Mountain NP, TN (elevation 793 m). 

Figure 4-20. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Southeast Subregion: 2008. 

(a) Sumatra 

CASTNet Site Sumatra, FL (SUM156) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Sumatra, FL (SUM156) MDA8: 2008

R2 = 0.3979

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 105 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) Cadiz 

CASTNet Site Cadiz, KY (CDZ171) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Cadiz, KY (CDZ171) MDA8: 2008

R2 = 0.5764

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(c) Sand Mountain 

CASTNet Site Sand Mountain, AL (SND152) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Sand Mountain, AL (SND152) MDA8: 2008

R2 = 0.556

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Great Smoky Mountain NP 

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2008

R2 = 0.6463

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

ICF International 106 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Time-series and scatter plots for the Northeast subregion are provided in Figure 4-21. The 

Northeast sites are Blackwater NWR, MD (elevation 4 m), Acadia NP, ME (elevation 158 m), 

Penn State, PA (elevation 378 m), and Huntington Wildlife Forest, NY (elevation 502 m). 

Figure 4-21. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for Selected 

CASTNet Sites in the Northeast Subregion: 2008. 

(a) Blackwater NWR 

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2008

R2 = 0.704

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120

Observed

Simulated

(b) Acadia NP 

CASTNet Site Acadia NP, ME (ACA416) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Acadia NP, ME (ACA416) MDA8: 2008

R2 = 0.3558

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Observed

Simulated

ICF International 107 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Penn State 

CASTNet Site Penn State, PA (PSU106) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Penn State, PA (PSU106) MDA8: 2008

R2 = 0.6704

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

(d) Huntington Wildlife Forest 

CASTNet Site Huntington Wildlife Forest, NY (HWF187) MDA8: 2008

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

Day

ppb

Obs Sim

CASTNet Site Huntington Wildlife Forest, NY (HWF187) MDA8: 2008

R2 = 0.4002

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Observed

Simulated

For the AQS sites, only 24-hour average ozone was considered. The results are similar to those 

for 2006 and 2007 in that the 24-hour ozone concentrations are overestimated for all subregions 

and periods. 

Table 4-20 lists the seasonal and annual performance metrics for 24-hour average ozone for the 

entire U.S. and each of the six model performance subregions for 2008, calculated using the 

AQS data. Normalized bias and error statistics are not included in this table. Units are ppb. 

ICF International 108 October 28, 2011 11-035
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Model Performance Evaluation Procedures & Results 

Table 4-20. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average Ozone Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario) Using AQS Data: 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 52,990 22.73 30.63 7.90 9.75 34.50 41.80 0.246 

Spring 

(Mar-May) 91,203 36.45 43.73 7.28 9.49 19.70 25.00 0.242 

Summer 

(Jun-Aug) 108,009 33.95 50.15 16.20 17.05 39.80 41.80 0.262 

Fall (SepNov) 83,873 25.46 41.23 15.77 16.25 49.40 51.00 0.286 

Annual 336,075 30.73 43.10 12.36 13.65 35.90 39.60 0.320 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 15,086 20.78 30.78 10.01 11.80 47.50 53.20 0.152 

Spring 

(Mar-May) 16,582 35.95 41.18 5.23 8.90 15.30 24.40 0.246 

Summer 

(Jun-Aug) 18,523 36.74 47.47 10.73 13.93 23.60 31.60 0.325 

Fall (SepNov) 16,919 27.01 42.45 15.44 16.43 48.30 50.90 0.280 

Annual 67,110 30.50 40.90 10.40 12.84 33.10 39.50 0.328 

ICF International 109 October 28, 2011 11-035
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Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 9,587 26.08 35.34 9.26 11.21 38.40 43.60 0.188 

Spring 

(Mar-May) 11,651 42.11 46.36 4.25 8.32 11.50 20.00 0.120 

Summer 

(Jun-Aug) 13,743 41.04 51.12 10.08 11.41 22.60 25.40 0.167 

Fall (SepNov) 11,765 28.84 43.01 14.17 14.68 43.00 44.40 0.163 

Annual 46,746 35.17 44.66 9.49 11.42 28.20 32.60 0.297 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 13,043 22.91 28.64 5.73 7.27 26.10 32.30 0.341 

Spring 

(Mar-May) 17,211 34.71 40.64 5.93 7.97 17.30 22.50 0.255 

Summer 

(Jun-Aug) 18,926 29.03 45.81 16.78 16.96 47.20 47.60 0.356 

Fall (SepNov) 16,001 25.69 38.78 13.09 13.53 41.90 43.50 0.314 

Annual 65,181 28.48 39.28 10.80 11.81 33.80 36.90 0.319 

ICF International 110 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 2,937 19.53 20.97 1.44 5.22 6.50 29.00 0.483 

Spring 

(Mar-May) 12,201 35.33 43.12 7.79 9.15 20.60 24.40 0.228 

Summer 

(Jun-Aug) 16,984 33.01 52.13 19.12 19.17 44.70 44.90 0.243 

Fall (SepNov) 9,796 23.67 39.55 15.88 16.22 49.90 51.70 0.430 

Annual 41,918 30.56 44.39 13.83 14.59 36.20 39.40 0.358 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,652 25.21 37.00 11.79 12.58 39.90 42.70 0.185 

Spring 

(Mar-May) 20,489 35.98 46.97 11.00 11.91 28.50 30.60 0.279 

Summer 

(Jun-Aug) 22,919 32.18 52.02 19.84 20.12 50.40 50.90 0.213 

Fall (SepNov) 17,453 25.42 45.13 19.71 19.82 58.20 58.60 0.262 

Annual 67,513 30.90 47.23 16.33 16.81 44.80 45.90 0.245 

ICF International 111 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,685 20.77 24.09 3.32 6.17 19.20 34.50 0.514 

Spring 

(Mar-May) 13,069 36.34 44.34 8.00 9.77 20.60 25.20 0.292 

Summer 

(Jun-Aug) 16,914 34.18 52.89 18.70 18.92 43.70 44.20 0.361 

Fall (SepNov) 11,939 21.34 36.87 15.53 15.95 53.70 55.70 0.309 

Annual 47,607 29.95 43.09 13.13 14.14 36.90 40.70 0.440 

NO2, SO2, CO, and PM2.5 

Table 4-21 lists the seasonal and annual performance metrics for 24-hour average NO2 for the 

entire U.S. and each of the six model performance subregions for 2008, calculated using the 

AQS data. Normalized bias and error statistics are not included in this table. Units are ppb. 

Table 4-21. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average NO2 Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario): 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 33,591 13.33 7.48 -5.85 7.04 -45.90 68.40 0.294 

Spring 

(Mar-May) 34,088 9.97 5.31 -4.66 5.68 -48.10 73.00 0.333 

Summer 

(Jun-Aug) 34,130 8.19 5.20 -2.99 4.50 -32.10 68.60 0.262 

Fall (SepNov) 32,825 11.74 6.30 -5.44 6.63 -45.60 73.10 0.296 

Annual 134,634 10.79 6.07 -4.72 5.95 -42.90 70.70 0.317 

ICF International 112 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 9,525 14.96 6.95 -8.02 8.57 -70.70 82.10 0.347 

Spring 

(Mar-May) 9,606 11.04 5.51 -5.53 6.32 -62.80 79.10 0.388 

Summer 

(Jun-Aug) 9,334 9.76 5.76 -4.00 5.00 -48.70 67.60 0.377 

Fall (SepNov) 9,337 15.34 6.47 -8.87 9.46 -73.50 85.50 0.340 

Annual 37,802 12.77 6.17 -6.61 7.34 -63.90 78.60 0.355 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 3,695 14.83 4.66 -10.18 10.49 -76.40 100.10 0.482 

Spring 

(Mar-May) 3,773 9.22 3.18 -6.04 6.32 -75.60 99.10 0.489 

Summer 

(Jun-Aug) 3,654 7.12 3.08 -4.03 4.41 -65.60 88.60 0.394 

Fall (SepNov) 3,668 11.35 3.98 -7.37 7.64 -72.40 99.00 0.497 

Annual 14,790 10.63 3.72 -6.91 7.22 -72.50 96.70 0.486 

ICF International 113 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 8,376 9.37 6.55 -2.82 4.53 -21.60 56.00 0.253 

Spring 

(Mar-May) 8,376 6.99 4.82 -2.17 3.67 -20.70 60.60 0.229 

Summer 

(Jun-Aug) 8,350 5.50 5.66 0.16 3.24 12.50 61.90 0.170 

Fall (SepNov) 7,920 8.44 5.91 -2.54 4.34 -15.60 61.60 0.284 

Annual 33,022 7.57 5.73 -1.84 3.94 -11.30 60.00 0.236 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 1,499 17.12 11.89 -5.23 6.34 -28.20 40.60 0.398 

Spring 

(Mar-May) 1,546 14.41 7.94 -6.47 7.31 -44.40 58.10 0.354 

Summer 

(Jun-Aug) 1,782 11.96 6.61 -5.36 5.98 -45.00 56.80 0.415 

Fall (SepNov) 1,572 15.00 9.70 -5.30 6.04 -36.80 47.30 0.457 

Annual 6,399 14.51 8.93 -5.58 6.40 -38.90 51.00 0.410 

ICF International 114 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,240 10.23 7.13 -3.10 4.87 -26.70 58.80 0.276 

Spring 

(Mar-May) 4,076 7.97 4.62 -3.35 4.49 -40.30 68.50 0.226 

Summer 

(Jun-Aug) 4,150 6.54 4.31 -2.23 3.80 -22.10 68.90 0.042 

Fall (SepNov) 4,047 8.33 5.61 -2.72 4.63 -21.90 67.30 0.169 

Annual 16,513 8.28 5.43 -2.85 4.45 -27.70 65.80 0.224 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,256 16.44 10.41 -6.03 7.68 -40.10 58.50 0.230 

Spring 

(Mar-May) 6,711 12.76 6.66 -6.10 7.25 -51.40 71.00 0.262 

Summer 

(Jun-Aug) 6,860 9.93 5.20 -4.73 5.45 -48.40 70.20 0.341 

Fall (SepNov) 6,281 12.14 7.48 -4.66 6.14 -43.90 64.30 0.257 

Annual 26,108 12.75 7.37 -5.38 6.61 -46.10 66.20 0.301 

For 2008, the 24-hour average NO2 concentrations are underestimated for all subregions and 

time periods. Model performance is worst for the West Coast and Intermountain West 

subregions. Elsewhere, the fractional bias indicates that the modeled values are well within a 

factor of two (67 percent) of the observed values. 

Table 4-22 lists the seasonal and annual performance metrics for 24-hour average SO2 for the 

entire U.S. and each of the six model performance subregions for 2008, calculated using the 

AQS data. Units are ppb. 

ICF International 115 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-22. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average SO2 Concentration 

(ppb) for the Base Case (Current Atmosphere Scenario): 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 36,649 3.41 5.78 2.37 3.96 58.30 95.30 0.121 

Spring 

(Mar-May) 40,166 2.72 4.23 1.50 3.10 57.20 99.00 0.098 

Summer 

(Jun-Aug) 39,697 2.45 4.35 1.89 3.17 68.10 103.10 0.093 

Fall (SepNov) 38,290 2.58 4.90 2.32 3.57 69.40 103.70 0.102 

Annual 157,802 2.79 4.81 2.02 3.45 63.20 100.20 0.110 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 3,737 1.05 1.46 0.41 0.96 37.40 92.80 0.121 

Spring 

(Mar-May) 3,607 1.01 1.23 0.21 0.81 33.70 91.80 0.158 

Summer 

(Jun-Aug) 3,617 1.01 1.42 0.41 0.84 48.80 89.10 0.160 

Fall (SepNov) 3,647 1.15 1.59 0.43 0.98 44.30 89.80 0.175 

Annual 14,608 1.06 1.42 0.37 0.90 41.00 90.90 0.153 

ICF International 116 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 2,576 2.13 0.89 -1.24 1.77 -6.60 106.90 0.018 

Spring 

(Mar-May) 2,599 1.62 0.64 -0.98 1.43 0.60 117.20 0.025 

Summer 

(Jun-Aug) 2,510 1.94 0.68 -1.26 1.68 2.20 117.90 0.115 

Fall (SepNov) 2,448 2.42 0.84 -1.58 2.02 -15.80 106.80 0.051 

Annual 10,133 2.02 0.76 -1.26 1.72 -4.80 112.20 0.047 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 8,832 2.27 3.21 0.94 2.83 72.90 106.50 0.013 

Spring 

(Mar-May) 9,118 1.94 2.39 0.45 2.35 75.10 117.60 0.017 

Summer 

(Jun-Aug) 8,792 1.58 2.83 1.24 2.53 93.50 123.80 0.008 

Fall (SepNov) 8,310 1.73 3.04 1.32 2.53 90.70 117.50 0.045 

Annual 35,052 1.88 2.86 0.98 2.56 82.90 116.30 0.017 

ICF International 117 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 6,777 4.21 8.58 4.38 5.69 83.70 97.20 0.049 

Spring 

(Mar-May) 6,821 3.66 6.54 2.88 4.43 77.90 94.30 0.053 

Summer 

(Jun-Aug) 6,767 3.19 6.55 3.36 4.49 83.30 99.50 0.028 

Fall (SepNov) 6,617 3.23 7.65 4.43 5.34 93.30 105.90 0.037 

Annual 26,982 3.57 7.33 3.76 4.98 84.50 99.20 0.045 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 8,154 3.82 7.23 3.41 4.84 78.70 94.30 0.067 

Spring 

(Mar-May) 8,189 3.48 5.51 2.03 3.80 66.90 90.10 0.075 

Summer 

(Jun-Aug) 8,247 3.28 5.64 2.36 4.01 76.10 98.60 0.051 

Fall (SepNov) 7,903 3.17 6.29 3.11 4.41 83.90 101.60 0.082 

Annual 32,493 3.44 6.16 2.72 4.26 76.30 96.10 0.069 

ICF International 118 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(ppb) 

Mean 

Residual 

(ppb) 

Mean 

Unsigned 

Error 

(ppb) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 9,573 5.06 8.70 3.64 5.13 51.40 75.30 0.132 

Spring 

(Mar-May) 9,832 3.15 5.79 2.64 3.72 59.10 85.70 0.142 

Summer 

(Jun-Aug) 9,764 2.77 5.60 2.83 3.55 66.30 87.80 0.207 

Fall (SepNov) 9,365 3.07 6.42 3.36 4.22 68.50 90.50 0.161 

Annual 38,534 3.51 6.62 3.11 4.15 61.30 84.80 0.181 

For 2008, model performance for 24-hour average SO2 concentrations varies by subregion. 

Again SO2 is underestimated in the Intermountain West, where observed values are low and 

overestimated elsewhere, including in the East where observed values are higher. 

Table 4-23 lists the seasonal and annual performance metrics for 24-hour average CO for the 

entire U.S. and each of the six model performance subregions for 2008, calculated using the 

AQS data. Units are ppm. 

Table 4-23. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average CO Concentration 

(ppm) for the Base Case (Current Atmosphere Scenario): 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 30,496 0.46 0.27 -0.19 0.26 -33.30 67.80 0.029 

Spring 

(Mar-May) 29,941 0.35 0.23 -0.12 0.18 -22.90 61.40 0.026 

Summer 

(Jun-Aug) 29,093 0.31 0.23 -0.08 0.18 -11.50 64.70 0.009 

Fall (SepNov) 29,651 0.39 0.23 -0.16 0.23 -27.30 69.50 0.039 

Annual 119,181 0.38 0.24 -0.14 0.21 -23.90 65.80 0.033 

ICF International 119 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 7,603 0.53 0.28 -0.25 0.29 -49.40 64.30 0.088 

Spring 

(Mar-May) 7,675 0.36 0.24 -0.11 0.17 -24.20 55.90 0.064 

Summer 

(Jun-Aug) 7,446 0.30 0.26 -0.05 0.16 -9.40 59.00 0.035 

Fall (SepNov) 7,274 0.46 0.27 -0.19 0.24 -38.80 63.10 0.125 

Annual 29,998 0.41 0.26 -0.15 0.22 -30.50 60.60 0.083 

(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,432 0.57 0.19 -0.38 0.39 -77.10 90.90 0.071 

Spring 

(Mar-May) 4,713 0.35 0.17 -0.18 0.21 -47.70 75.70 0.078 

Summer 

(Jun-Aug) 3,965 0.33 0.16 -0.17 0.20 -46.40 76.80 0.003 

Fall (SepNov) 5,087 0.46 0.17 -0.29 0.31 -65.80 85.70 0.044 

Annual 19,197 0.44 0.17 -0.26 0.29 -60.60 82.90 0.073 

ICF International 120 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,670 0.31 0.21 -0.09 0.19 -10.70 67.80 0.000 

Spring 

(Mar-May) 4,526 0.27 0.18 -0.09 0.14 -18.00 60.20 0.013 

Summer 

(Jun-Aug) 4,497 0.22 0.17 -0.05 0.12 -1.50 62.30 0.048 

Fall (SepNov) 4,476 0.29 0.18 -0.10 0.17 -12.50 70.30 0.028 

Annual 18,169 0.27 0.19 -0.09 0.16 -10.70 65.20 0.016 

(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 2,526 0.45 0.32 -0.13 0.23 -20.20 56.80 0.016 

Spring 

(Mar-May) 2,589 0.44 0.26 -0.18 0.23 -33.60 57.70 0.024 

Summer 

(Jun-Aug) 2,705 0.36 0.25 -0.11 0.18 -18.80 56.90 0.000 

Fall (SepNov) 2,661 0.39 0.26 -0.13 0.21 -21.70 60.30 0.016 

Annual 10,481 0.41 0.27 -0.14 0.21 -23.50 57.90 0.019 

ICF International 121 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,351 0.44 0.27 -0.17 0.24 -28.10 64.00 0.006 

Spring 

(Mar-May) 4,306 0.38 0.22 -0.15 0.20 -28.20 63.00 0.003 

Summer 

(Jun-Aug) 4,332 0.37 0.23 -0.14 0.21 -26.30 66.40 0.000 

Fall (SepNov) 4,078 0.38 0.23 -0.15 0.22 -26.80 66.80 0.009 

Annual 17,067 0.39 0.24 -0.15 0.22 -27.40 65.00 0.006 

(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(ppm) 

Mean 

Simulated 

(ppm) 

Mean 

Residual 

(ppm) 

Mean 

Unsigned 

Error 

(ppm) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,914 0.41 0.35 -0.05 0.20 3.90 55.80 0.132 

Spring 

(Mar-May) 6,132 0.31 0.28 -0.04 0.15 7.50 54.70 0.103 

Summer 

(Jun-Aug) 6,148 0.29 0.28 -0.01 0.16 21.30 63.50 0.084 

Fall (SepNov) 6,075 0.32 0.27 -0.05 0.18 11.10 64.90 0.119 

Annual 24,269 0.33 0.30 -0.04 0.17 11.00 59.80 0.130 

For 2008, CO concentrations are underestimated for all regions and time periods. 

Table 4-24 lists the seasonal and annual performance metrics for 24-hour average PM2.5 for the 

entire U.S. and each of the six model performance subregions for 2008, calculated using the 

AQS data. Units are μgm-3. 

ICF International 122 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Table 4-24. Seasonal and Annual GEOS-CHEM Model Performance Statistics for 24-hour Average PM2.5 Concentration 

(μgm-3) for the Base Case (Current Atmosphere Scenario): 2008. 

(a) Entire Continental U.S. 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 35,672 11.12 10.03 -1.09 4.80 -10.60 45.60 0.197 

Spring 

(Mar-May) 36,306 9.86 11.92 2.06 4.33 17.90 39.40 0.202 

Summer 

(Jun-Aug) 36,395 12.68 10.50 -2.18 6.37 -24.20 53.20 0.055 

Fall (SepNov) 36,686 10.29 10.16 -0.13 4.75 -2.50 47.20 0.175 

Annual 145,059 10.99 10.66 -0.33 5.07 -4.80 46.40 0.122 

(b) West Coast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 4,039 13.56 5.68 -7.88 8.67 -60.10 75.00 0.091 

Spring 

(Mar-May) 3,772 8.87 10.85 1.98 4.96 17.50 47.30 0.141 

Summer 

(Jun-Aug) 3,501 12.53 8.74 -3.79 6.46 -24.10 51.80 0.040 

Fall (SepNov) 3,963 12.89 7.62 -5.27 7.07 -41.50 60.80 0.050 

Annual 15,275 11.99 8.16 -3.83 6.83 -27.90 59.20 0.027 

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(c) Intermountain West 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 3,544 10.12 4.03 -6.09 6.77 -64.70 78.30 0.118 

Spring 

(Mar-May) 3,618 7.18 11.62 4.44 6.28 33.10 54.50 0.095 

Summer 

(Jun-Aug) 3,730 7.89 6.61 -1.28 5.45 -31.40 64.00 0.005 

Fall (SepNov) 3,768 7.97 4.35 -3.63 4.44 -48.10 63.60 0.028 

Annual 14,660 8.28 6.64 -1.63 5.72 -27.80 65.00 0.009 

(d) Central States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,368 10.37 11.11 0.75 3.75 6.10 36.00 0.442 

Spring 

(Mar-May) 5,560 9.90 11.06 1.16 3.75 13.40 36.40 0.177 

Summer 

(Jun-Aug) 5,398 11.25 8.42 -2.83 5.83 -35.90 60.00 0.066 

Fall (SepNov) 5,386 10.22 10.09 -0.13 4.21 -0.10 43.00 0.300 

Annual 21,712 10.43 10.18 -0.25 4.38 -4.00 43.80 0.213 

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(e) Great Lakes States 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 5,623 13.31 13.84 0.53 4.57 4.80 34.60 0.328 

Spring 

(Mar-May) 5,803 10.85 13.44 2.59 4.36 24.80 37.80 0.389 

Summer 

(Jun-Aug) 5,894 12.81 11.87 -0.94 5.17 -13.10 44.30 0.228 

Fall (SepNov) 5,890 11.25 13.16 1.90 4.59 21.50 41.50 0.412 

Annual 23,210 12.05 13.06 1.02 4.68 9.50 39.60 0.316 

(f) Southeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 9,828 9.29 9.47 0.17 3.51 1.40 38.00 0.193 

Spring 

(Mar-May) 10,109 10.95 11.04 0.09 3.21 1.20 30.00 0.276 

Summer 

(Jun-Aug) 10,236 14.20 10.22 -3.97 7.40 -37.80 56.90 0.010 

Fall (SepNov) 10,270 10.55 9.92 -0.62 3.86 -7.50 39.10 0.248 

Annual 40,443 11.27 10.17 -1.10 4.51 -10.80 41.10 0.082 

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(g) Northeast 

Time 

Period 

# Data 

Pairs 

Mean 

Observed 

(μgm-3) 

Mean 

Simulated 

(μgm-3) 

Mean 

Residual 

(μgm-3) 

Mean 

Unsigned 

Error 

(μgm-3) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Winter 

(Dec-Feb) 7,270 11.84 12.75 0.91 4.35 6.60 36.20 0.461 

Spring 

(Mar-May) 7,444 9.58 13.43 3.86 4.87 33.80 42.90 0.406 

Summer 

(Jun-Aug) 7,636 14.35 13.38 -0.97 6.09 -6.90 43.80 0.170 

Fall (SepNov) 7,409 9.22 12.80 3.58 5.46 31.60 48.30 0.202 

Annual 29,759 11.27 13.09 1.83 5.20 16.20 42.80 0.262 

For 2008, model performance for 24-hour average PM2.5 is a mix of over and underestimation of 

the observed values. Again the annual cycle of PM2.5 concentrations indicated by the 

observations is not replicated by the model, but for many of the regions and time periods, the 

bias and error statistics indicate good model performance. 

Dry Deposition 

Table 4-25 compares the simulated and observed annual deposition amounts and provides 

summary annual performance metrics for dry deposition species for the entire U.S. for 2008 

Table 4-25. Annual GEOS-CHEM Model Performance Statistics for Dry Deposition (kg/ha) for the Base Case (Current 

Atmosphere Scenario) for the Continental U.S.: 2008. 

Time Period 

# Data 

Pairs 

Mean 

Observed 

(kg/ha) 

Mean 

Simulated 

(kg/ha) 

Mean 

Residual 

(kg/ha) 

Mean 

Unsigned 

Error 

(kg/ha) 

Fractional 

Bias (%) 

Fractional 

Error (%) R2

Nitric acid 

(HNO3) 82 4.34 6.27 1.93 2.73 30.70 53.00 0.294 

Ammonium 

(NH4) 82 0.30 0.24 -0.06 0.10 -26.10 46.40 0.546 

Particulate 

nitrate (NO3) 82 0.27 0.34 0.06 0.19 11.20 79.40 0.246 

Ozone (O3) 80 37.81 35.39 -2.42 11.96 -6.20 34.90 0.084 

Sulfur dioxide 

(SO2) 82 2.77 4.38 1.61 1.92 52.80 61.00 0.453 

Sulfate (SO4) 82 0.87 0.46 -0.41 0.44 -62.60 69.40 0.491 

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Overall model performance for dry deposition for 2008 is slightly better than for the other two 

years. Again dry deposition of ozone is underestimated. The modeled results are about the 

same for all three years but the observed values are different. 

Year-to-Year Variations in Model Performance 

Model performance for ozone is similar for all three years. Both 24-hour average and MDA8 

ozone concentrations are overestimated for most regions and time periods. Statistics for MDA8 

are similar for 2006 and 2007, and very slightly worse overall for 2008. For all three years, the 

MDA8 bias and error statistics suggest reasonably good performance and the best performance 

for ozone is achieved for the western subregions. 

Model performance for NO2, SO2, and CO is also similar among the years. For all three years, 

24-hour average NO2, and CO concentrations are generally underestimated while SO2

concentrations are generally overestimated (except for the Intermountain West subregion). 

Model performance for 24-hour average PM2.5 is a mix of over and underestimation of the 

observed values and the bias and error statistics indicate good model performance for many of 

the subregions for all three years. The annual cycle of PM2.5 concentrations indicated by the 

observations is not replicated by the model and this finding is also consistent among the 

modeled years. 

Model performance for dry deposition is similar for all three years (slightly better for 2008). The 

statistical measures indicate generally good model performance. Dry deposition for ozone is 

underestimated for all three years, but to varying degrees. The modeled results are about the 

same for all three years but the observed values are different. 

4.2. Case Studies 

A more detailed examination of the GEOS-Chem results and the vertical distribution of ozone 

simulated by GEOS-Chem was conducted for selected case study periods. One objective was to 

examine how well the GEOS-Chem model is able to represent the vertical distribution of ozone in 

the troposphere under a variety of conditions, including, possibly, conditions associated with the 

stratospheric intrusion of ozone. Thus, this component of the model evaluation focused primarily 

on the western U.S., since stratospheric ozone intrusion is most likely to occur in this region and 

influence the vertical ozone profiles (Wimmers et al., 2003). The case study periods were selected 

based the availability of ozonesonde data from the INTEX-B (Thompson et al., 2008 and 

Thompson et al., 2011) and ARCTAS (ARC-IONS) (Thompson et al., 2011) programs with 

emphasis on data availability for the western U.S. and the simultaneous occurrence of high ozone 

concentrations at one or more of the CASTNet sites in the West Coast and Intermountain West 

subregions. Five periods were selected. At least one springtime period was desired, since 

springtime conditions can be conducive to stratospheric intrusion. One springtime period met the 

selection criteria given above. The remaining periods are summer periods that are primarily 

characterized by synoptic-scale high-pressure and stagnation over the western U.S. For each 

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ICF International 128 October 28, 2011 11-035

case study period, the reasonableness of the GEOS-Chem ozone concentrations at both the 

surface and aloft, relative to the observed data, was examined. 

Case Study #1: 21 April 2006 

Several CASTNet sites in California experienced relatively high ozone concentrations on 20 and 

21 April. Ozonesonde data from the INTEX-B program are available for 21 April for Trinidad 

Head, CA. This day is the focus of the first case study analysis. 

Regional weather conditions over California on 21 April 2006 were influenced by an upper-level 

ridge centered over the western states and an upper-level trough located off southern California, 

with moderate southwesterly winds aloft over the Trinidad Head area. A weak surface cold front 

moved across northern California throughout the day as the upper-level low moved inland over 

central California, resulting in very light precipitation along the California/Oregon border. 

The nearest CASTNet site to Trinidad Head is Lassen Volcanic NP. Figure 4-22 illustrates the 

simulated and observed MDA8 ozone for a multi-day day period that includes the case study day. 

Figure 4-22. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for the Lassen 

NP CASTNet Site for 11-29 April 2006. 

CASTNet Site Lassen Vol NP, CA (LAV410) MDA8: April 2006

0

10

20

30

40

50

60

70

80

90

100

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Date 

ppb

Obs Sim

The model shows reasonably good skill in replicating the daily ozone concentrations, including 

the higher values during 19-21 April. 

Figure 4-23 compares the simulated and observed vertical concentration profiles for Trinidad 

Head for 21 August. The simulated values match the time of the sounding. The ozonesonde 

data are plotted in blue and the GEOS-Chem-derived values for each model layer are plotted in 

gold. Note that in the output files used for this analysis, GEOS-Chem values are only available 

below the tropopause. The ozonesonde data are from the INTEX-B program data archive 

(Thompson et al., 2008 and Thompson et al., 2011). 
Modeling for North American Background Concentrations 

Model Performance Evaluation Procedures & Results 

Figure 4-23. Vertical Profiles of Simulated and Observed Ozone Concentrations (ppb) for Trinidad Head for 21 April 

2006. Observed Profile is Based on Ozonesonde Data from the INTEX-B Program. 

Ozone Profile for Trinidad Head, CA:

 21 April 2006 (2000-2100 GMT)

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ozone (ppb)

Alt (km)

The simulated values are similar to the observed values between approximately 1 and 3 km in 

the vertical, and greater than the observed values at other levels, including near the surface. 

One might surmise from the observed ozone profile that high ozone concentrations in the upper 

troposphere may be associated with stratospheric ozone. The simulated ozone concentration 

profile does not follow the observed profile at these levels. 

Case Study #2: 14 July 2006 

CASTNet sites in Arizona and Colorado experienced relatively high ozone concentrations during 

13-15 July. Ozonesonde data from the INTEX-B program are available for 14 July for Boulder, 

CO. This day is the focus of the second case study analysis. 

Regional weather over Colorado on 14 July 2006 was influenced by a broad, upper-level ridge 

centered over the Utah/Colorado area that covered the majority of the western U.S., resulting in 

very light and variable winds aloft. At the surface, the area was influenced by a large high 

pressure system centered over Colorado with light winds, above normal temperatures, and no 

precipitation. 

The nearest CASTNet site to Boulder is Rocky Mountain NP. Figure 4-24 illustrates the simulated 

and observed MDA8 ozone for a multi-day day period that includes the case study day. 

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Figure 4-24. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for the Rocky 

Mountain NP CASTNet Site for 8-20 July 2006. 

CASTNet Site Rocky Mtn NP, CO (ROM406) MDA8: July 2006

0

10

20

30

40

50

60

70

80

90

100

8 9 10 11 12 13 14 15 16 17 18 19 20

Date

ppb

Obs Sim

The model overestimates the observed ozone concentrations by quite a lot but captures the 

day-to-day variations in ozone, especially during the days of interest for this case study. 

Figure 4-25 compares the simulated and observed vertical concentration profiles for Boulder for 

14 July. Again, the simulated values match the time of the sounding. The ozonesonde data are 

plotted in blue and the GEOS-Chem-derived values for each model layer are plotted in gold. 

The ozonesonde data are from the INTEX-B program data archive (Thompson et al., 2008 and 

Thompson et al., 2011). 

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Figure 4-25. Vertical Profiles of Simulated and Observed Ozone Concentrations (ppb) for Boulder for 14 July 2006. 

Observed Profile is Based on Ozonesonde Data from the INTEX-B Program. 

Ozone Profile for Boulder, CO:

 14 July 2006 (1800-1900 GMT)

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ozone (ppb)

Alt (km)

The simulated values are lower than observed between approximately 3 and 5 km in the vertical 

and greater than the observed values further aloft. The simulated ozone concentration profile 

does not resemble the observed profile. 

Case Study #3: 17-20 August 2006 

CASTNet sites in California experienced relatively high ozone concentrations during midAugust. Ozonesonde data from the INTEX-B program are available for several areas and 

several days in August. Here we focus on 17- 20 August for Trinidad Head. 

Weather conditions over the western U.S. during this period were influenced by a weak upperlevel trough situated over the Pacific Northwest and a weak upper-level ridge centered over the 

southeast U.S. By the end of the period, the weak trough had filled and moved eastward, 

replaced by a broad weak ridge that was centered over the central U.S. Over California, the 

winds aloft were moderate and westerly at the beginning of the period, and weak and 

southwesterly at the end of the period, with only a trace of rain measured along the coastal area 

of the California/Oregon border on the last day of the period. 

The nearest CASTNet site to Trinidad Head is Lassen Volcanic NP. However, the highest ozone 

was observed at Pinnacles NM. Figure 4-26 illustrates the simulated and observed MDA8 ozone 

for both sites for a multi-day day period that includes 17-20 August. 

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Figure 4-26. Comparison of the Mean Simulated and Mean Observed MDA8 Ozone Concentrations (ppb) for the Lassen 

Volcanic NP and Pinnacles NM CASTNet Sites for 12-25 August 2006. 

(a) Lassen Volcanic NP (b) Pinnacles NM 

CASTNet Site Lassen Vol NP, CA (LAV410) MDA8: Aug 2006

0

10

20

30

40

50

60

70

80

90

100

12 13 14 15 16 17 18 19 20 21 22 23 24 25

Date

ppb

Obs Sim

CASTNet Site Pinnacles NM, CA (PIN414) MDA8: Aug 06

0

10

20

30

40

50

60

70

80

90

100

12 13 14 15 16 17 18 19 20 21 22 23 24 25

Date

ppb

Obs Sim

Model performance at the surface is very good for both sites and captures the (slight) increase 

in ozone at Lassen NP on the 20th and the more dramatic increase at Pinnacles NM during 18–

20 August. 

Figure 4-27 compares the simulated and observed vertical concentration profiles for Trinidad 

Head for each of the four days. The simulated values match the time of the sounding. The 

ozonesonde data are plotted in blue and the GEOS-Chem-derived values for each model layer 

are plotted in gold. The ozonesonde data are from the INTEX-B program data archive 

(Thompson et al., 2008 and Thompson et al., 2011). 

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Figure 4-27. Vertical Profiles of Simulated and Observed Ozone Concentrations (ppb) for Trinidad Head for 17-20 

August 2006. Observed Profile is Based on Ozonesonde Data from the INTEX-B Program. 

(a) 17 August (b) 18 August 

Ozone Profile for Trinidad Head, CA:

 17 August 2006 (2000-2100 GMT)

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ozone (ppb)

Alt (km)

Ozone Profile for Trinidad Head, CA:

 18 August 2006 (2000-2100 GMT)

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ozone (ppb)

Alt (km)

(a) 19 August (b) 20 August 

Ozone Profile for Trinidad Head, CA:

 19 August 2006 (1900-2000 GMT)

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ozone (ppb)

Alt (km)

Ozone Profile for Trinidad Head, CA:

20 August 2006 (1900-2000 GMT)

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ozone (ppb)

Alt (km)

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Both the simulated and observed ozone profiles are quite complicated. While the detailed 

vertical structure of observed profiles is not well simulated, the model results show layers of 

high ozone in the middle troposphere on all four days, consistent with the observations. 

Additional Case Studies 

Two additional case studies were examined: 26-30 June and 8-9 July 2008. The ozonesonde 

data for 2008 are from the ARCTAS (ARC-IONS) program data archive (Thompson et al., 

2011). The results are within the range of those presented in the previous sections and are 

therefore not shown here. Overall, the simulated vertical profiles show poor agreement with the 

observations for these days, most similar to the July 2006 example. 

4.3. Summary of Model Performance 

A variety of graphical and statistical methods were used to examine GEOS-Chem model 

performance for the three-year period 2006, 2007 and 2008. The performance evaluation 

focused on ozone but also included a variety of other air concentration and deposition species. 

Key findings from the model performance evaluation are: 

 Model performance for all species is consistent among the three modeled years. 

 Ozone concentrations are overestimated by the GEOS-Chem model for all three years and 

nearly all regions and time periods considered in this analysis. 

 Overestimation is greater for 24-hour average ozone than for 8-hour average ozone (MDA8). 

 Model performance for ozone varies by season and by subregion. 

 As expected, given the grid resolution, ozone concentrations are better represented for the 

more rural CASTNet sites compared to the more urban AQS sites. 

 Based on comparison with the CASTNet data, the bias and error statistics for MDA8 ozone 

suggest reasonably good performance. Overestimation of ozone during the summer and 

autumn months is most prevalent in the eastern U.S. (Central States, Great Lakes, 

Southeast, and Northeast subregions). For the western subregions, there is less 

overestimation of ozone and overall good agreement with the observations. The best 

agreement with the observations is achieved for the Intermountain West subregion. 

 Site-specific model performance for MDA8 ozone concentration (again for the CASTNet 

sites) indicates that for some sites, especially those in the west, the annual variation in 

ozone concentration is very well replicated. For most of the subregions, model performance 

improves with site elevation. 

 The spatial and temporal variability of NO2, SO2, CO data from the AQS network are not well 

characterized. NO2 concentrations are underestimated for all subregions and time periods. 

SO2 is underestimated in the Intermountain West, where observed values are low and 

overestimated elsewhere, including in the East where observed values are higher. 

Simulated CO is only weakly correlated with the observed concentrations, and 

concentrations are underestimated for all regions and time periods. The modeled values 

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Model Performance Evaluation Procedures & Results 

exhibit less seasonal variation than the observed values. These results are perhaps 

understandable, especially for NO2 and CO, given the coarse grid resolution and the 

probable strong response of AQS monitors to local emissions. 

 Model performance for 24-hour average PM2.5 is mixed. For many of the regions and time 

periods, the bias and error statistics indicate good model performance. However, the 

seasonal variation in PM2.5 that is characterized by higher concentrations during the summer 

months is not replicated by the model. 

 Overall, model performance for dry deposition is quite good. Dry deposition of ozone is 

underestimated by the model, which could contribute to overestimation of the ozone 

concentrations. 

 Comparison of simulated and observed ozone profiles for five case-study periods (a total of 

12 days) and several locations within the western U.S. gives mixed results. For some cases, 

the simulated vertical profiles show poor agreement with the observations. For other cases 

the results are more promising in that although the detailed vertical structure of observed 

profiles is not well simulated, the model results show layers of high ozone in the middle 

troposphere, consistent with the observations. 

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ICF International 136 October 28, 2011 11-035
5. Estimation of North American Background Ozone (NAB 

Scenario) 

The North American background scenario for which the emission inventory does not include 

anthropogenic emissions from the U.S., Canada, and Mexico provides an estimate of NAB O3. 

Figure 5-1 displays the mean MDA8 ozone concentration for the base (current atmosphere) and 

North American background scenarios, based on all three simulation years. The concentrations 

for the North American background scenario are the estimated NAB O3 values relevant to the 

MDA8 metric. 

Figure 5-1. Mean MDA8 Ozone Concentrations (ppb) for the Base (Current Atmosphere) and North American 

Background Scenarios, Based on the 2006, 2007, and 2008 Simulation Period. 

(a) Current Atmosphere Scenario (b) North American Background Scenario 

For the current atmosphere scenario, mean MDA8 ozone ranges from 21.3 to 82.6 ppb within 

the modeling grid. For the North American background scenario, mean MDA8 ozone ranges 

from 17.4 to 42.9 ppb. Thus on an annual basis and within the full modeling grid, NAB ozone 

ranges from 17.4 to 42.9 ppb, depending upon the location. It is of interest that the 

concentration patterns differ between the two scenarios and the highest NAB O3 values do not 

necessarily occur in the areas with the highest overall ozone concentrations. The highest NAB 

O3 values are found over the western U.S. 

The modeling results for NAB O3 vary by season, as illustrated in Figure 5-2. 

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Figure 5-2. Mean MDA8 Ozone Concentrations (ppb) for the North American Background Scenario, Based on the 2006, 

2007, and 2008 Simulation Period. 

(a) Winter (December–February) (b) Spring (March–May) 

(c) Summer (June–August) (d) Fall (September–November) 

For most areas within the domain, the highest NAB O3 values tend to occur during the spring 

period (March–May). High values also occur during the summer months in the western U.S., 

especially over the more mountainous regions. For all four periods, the highest NAB O3 values 

are simulated over (approximately) this same area. 

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Estimation of North American Background Ozone (NAB Scenario) 

Table 5-1 lists the seasonal and annual mean NAB O3 values relevant to 24-hour average 

ozone for the entire U.S. and each of the six model performance subregions, based on the 

CASTNet sites and the three-year simulation period. Averages were computed over all 

CASTNet sites in the region and each day of the three-year simulation period that corresponds 

to the averaging period. For comparison purposes, the mean observed and base-case (current 

atmosphere) concentrations are also included in the table. Units are ppb. 

Table 5-1. GEOS-Chem-Derived Seasonal and Annual Mean 24-Hour Average Observed, Current Atmosphere, and NAB 

Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. 

(a) Entire Continental U.S. 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 29.03 30.46 29.21 

Spring 

(Mar-May) 42.04 45.20 29.05 

Summer 

(Jun-Aug) 40.04 51.52 24.73 

Fall (SepNov) 30.70 40.17 24.72 

Annual 35.50 41.88 26.93 

(b) West Coast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 33.33 35.94 32.00 

Spring 

(Mar-May) 44.70 45.40 33.07 

Summer 

(Jun-Aug) 49.59 53.08 27.50 

Fall (SepNov) 37.10 44.09 30.21 

Annual 41.24 44.63 30.74 

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(c) Intermountain West 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 39.97 39.28 33.93 

Spring 

(Mar-May) 49.62 47.75 36.75 

Summer 

(Jun-Aug) 47.55 52.10 34.91 

Fall (SepNov) 38.96 43.34 31.75 

Annual 44.04 45.65 34.33 

(d) Central States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 29.14 31.33 26.70 

Spring 

(Mar-May) 39.17 42.21 27.84 

Summer 

(Jun-Aug) 37.29 47.06 27.90 

Fall (SepNov) 30.07 37.18 23.21 

Annual 33.88 39.42 26.37 

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(e) Great Lakes States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 23.13 21.76 27.47 

Spring 

(Mar-May) 38.01 41.96 27.30 

Summer 

(Jun-Aug) 37.11 54.01 22.93 

Fall (SepNov) 25.61 35.11 22.45 

Annual 31.01 38.20 25.07 

(f) Southeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 27.32 31.18 27.46 

Spring 

(Mar-May) 42.29 47.95 25.56 

Summer 

(Jun-Aug) 38.83 52.46 21.72 

Fall (SepNov) 30.66 43.24 22.29 

Annual 34.85 43.78 24.25 

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Estimation of North American Background Ozone (NAB Scenario) 

(g) Northeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 24.82 25.79 29.06 

Spring 

(Mar-May) 38.85 43.49 27.37 

Summer 

(Jun-Aug) 34.25 49.45 18.43 

Fall (SepNov) 24.61 36.97 21.84 

Annual 30.70 38.99 24.18 

The annual average NAB O3 concentration for the entire U.S. (relevant to 24-hour average 

ozone concentration) is estimated to be 26.9 ppb. This varies among the regions. For the 

western U.S. (West Coast and Intermountain West subregions), the estimated range is 30.7 to 

34.3 ppb. For the eastern U.S. (Central States, Great Lakes, Southeast, and Northeast 

subregions), the estimated range is 24.2 to 26.4 ppb. 

For the spring months (March–May), the average NAB O3 concentration relevant to 24-hour 

average ozone is estimated to be 29.1 ppb for the entire U.S.; it ranges from 33.1 to 36.8 ppb 

for the western U.S. and from 25.6 to 27.8 ppb for the eastern U.S. 

For the summer months (June–August), the average NAB O3 concentration relevant to 24-hour 

average ozone is estimated to be 24.7 ppb for the entire U.S.; it ranges from 27.5 to 34.9 ppb 

for the western U.S. and from 18.4 to 27.9 ppb for the eastern U.S. 

Table 5-2 lists the seasonal and annual mean NAB O3 values relevant to MDA8 ozone for the 

entire U.S. and each of the six model performance subregions, based on the CASTNet sites 

and the three-year simulation period. The mean observed and base-case (current atmosphere) 

concentrations are also included in the table. Units are ppb. 

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Estimation of North American Background Ozone (NAB Scenario) 

Table 5-2. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, and NAB Ozone 

Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. 

(a) Entire Continental U.S. 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 34.66 37.21 32.23 

Spring 

(Mar-May) 50.93 54.83 33.20 

Summer 

(Jun-Aug) 52.03 64.11 30.04 

Fall (SepNov) 39.50 50.00 28.68 

Annual 44.37 51.63 31.04 

(b) West Coast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 38.87 42.20 35.39 

Spring 

(Mar-May) 54.10 55.02 37.77 

Summer 

(Jun-Aug) 64.14 67.50 33.86 

Fall (SepNov) 46.31 53.62 34.61 

Annual 50.94 54.61 35.45 

ICF International 143 October 28, 2011 11-035
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Estimation of North American Background Ozone (NAB Scenario) 

(c) Intermountain West 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 43.69 43.90 37.16 

Spring 

(Mar-May) 55.59 54.69 41.73 

Summer 

(Jun-Aug) 55.48 60.34 40.39 

Fall (SepNov) 44.62 50.30 36.42 

Annual 49.89 52.37 38.94 

(d) Central States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 34.85 37.36 30.27 

Spring 

(Mar-May) 48.16 50.40 32.60 

Summer 

(Jun-Aug) 47.88 57.92 34.23 

Fall (SepNov) 38.88 45.77 27.80 

Annual 42.44 47.87 31.18 

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Estimation of North American Background Ozone (NAB Scenario) 

(e) Great Lakes States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 28.69 29.34 30.42 

Spring 

(Mar-May) 47.80 53.78 31.27 

Summer 

(Jun-Aug) 50.91 69.51 27.75 

Fall (SepNov) 35.43 47.39 26.40 

Annual 40.77 50.03 28.98 

(f) Southeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 34.26 38.92 30.32 

Spring 

(Mar-May) 52.44 57.97 29.31 

Summer 

(Jun-Aug) 51.22 64.46 27.03 

Fall (SepNov) 40.63 53.20 25.80 

Annual 44.76 53.77 28.11 

ICF International 145 October 28, 2011 11-035
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Estimation of North American Background Ozone (NAB Scenario) 

(g) Northeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Winter 

(Dec-Feb) 30.17 32.84 31.59 

Spring 

(Mar-May) 47.27 53.86 30.83 

Summer 

(Jun-Aug) 47.19 64.28 22.88 

Fall (SepNov) 33.47 48.05 25.19 

Annual 39.63 49.89 27.61 

The annual average NAB O3 concentration for the entire U.S. (relevant to MDA8 ozone 

concentration) is estimated to be 31.0 ppb. This varies among the regions. For the western 

U.S., the estimated range is 35.5 to 38.9 ppb. For the eastern U.S., the estimated range is 27.6 

to 31.2 ppb. 

For the spring months (March–May), the average NAB O3 concentration relevant to MDA8 

ozone is estimated to be 33.2 ppb for the entire U.S.; it ranges from 37.8 to 41.7 ppb for the 

western U.S. and from 29.3 to 32.6 ppb for the eastern U.S. 

For the summer months (June–August), the average NAB O3 concentration relevant to 24-hour 

average ozone is estimated to be 30.0 ppb for the entire U.S.; it ranges from 33.9 to 40.4 ppb 

for the western U.S. and from 22.9 to 34.2 ppb for the eastern U.S. 

Plots comparing MDA8 ozone for the current atmosphere and North American background 

scenarios for selected CASTNet sites are provided in Figure 5-3 through Figure 5-8. 

ICF International 146 October 28, 2011 11-035
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Estimation of North American Background Ozone (NAB Scenario) 

ICF International 147 October 28, 2011 11-035

Figure 5-3. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Lassen Volcanic NP, CA. 

 (a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2006

Base NAB

 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2007

Base NAB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2008

Base NAB
Modeling for North American Background Concentrations 

Estimation of North American Background Ozone (NAB Scenario) 

ICF International 148 October 28, 2011 11-035

Figure 5-4. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Pinedale, WY. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Pinedale, WY (PND165) MDA8: 2006

Base NAB

 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Pinedale, WY (PND165) MDA8: 2007

Base NAB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Pinedale, WY (PND165) MDA8: 2008

Base NAB
Modeling for North American Background Concentrations 

Estimation of North American Background Ozone (NAB Scenario) 

Figure 5-5. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Big Bend NP, TX. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2006

Base PRB

 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2007

Base PRB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2008

Base PRB

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Figure 5-6. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Perkinstown, WI. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2006

Base PRB

 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2007

Base PRB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2008

Base PRB

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Figure 5-7. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Great Smoky Mountains NP, TN. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2006

Base NAB

 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2007

Base NAB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2008

Base NAB

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Figure 5-8. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Blackwater NWR, MD. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2006

Base NAB

 

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2007

Base NAB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2008

Base NAB

Site-specific estimates of NAB ozone, relevant to MDA8 ozone, are summarized in Table 5-3. In 

this table mean MDA8 ozone is presented for each of the CASTNet sites highlighted in Section 

4 of this report. Mean values of observed, base (current atmosphere), and NAB O3 (North 

American background) concentrations, based on the three-year period are provided. Standard 

deviation values associated with the means are also provided. 

ICF International 152 October 28, 2011 11-035
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Estimation of North American Background Ozone (NAB Scenario) 

Table 5-3. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, and NAB Ozone Concentrations (ppb) fo

Based on the 2006, 2007 and 2008 Simulation Period. 

Site 

Winter (Dec–Feb) Spring (Mar–May) Summer (Jun–Aug) Fall (Sep–Nov) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Mount Rainier NP, WA 

Avg 26.41 35.36 33.03 39.11 43.31 34.52 33.69 39.18 23.69 24.19 37.57 30.72 

SDev 8.29 4.60 3.70 8.01 5.93 5.32 12.69 9.47 5.40 8.95 6.38 4.82 

Pinnacles NM, CA 

Avg 37.74 42.13 33.13 51.76 50.74 34.73 57.13 50.96 28.50 48.02 52.65 32.54 

SDev 5.92 5.92 3.11 9.16 7.92 4.79 12.12 12.71 7.12 11.72 11.36 4.94 

Joshua Tree NM, CA 

Avg 47.21 45.25 35.70 67.46 68.08 40.61 82.85 98.85 38.78 56.56 65.19 36.41 

SDev 5.44 5.94 3.65 11.41 16.66 5.53 12.73 15.15 8.90 10.68 17.51 5.23 

Lassen NP, CA Avg 43.60 40.30 34.65 52.20 48.68 37.22 57.99 53.38 37.95 45.72 48.63 35.46 

SDev 3.87 4.75 3.68 7.11 7.37 6.39 10.52 10.33 8.23 10.27 9.79 5.43 

Chiricahua NM, AZ 

Avg 44.60 46.21 36.18 56.27 55.74 42.57 54.51 59.94 40.18 44.61 49.37 33.90 

SDev 4.71 5.01 4.75 7.10 6.52 5.39 8.52 7.63 7.91 6.15 7.39 6.26 

Great Basin NP, NV 

Avg 42.82 42.12 36.01 55.15 52.55 41.70 58.41 61.95 42.13 45.18 49.10 36.42 

SDev 4.20 4.39 4.28 7.25 7.11 5.38 7.00 9.13 6.74 7.46 9.07 5.63 

Pinedale, WY Avg 45.46 43.80 38.23 55.53 53.81 41.42 56.54 57.33 40.80 45.33 49.71 37.28 

SDev 5.50 4.28 4.06 7.62 6.01 5.54 6.02 8.28 6.69 6.85 7.81 5.77 

Rocky Mtn NP, CO 

Avg 47.33 45.80 39.77 56.42 56.91 42.83 60.32 65.34 39.95 47.01 52.70 37.63 

SDev 5.54 4.42 3.85 7.20 6.04 5.21 8.27 8.05 5.64 6.74 7.91 5.43 

Caddo Valley, AR Avg 30.42 39.90 27.91 42.92 50.29 27.72 43.18 57.60 29.18 36.09 49.77 24.02 

SDev 8.68 5.56 4.44 10.04 6.40 4.67 13.05 8.69 6.57 11.61 9.03 4.12 

Konza Prairie, KS Avg 33.12 32.96 28.92 49.67 52.01 32.18 53.37 67.54 36.60 40.55 45.84 27.44 

SDev 6.34 6.56 5.39 10.23 8.32 5.06 10.12 8.23 6.70 11.47 12.00 4.65 

Theodore Roosevelt NP, ND 

Avg 34.44 32.77 30.27 47.22 45.69 33.71 48.44 53.04 35.88 35.18 38.40 29.58 

SDev 5.04 6.98 5.92 7.38 5.96 4.46 8.17 8.96 6.56 7.53 9.25 6.06 

Big Bend NP, TX Avg 42.99 43.86 32.74 54.46 53.10 39.08 46.35 55.20 39.56 43.61 47.77 30.49 

SDev 5.95 4.95 6.07 6.73 5.28 6.35 9.01 8.95 7.70 7.78 6.94 5.68 

ICF International 153 11-035
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ICF International 154 11-035

Site 

Winter (Dec–Feb) Spring (Mar–May) Summer (Jun–Aug) Fall (Sep–Nov) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

Unionville (Ann Arbor), MI 

Avg 28.42 26.59 30.19 46.56 50.64 31.76 48.36 68.57 27.52 32.49 44.95 26.17 

SDev 7.74 9.28 6.05 10.32 13.40 5.33 11.45 14.98 4.79 10.98 17.30 4.99 

Salamonie Reservoir, IN 

Avg 27.11 27.98 29.53 47.26 54.30 31.10 51.33 72.50 28.34 36.11 48.05 26.09 

SDev 7.07 7.98 5.58 10.86 13.49 5.54 10.48 11.05 4.98 13.43 17.66 4.91 

Oxford, OH Avg 27.42 27.22 29.96 48.48 56.08 30.56 53.30 75.92 27.96 37.63 50.69 26.37 

SDev 7.45 8.99 4.98 11.85 15.46 5.50 11.82 11.48 5.48 15.79 19.98 4.91 

Perkinstown, WI Avg 31.95 32.75 31.95 48.67 49.82 32.77 44.79 52.64 27.50 33.77 42.73 27.08 

SDev 6.83 6.94 4.81 8.30 8.90 5.11 10.07 12.47 5.14 9.06 11.13 4.63 

Sumatra, FL Avg 35.13 47.40 30.19 49.05 58.29 32.06 36.22 58.40 33.19 36.63 53.56 27.32 

SDev 8.75 5.96 4.05 10.21 7.70 4.53 12.19 9.01 6.66 9.86 7.83 4.62 

Cadiz, KY Avg 31.07 33.66 29.35 51.16 57.71 28.41 53.95 69.32 27.62 43.04 53.76 25.48 

SDev 8.92 8.17 4.48 10.69 9.40 5.11 11.73 7.97 4.97 13.41 14.43 4.47 

Sand Mountain, AL 

Avg 33.72 40.18 29.15 53.56 58.71 29.07 57.75 66.20 27.84 43.46 56.22 25.10 

SDev 8.72 7.57 4.48 9.84 6.71 4.63 11.81 7.47 6.71 12.26 11.64 4.38 

Great Smoky Mtns. NP, TN 

Avg 39.73 39.61 30.38 59.99 59.19 29.42 65.01 65.23 26.93 48.47 54.41 26.16 

SDev 7.35 7.08 4.35 10.84 7.00 4.97 11.56 7.09 6.80 11.70 10.55 4.95 

Blackwater NWR, MD 

Avg 30.80 34.84 30.18 51.90 61.25 29.35 60.55 77.05 25.41 39.49 56.20 25.96 

SDev 6.91 7.46 4.67 10.60 13.96 4.98 13.12 13.17 6.64 11.60 16.53 4.57 

Acadia NP, ME Avg 35.63 41.99 33.11 47.49 56.57 31.47 44.25 66.73 19.85 34.92 52.35 25.91 

SDev 6.08 9.00 3.93 8.45 16.17 5.33 13.57 23.83 4.26 9.48 17.82 4.96 

Penn State, PA Avg 29.12 29.34 31.35 48.88 55.55 30.94 52.10 65.56 24.09 36.28 48.79 25.47 

SDev 7.70 9.05 5.06 10.30 12.61 5.41 12.39 13.86 5.56 12.02 14.98 4.70 

Huntington Wildlife Forest, NY 

Avg 31.83 35.58 32.59 44.38 49.61 32.08 36.36 50.46 22.24 29.87 43.68 25.36 

SDev 5.75 5.67 4.01 9.61 8.64 4.99 10.88 13.99 4.79 9.23 10.59 5.19 
6. Additional Scenarios and Comparisons 

6.1. U.S. and Natural Background Scenarios 

The results for two additional scenarios are presented and discussed in this section. These are 

1) the U.S. background scenario for which the emission inventory consists of the current 

atmosphere emissions minus anthropogenic emissions for sources in the onshore areas of the 

U.S. and 2) the natural background scenario which includes only natural emissions (for all 

sources throughout the modeling domain). In the remainder of this section these are referred to 

as USB and NB respectively. For ease of comparison, many of the graphical and tabular 

summaries include the results for the base-case (current atmosphere), NAB, USB, and NB 

scenarios. 

Figure 6-1 displays the mean MDA8 ozone concentrations for the base (current atmosphere), 

NAB, USB, and NB scenarios, based on all three simulation years. 

Figure 6-1. Mean MDA8 Ozone Concentrations (ppb) for the Base (Current Atmosphere), NAB, USB and NB Scenarios, 

Based on the 2006, 2007, and 2008 Simulation Period. 

(a) Current Atmosphere Scenario (b) North American Background Scenario 

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Additional Scenarios and Comparisons 

(c) U.S. Background Scenario (d) Natural Background Scenario 

For the current atmosphere scenario, mean MDA8 ozone ranges from 21.3 to 82.6 ppb within 

the modeling grid. For the NAB scenario, mean MDA8 ozone ranges from 17.4 to 42.9 ppb. For 

the USB scenario, mean MDA8 ozone ranges from 19.9 to 73.4 ppb. Compared to the NAB 

values, the simulated USB values are higher throughout the domain including over the 

continental U.S. This increase is attributable to Canadian, Mexican and offshore emissions. For 

the NB scenario, the concentrations are quite a bit lower and mean MDA8 ozone ranges from 

12.8 to 30.7 ppb. Within the U.S., the simulated NB ozone concentrations are very low along the 

northeast corridor and the highest values are found over Colorado. 

As for NAB O3 (see Figure 5-2), the modeling results for USB and NB vary by season, as 

illustrated in Figures 6-2 and 6-3. 

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Figure 6-2. Mean MDA8 Ozone Concentrations (ppb) for the U.S. Background Scenario, Based on the 2006, 2007, and 

2008 Simulation Period. 

(a) Winter (December–February) (b) Spring (March–May) 

(c) Summer (June–August) (b) Fall (September–November) 

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Figure 6-3. Mean MDA8 Ozone Concentrations (ppb) for the Natural Background Scenario, Based on the 2006, 2007, and 

2008 Simulation Period. 

(a) Winter (December–February) (b) Spring (March–May) 

(c) Summer (June–August) (b) Fall (September–November) 

Depending upon locations, the highest USB and NB values tend to occur either during the 

spring or summer period. In general, the highest values occur offshore (USB only) and over the 

western U.S. (USB and NB). 

Table 5-1 lists the seasonal and annual mean NAB, USB and NB O3 values relevant to 24-hour 

average ozone for the entire U.S. and each of the six model performance subregions, based on 

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the CASTNet sites and the three-year simulation period. Averages were computed over all 

CASTNet sites in the region and each day of the three-year simulation period that corresponds 

to the averaging period. For comparison purposes, the mean observed and base-case (current 

atmosphere) concentrations are also included in the table. Units are ppb. 

Table 6-1. GEOS-Chem-Derived Seasonal and Annual Mean 24-Hour Average Observed, Current Atmosphere, NAB, 

USB, and NB Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. 

(a) Entire Continental U.S. 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 29.03 30.46 29.21 31.17 19.41 

Spring 

(Mar-May) 42.04 45.20 29.05 32.51 19.88 

Summer 

(Jun-Aug) 40.04 51.52 24.73 28.44 19.55 

Fall (SepNov) 30.70 40.17 24.72 27.98 17.94 

Annual 35.50 41.88 26.93 30.03 19.20 

(b) West Coast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 33.33 35.94 32.00 33.56 21.13 

Spring 

(Mar-May) 44.70 45.40 33.07 34.76 21.60 

Summer 

(Jun-Aug) 49.59 53.08 27.50 30.25 20.25 

Fall (SepNov) 37.10 44.09 30.21 32.68 21.53 

Annual 41.24 44.63 30.74 32.85 21.13 

ICF International 159 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

(c) Intermountain West 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 39.97 39.28 33.93 35.66 22.75 

Spring 

(Mar-May) 49.62 47.75 36.75 38.85 25.32 

Summer 

(Jun-Aug) 47.55 52.10 34.91 37.79 27.30 

Fall (SepNov) 38.96 43.34 31.75 34.44 22.91 

Annual 44.04 45.65 34.33 36.69 24.58 

(d) Central States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 29.14 31.33 26.70 28.68 17.87 

Spring 

(Mar-May) 39.17 42.21 27.84 31.39 19.79 

Summer 

(Jun-Aug) 37.29 47.06 27.90 31.32 22.86 

Fall (SepNov) 30.07 37.18 23.21 26.29 17.33 

Annual 33.88 39.42 26.37 29.38 19.43 

ICF International 160 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

(e) Great Lakes States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 23.13 21.76 27.47 28.89 18.06 

Spring 

(Mar-May) 38.01 41.96 27.30 30.58 18.73 

Summer 

(Jun-Aug) 37.11 54.01 22.93 26.59 18.53 

Fall (SepNov) 25.61 35.11 22.45 25.18 16.43 

Annual 31.01 38.20 25.07 27.84 17.95 

(f) Southeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 27.32 31.18 27.46 29.83 18.39 

Spring 

(Mar-May) 42.29 47.95 25.56 28.77 17.86 

Summer 

(Jun-Aug) 38.83 52.46 21.72 24.15 17.38 

Fall (SepNov) 30.66 43.24 22.29 25.16 16.23 

Annual 34.85 43.78 24.25 26.98 17.46 

ICF International 161 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

(g) Northeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 24.82 25.79 29.06 31.33 19.08 

Spring 

(Mar-May) 38.85 43.49 27.37 33.48 18.20 

Summer 

(Jun-Aug) 34.25 49.45 18.43 25.43 14.56 

Fall (SepNov) 24.61 36.97 21.84 27.20 15.70 

Annual 30.70 38.99 24.18 29.38 16.88 

The annual average USB ozone concentration for the entire U.S. (relevant to 24-hour average 

ozone concentration) is estimated to be 30.0 ppb and the NB concentration is estimated to be 

19.2 ppb. 

For the western U.S. (West Coast and Intermountain West subregions), the estimated range in 

annual average USB concentration is 32.9 to 36.7 ppb and the estimated range in NB 

concentration is 21.1 to 24.6 ppb. For the eastern U.S. (Central States, Great Lakes, Southeast, 

and Northeast subregions) the values are lower and the estimated range in USB concentration 

is 27.0 to 29.4 ppb and the estimated range in NB concentration is 16.9 to 19.4 ppb. 

For the spring months, the average USB ozone concentration relevant to 24-hour average 

ozone is estimated to be 32.5 ppb for the entire U.S.; the corresponding NB concentration is 

19.9 ppb. 

For the summer months, the average USB ozone concentration relevant to 24-hour average 

ozone is estimated to be 28.4 ppb for the entire U.S. The NB concentrations are characterized 

by less variation among the seasons than either the USB or the NAB and the corresponding NB 

concentration is 19.6 ppb. 

Table 5-2 lists the seasonal and annual mean NAB, USB, and NB O3 values relevant to MDA8 

ozone for the entire U.S. and each of the six model performance subregions, based on the 

CASTNet sites and the three-year simulation period. The mean observed and base-case 

(current atmosphere) concentrations are also included in the table. Units are ppb. 

ICF International 162 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

Table 6-2. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, NAB, USB, and NB 

Ozone Concentrations (ppb), Based on the 2006, 2007 and 2008 Simulation Period. 

(a) Entire Continental U.S. 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 34.66 37.21 32.23 34.48 21.39 

Spring 

(Mar-May) 50.93 54.83 33.20 37.05 22.84 

Summer 

(Jun-Aug) 52.03 64.11 30.04 34.31 23.84 

Fall (SepNov) 39.50 50.00 28.68 32.34 20.80 

Annual 44.37 51.63 31.04 34.56 22.23 

(b) West Coast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 38.87 42.20 35.39 37.02 23.47 

Spring 

(Mar-May) 54.10 55.02 37.77 39.62 24.95 

Summer 

(Jun-Aug) 64.14 67.50 33.86 37.17 25.16 

Fall (SepNov) 46.31 53.62 34.61 37.23 24.70 

Annual 50.94 54.61 35.45 37.79 24.57 

ICF International 163 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

(c) Intermountain West 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 43.69 43.90 37.16 38.94 25.00 

Spring 

(Mar-May) 55.59 54.69 41.73 43.91 28.99 

Summer 

(Jun-Aug) 55.48 60.34 40.39 43.38 31.53 

Fall (SepNov) 44.62 50.30 36.42 39.18 26.17 

Annual 49.89 52.37 38.94 41.37 27.94 

(d) Central States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 34.85 37.36 30.27 32.60 20.23 

Spring 

(Mar-May) 48.16 50.40 32.60 36.50 23.28 

Summer 

(Jun-Aug) 47.88 57.92 34.23 37.86 28.18 

Fall (SepNov) 38.88 45.77 27.80 31.28 20.81 

Annual 42.44 47.87 31.18 34.52 23.10 

ICF International 164 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

(e) Great Lakes States 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 28.69 29.34 30.42 32.08 19.92 

Spring 

(Mar-May) 47.80 53.78 31.27 34.94 21.52 

Summer 

(Jun-Aug) 50.91 69.51 27.75 31.96 22.61 

Fall (SepNov) 35.43 47.39 26.40 29.48 19.36 

Annual 40.77 50.03 28.98 32.14 20.86 

(f) Southeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 34.26 38.92 30.32 32.90 20.24 

Spring 

(Mar-May) 52.44 57.97 29.31 32.78 20.55 

Summer 

(Jun-Aug) 51.22 64.46 27.03 29.76 21.69 

Fall (SepNov) 40.63 53.20 25.80 28.91 18.75 

Annual 44.76 53.77 28.11 31.09 20.31 

ICF International 165 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

(g) Northeast 

Time 

Period 

Mean 

Observed 

(ppb) 

Mean 

Simulated 

(Base) 

(ppb) 

Mean 

Simulated 

(NAB) 

(ppb) 

Mean 

Simulated 

(USB) 

(ppb) 

Mean 

Simulated 

(NB) 

(ppb) 

Winter 

(Dec-Feb) 30.17 32.84 31.59 34.62 20.68 

Spring 

(Mar-May) 47.27 53.86 30.83 37.98 20.51 

Summer 

(Jun-Aug) 47.19 64.28 22.88 31.46 18.12 

Fall (SepNov) 33.47 48.05 25.19 31.67 18.08 

Annual 39.63 49.89 27.61 33.95 19.35 

The annual average USB ozone concentration for the entire U.S. (relevant to MDA8 ozone 

concentration) is estimated to be 34.6 ppb and the NB concentration is estimated to be 22.2 ppb. 

For the western U.S. (West Coast and Intermountain West subregions), the estimated range in 

annual average USB concentration is 37.8 to 41.4 ppb and the estimated range in NB 

concentration is 24.6 to 27.9 ppb. For the eastern U.S. (Central States, Great Lakes, Southeast, 

and Northeast subregions) the values are lower and the estimated range in USB concentration 

is 31.1 to 34.5 ppb and the estimated range in NB concentration is 19.4 to 23.1 ppb. 

For the spring months, the average USB ozone concentration relevant to MDA8 ozone is 

estimated to be 37.1 ppb for the entire US; the corresponding NB concentration is 22.8 ppb. 

For the summer months, the average USB ozone concentration relevant to MDA8 ozone is 

estimated to be 34.3 ppb for the entire U.S. The corresponding NB concentration is 23.8 ppb. 

Plots comparing MDA8 ozone for the current atmosphere, NAB, USB, and NB scenarios for 

selected CASTNet sites are provided in Figure 6-3 through Figure 6-9. Note that the NAB 

(green) and USB (gray) lines frequently overlap. 

ICF International 166 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

ICF International 167 October 28, 2011 11-035

Figure 6-4. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Lassen Volcanic NP, CA. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2006

Base NAB USB NB

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Lassen Volcanic NP, CA (LAV410) MDA8: 2007

Base NAB USB NB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Lassen Vocalnic NP, CA (LAV410) MDA8: 2008

Base NAB USB NB
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

ICF International 168 October 28, 2011 11-035

Figure 6-5. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Pinedale, WY. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Pinedale, WY (PND165) MDA8: 2006

Base NAB USB NB

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Pinedale, WY (PND165) MDA8: 2007

Base NAB USB NB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Pinedale, WY (PND165) MDA8: 2008

Base NAB USB NB
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

Figure 6-6, Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Big Bend NP, TX. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2006

Base NAB USB NB

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2007

Base NAB USB NB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Big Bend NP, TX (BBE401) MDA8: 2008

Base NAB USB NB

ICF International 169 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

Figure 6-7. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Perkinstown, WI. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2006

Base NAB USB NB

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2007

Base NAB USB NB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Perkinstown, WI (PRK134) MDA8: 2008

Base NAB USB NB

ICF International 170 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

Figure 6-8. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Great Smoky Mountains NP, TN. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2006

Base NAB USB NB

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2007

Base NAB USB NB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Great Smoky NP, TN (GRS420) MDA8: 2008

Base NAB USB NB

ICF International 171 October 28, 2011 11-035
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Additional Scenarios and Comparisons 

Figure 6-9. Mean MDA8 Ozone Concentrations (ppb) for the Current Atmosphere and North American Background 

Scenarios for Blackwater NWR, MD. 

(a) 2006 (b) 2007 

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2006

Base NAB USB NB

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2007

Base NAB USB NB

(c) 2008 

0

10

20

30

40

50

60

70

80

90

100

110

120

1 31 61 91 121 151 181 211 241 271 301 331 361

ppb

Day

CASTNet Site Blackwater NWR, MD (BWR139) MDA8: 2008

Base NAB USB NB

Site-specific estimates of NAB ozone, relevant to MDA8 ozone are summarized in Table 5-3. In 

this table mean MDA8 ozone is presented for each of the CASTNet sites highlighted in Section 4 

of this report. Mean values of observed, base (current atmosphere), NAB, USB, and NB ozone 

concentrations for the spring, summer, and annual periods, based on three years of modeling 

results are provided. Standard deviation values associated with the means are also provided. 

ICF International 172 October 28, 2011 11-035
Table 6-3. GEOS-Chem-Derived Seasonal and Annual Mean MDA8 Observed, Current Atmosphere, NAB, USB, and NB Ozone Concentrat

Summer, and Annual Periods for Selected CASTNet Sites, Based on the 2006, 2007 and 2008 Simulation Period. 

Site 

Spring (Mar–May) Summer (Jun–Aug) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

USB 

(ppb) 

NB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

USB 

(ppb) 

NB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

Mount Rainier NP, WA 

Avg 39.11 43.31 34.52 36.46 21.69 33.69 39.18 23.69 26.55 16.20 30.89 38.88 

SDev 8.01 5.93 5.32 5.18 3.58 12.69 9.47 5.40 6.71 4.09 11.34 7.43 

Pinnacles NM, CA 

Avg 51.76 50.74 34.73 36.23 22.14 57.13 50.96 28.50 29.94 20.35 48.71 49.15 

SDev 9.16 7.92 4.79 4.70 3.31 12.12 12.71 7.12 7.19 5.76 12.28 10.67 

Joshua Tree NM, CA 

Avg 67.46 68.08 40.61 43.59 27.62 82.85 98.85 38.78 45.77 30.01 63.58 69.49 

SDev 11.41 16.66 5.53 6.04 4.31 12.73 15.15 8.90 7.81 6.23 16.87 24.08 

Lassen NP, CA 

Avg 52.20 48.68 37.22 38.39 24.26 57.99 53.38 37.95 39.04 27.98 49.95 47.79 

SDev 7.11 7.37 6.39 6.34 4.44 10.52 10.33 8.23 8.26 6.78 10.13 9.59 

Chiricahua NM, AZ 

Avg 56.27 55.74 42.57 46.33 30.17 54.51 59.94 40.18 46.53 32.84 50.06 52.86 

SDev 7.10 6.52 5.39 4.92 4.01 8.52 7.63 7.91 7.65 6.28 8.67 8.59 

Great Basin NP, NV 

Avg 55.15 52.55 41.70 43.28 28.66 58.41 61.95 42.13 45.57 30.84 50.56 51.49 

SDev 7.25 7.11 5.38 5.26 4.22 7.00 9.13 6.74 6.97 5.46 9.31 10.48 

Pinedale, WY Avg 55.53 53.81 41.42 43.22 28.36 56.54 57.33 40.80 42.69 31.05 50.56 51.21 

SDev 7.62 6.01 5.54 5.50 4.09 6.02 8.28 6.69 6.53 5.43 8.43 8.44 

Rocky Mtn NP, CO 

Avg 56.42 56.91 42.83 44.66 30.07 60.32 65.34 39.95 41.93 31.30 52.82 55.25 

SDev 7.20 6.04 5.21 5.08 4.06 8.27 8.05 5.64 5.78 4.70 9.08 9.79 

Caddo Valley, AR 

Avg 42.92 50.29 27.72 30.64 19.93 43.18 57.60 29.18 31.07 23.48 38.21 49.44 

SDev 10.04 6.40 4.67 4.40 3.28 13.05 8.69 6.57 6.57 5.27 12.14 9.83 

Konza Prairie, KS 

Avg 49.67 52.01 32.18 35.24 23.48 53.37 67.54 36.60 39.13 31.14 44.25 49.69 

SDev 10.23 8.32 5.06 5.25 4.18 10.12 8.23 6.70 6.48 6.15 12.53 15.34 

Theodore Roosevelt NP, ND 

Avg 47.22 45.69 33.71 37.97 22.98 48.44 53.04 35.88 40.89 29.49 41.20 42.54 

SDev 7.38 5.96 4.46 5.01 3.31 8.17 8.96 6.56 5.88 6.78 9.63 10.98 

Big Bend NP, TX 

Avg 54.46 53.10 39.08 47.04 29.36 46.35 55.20 39.56 47.70 32.37 46.88 50.02 

SDev 6.73 5.28 6.35 5.45 4.59 9.01 8.95 7.70 6.62 6.26 8.75 8.06 

ICF International 173 11-035
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Additional Scenarios and Comparisons 

ICF International 174 11-035

Site 

Spring (Mar–May) Summer (Jun–Aug) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

USB 

(ppb) 

NB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

NAB 

(ppb) 

USB 

(ppb) 

NB 

(ppb) 

Obs 

(ppb) 

Base 

(ppb) 

Unionville (Ann Arbor), MI 

Avg 46.56 50.64 31.76 36.19 21.68 48.36 68.57 27.52 32.81 22.17 39.01 47.81 

SDev 10.32 13.40 5.33 5.42 3.73 11.45 14.98 4.79 6.95 4.17 13.38 20.52 

Salamonie Reservoir, IN 

Avg 47.26 54.30 31.10 34.58 21.60 51.33 72.50 28.34 32.39 23.36 40.69 50.84 

SDev 10.86 13.49 5.54 5.44 3.95 10.48 11.05 4.98 6.02 4.42 14.28 20.54 

Oxford, OH Avg 48.48 56.08 30.56 33.92 21.23 53.30 75.92 27.96 32.03 22.92 41.73 52.62 

SDev 11.85 15.46 5.50 5.55 3.82 11.82 11.48 5.48 6.48 4.65 15.69 22.64 

Perkinstown, WI 

Avg 48.67 49.82 32.77 36.33 22.06 44.79 52.64 27.50 31.15 21.93 39.68 44.55 

SDev 8.30 8.90 5.11 4.77 3.20 10.07 12.47 5.14 5.19 4.52 11.16 12.66 

Sumatra, FL Avg 49.05 58.29 32.06 36.65 22.83 36.22 58.40 33.19 35.86 26.44 39.32 54.45 

SDev 10.21 7.70 4.53 4.47 3.13 12.19 9.01 6.66 6.56 5.46 11.79 8.91 

Cadiz, KY Avg 51.16 57.71 28.41 31.19 20.08 53.95 69.32 27.62 29.99 22.51 45.27 53.72 

SDev 10.69 9.40 5.11 5.19 3.35 11.73 7.97 4.97 5.14 4.20 14.28 16.47 

Sand Mountain, AL 

Avg 53.56 58.71 29.07 32.01 20.65 57.75 66.20 27.84 29.65 22.42 47.21 55.41 

SDev 9.84 6.71 4.63 4.50 3.12 11.81 7.47 6.71 6.77 5.55 14.22 12.75 

Great Smoky Mtns NP, TN 

Avg 59.99 59.19 29.42 32.36 20.75 65.01 65.23 26.93 28.86 21.54 53.39 54.69 

SDev 10.84 7.00 4.97 4.92 3.16 11.56 7.09 6.80 6.93 5.63 14.41 12.42 

Blackwater NWR, MD 

Avg 51.90 61.25 29.35 35.57 19.86 60.55 77.05 25.41 30.33 20.38 45.55 57.46 

SDev 10.60 13.96 4.98 5.85 2.83 13.12 13.17 6.64 7.10 5.63 15.62 20.03 

Acadia NP, ME 

Avg 47.49 56.57 31.47 47.94 20.28 44.25 66.73 19.85 47.33 15.21 41.11 54.49 

SDev 8.45 16.17 5.33 13.37 3.16 13.57 23.83 4.26 17.88 3.05 11.27 19.65 

Penn State, PA 

Avg 48.88 55.55 30.94 35.97 20.83 52.10 65.56 24.09 29.28 19.18 41.90 49.93 

SDev 10.30 12.61 5.41 5.38 3.36 12.39 13.86 5.56 6.67 4.72 14.23 18.40 

Huntington Wildlife Forest, NY 

Avg 44.38 49.61 32.08 39.42 21.18 36.36 50.46 22.24 32.95 17.56 35.68 44.89 

SDev 9.61 8.64 4.99 4.29 2.94 10.88 13.99 4.79 7.89 4.00 10.62 11.79 
6.2. Comparison of North American and Natural Background Ozone 

Concentrations 

As discussed in the previous sections, there are spatial and seasonal variations in the NAB 

concentrations and to a lesser extent the NB concentrations. Figures 6-10 through 6-13 show 

that estimated NAB also varies with observed concentration level, and that this variation is more 

pronounced for certain regions. NB, on the other hand, is fairly consistent for the range of 

concentrations simulated for all regions. 

Figure 6-10 illustrates this for the Intermountain West subregion, using observed and simulated 

values for the CASTNet sites within this subregion for all three simulation years. Results for 

Spring (March – May) and Summer (June – August) are shown in the left and right columns, 

respectively. The top plot in each column compares simulated versus observed MDA8 ozone 

Concentrations for all sites in the subregion. The box-and-whisker plots that follow summarize 

NAB and NB for 10-ppb bins of observed ozone concentration. The box-and-whisker plots 

illustrate the 25th, 50th, and 75th percentile values (lines at the bottom of the box, within the box, 

and at the top of the box), as well as minimum and maximum values (lowest and highest values 

indicated by the whiskers). For this region, the NAB, and to a lesser extent, the NB values 

increase with concentration. 

Figures 6-11 through 6-13 provide the same information for the Great Lakes, Southeast, and 

Northeast subregions, and indicate less variation in both NAB and NB with observed 

concentration, especially for the Southeast subregion (Figure 6-12). 

ICF International 175 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

Figure 6-10. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Intermountain West CASTNet sites. The 

Box-and-Whisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB and NB for 10-ppb Bins of 

Observed Ozone Concentration: 2006-2008. 

Spring (March–May) Summer (June–August) 

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Intermountain Sites - Spring 2006-2008

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Intermountain Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Intermountain Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Intermountain Sites - Summer 2006

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Intermountain Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Intermountain Sites - Summer 2006-2008

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Additional Scenarios and Comparisons 

Figure 6-11. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Great Lakes CASTNet sites. The Boxand-Whisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB and NB for 10-ppb Bins of 

Observed Ozone Concentration: 2006-2008. 

Spring (March–May) Summer (June–August) 

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Great Lakes Sites - Spring 2006-2008

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Great Lakes Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Great Lakes Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Great Lakes Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Great Lakes Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Great Lakes Sites - Summer 2006-2008

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Additional Scenarios and Comparisons 

Figure 6-12. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Southeast CASTNet sites. The Box-andWhisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB and NB for 10-ppb Bins of 

Observed Ozone Concentration: 2006-2008. 

Spring (March–May) Summer (June–August) 

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Southeast Sites - Spring 2006-2008

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Southeast Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Southeast Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Southeast Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Southeast Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Southeast Sites - Summer 2006-2008

ICF International 178 October 28, 2011 11-035
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

Figure 6-13. Simulated vs. Observed MDA8 Ozone Concentrations (ppb) for the Northeast CASTNet sites. The Box-andWhisker Plots (Minimum, 25th, 50th, 75th Percentiles, and Maximum) Summarize NAB and NB for 10-ppb Bins of 

Observed Ozone Concentration: 2006-2008. 

Spring (March–May) Summer (June–August) 

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Northeast Sites - Spring 2006-2008

0

20

40

60

80

100

0 20 40 60 80 100

Base Simulated (ppb)

Observed (ppb)

CASTNet MDA8 for Northeast Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Northeast Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

PRB Concentration (ppb)

Obs Concentration Range (ppb)

NAB Summary for Northeast Sites - Summer 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Northeast Sites - Spring 2006-2008

0

20

40

60

80

100

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100

NB Concentration (ppb)

Obs Concentration Range (ppb)

NB Summary for Northeast Sites - Summer 2006-2008

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Additional Scenarios and Comparisons 

ICF International 180 October 28, 2011 11-035

An additional comparison highlights the frequency distribution of the observed and simulated 

ozone concentrations for sites at different elevations. In this case, the sites are separated into 

two groups according to site elevation. CASTNet sites with elevations less than 1.5 km are in 

one group and sites with elevations greater than 1.5 km are in another group. Figure 6-14

illustrates the frequency distribution of observed, base, NAB, and NB concentrations for each 

group of sites. For each 10-ppb bin, frequency is calculated as the number of days with MDA8 

concentrations within that bin divided by the total number of days included in the distribution. All 

days within the three year simulation period were included. A five-point smoother was applied to 

each distribution for plotting. 

Figure 6-14. Frequency Distributions of MDA8 Ozone Concentrations (ppb) for March – August 2006, 2007 & 2008 for 

Low-Elevation (<1.5 km) and High-Elevation (>1.5 km) CASTNet Sites. Observed Values are in Black; Model Results are 

in Red, NAB is in Blue, and NB is in Green (Dashed). 

Low-Elevation Sites

0

0.02

0.04

0.06

0.08

0.1

0 10 20 30 40 50 60 70 80 90 100 110 120 130 14

Frequency

O3 Conc (ppb)

Frequency Distributions of MDA8 of O3 Conc in March-August 2006-2008: Low Altitude Sites ( <1.5 km)

0

Observed GEOS-Chem NAB NB

High-Elevation Sites

0

0.02

0.04

0.06

0.08

0.1

0 10 20 30 40 50 60 70 80 90 100 110 120 130 14

Frequency

O3 Conc (ppb)

Frequency Distributions of MDA8 of O3 Conc in March-August 2006-2008: High Altitude Sites ( > 1.5 km)

0

Observed GEOS-Chem NAB NB
Modeling for North American Background Concentrations 

Additional Scenarios and Comparisons 

The overall frequency distribution of the observed concentrations is better represented at the 

high-elevations sites, compared to the lower-elevations sites. The offset for the lower-elevation 

sites indicates that much of the overestimation of ozone indicated by the statistical model 

performance evaluation is associated with the lower elevation sites. 

The distributions are also quite different between the high- and low-elevations sites, with a clear 

tendency for higher concentrations, including NAB and NB concentrations, at the high-elevation 

sites. 

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Additional Scenarios and Comparisons 

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ICF International 182 October 28, 2011 11-035
7. Attributes and Limitations 

The GEOS-Chem tool allows the estimation of contributions to background ozone concentrations 

from a variety of sources and source regions, and the non-linear effects associated with the 

removal of emissions from selected sources and source regions has been shown to be small. 

Attributes and limitations include: 

 The grid resolution required for a global simulation is not expected to resolve regional- and 

urban-scale ozone production in the U.S. and elsewhere and thus may under- or overestimate the anthropogenic contribution to long-range ozone transport. 

 The GEOS-CHEM chemical mechanism includes a relatively detailed representation of the 

reactions and species involved in the production of ozone in the atmosphere. Details of the 

mechanism are presented in Evans et al. (2003). The mechanism includes hundreds of 

reactions and more than 80 species. In a comparison of chemical mechanisms, Emmerson and 

Evans (2008) noted that the GEOS-CHEM mechanism (and the other mechanisms evaluated in 

their paper) should be able to represent the atmospheric chemistry in the troposphere. 

Nevertheless, all chemical mechanisms suffer from the necessity to limit the number of reactions 

and species to a finite set rather than the many thousands of reactions and species actually 

taking part in the chemistry of the troposphere. Perhaps even more important, limitations in grid 

resolution of the model can limit the model’s ability to properly represent the relative proportions 

of species. This limitation could result in alterations in the estimates of ozone production rates 

compared to what would be simulated with higher grid resolution. Good performance at monitors 

will not guarantee that alterations in the chemical mix (e.g., by removing a category of 

emissions) will produce the correct response in ozone production. Hence, the chemical 

mechanism and the interaction of the chemical mechanism with grid resolution must be 

considered to be potential sources of uncertainty in the model results. 

 Emissions estimates for GEOS-CHEM are based on data available to the model developers. 

These data are more readily available for some parts of the world (e.g., the U.S., Europe) 

than others (e.g., developing nations). The magnitude and distribution of emissions are 

therefore sources of uncertainty in the model runs, and this uncertainty may be difficult to 

quantify for many parts of the world. Although the chemical mechanism in GEOS-CHEM 

includes a wide range of hydrocarbons, the number of emitted species included in the model 

is much smaller. The speciation of emissions into constituent hydrocarbons is difficult even 

for domestic US emissions, where relatively robust data are available for making these 

estimates. For parts of the world where data are lacking, even greater uncertainty is present. 

The use of a more limited set of emitted species is likely appropriate given that a more 

detailed speciation of hydrocarbons would necessarily involve some guess work. 

Uncertainties in the speciation of hydrocarbons introduce another uncertainty into the 

GEOS-CHEM simulation results. 

 Considering the scale and resolution of the modeling domain, parameterizations of smallscale processes, such as boundary layer ventilation and downward mixing of free 

tropospheric and surface air are key sources of uncertainty in any global model application, 

including this application of GEOS-Chem. 

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Attributes and Limitations 

ICF International 184 October 28, 2011 11-035

 It is difficult to fully evaluate model performance, given the grid resolution and the available 

data. In particular, it is difficult to confirm that the model reliably simulates the vertical 

distribution (and transport) of ozone aloft as well as the various processes, such as vertical 

mixing within the troposphere and stratospheric ozone intrusion, that influence background 

ozone levels. 

The assessment of model performance for this application reveals consistent overestimation of 

ozone concentrations throughout the U.S., but especially in the eastern U.S. Comparison of 

simulated and observed ozone profiles for five case-study periods (a total of 12 days) and 

several locations within the western U.S. gives mixed results. For some cases, the simulated 

vertical profiles show poor agreement with the observations. For other cases the results are 

more promising in that although the detailed vertical structure of observed profiles is not well 

simulated, the model results show layers of high ozone in the middle troposphere, consistent 

with the observations. 
8. References 

Bey, I., D.J. Jacob, R.M. Yantosca, J.A. Logan, B.D. Field, A.M. Fiore, Q. Li, H.Y. Liu, L.J. 

Mickley, and M. Schultz. 2001. Global modeling of tropospheric chemistry with assimilated 

meteorology: Model description and evaluation. J. Geophys. Res., 106, 23,073– 23,096. 

K.M. Emmerson and M.J. Evans. 2008. Comparison of tropospheric chemistry schemes for use 

within global models. Atmos. Chem. Phys. Discuss., 8, 19957–19987. 

M. Evans, A. Fiore, and D.J. Jacob. 2003. The GEOS-Chem Chemical Mechanism Version 5-7-

08. http://acmg.seas.harvard.edu/geos/wiki_docs/chemistry/geoschem_mech.pdf. 

Fiore A., D.J. Jacob, H. Liu, R.M. Yantosca, T.D., Fairlie, and Q. Li. 2003. Variability in surface 

ozone background over the United States: Implications for air quality policy. J. Geophys. 

Res., 108(D24), 4787. 

Liu H., D.J. Jacob, I. Bey, and R.M. Yantosca. 2001. Constraints from 210Pb and 7

Be on wet 

deposition and transport in a global three-dimensional chemical tracer model driven by 

assimilated meteorological fields. J. Geophys. Res., 106(12), 109-128. 

Mao, J., D.J. Jacob, M.J. Evans, J.R. Olson, X. Ren, H.W. Brune, J.M. Clair, J.D. St., Crounse, 

K.M. Spencer, M.R. Beaver, P.O. Wennberg, M.J. Cubison, J.L. Jimenez,., A. Fried, P. 

Weibring, J.G. Walega, S.R. Hall, A.J. Weinheimer, R.C. Cohen, G. Chen, J.H. Crawford, 

C. McNaughton, A.D. Clarke, L. Jaegl ́e, J.A. Fisher, R.M. Yantosca, P. Le Sager, and C. 

Carouge. 2010. Chemistry of hydrogen oxide radicals (HOx) in the Arctic troposphere in 

spring, Atmos. Chem. Phys., 10, 5823–5838. 

McLinden, C.A, C.S. Olsen, B. Hannegan, O. Wild, M.J. Prather, and O. Sundet. 2000. 

Stratospheric ozone in 3-D models: A simple chemistry and the cross-tropopause ozone 

flux. J. Geophys. Res., 105(14), 653-655. 

Thompson, A.M., J.E. Yorks, S.K. Miller, J.C. Whitte, K.M. Dougherty, G.A. Morris, D. 

Baumgardner, L. Ladino, and B. Rappengluck. 2008. Tropospheric ozone sources and 

wave activity over Mexico City and Houston during MILAGRO/Intercontinental Transport 

Experiment (INTEX-B) Ozonesonde Network Study, 2006 (IONS-06). Atmos. Chem. Phys., 

8, 5113-5125. 

Thompson, A.M., S.J. Oltmans, D.W. Tarasick, P. von der Gather, H.G.J. Smit, J.C. Witte. 2011. 

Strategic ozone sounding networks: Review of design and accomplishments. Atmos. 

Environ., 45, 2145-2163. 

Wang, Y., D.J. Jacob, and J.A. Logan. 1998. Global simulation of tropospheric O3-NOxhydrocarbon chemistry, 1. Model formulation. J. Geophys. Res., 103(D9), 10,713-10,726. 

Wang, H., D.J. Jacob, P. Le Sager, D.G. Streets, R.J. Park, A.B. Gilliland, and A. van 

Donkelaar. 2009. Surface ozone background in the United States: Canadian and Mexican 

pollution influences, Atmos. Environ., 43(1), 310-1,319. 

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Wesley, M.L. 1989. Parameterization of surface resistance to gaseous dry deposition in 

regional-scale numerical models. Atmos. Environ., 23, 1293-1304. 

Wimmers, A.J., J.L. Moody, E.V. Browell, J.W. Hair, W.B. Grant, C.F. Butler, M.A. Fenn, C.C. 

Schmidt, J. Li, and B.A. Ridley. 2002. Signatures of tropopause folding in satellite imagery, 

J. Geosphys. Res., 108(D4), 8360-8371. 

Yantosca and Carouge. 2011. GEOS–Chem v9–01–01 Online User's Guide. 

http://acmg.seas.harvard.edu/geos/doc/man. 

Zhang, L. 2011. Personal communication with Dr. Lin Zhang of the Harvard University 

Atmospheric Chemistry Modeling (ACM) Group. 

Zhang L. and D. Jacob. 2011. Presentation entitled “GEOS-Chem Estimates of PRB Ozone 

(with focus on Intermountain West).” Prepared for BP, EPRI & NASA. 

Zhang, L., D.J. Jacob, N.V. Smith-Downey, D.A. Wood, D. Blewitt, C.C. Carouge, A. van 

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1/ 2°•2/3° horizontal resolution over North America. Accepted for publication in 

Atmospheric Environment.