Final Report

SENSITIVITY ANALYSIS OF PVMRM AND OLM 

IN AERMOD 

Alaska DEC Contract No. 18-8018-04

Submitted to:

Alaska Department of Environmental Conservation 

Division of Air Quality 

410 Willoughby Avenue, Suite 303 

Juneau, Alaska 99801-1795 

September 2004

FINAL REPORT 

SENSITIVITY ANALYSIS OF PVMRM AND OLM 

IN AERMOD

Alaska DEC Contract No. 18-8018-04 

Prepared for 

Alan E. Schuler, P.E. 

Alaska Department of Environmental Conservation 

Division of Air Quality 

410 Willoughby Avenue, Suite 303 

Juneau, Alaska 99801-1795 

September 2004 

Submitted by 

MACTEC Federal Programs, Inc. 

5001 S. Miami Blvd., Suite 300 

P.O. Box 12077 

Research Triangle Park, NC 27709-2077 

(919) 941-0333 FAX (919) 941-0234 

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iii

ACKNOWLEDGMENTS 

 This report of the sensitivity analysis of the PVMRM and OLM options in AERMOD 

was prepared by Roger W. Brode of MACTEC Federal Programs, Inc., Research Triangle Park, 

NC, under Alaska Department of Environmental Conservation (ADEC) Contract No. 18-8018-

04, with Alan Schuler as the ADEC project manager. The sensitivity analysis was also funded in 

part by BP Exploration (Alaska), Inc (BPXA), with Alison Cooke as the BPXA project manager. 

MACTEC also appreciates the comments of Pat Hanrahan, formerly of Oregon DEQ, and Rob 

Wilson and Herman Wong of EPA Region 10. 
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TABLE OF CONTENTS 

Page

ACKNOWLEDGMENTS ...........................................................................................................iii 

TABLE OF CONTENTS ............................................................................................................. v 

LIST OF FIGURES ..................................................................................................................... vi 

LIST OF TABLES .....................................................................................................................viii 

1.0 INTRODUCTION.................................................................................................................. 1 

2.0 DESCRIPTION OF SENSITIVITY TESTS....................................................................... 3 

2.1 SOURCE DESCRIPTIONS............................................................................................ 3 

2.2 MODELING OPTIONS ................................................................................................. 4 

2.3 METEOROLOGICAL AND OZONE DATA ............................................................... 5 

2.4 RECEPTOR DATA........................................................................................................ 6 

3.0 SENSITIVITY ANALYSIS RESULTS ............................................................................... 9 

3.1 SINGLE SOURCE SCENARIOS .................................................................................. 9 

3.2 MULTIPLE-SOURCE SCENARIO............................................................................. 15 

3.3 POINT VS. VOLUME AND AREA SOURCE COMPARISONS.............................. 17 

4.0 SUMMARY AND CONCLUSIONS .................................................................................. 55 

5.0 REFERENCES..................................................................................................................... 57 
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LIST OF FIGURES 

Figure Page

2.1. Anchorage Area Terrain, Source and Receptors for Complex Terrain Scenarios............. 7 

2.2. Receptor and Near-field Source Locations for Multiple Source Scenario ......................... 8 

3.1. Ratios of Maximum Annual NO2 Concentrations to Full Conversion ............................. 22 

3.2. Ratios of Maximum 1-Hour NO2 Concentrations to Full Conversion ............................. 22 

3.3. Terrain Elevation vs. Downwind Distance for Maximum Annual NO2 Averages for 

Diesel Generator (top) and Gas Turbine (bottom) Sources .............................................. 23 

3.4. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 1g/s Emission Rate ................. 24 

3.5. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 50g/s Emission Rate ............... 25 

3.6. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with No Downwash ................ 26 

3.7. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Downwash ...................... 27 

3.8. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Flat Terrain ..................... 28 

3.9. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Complex Terrain............. 29 

3.10. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Rural Dispersion ............. 30 

3.11. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Urban Dispersion............ 31 

3.12. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with No Downwash........................ 32 

3.13. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Downwash.............................. 33 

3.14. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Flat Terrain............................. 34 

3.15. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Complex Terrain .................... 35 

3.16. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Rural Dispersion .................... 36 

3.17. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Urban Dispersion ................... 37 

3.18. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 1g/s Emission Rate ................. 38 

3.19. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 50g/s Emission Rate ............... 39 

3.20. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with No Downwash ................ 40 
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3.21. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Downwash ...................... 41 

3.22. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Flat Terrain ..................... 42 

3.23. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Complex Terrain............. 43 

3.24. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Rural Dispersion ............. 44 

3.25. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Urban Dispersion............ 45 

3.26. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with No Downwash........................ 46 

3.27. 1-Hour Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Downwash.............................. 47 

3.28. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Flat Terrain............................. 48 

3.29. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Complex Terrain .................... 49 

3.30. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Rural Dispersion .................... 50 

3.31. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Urban Dispersion ................... 51 

3.32. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 10m Non-buoyant Point Source...................................... 52 

3.33. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 10m Volume Source with Initial Sigmas=4.65m............ 53 

3.34. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 10m Area Source with Initial Sigma-z=4.65m ............... 54 
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LIST OF TABLES 

Table Page

2.1. Source Characteristics for AERMOD PVMRM Sensitivity Analysis................................ 4 

2.2. Modeling Options Analyzed by Source Type..................................................................... 5 

3.1. Comparison of Maximum Annual NO2 Averages and Distance to Maximum for Full 

Conversion, ARM, OLM, and PVMRM for Single Source Scenarios............................. 12 

3.2. Comparison of Maximum 1-Hour NO2 Averages and Distance to Maximum for Full 

Conversion, ARM, OLM, and PVMRM for Single Source Scenarios............................. 13 

3.3. Comparison of Maximum Annual and 1-Hour NO2 Averages for Full Conversion, 

ARM, OLM, and PVMRM for Multiple-Source Scenario............................................... 16 

3.4. Comparison of Maximum Annual NO2 Averages and Distance to Maximum for Full 

Conversion, ARM, OLM, and PVMRM for 10m Point, Volume and Area Sources ....... 18 

3.5. Comparison of Maximum 1-Hour NO2 Averages and Distance to Maximum for Full 

Conversion, ARM, OLM, and PVMRM for 10m Point, Volume and Area Sources ....... 19 
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1.0 INTRODUCTION 

 The purpose of this report is to document the results of a sensitivity analysis performed 

for the Plume Volume Molar Ratio Method (PVMRM) and Ozone Limiting Method (OLM) 

options in the AERMOD dispersion model. The PVMRM and OLM options for modeling the 

conversion of NOx emissions to NO2 have been incorporated into the AERMOD dispersion 

model. The PVMRM approach was originally developed as a post-processor for the ISCST3 

model (Hanrahan, 1999a). A technical description of the implementation of PVMRM in 

AERMOD is provided in the Addendum to the AERMOD Model Formulation Document 

(Cimorelli, et al., 2002), and user instructions on the application of the PVMRM and OLM 

options in AERMOD are provided in the Addendum to the AERMOD User’s Guide (EPA, 

2002). 

The sensitivity analysis includes a comparison of NO2 concentrations estimated by the 

PVMRM option to NO2 concentrations based on the OLM option, Tier 1 screening assumption 

(full conversion of NOx to NO2), and Tier 2 screening (75 percent Ambient Ratio Method, ARM) 

across a range of meteorology and a range of source characteristics typical of NOx sources. A 

description of the sensitivity tests is provided in Section 2. Results of the sensitivity analysis are 

presented in Section 3, and Section 4 provides a summary and conclusions.
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2.0 DESCRIPTION OF SENSITIVITY TESTS 

 This section includes a description of the sensitivity tests performed on the PVMRM and 

OLM options in the AERMOD dispersion model. The source parameters, modeling options, 

meteorological data and receptor data used in the sensitivity analysis are described in detail. The 

sensitivity analysis includes several tests involving individual sources, and a multiple-source 

scenario. 

2.1 SOURCE DESCRIPTIONS 

A total of six different individual sources were modeled, including a non-buoyant 10m 

point source, a standard 35m buoyant point source (used in EPA consequence analyses), a typical 

diesel generator source, a typical gas turbine source, a 10m volume source, and a 10m circular 

area source. The source parameters used for these single source scenarios are summarized in 

Table 2.1. A range of buoyancy and momentum fluxes are represented by the point sources, 

with the diesel generator representing a relatively low buoyancy/low momentum release, the 

35m stack representing medium buoyancy and momentum, and the gas turbine representing 

relatively high buoyancy and momentum. The emission rates for the diesel generator and gas 

turbine sources are considered typical for those source types. The 35m stack was modeled with 

two different emission rates to test the sensitivity of the PVMRM and OLM algorithms to the 

NOx emission rate. 

In addition to the single source scenarios, a hypothetical multiple-source scenario based 

in part on an actual permit application was included in the sensitivity analysis to test the plume 

merging algorithm contained within PVMRM. The multiple-source scenario included a total of 

65 point sources and 13 area sources, with 1,598 receptors. 
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Table 2.1. Source Characteristics for AERMOD PVMRM Sensitivity Analysis

Point Sources: 

Source 

Description 

Emission 

Rate (g/s) 

Stack 

Height (m) 

Exit 

Velocity 

(m/s) 

Exit 

Temp. 

(K) 

Stack 

Diameter 

(m) 

Buoyancy 

Fluxa

(m4

/s3) 

Momentum 

Fluxa

(m4

/s2) 

10m Nonbuoyant Point 10 10 0.0 Ambient 0.0 0.0 0.0 

35m Buoyant 

Stack 1 35 11.7 432 2.4 60.71 124.64 

35m Buoyant 

Stack 50 35 11.7 432 2.4 60.71 124.64 

Diesel Engine 5 26 50.0 727 0.3 6.88 21.14 

Gas Turbine 30 31 63.0 750 2.4 565.25 2081.61 

Volume Sources: 

Source 

Description 

Emission 

Rate (g/s) 

Release 

Height (m) 

Initial 

Sigma-y 

(m) 

Initial 

Sigma-z 

(m) 

 

10m Volume 10 10 0.1 to 9.3 0.1 to 9.3 

Area Source: 

Source 

Description 

Emission 

Rate (g/s) 

Release 

Height (m) 

Radius of 

Area (m) 

Initial 

Sigma-z 

(m) 

 

10m Circular 

Area 10 10 10 4.65 

a

 Buoyancy and momentum fluxes are based on an ambient temperature of 273.16 K. 

The 10m non-buoyant point source and 10m volume source were included in the analysis 

to test the sensitivity on the algorithms to varying the initial size of the volume source and to 

verify that the results for the volume source converged to the results for the point source as the 

initial dispersion parameters were decreased. A 10m circular area source was also included with 

an initial sigma-z of 4.65 for comparison with the 10m volume source of similar dimensions. 

The 20m width of the area source is comparable to a volume source with an initial sigma-y of 

4.65m. 

2.2 MODELING OPTIONS 

 The single sources were tested with various modeling options, including building 

downwash vs. no downwash, rural vs. urban dispersion, and flat vs. complex terrain. Not all 

sources were modeled for each modeling option. Table 2.2 summarizes the modeling options 
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examined for each of the individual source types. A population of 250,000, which is the 

approximate population for Anchorage, Alaska, was used for the urban option. The complex 

terrain options were based on terrain in the Anchorage, Alaska area. The flat vs. complex terrain 

sources were located about 10km southeast of Anchorage, with relatively flat terrain (including 

terrain below stack base) to the north and west of the source, and terrain rising fairly steeply 

within about 1.5 to 2km east of the source. The terrain for the Anchorage area is shown in 

Figure 2.1. 

Table 2.2. Modeling Options Analyzed by Source Type

Source Description Modeling Options Meteorological 

Data 

10m Non-buoyant Point Rural, Flat Terrain, No Downwash Anchorage 1999 

10m Volume Source, 

Varying Initial Sigmas 

Rural, Flat Terrain Anchorage 1999 

10m Circular Area 

Source 

Rural, Flat Terrain Anchorage 1999 

35m Buoyant Stack, 

Varying Emission Rate 

Rural, Flat Terrain, No Downwash PadA 1991-95 

Diesel Generator No Downwash vs. Downwash (Rural/Flat Terrain) 

Rural vs. Urban Dispersion (Flat Terrain/No Downwash) 

Flat Terrain vs. Complex Terrain (Rural/No Downwash) 

PadA 1991-95 

PadA 1991-95 

Anchorage 1999 

Gas Turbine No Downwash vs. Downwash (Rural/Flat Terrain) 

Rural vs. Urban Dispersion (Flat Terrain/No Downwash) 

Flat Terrain vs. Complex Terrain (Rural/No Downwash) 

PadA 1991-95 

PadA 1991-95 

Anchorage 1999 

2.3 METEOROLOGICAL AND OZONE DATA 

Three different sets of meteorological data were used, depending on the scenario. The 

basic single source scenarios were run using five years of site-specific meteorological data from 

the PadA installation on the North Slope of Alaska near Prudhoe Bay for the period 1991 

through 1995. The five years of site-specific meteorological data included 10m wind speed, 
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wind direction and temperature data, and were processed through AERMET together with 

National Weather Service (NWS) surface and upper air data for Barrow, Alaska. The multisource scenario was run using one year of site-specific data from the same site in Prudhoe Bay 

for 1991, but differed from 1991 of the five-year dataset in that 10m sigma-theta and solar 

radiation data were also included. The third meteorological data set used for comparisons 

between flat and complex terrain consisted of one year (1999) of NWS surface and upper air 

meteorological data for Anchorage, Alaska. The 1999 Anchorage meteorological data were also 

used for the comparisons between the 10m non-buoyant point source and the volume and area 

sources. The meteorological data used for the single source scenarios are summarized in 

Table 2.2. 

The site-specific meteorological data from PadA were processed using the most recent 

version of AERMET available from the SCRAM website, dated 03273. The following annual 

surface characteristics were used in processing the PadA meteorological data, and were assumed 

to be constant across all sectors: albedo = 0.30; Bowen ratio = 1.5; surface roughness = 0.01m. 

The following annual surface characteristics were used in processing the Anchorage 

meteorological data, and were also assumed to be constant across all sectors: albedo = 0.25; 

Bowen ratio = 1.5; surface roughness = 0.10m.

The hourly ozone data used with the PadA meteorological data were collected at the same 

location as the meteorological data. The hourly ozone data used with the 1999 Anchorage 

meteorological data were obtained from the U.S. EPA CASTNET website for the Denali 

National Park site located about 250 km north of Anchorage. A value of 40 ppb was used to 

substitute for missing ozone data for both databases. 

2.4 RECEPTOR DATA 

 The individual source scenarios were modeled with a polar grid of receptors centered on 

the source, with distances (in meters) of 50, 100, 150, 200, 300, 500, 750, 1000, 1500, 2000, 

3000, 5000, 10000, 20000, and 30000, and 36 radials, for a total of 540 receptors. The use of a 

polar grid allowed for the results to be summarized as a function of distance downwind from the 
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source. The receptor elevations and hill height scales for the complex terrain scenarios were 

calculated using the most recent version of AERMAP (dated 03107) and 1-degree Digital 

Elevation Model (DEM) data for the Anchorage, Alaska area. Figure 2.1 shows the source and 

receptor locations and topography of the Anchorage area used for the complex terrain scenarios, 

out to a distance of about 10,000 meters from the source. The receptor network used for the 

multiple-source scenario included discrete receptors located at 25m intervals along the property 

boundary and gridded receptors at 100m intervals across a 4km-by-4km domain. A total of 

1,598 receptors were included in the multiple-source scenario. Figure 2.2 shows the receptor and 

near-field source locations used in the multiple-source scenario. Additional background sources 

located beyond the 4km-by-4km receptor domain are not shown in Figure 2.2. 

STACK

342000 344000 346000 348000 350000 352000 354000 356000 358000 360000 362000 364000

6776000 6778000 6780000 6782000 6784000 6786000 6788000 6790000 6792000 6794000

Figure 2.1. Anchorage Area Terrain, Source and Receptors for 

Complex Terrain Scenarios 
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441500 442000 442500 443000 443500 444000 444500 445000 445500

7801000 7801500 7802000 7802500 7803000 7803500 7804000

Figure 2.2. Receptor and Near-field Source Locations for Multiple Source Scenario 
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3.0 SENSITIVITY ANALYSIS RESULTS 

 The results of the PVMRM sensitivity analysis are presented in this section. The results 

for the single source scenarios are presented first, and include comparisons of design values 

based on the highest annual and 1-hour average NO2 concentrations, and comparisons of 

maximum concentrations as a function of distance downwind from the source. 

3.1 SINGLE SOURCE SCENARIOS 

 Table 3.1 includes a comparison of maximum annual average NO2 concentrations for 

each of the single source scenarios involving the 35m stack, diesel generator and gas turbine 

sources. Results are presented for full conversion, ARM (0.75), OLM and PVMRM. The 

average OLM/FULL ratio for the single source annual averages is 0.88, and the average 

PVMRM/FULL ratio for annual averages is 0.55. In many cases, the distance to maximum 

annual average is further downwind from the source for PVMRM than for full conversion. This 

is due to the lower NO2/NOx ratios close to the source for PVMRM. 

Table 3.2 shows comparisons of maximum 1-hour average NO2 concentrations for each 

of the single source scenarios presented in Table 3.1. The average OLM/FULL ratio for the 

single source 1-hour averages is 0.68, and the average PVMRM/FULL ratio for 1-hour averages 

is 0.56. The conversion ratios for OLM and PVMRM are also summarized in the form of bar 

charts in Figures 3.1 and 3.2 for annual averages and 1-hour averages, respectively. 

 A comparison of the 35m stack with 1g/s emission rate vs. 50g/s emission rate clearly 

shows the sensitivity of both OLM and PVMRM to the level of NOx emissions. For the 1g/s 

source, the OLM option exhibited no ozone limiting effect, giving a ratio to full conversion of 

1.0. The PVMRM option showed some effects of ozone limiting, but the ratio of 0.892 is very 

close to the equilibrium ratio of 0.90 for PVMRM. Both options showed significantly more 
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ozone limiting with the 50g/s source, as expected, with the OLM option providing about the 

same result as ARM, and the PVMRM option showing a more significant reduction. 

 Comparing the results for no downwash vs. downwash for the diesel generator and gas 

turbine sources, the maximum concentrations are much higher with downwash, as expected, and 

the conversion ratios are lower. The lower conversion ratios for OLM are most likely due to the 

higher ground-level NOx concentrations with downwash. The lower conversion ratios for 

PVMRM are probably due to the fact that the maximum NOx concentrations occur much closer 

to the source for the downwash cases, and the plume volume corresponding to the maximum 

concentrations will therefore be smaller. The urban results (no downwash) tend to be slightly 

lower for the diesel generator annual averages than the rural results, except for the PVMRM 

option, whereas the urban results tend to be higher than rural results for the gas turbine. This is 

probably due to the difference in plume buoyancy between the two sources. The plume rise for 

the diesel generator is expected to be much lower than for the gas turbine, and therefore the 

diesel generator is less likely than the gas turbine to experience limited mixing under stable 

conditions due to the urban mixing height. 

 The maximum annual average concentrations tend to be somewhat lower for the complex 

terrain scenario compared to flat terrain, with conversion rates being similar between flat and 

complex terrain. The lower annual averages for complex terrain are due to the fact that the 

maximum ground-level NO2 concentrations occur relatively close to the source (300 meters for 

the FULL and OLM options), where the terrain is at or below stack base. The terrain elevations 

associated with the maximum ground-level concentrations as a function of downwind receptor 

distance are shown in Figure 3.3. For all three options (FULL, OLM and PVMRM), the terrain 

is below stack base at the 300 and 500 meters receptor distances for the diesel generator. The 

effect of terrain below stack base is to increase the effective height of the plume above local 

ground, and thereby reduce the ground-level concentrations. The PVMRM option for the gas 

turbine source is the only exception to this pattern. In this case, the maximum annual average is 

slightly higher for complex terrain than for flat terrain, and occurs as a downwind distance of 

5,000 meters compared to 1,000 meters for the flat terrain maximum. Figure 3.3 shows that 
FINAL 09/30/2004 

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receptors are below stack base at the 1,000 meter distance for the FULL and OLM options, and 

well above stack base at the 5,000 meter distance for the PVMRM option. 

The effect of complex terrain is more clearly evident in the maximum 1-hour averages 

(Table 3.2), with the complex terrain results being much higher than the flat terrain results. The 

PVMRM conversion ratios for 1-hour averages tend to be lower than for annual averages, which 

may be explained by the fact that the maximum 1-hour averages are controlled by stable 

conditions, during which the plume volume tends to be smaller. The OLM conversion ratios for 

1-hour averages are also lower due to the much higher ground-level concentrations. 

 The distance to maximum concentration for the PVMRM option tends to be at or beyond 

the distance to maximum for full conversion. This is consistent with the fact that conversion 

ratios are lowest close to the source and increase with downwind distance for PVMRM. The 

distance to maximum concentration for the OLM option is more likely to match the distance to 

maximum for full conversion, and is closer to the source in a few cases. 

 

 Figures 3.4 and 3.5 show the annual average NO2 concentrations (top pane) and 

conversion ratios (bottom pane) as a function of distance downwind from the source for the 35m 

1g/s and 50g/s sources, respectively. These figures clearly show the dependence of the OLM 

conversion ratio on the emission rate and ground-level NOx concentration, with no ozone 

limiting occurring for the 1g/s source, and with the dip in conversion ratio between 750-1000m 

roughly mirroring the increase in concentrations with distance for the 50g/s source. The 

dependence of the PVMRM conversion ratio on emission rate is also shown, with much lower 

ratios (more ozone limiting) for the 50g/s source. However, the dependence of the PVMRM 

conversion ratio on distance from the source is clearly shown in Figure 3.5 for the 50g/s source, 

with much smaller ratios close to the source where the plume volume is relatively small, and 

increasing with distance to the equilibrium ratio of 0.90 beyond about 10 km. 
FINAL 09/30/2004 

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Table 3.1. Comparison of Maximum Annual NO2 Averages and Distance to Maximum for 

Full Conversion, ARM, OLM, and PVMRM for Single Source Scenarios 

Source Scenario 

Conversion 

Option 

Maximum NO2

Concentration 

(μg/m3

) 

Distance to 

Maximum (m) 

Ratio to Full 

Conversion 

 FULL 0.112 750. 1.000 

 ARM 0.084 750. 0.750 

 OLM 0.112 750. 1.000 

35m Stack, 1g/s 

Rural, No Downwash 

 PVMRM 0.100 750. 0.892 

 FULL 5.601 750. 1.000 

 ARM 4.201 750. 0.750 

 OLM 4.178 1000. 0.746 

35m Stack, 50g/s 

Rural, No Downwash 

 PVMRM 1.537 3000. 0.274 

 FULL 2.927 500. 1.000 

 ARM 2.195 500. 0.750 

 OLM 2.807 500. 0.959 

Diesel Generator 

Rural, No Downwash 

 PVMRM 1.236 1500. 0.422 

 FULL 54.375 50. 1.000 

 ARM 40.781 50. 0.750 

 OLM 21.498 50. 0.395 

Diesel Generator 

Rural, Downwash 

 PVMRM 11.164 50. 0.205 

 FULL 2.338 500. 1.000 

 ARM 1.754 500. 0.750 

 OLM 2.226 500. 0.952 

Diesel Generator 

Urban, No Downwash 

 PVMRM 1.317 2000. 0.563 

 FULL 3.682 300. 1.000 

 ARM 2.761 300. 0.750 

 OLM 3.604 300. 0.979 

Diesel Generator 

Flat Terrain 

 PVMRM 1.606 1500. 0.436 

 FULL 3.228 300. 1.000 

 ARM 2.421 300. 0.750 

 OLM 3.179 300. 0.985 

Diesel Generator 

Complex Terrain 

 PVMRM 1.511 1500. 0.468 

 FULL 0.441 1000. 1.000 

 ARM 0.331 1000. 0.750 

 OLM 0.440 1000. 0.997

Gas Turbine 

Rural, No Downwash 

 PVMRM 0.303 1500. 0.688

 FULL 13.984 50. 1.000

 ARM 10.488 50. 0.750

 OLM 8.482 50. 0.607

Gas Turbine 

Rural, Downwash 

 PVMRM 2.849 750. 0.204

 FULL 0.704 1500. 1.000

 ARM 0.528 1500. 0.750

 OLM 0.700 2000. 0.994

Gas Turbine 

Urban, No Downwash 

 PVMRM 0.591 3000. 0.839

 FULL 0.636 750. 1.000

 ARM 0.477 750. 0.750

 OLM 0.636 750. 1.000

Gas Turbine 

Flat Terrain 

 PVMRM 0.465 1000. 0.732

 FULL 0.571 750. 1.000

 ARM 0.428 750. 0.750

 OLM 0.571 750. 1.000

Gas Turbine 

Complex Terrain 

 PVMRM 0.479 5000. 0.838
FINAL 09/30/2004 

13

Table 3.2. Comparison of Maximum 1-Hour NO2 Averages and Distance to Maximum for 

Full Conversion, ARM, OLM, and PVMRM for Single Source Scenarios 

Source Scenario 

Conversion 

Option 

Maximum NO2

Concentration 

(μg/m3

) 

Distance to 

Maximum (m) 

Ratio to Full 

Conversion 

 FULL 5.398 1000. 1.000 

 ARM 4.048 1000. 0.750 

 OLM 5.398 1000. 1.000 

35m Stack, 1g/s 

Rural, No Downwash 

 PVMRM 4.858 1000. 0.900 

 FULL 269.899 1000. 1.000 

 ARM 202.424 1000. 0.750 

 OLM 151.088 3000. 0.560 

35m Stack, 50g/s 

Rural, No Downwash 

 PVMRM 131.033 3000. 0.485 

 FULL 98.418 750. 1.000 

 ARM 73.813 750. 0.750 

 OLM 68.636 200. 0.697 

Diesel Generator 

Rural, No Downwash 

 PVMRM 73.259 1000. 0.744 

 FULL 427.190 50. 1.000 

 ARM 320.393 50. 0.750 

 OLM 185.885 50. 0.435 

Diesel Generator 

Rural, Downwash 

 PVMRM 147.120 50. 0.344 

 FULL 98.418 750. 1.000 

 ARM 73.813 750. 0.750 

 OLM 68.636 200. 0.697 

Diesel Generator 

Urban, No Downwash 

 PVMRM 73.259 1000. 0.744 

 FULL 73.535 200. 1.000 

 ARM 55.151 200. 0.750 

 OLM 73.093 200. 0.994 

Diesel Generator 

Flat Terrain 

 PVMRM 34.056 1000. 0.463 

 FULL 357.518 1500. 1.000 

 ARM 268.138 1500. 0.750 

 OLM 110.885 1500. 0.310 

Diesel Generator 

Complex Terrain 

 PVMRM 94.448 5000. 0.264 

 FULL 51.910 3000. 1.000 

 ARM 38.933 3000. 0.750 

 OLM 35.904 2000. 0.692

Gas Turbine 

Rural, No Downwash 

 PVMRM 38.187 5000. 0.736

 FULL 443.448 100. 1.000

 ARM 332.586 100. 0.750

 OLM 177.423 50. 0.400

Gas Turbine 

Rural, Downwash 

 PVMRM 138.667 750. 0.313

 FULL 61.648 1500. 1.000

 ARM 46.236 1500. 0.750

 OLM 61.395 1500. 0.996

Gas Turbine 

Urban, No Downwash 

 PVMRM 55.025 1500. 0.893

 FULL 38.605 750. 1.000

 ARM 28.954 750. 0.750

 OLM 38.605 750. 1.000

Gas Turbine 

Flat Terrain 

 PVMRM 18.481 750. 0.479

 FULL 378.023 2000. 1.000

 ARM 283.518 2000. 0.750

 OLM 120.449 2000. 0.319

Gas Turbine 

Complex Terrain 

 PVMRM 150.036 2000. 0.397
FINAL 09/30/2004 

14

 Figures 3.6 through 3.11 show the annual concentrations and conversion ratios vs. 

distance for the diesel generator scenarios, and Figures 3.12 through 3.17 show the results for the 

gas turbine scenarios. These figures show similar overall patterns for the dependence of 

conversion ratio on distance, with the OLM ratio being most dependent on the ground-level NOx

concentration, and the PVMRM ratio showing dependence on plume volume. 

The effect of complex terrain is more noticeable for the gas turbine source (Figure 3.15) 

than for the diesel generator source (Figure 3.9). The gas turbine source shows a secondary 

maximum in the concentration plot for full conversion at a distance of 2,000 meters associated 

with plume impaction on the terrain to the east of the source (see Figure 3.3). All options show 

another maximum at a distance of 5,000 meters, also associated with complex terrain impacts. 

The deviation of the OLM value from the full conversion value at the 2,000 meter 

receptor distance for the gas turbine (Figure 3.15) bears explanation. As shown in the 

conversion ratio plot in Figure 3.15, the conversion ratio for OLM drops to about 0.73 at that 

distance. The maximum NO2 concentration for the OLM option occurs for terrain below stack 

base north-northwest (NNW) of the source, while the maximum NO2 concentrations for full 

conversion and PVMRM occur for complex terrain about 126 meters above stack base eastsoutheast (ESE) of the source (see Figure 3.3). The average concentration for the receptor to the 

NNW is based on a larger number of values associated with one of the peaks in the annual wind 

rose. The average concentration for the receptor to the ESE is based on a smaller number of 

values with higher concentrations due to the complex terrain impact. Since the OLM conversion 

ratio is strongly dependent on the ground-level NOx concentration, the hourly conversion ratios 

are lower for the complex terrain impacts to the ESE than for the impacts to the NNW. The NO2

concentrations for the OLM option are nearly equal for the NNW and ESE receptors, but the 

conversion ratio is 0.71 for the complex terrain receptor to the ESE and 1.0 for the receptor 

below stack base to the NNW. 

 There is also a noticeable dip in the conversion ratio for PVMRM at the 2,000 meter 

distance shown in Figure 3.15. This can be explained by the fact that the maximum impacts 

occurring on complex terrain for receptors at and beyond 2,000 meters are associated with stable 
FINAL 09/30/2004 

15

atmospheric conditions, whereas the maximum impacts for receptors located at or below stack 

base out to about 1,500 meters are associated with convective atmospheric conditions. The 

plume volume will tend to be much smaller for stable plumes than for convective plumes, 

resulting in lower conversion ratios for the complex terrain impacts. 

Figures 3.18 through 3.31 show the 1-hour average concentrations and conversion ratios 

for the 35m stack, diesel generator and gas turbine single source scenarios. These figures show 

some similar trends as the annual averages, but the curves tend to be less smooth as the results 

are more sensitive to variations in the controlling meteorological conditions and hourly ozone 

data at each distance. The effects of complex terrain are more noticeable than for annual 

averages for both the diesel engine and gas turbine sources, with sharp peaks occurring at the 

distance where the plume impacts the terrain to the east. The peak appears to be slightly further 

downwind for the gas turbine than for the diesel generator. This is consistent with the higher 

plume rise for the gas turbine, resulting in the plume impacting the terrain further away. 

3.2 MULTIPLE-SOURCE SCENARIO 

 Table 3.3 provides the results of the multiple-source scenario, showing the maximum 

annual average NO2 concentrations and conversion ratios, and maximum 1-hour NO2 averages 

and ratios for each of the conversion options. Table 3.3 includes results for OLM based on 

individual plumes, and results for OLM based on combining plumes for six groups of sources 

based on proximity of the sources and the likelihood that plumes from those sources would 

quickly merge. The PVMRM option shows the lowest conversion ratio for the annual averages, 

0.62, which is slightly lower than the conversion ratio for the OLM-Group option of 0.65. The 

OLM-Group ratio is somewhat lower than the OLM-Individual ratio, as expected. The result for 

the OLM option for individual plumes is similar to the result based on ARM. 

 It should be noted that while the annual average NO2 concentrations presented in 

Table 3.3 exceed the 100 μg/m3

 National Ambient Air Quality Standard (NAAQS) for NO2, 

these values do not reflect predicted impacts from an actual facility. While the multiple-source 

scenario was based in part on actual sources, several changes and assumptions were made to 
FINAL 09/30/2004 

16

simplify the sensitivity analysis. Therefore, the results do not reflect the impacts that would 

likely occur based on a more rigorous assessment. 

Table 3.3. Comparison of Maximum Annual and 1-Hour NO2 Averages for Full 

Conversion, ARM, OLM, and PVMRM for Multiple-Source Scenario 

Averaging Period Conversion Option 

Maximum NO2

Concentration 

(μg/m3

) 

Ratio to Full 

Conversion 

Annual Full Conversion 

ARM 

OLM-Individual 

OLM-Group 

PVMRM 

166.6 

125.0 

126.3 

108.0 

103.8 

1.00 

0.75 

0.76 

0.65 

0.62 

1-Hour Full Conversion 

ARM 

OLM-Individual 

OLM-Group 

PVMRM 

5,290.3 

3,967.7 

2,079.9 

1,822.2 

3,196.9 

1.00 

0.75 

0.39 

0.34 

0.60 

The results in Table 3.3 for 1-hour averages from the multiple-source scenario show 

much lower conversion ratios for the OLM options (0.39 for individual plumes and 0.34 with 

combined plumes) than for the PVMRM option (0.60). This is not unexpected given the fact that 

the ozone limiting potential under the OLM options is strongly related to the maximum groundlevel concentration of NOx, whereas the PVMRM ratio is related to the volume of the plume. As 

an example, for an elevated source the maximum 1-hour average concentration is typically 

associated with light wind, convective conditions. The OLM ratio will tend to be low for this 

case because of the high ground-level concentration, whereas the PVMRM ratio will tend to be 

high due to the relatively large volume of the plume under convective conditions. It is worth 

noting that the 1-hour conversion ratio for PVMRM is similar to the ratio for annual averages, 

whereas the OLM ratios are quite dissimilar between annual and 1-hour averages. 
FINAL 09/30/2004 

17

3.3 POINT VS. VOLUME AND AREA SOURCE COMPARISONS 

Additional comparisons were made between a 10m non-buoyant point source, a 10m 

volume source with varying initial dispersion coefficients (sigmas), and a 10m circular area 

source with an initial vertical dispersion coefficient (sigma-z). These comparisons provide 

additional sensitivity results for relatively low-level sources. They also provide a test of whether 

the volume source results converge to the point source result as the initial dispersion coefficients 

are decreased, and provide a comparison of a circular area source to a volume source of similar 

dimensions. 

The results for the point vs. volume and area source comparisons are provided in Table 

3.4 for annual NO2 averages and in Table 3.5 for 1-hour NO2 averages. The initial sigma values 

for the volume sources apply to both lateral (sigma-y) and vertical (sigma-z) dispersion 

coefficients. For both averaging periods the results for the volume source with initial sigmas of 

0.1m are nearly identical to the point source results. As the initial sigmas are increased, up to 

9.3m, the maximum concentrations for the volume sources also increase. This result is 

reasonable as the larger initial sigmas will bring the plume to the ground quicker. For the two 

largest values of initial sigma, the distance to maximum concentration also drops from 100m to 

50m. 

The OLM annual average conversion ratios for the volume sources get smaller as the 

ground-level concentration increases, as expected. The PVMRM ratios are nearly the same for 

the point source and the first three volume sources (0.1, 1.0, and 2.3m initial sigmas). The 

PVMRM ratio drops slightly between the 2.3m and 4.65m initial sigmas, due to the closer 

distance to maximum concentration for the 4.65m case, resulting in a smaller plume volume. 

The largest change in PVMRM conversion ratio occurs between the initial sigmas of 4.65m and 

9.3m. The ratio increases for the larger initial sigma due to the corresponding increase in the 

plume volume resulting in more ozone available for conversion. The plume volume for volume 

sources incorporates the initial volume of the source defined by the initial sigmas in addition to 

the plume volume resulting from relative dispersion. 
FINAL 09/30/2004 

18

Table 3.4. Comparison of Maximum Annual NO2 Averages and Distance to Maximum for 

Full Conversion, ARM, OLM, and PVMRM for 10m Point, Volume and Area Sources 

Source Scenario 

Conversion 

Option 

Maximum NO2

Concentration 

(μg/m3

) 

Distance to 

Maximum (m) 

Ratio to Full 

Conversion 

 FULL 376.871 100. 1.000 

 ARM 282.654 100. 0.750 

 OLM 66.148 100. 0.176 

10m Point 

 

 PVMRM 47.685 100. 0.127 

 FULL 376.974 100. 1.000 

 ARM 282.731 100. 0.750 

 OLM 66.162 100. 0.176 

10m Volume 

Initial sigma = 0.1m 

 

 PVMRM 47.698 100. 0.127 

 FULL 383.590 100. 1.000 

 ARM 287.693 100. 0.750 

 OLM 67.036 100. 0.175 

10m Volume 

Initial sigma = 1.0m 

 

 PVMRM 48.504 100. 0.126

 FULL 409.933 100. 1.000

 ARM 307.450 100. 0.750

 OLM 71.006 100. 0.173

10m Volume 

Initial sigma = 2.3m 

 

 PVMRM 51.694 100. 0.126

 FULL 742.539 50. 1.000

 ARM 556.904 50. 0.750

 OLM 114.185 50. 0.154

10m Volume 

Initial sigma = 4.65m 

 

 PVMRM 91.998 50. 0.124

 FULL 1,085.476 50. 1.000

 ARM 814.107 50. 0.750

 OLM 156.531 50. 0.144

10m Volume 

Initial sigma = 9.3m 

 

 PVMRM 182.709 50. 0.168

 FULL 751.711 50. 1.000

 ARM 563.782 50. 0.750

 OLM 97.653 50. 0.130

10m Area 

Initial sigmaz = 4.65m 

20m wide circular area 

 PVMRM 104.517 50. 0.139
FINAL 09/30/2004 

19

Table 3.5. Comparison of Maximum 1-Hour NO2 Averages and Distance to Maximum for 

Full Conversion, ARM, OLM, and PVMRM for 10m Point, Volume and Area Sources 

Source Scenario 

Conversion 

Option 

Maximum NO2

Concentration 

(μg/m3

) 

Distance to 

Maximum (m) 

Ratio to Full 

Conversion 

 FULL 11,106.540 50. 1.000 

 ARM 8,329.905 50. 0.750 

 OLM 1,178.703 50. 0.106 

10m Point 

 

 

 PVMRM 1,258.679 50. 0.113 

 FULL 11,119.789 50. 1.000 

 ARM 8,339.842 50. 0.750 

 OLM 1,180.025 50. 0.106 

10m Volume 

Initial sigma = 0.1m 

 

 PVMRM 1,260.190 50. 0.113 

 FULL 11,138.263 50. 1.000 

 ARM 8,353.697 50. 0.750 

 OLM 1,182.047 50. 0.106 

10m Volume 

Initial sigma = 1.0m 

 

 PVMRM 1,262.501 50. 0.113 

 FULL 11,408.523 50. 1.000

 ARM 8,556.393 50. 0.750

 OLM 1,219.742 50. 0.107

10m Volume 

Initial sigma = 2.3m 

 

 PVMRM 1,295.209 50. 0.114

 FULL 11,804.340 50. 1.000

 ARM 8,853.255 50. 0.750

 OLM 1,247.224 50. 0.106

10m Volume 

Initial sigma = 4.65m 

 

 PVMRM 1,326.409 50. 0.112

 FULL 11,804.340 50. 1.000

 ARM 8,853.255 50. 0.750

 OLM 1,247.224 50. 0.106

10m Volume 

Initial sigma = 9.3m 

 

 PVMRM 1,326.409 50. 0.112

 FULL 12,106.910 50. 1.000

 ARM 9,080.184 50. 0.750

 OLM 1,286.426 50. 0.106

10m Area 

Initial sigmaz = 4.65m 

20m wide circular area 

 PVMRM 1,474.794 50. 0.122
FINAL 09/30/2004 

20

The results for the area source with an initial sigma-z of 4.65m are similar to the results 

for the volume source with initial sigmas of 4.65m. The full conversion results for the area 

source are about 1 percent higher than for the comparable volume source for the annual averages. 

This difference is reasonable since the area source simulates a uniform emission rate over the 

20m wide circular area, whereas the volume source is treated as a virtual point source with a 

peak emission density at the center of the source. This creates a higher emission density for the 

area source for the downwind portion of the source closest to the receptors. The slightly higher 

ground-level NOx concentrations may contribute to the slightly lower NO2 concentrations for the 

OLM option for the area source, with a lower conversion ratio reflecting more ozone limiting 

conditions. 

The PVMRM option gives higher average NO2 concentrations for the area source than 

for the corresponding volume source for both annual and 1-hour averages, by about 10 percent. 

This is related to the differences in the definition of the plume volume for area sources vs. 

volume sources. Since the area source simulates a uniform emission density across the source, 

the lateral dimension of the plume is defined as the projected width of the area source (which 

equals 20m for the circular area) plus 2 times the relative dispersion coefficient to account for 

lateral plume spread beyond the edges of the source. The lateral dimension of the volume source 

plume is based on combining the initial sigma-y with the relative dispersion coefficient. This 

difference will result in slightly larger plume volumes for area sources than for volume sources 

with similar dimensions, which will result in higher NO2/NOx conversion ratios for PVMRM. 

Figures 3.32, 3.33, and 3.34 compare the annual NO2 concentration and conversion ratio 

plots vs. distance for the 10m point source, the 10m volume source with initial sigmas of 4.65m, 

and the 10m circular area source, respectively. Since these are low-level sources with peak 

impacts close to the source, the conversion ratios for OLM and PVMRM show a more similar 

pattern than for the other sources with elevated plumes, with low conversion ratios close to the 

source and ratios increasing relatively smoothly with increasing distance. However, the 

PVMRM conversion ratio increases more gradually with distance than OLM out to a distance of 

about 500m, with a steeper increase in the ratio beyond that distance. As noted in previous 

discussions, the change in conversion ratio with distance for PVMRM is primarily controlled by 
FINAL 09/30/2004 

21

the volume of the plume, whereas the conversion ratio for OLM is primarily controlled by the 

ground-level NOx concentration. 

 
FINAL 09/30/2004 

22

Figure 3.1. Ratios of Maximum Annual NO2 Concentrations to Full Conversion

0

0.2

0.4

0.6

0.8

1

1.2

35m Stack, 1g/s 

35m Stack, 50g/s 

Diesel Generator No DW 

Diesel Generator DW

Diesel Generator URB 

Diesel Generator Flat 

Diesel Generator CT 

Gas Turbine No DW

Gas Turbine DW 

Gas Turbine URB 

Gas Turbine Flat 

Gas Turbine CT 

Scenario

Ratio

OLM

PVMRM

Figure 3.2. Ratios of Maximum 1-Hour NO2 Concentrations to Full Conversion

0

0.2

0.4

0.6

0.8

1

1.2

35m Stack, 1g/s

35m Stack, 50g/s 

Diesel Generator No DW 

Diesel Generator DW

Diesel Generator URB 

Diesel Generator Flat

Diesel Generator CT 

Gas Turbine No DW

Gas Turbine DW

Gas Turbine URB 

Gas Turbine Flat 

Gas Turbine CT 

Scenario

Ratio

OLM

PVMRM
FINAL 09/30/2004 

23

Terrain Elevation vs. Distance for Diesel Generator Annual Averages 

0

50

100

150

200

1 10 100 1000 10000 100000

Distance (meters)

Terrain Elevation (meters)

FULL

OLM

PVMRM

Stack Base

Elevation = 115m

Terrain Elevation vs. Distance for Gas Turbine Annual Averages

0

50

100

150

200

250

300

1 10 100 1000 10000 100000

Distance (meters)

Terrain Elevation (meters)

FULL

OLM

PVMRM Stack Base

Elevation = 115m

Figure 3.3. Terrain Elevation vs. Downwind Distance for Maximum Annual NO2 Averages 

for Diesel Generator (top) and Gas Turbine (bottom) Sources
FINAL 09/30/2004 24

35m Stack, 1g/s - No Downwash - Annual

0.0001

0.001

0.01

0.1

1

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

35m Stack, 1g/s - No Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.4. Annual Average NO2 Concentration (μg/m3

) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 1g/s Emission Rate
FINAL 09/30/2004 25

35m Stack, 50g/s - No Downwash - Annual

0.001

0.01

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

35m Stack, 50g/s - No Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.5. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 50g/s Emission Rate
FINAL 09/30/2004 26

Diesel Generator - No Downwash - Annual

0.001

0.01

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - No Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.6. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with No Downwash
FINAL 09/30/2004 27

Diesel Generator - Downwash - Annual

0.01

0.1

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.7. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Downwash
FINAL 09/30/2004 28

Diesel Generator - Flat Terrain - Annual

0.0001

0.001

0.01

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Flat Terrain - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.8. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Flat Terrain
FINAL 09/30/2004 29

Diesel Generator - Complex Terrain - Annual

0.0001

0.001

0.01

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Complex Terrain - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.9. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Complex Terrain
FINAL 09/30/2004 30

Diesel Generator - Rural - Annual

0.001

0.01

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Rural - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.10. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Rural Dispersion
FINAL 09/30/2004 31

Diesel Generator - Urban - Annual

0.001

0.01

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Urban - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.11. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Urban Dispersion
FINAL 09/30/2004 32

Gas Turbine - No Downwash - Annual

0.001

0.01

0.1

1

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - No Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.12. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with No Downwash
FINAL 09/30/2004 33

Gas Turbine - Downwash - Annual

0.01

0.1

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.13. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Downwash
FINAL 09/30/2004 34

Gas Turbine - Flat Terrain - Annual

0.001

0.01

0.1

1

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Flat Terrain - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.14. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Flat Terrain
FINAL 09/30/2004 35

Gas Turbine - Complex Terrain - Annual

0.001

0.01

0.1

1

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Complex Terrain - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.15. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Complex Terrain
FINAL 09/30/2004 36

Gas Turbine - Rural - Annual

0.001

0.01

0.1

1

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Rural - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.16. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Rural Dispersion
FINAL 09/30/2004 37

Gas Turbine - Urban - Annual

0.001

0.01

0.1

1

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Urban - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.17. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Urban Dispersion
FINAL 09/30/2004 38

35m Stack, 1g/s - No Downwash - 1hr

0.1

1

10

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

35m Stack, 1g/s - No Downwash - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.18. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 1g/s Emission Rate
FINAL 09/30/2004 39

35m Stack, 50g/s - No Downwash - 1hr

1

10

100

1000

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

35m Stack, 50g/s - No Downwash - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.19. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 35m Buoyant Stack with 50g/s Emission Rate
FINAL 09/30/2004 40

Diesel Generator - No Downwash - 1hr

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - No Downwash - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.20. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with No Downwash
FINAL 09/30/2004 41

Diesel Generator - Downwash - 1hr

1

10

100

1000

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Downwash - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.21. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Downwash
FINAL 09/30/2004 42

Diesel Generator - Flat Terrain - 1hr

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Flat Terrain - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.22. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Flat Terrain
FINAL 09/30/2004 43

Diesel Generator - Complex Terrain - 1hr

1

10

100

1000

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Complex Terrain - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.23. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Complex Terrain
FINAL 09/30/2004 44

Diesel Generator - Rural - 1hr

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Rural - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.24. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Rural Dispersion
FINAL 09/30/2004 45

Diesel Generator - Urban - 1hr

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Diesel Generator - Urban - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.25. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Diesel Generator Source with Urban Dispersion
FINAL 09/30/2004 46

Gas Turbine - No Downwash - 1hr

0.1

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - No Downwash - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.26. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with No Downwash
FINAL 09/30/2004 47

Gas Turbine - Downwash - 1hr

1

10

100

1000

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Downwash - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.27. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Downwash
FINAL 09/30/2004 48

Gas Turbine - Flat Terrain - 1hr

0.1

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Flat Terrain - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.28. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Flat Terrain
FINAL 09/30/2004 49

Gas Turbine - Complex Terrain - 1hr

0.1

1

10

100

1000

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Complex Terrain - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.29. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Complex Terrain
FINAL 09/30/2004 50

Gas Turbine - Rural - 1hr

0.1

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Rural - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.30. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Rural Dispersion
FINAL 09/30/2004 51

Gas Turbine - Urban - 1hr

0.1

1

10

100

1 10 100 1000 10000 100000

Distance (meters)

Concentration (ug/m^3)

FULL

OLM

PVMRM

Gas Turbine - Urban - 1hr

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (meters)

Ratio

OLM/FULL

PVM/FULL

Figure 3.31. 1-Hour Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for Gas Turbine Source with Urban Dispersion
FINAL 09/30/2004 52

10m Non-buoyant Point Source - No Downwash - Annual

0.1

1

10

100

1000

1 10 100 1000 10000 100000

Distance (m)

Concentration (ug/m^3)

FULL

OLM

PVMRM

10m Non-buoyant Point Source - No Downwash - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (m)

Ratio

OLM/FULL

PVM/FULL

Figure 3.32. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 10m Non-buoyant Point Source
FINAL 09/30/2004 53

10m Volume Source - Initial Sigmas = 4.65m - Annual

0.1

1

10

100

1000

1 10 100 1000 10000 100000

Distance (m)

Concentration (ug/m^3)

FULL

OLM

PVMRM

10m Volume Source - Initial Sigmas = 4.65m - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (m)

Ratio

OLM/FULL

PVM/FULL

Figure 3.33. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 10m Volume Source with Initial Sigmas=4.65m
FINAL 09/30/2004 54

10m Area Source - Initial Sigma-z = 4.65m - Annual

0.1

1

10

100

1000

1 10 100 1000 10000 100000

Distance (m)

Concentration (ug/m^3)

FULL

OLM

PVMRM

10m Area Source - Initial Sigma-z = 4.65m - Annual

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000 10000 100000

Distance (m)

Ratio

OLM/FULL

PVM/FULL

Figure 3.34. Annual Average NO2 Concentration (μg/m3) vs. Downwind Distance (top) and 

Conversion Ratio (bottom) for 10m Area Source with Initial Sigma-z=4.65m
FINAL 09/30/2004 55

4.0 SUMMARY AND CONCLUSIONS 

 This report presents results of a sensitivity analysis of the PVMRM and OLM options for 

NOx to NO2 conversion in the AERMOD dispersion model. Several single source scenarios 

were examined as well as a multiple-source scenario. The average conversion ratios of NO2/NOx

for the PVMRM option tend to be lower than for the OLM option and for the Tier 2 option of 

0.75 ARM. The sensitivity of the PVMRM and OLM options to emission rate, source 

parameters and modeling options appear to be reasonable and are as expected based on the 

formulations of the two methods. For a given NOx emission rate and ambient ozone 

concentration, the NO2/NOx conversion ratio for PVMRM is primarily controlled by the volume 

of the plume, whereas the conversion ratio for OLM is primarily controlled by the ground-level 

NOx concentration. 

Overall the PVMRM option appears to provide a more realistic treatment of the 

conversion of NOx to NO2 as a function of distance downwind from the source than OLM or the 

other NO2 screening options (Hanrahan, 1999a; Hanrahan, 1999b). No anomalous behavior of 

the PVMRM or OLM options was identified as a result of these sensitivity tests. 
FINAL 09/30/2004 56
FINAL 09/30/2004 57

5.0 REFERENCES 

Cimorelli, A. J., S. G. Perry, A. Venkatram, J. C. Weil, R. J. Paine, R. B. Wilson, R. F. Lee, W. 

D. Peters, R. W. Brode, and J. O. Paumier, 2002: AERMOD: Description of Model 

Formulation (Version 02222). EPA 454/R-02-002d. U. S. Environmental Protection 

Agency, Research Triangle Park, NC. 

Environmental Protection Agency, 2002: User’s Guide for the AMS/EPA Regulatory Model - 

AERMOD. EPA-454/R-02-001. U.S. Environmental Protection Agency, Research 

Triangle Park, North Carolina 27711. 

Hanrahan, P.L., 1999a. “The plume volume molar ratio method for determining NO2/NOx ratios 

in modeling. Part I: Methodology,” J. Air & Waste Manage. Assoc., 49, 1324-1331. 

Hanrahan, P.L., 1999b. “The plume volume molar ratio method for determining NO2/NOx ratios 

in modeling. Part II: Evaluation Studies,” J. Air & Waste Manage. Assoc., 49, 1332-

1338.