E

C

/R Incorporated Providing Environmental Technical Support Since 1989

501 Eastowne Drive, Suite 250  Chapel Hill, North Carolina 27514 

Telephone: (919)484-0222  Fax: (919) 484-0122 

MEMORANDUM

Date: September 3, 2010 

To: Anita Lee, U.S. EPA/Region 9 

From: Nancy Jones, EC/R Incorporated 

Subject: Ammonia Mobile Source Risk Estimates for Four Corners Power Plant 

1. Introduction 

 This memo describes the air dispersion modeling analysis conducted in response to 

comments EPA received on the Advanced Notice of Proposed Rulemaking (ANPR) of the Best 

Available Retrofit Technology (BART) for the Four Corners Power Plant (FCPP) located on the 

Navajo Nation, near Farmington New Mexico. The commenters noted that the increased 

pollutants released in the transportation of ammonia to the facility to be used for selective 

catalytic reduction (SCR) of NOx emissions would generate emissions of NOx, CO, VOC, PM10, 

and other pollutants. The modeling study was conducted using the Human Exposure Model 

Version 3 (HEM3) that contains as its air dispersion model the American Meteorological Society 

(AMS) and EPA Regulatory Model (AERMOD). 

 As shown in Figure 1, the roadway emissions were modeled as five area sources 

approximately overlying the roads. Air concentrations at census blocks near the roadway were 

estimated based on the pollutants emissions rates and the site-specific meteorology and terrain 

input into the model. The census block locations are shown in Figure 2. Detailed model inputs 

are presented in Attachment 2. Potential risks from toxic air pollutants were estimated by 

comparing the modeled air concentrations to OAQPS’ dose response values contained within 

HEM3. Criteria pollutant air concentration estimates were compared to their National Ambient 

Air Quality Standards (NAAQS) and Class I increments. 

2. Emissions Estimates 

 Emissions were estimated from the estimated 15 additional trucks per week needed to 

transport the ammonia in the form of urea crystals from the railway station in Gallup, New 

Mexico to FCPP (approximately 100 miles each way). The number of trucks needed to transport 

urea to the FCPP was calculated based on the size of the plant using data presented in a technical 

and economic comparison of SCR ammonia options 

(http://www.mcilvainecompany.com/NOx_Decision_Tree/subscriber/Tree/DescriptionTextLinks

/CostAnalyWashGrp.pdf). The report provided data on the number of trucks or railcars that 

would be needed to transport various forms of ammonia and estimated it would take 19 truck 

deliveries to deliver 370 tons of urea or 19.5 tons per truckload. 
Ammonia Mobile Source Risk Estimates 

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Figure 1. Approximate Locations of Line Sources from Gallup, New Mexico 

 to the Four Corners Power Plant 

 The FCPP BART analysis provided updated annual reagent costs for each of the five 

units. The reagent costs for each of the units were converted to annual tons using the estimated 

cost for urea presented in the BART analysis, as $450 per ton of urea. The number of truck 

deliveries was then estimated using the annual tons of urea needed per unit and the amount of 

urea per truck provided in the ammonia optimization report. A summary of the results is 

presented in Table 1. 

 Emissions estimates for criteria pollutants were developed from the most conservative 

EMFAC2007 (version 2.3) Emission Factors for On-Road Heavy-Heavy-Duty Diesel Trucks for 

2010 from South Coast Air Quality Management District (SCAQMD) 

(http://www.aqmd.gov/ceqa/handbook/onroad/onroad.html). Toxic air pollutant emission 

factors were developed from the reactive organic gas (ROG) and PM2.5 emission factors using 

EPA/OAQPS’ Speciate Database (http://cfpub.epa.gov/si/speciate/). Table 2 presents the 

SCAQMD emissions factors and the criteria pollutant emissions estimates used in the analysis. 

Table 3 presents the toxic air pollutant emissions estimates developed using the speciate factors. 
Ammonia Mobile Source Risk Estimates 

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Figure 2. Approximate Locations of Line Sources in Red and Census Block 

 Receptor Locations in Green 

Table 1. Development of Estimates of Urea Truck Traffic for SCR at FCPP 

Four Corners Unit 

Number 

Annual Reagent Cost 

($/yr)1

Annual Tons of Urea 

(Tons/yr)2

Number of Truck 

Deliveries 

(Deliveries/yr)3

Unit 1 $871,000 1,936 99.4 

Unit 2 $713,000 1,584 81.3 

Unit 3 $842,000 1,871 96.1 

Unit 4 $2,231,000 4,958 254.6 

Unit 5 $2,231,000 4,958 254.6 

Total $6,888,000 15,307 786 

1

 Annual reagent costs obtained from APS FCPP BART Analysis submitted 8/19/2008. 

2

 The annual tons of urea were calculated by dividing the annual reagent cost by the cost of urea from the FCPP 

BART Analysis, $450/Ton. 

3

 Number of truck deliveries calculated using the annual tons of urea and the tons of urea per truckload. 
Ammonia Mobile Source Risk Estimates 

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Table 2. Criteria Pollutant Emission Rates Used to Estimate Risk from Transporting Urea 

to FCPP for SCR

Pollutant 

SCAQMD EF 

(lb/mi) 

Emissions 786 trucks/yr, 200 miles 

roundtrip (TPY) 

Reactive Organic Gases (ROG) 3.04E-03 2.36E-01 

CO 1.20E-02 9.32E-01 

NOX 3.82E-02 2.98E+00 

SOX 4.13E-05 3.22E-03 

PM10 1.83E-03 1.43E-01 

PM2.5 including tires & brakes 1.60E-03 1.25E-01 

PM2.5 engine exhaust 1.55E-03 1.21E-01 

Table 3. Toxic Air Pollutant Emission Rates Used to Estimate Risk from Transporting 

Urea to FCPP for SCR 

Pollutant 

SCAQMD 

EF 

(lb/mi) 

Weight Percent of 

VOC from 

Speciate 

Calculated HAP 

Emission Factor 

(lb/mile) 

Emissions 786 

trucks/yr, 200 miles 

roundtrip (TPY) 

Gas Phase Speciate Database Record # 4674: Diesel Exhaust - Medium Duty Trucks 

ROG* 3.04E-03 

1,3-Butadiene 1.18E-01 3.60E-04 2.80E-02 

2,2,4-Trimethylpentane 4.73E-01 1.44E-03 1.12E-01

Acetaldehyde 1.59E+01 4.85E-02 3.78E+00 

Acrolein 1.30E+00 3.94E-03 3.08E-01 

Benzene 1.05E+00 3.18E-03 2.48E-01 

Ethyl benzene 1.79E-01 5.45E-04 4.25E-02 

Formaldehyde 8.51E+00 2.59E-02 2.02E+00 

M & p-xylene 8.89E-01 2.70E-03 2.11E-1 

Naphthalene 2.35E-01 7.16E-04 5.58E-02 

O-xylene 3.17E-01 9.63E-04 7.51E-2 

Propionaldehyde 5.34E+00 1.62E-02 1.27E+00 

Toluene 1.52E+00 4.62E-03 3.60E-01 

Dibenzofuran 1.09E-02 3.33E-05 2.60E-03 

Acetophenone 1.95E+00 5.92E-03 4.61E-01 

Particulate Phase Speciate Database Record # 322082.5: Heavy Duty Diesel Trucks 

Diesel engine emissions 1.55E-03 1.21E-01 

Arsenic compounds 1.50E-02 2.33E-05 1.82E-03 

Manganese compounds 9.00E-03 1.40E-05 1.09E-03 

Nickel compounds 2.70E-02 4.20E-05 3.27E-03 

* Assumed that reactive organic gases (ROG) are equivalent to volatile organic compounds (VOC). 
Ammonia Mobile Source Risk Estimates 

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3. Impacts of Criteria Air Pollutant Emissions 

 In order to place into perspective peoples’ exposure to criteria air pollutants, the modeled 

air concentration from the census blocks was compared to the NAAQS and Class I increments 

(when applicable) for the pollutant. The NAAQS and Class I increments for the criteria 

pollutants include both short-term and long-term average air concentrations (see Attachment 1). 

Table 4 presents the NAAQS levels used in this analysis, with the concentrations converted as 

necessary from expression as parts per million or billion to milligrams per cubic meter (mg/m3). 

As shown in Table 5, the annual average air concentrations modeled at the most impacted census 

block (the populated census block with the highest modeled air concentration) are well below the 

NAAQS and Class I increments. The population at this census block in San Juan County, New 

Mexico is seven individuals. As shown in Table 6, the short-term average air concentrations are 

also well below the NAAQS and Class I increments with the one-hour highest average 

concentrations occurring at a census block with a population of seven in McKinley County, the 

highest eight-hour and 24-hour average concentrations occurring at a census block with a 

population of nine in San Juan County. 

Table 4. NAAQS and Class I Increments Used in Criteria Pollutant Comparison 

Pollutant NAAQS Level 

as Codified

NAAQS Level for 

Comparison (mg/m3)

Class I Increments 

(mg/m3) 

Averaging 

time

CO 10 mg/m310 8-hour 

40 mg/m340 1-hour 

NO2 53 ppb 0.1 0.0025 Annual

100 ppb 0.19 1-hour 

PM10 0.004 Annual 

0.15 mg/m30.15 0.008 24-hour 

PM2.5 0.015 mg/m30.015 0.001 (proposed) Annual

0.035 mg/m30.035 0.002 (proposed) 24-hour

SO2 0.03 ppm 0.08 0.002 Annual

0.005 24-hour 

0.025 3-hour 

75 ppb 0.2 1-hour

Table 5. Comparison of Modeled Annual Averages to NAAQS and Class I Increments 

Pollutant Annual Average 

NAAQS Level 

(mg/m3)

Class I Annual Average 

Increments 

(mg/m3) 

Annual Average Modeled 

Air Concentration 

(mg/m3)

CO NA NA 0.000020

NO2 0.1 0.0025 0.000065 

PM10 NA 0.004 0.0000031

PM2.5 0.015 0.001 (proposed) 0.0000027

SO2 0.08 0.002 0.00000007
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Table 6. Comparison of Modeled Short Term Concentrations to NAAQS and Class I 

Increments 

Pollutant Short-Term Average 

NAAQS Level 

(mg/m3

)

Class I Short-Term 

Average Increments 

(mg/m3

) 

Short-term Average 

Modeled Air Concentration

(mg/m3)

1-hour Average Air Concentrations 

CO 40. NA 0.0054 

NO2 0.19 NA 0.017 

SO2 0.2 NA 0.000019 

8-hour Average Air Concentrations

CO 10. NA 0.00041 

24-hour Average Air Concentrations

PM10 0.15 0.008 0.00063 

PM2.5 0.035 0.002 (proposed) 0.000054 

SO2 NA 0.005 0.0000014 

4. Impacts of Toxic Air Pollutants 

 NAAQS have not been established for the hundreds of toxic air pollutants that have the 

potential to cause cancer and other non-cancer adverse health effects. Toxicological dose 

response values have been established for use in estimating many toxic air pollutants’ adverse 

health effects. OAQPS has established a table of pollutant-specific values that are used with 

modeled or measured air concentrations in order to quantify the impact of exposure to these 

pollutants. The long-term or chronic dose response values used in the analysis can be found at 

http://www.epa.gov/ttn/atw/toxsource/summary.html in Table 1. As shown in Table 7 below, the 

estimated cancer risk summed over all modeled toxic air pollutants is less than one in one million 

and the hazard quotient is well below one. This value is for the highest impacted census block 

located in San Juan County, New Mexico with a population of 36 individuals. For the other 

impacted census blocks, the risks are estimated to be as low as 1 x 10-10 and the hazard quotients 

are estimated to be as low as 0.00001. 

Table 7. Modeled Risk Estimates for Toxic Air Pollutants

Cancer Risk 5 x 10-7 Five excess cancer case over a lifetime exposure 

per 10 million people exposed

Non-Cancer Hazard Quotient 0.2

A value of the HQ at or below one indicates that 

the exposure is not likely to result in adverse health 

effects.

5. Comparison of Modeled Results to Background Concentrations and the 2002 NATA 

 In addition to estimating impacts by comparing modeled air concentrations to levels such 

as the NAAQS or toxicological dose response values, another way to characterize risk is to 

compare estimates to other values such as monitored concentrations or other estimated values. 

For the criteria pollutants, a large monitoring network has been established for the purpose of 
Ammonia Mobile Source Risk Estimates 

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determining whether or not an area has attained the NAAQS. While the number of monitors for 

air toxics has increased in recent years, few areas of the nation are covered, and no air toxics 

monitors are sited in the area of interest in New Mexico. Thus, for this analysis, we used the 

most recent NATA analysis for comparisons. The NATA analysis is a county-level air toxics 

study that provides modeled air concentration and risk estimates for the entire United States and 

that is completed every three years. The results of the 2002 NATA are presented for comparison 

here, since the 2005 NATA has not been finalized at this time. 

 As shown in Table 8, the expected impact of the increased truck traffic necessary to 

deliver urea to the FCPP to operate the SCR for NOx control is very low when compared to 

nationally monitored concentrations (http://www.epa.gov/airtrends/). Additionally, the total NOx

estimated to be emitted from this mobile source is 3 tons per year, while at 80 percent emissions 

reduction, the emissions reduction anticipated from SCR is over 33,000 tons at the FCPP (Four 

Corners BART Analysis – Cost Analysis, 8/19/2008). Tables 9 and 10 show that for air toxics, 

the impact from the transportation of urea to the FCPP is well below the impact estimated in 

NATA and in fact, for pollutants where background values are available, the modeled estimates 

are well below background levels. 

Table 8. Comparison of Annual Average Modeled Criteria Pollutant Concentrations to 

National Monitored Values

 FCPP 2008 Range of Monitored Values (μg/m3

) 

 Poll. 

Avg. 

Period 

Modeled 

Air Conc. 

(μg/m3) Poll. 

Avg. Period 

10th 

%ile 

Arithmetic 

Avg. 

90th 

%ile 

# of 

sites 

CO Highest 

8-hour 

0.41 CO 2nd Highest 

8-hour 

1,200 2,100 3,100 206

NO2 Annual 0.065 NO2 Annual 8.7 25. 40. 151

PM10 Annual 0.0031 PM10 Seasonal wt. 

annual 

33. 56. 88. 325

PM2.5 Highest 

24-hour 

0.055 PM2.5 2nd Highest 

24-hour 

7.4 11. 14. 728

SO2 Annual 0.00007 SO2 Annual 3.7 9.1 15.5 141

Table 9. Comparison of Modeled Total Air Toxic Risks to NATA Estimates for San Juan 

County, NM Total Risks

Toxic Air Pollutant FCPP 2002 NATA Analysis

Modeled Risk Total Background 

Cancer Risk 5 x 10-7 2 x 10-6 1.5 x 10-6

Non-Cancer Hazard Quotient 0.2 2 na

 
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Table 10. Comparison of Modeled Air Concentrations of Risk Driver Air Toxics to NATA 

Estimates for These Pollutants for San Juan County, NM 

Toxic Air Pollutant 

FCPP 2002 NATA Analysis

Modeled Annual 

Average Air Conc. 

(μg/m3)

Total Annual 

Average Air 

Conc. (μg/m3)

Background 

Annual Average 

Air Conc. (μg/m3)

Acetaldehyde 0.031 28.00 0.574

Acrolein 0.0025 6.20 NA

Arsenic Compounds 0.000015 0.0053 NA

Diesel Engine Emissions 0.00098 0.1094 NA

Formaldehyde 0.016 110.75 0.689
Attachment 1 

National Ambient Air Quality Standards 

National Ambient Air Quality Standards (NAAQS) (Source: http://epa.gov/air/criteria.html) 

 Primary Standards Secondary Standards 

Pollutant Level Averaging Time Level Averaging Time

Carbon 

Monoxide

9 ppm 

(10 mg/m3) 

8-hour (1) None 

35 ppm 

(40 mg/m3) 

1-hour (1)

Lead 0.15 μg/m3 (2) Rolling 3-Month Average Same as Primary 

1.5 μg/m3 Quarterly Average Same as Primary 

Nitrogen 

Dioxide

53 ppb (3) Annual 

(Arithmetic Average) 

Same as Primary 

100 ppb 1-hour (4) None 

Particulate 

Matter (PM10) 

150 μg/m3

 24-hour (5) Same as Primary 

Particulate 

Matter (PM2.5) 

15.0 μg/m3

 Annual (6) 

(Arithmetic Average) 

Same as Primary 

35 μg/m3 24-hour (7) Same as Primary 

Ozone 0.075 ppm 

(2008 std) 

8-hour (8) Same as Primary 

0.08 ppm 

(1997 std) 

8-hour (9) Same as Primary 

0.12 ppm 1-hour (10) Same as Primary 

Sulfur 

Dioxide

0.03 ppm Annual 

(Arithmetic Average) 0.5 ppm 3-hour (1)

 

0.14 ppm 24-hour (1)

75 ppb (11) 1-hour None 

(1) Not to be exceeded more than once per year. 

(2) Final rule signed October 15, 2008. 

(3) The official level of the annual NO2 standard is 0.053 ppm, equal to 53 ppb, which is shown here for the purpose 

of clearer comparison to the 1-hour standard 

(4) To attain this standard, the 3-year average of the 98th percentile of the daily maximum 1-hour average at each 

monitor within an area must not exceed 100 ppb (effective January 22, 2010). 

(5) Not to be exceeded more than once per year on average over 3 years. 

(6) To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or 

multiple community-oriented monitors must not exceed 15.0 μg/m3. 

(7) To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each populationoriented monitor within an area must not exceed 35 μg/m3 (effective December 17, 2006). 

(8) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone 

concentrations measured at each monitor within an area over each year must not exceed 0.075 ppm. (effective May 

27, 2008) 

(9) (a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone 

concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm. 

 (b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation 

purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone 

standard. 

 (c) EPA is in the process of reconsidering these standards (set in March 2008). 

(10) (a) EPA revoked the 1-hour ozone standard in all areas, although some areas have continuing obligations under 

that standard ("anti-backsliding"). 

 (b) The standard is attained when the expected number of days per calendar year with maximum hourly average 

concentrations above 0.12 ppm is < 1. 

(11) (a) Final rule signed June 2, 2010. To attain this standard, the 3-year average of the 99th percentile of the daily 

maximum 1-hour average at each monitor within an area must not exceed 75 ppb. 
 
Attachment 2 

Air Modeling Inputs 
 
Table 1.1 Modeling Options for the HEM3 Air Dispersion Modeling

Options Value 

Terrain used, rural land use, no deposition 

or depletion 

Site specific terrain values for locations and hill heights 

from HEM3. 

Receptors Census blocks within 25 km of center of each line (see 

memorandum Figure 2). 

13 rings of radial receptors, every 30 degrees from 25 to 

50 km of center. 

Meteorological Data Albuquerque, NM 1991 AERMET file contained within 

HEM3. 

Acute Emissions Multiplier Annual emissions multiplied by 10 to account for 

variations in short-term emissions. 

Table 1.2 Source Input Data for the HEM3 Air Dispersion Modeling

Source 

ID 

UTM East 

(m) SW 

corner for 

area source 

UTM 

North (m), 

SW corner 

for area 

source 

UTM 

zone 

Length in 

x direction 

(m) 

Length in 

y direction 

(m) 

Rotation 

Angle 

(degrees)1

Release 

height 

(m) 

line1 3,933,935 703,077 12 10 36,441 0 3 

line2 3,970,375 702,889 12 10 10,095 50 3 

line3 3,975,446 711,618 12 10 98,194 0 3 

line4 706,555 4,073,510 12 25,012 10 5 3 

line5 727,081 4,063,284 12 8,500 10 60 3 

1

 HEM3 requires that the rotation angle be 90 degrees or less. As a result, area sources 4 and 5 had to be 

rotated and the X and Y coordinates for a new southwest corner developed for these sources.