June 2009 www.aecom.com

Page 1 of 6

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station 

June 18, 2009 

Robert Paine, AECOM Environment1

Ivar Tombach, Consultant2

 

1

 AECOM (d/b/a AECOM Environment), 2 Technology Park Drive, Westford, MA. 01886 

2

 753 Grada Avenue, Camariilo, CA 93010 

INTRODUCTION AND RATIONALE 

Ammonia gas in the ambient atmosphere near Navajo Generating Station (NGS) and surrounding Class I 

areas plays an important role in the formation of visibility-impairing ammoniated sulfate and ammonium nitrate 

particles from SO2 and NOx emissions from NGS. The amount of available ammonia (NH3) governs the 

acidity of the sulfate aerosol and the availability of remaining NH3 can govern the amount of ammonium nitrate 

(NH4HNO3) that can be produced from the NOx. For these reasons, it is important to know the free ammonia 

concentration (i.e., the concentration of gaseous ammonia in the ambient atmosphere) that could interact with 

the NGS emissions. 

The geographic area of interest is shown in Figure 1. NGS is in the middle of the map, surrounded by 11 

Class 1 areas that lie wholly or partially within 300 km of the plant. 

To the best of our knowledge, only a few measurements of concentrations of ambient ammonia have been 

made within 200 km of NGS. There were daily measurements provided by Dr. Jeff Collett for May 2003 in the 

Grand Canyon, which averaged about 0.56 ppb; these were provided to the FLMs in support of the values 

used in the Desert Rock PSD permitting. Limited measurements have been made by the U.S. EPA in the Four 

Corners area of New Mexico and Colorado, including at Mesa Verde National Park, some 260 km from NGS 

(Sather et al., 2008). Three-week integrated samples were taken for a year (December 2006 to December 

2007) with Ogawa passive samplers. Those samples showed the annual mean ammonia concentration at 

Mesa Verde NP, the most rural sampling location, to be 0.2 ppb, with a standard deviation of 0.2 ppb. During 

the coldest months (December-February), 3-week average concentrations never exceeded 0.2 ppb. 

Most of the area shown in Figure 1, especially along the paths from NGS to the closest Class I areas, is even 

more arid and less populated than the vicinity of Mesa Verde NP. Therefore, it is reasonable to expect that the 

ammonia concentrations in the area around NGS will be similar to those measured at Mesa Verde, or perhaps 

even less. This expectation cannot be verified without measurements, however. 

The background ammonia concentration is a critical input to the EPA CALPUFF model, which is used to 

predict visibility impacts from NGS. In the Best Available Retrofit Technology (BART) analysis for NGS, EPA 

is proposing to utilize significantly higher ammonia background concentrations as input into the CALPUFF 

model than the values proposed by SRP, which are the same as those used in the Desert Rock PSD 

permitting. Since the background ammonia has a significant influence on the magnitude of visibility modeling 
AECOM Environment

June 2009 www.aecom.com

Page 2 of 6

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station

results, which could affect the outcome of the pending EPA BART determination, SRP requested a proposal 

from AECOM to measure ambient ammonia concentrations. 

This memo presents a conceptual plan for such monitoring. 

SOURCES OF AMMONIA 

Figure 2 shows sources of ammonia emissions that lie within the area of Figure 1 or near it. The emissions 

shown in Figure 2 were obtained from the US EPA’s AirData web site for 2002. Although ammonia emissions 

from NGS are listed on the AirData web site, there are no ammonia sources at NGS. Therefore, the value in 

EPA’s database for ammonia emissions has been adjusted accordingly. It is evident that there are no 

quantified ammonia sources along transport routes from NGS to the nearby Class I areas, which implies 

ammonia concentrations are likely to be very low and quite uniform in that area. 

SCOPE OF MONITORING PROGRAM 

Measurements at a representative regional site should give a good picture of the ambient ammonia 

concentrations that influence the impacts of NGS on haze in the closest and, potentially, the most impacted 

Class I areas, within 100 km of NGS. 

The specific locations of the regional monitoring site will be selected considering the following scientific and 

logistical factors: 

(1) In most circumstances, the ammonia concentration at the Class I area will determine 

the nitrate concentration (and, hence, the haze) affecting the Class I area. However, 

there are situations where the chemical reactions are slow enough that the ammonia 

concentration somewhat upwind of the Class I area will be more representative (and 

the Park Service seems to desire a location that is not at the receptor end of the 

trajectory). Since we do not expect much spatial variation in the ammonia 

concentrations, the exact location of the site is not critical as long as it is not near any 

local ammonia source. 

(2) The site(s) will need to have electric power, have adequate security, and be reasonably 

accessible for servicing in all weather conditions during the study period. In this 

desolate study area, one location that meets these criteria is the IMPROVE monitoring 

site at each Class I area, but other suitable locations may also be found. 

Measurements will be taken during the winter months. During other months of the year, the warm 

temperatures result in very little nitrate production no matter what the ammonia levels. Although ammonia 

concentrations are highest during the summer, the lower concentrations in the cold and cool months are of 

interest here because the lack of available ammonia can limit the formation of nitrates during those critical 

months. 

Samples will be collected with coated annular diffusion denuders that are analyzed for NH3 and related gases 

and particles at Colorado State University (CSU). Experience with this measurement method in Wyoming (see 

Attachment A) suggests that sample durations will have to be about three days long in order to quantify 

concentrations below the 0.2 ppb that is likely to occur during winter. 

The study will be managed by AECOM Environment with the consulting assistance of Dr. Ivar Tombach. Field 

sampling will be carried out by Air Resource Specialists (ARS), and/or SRP personnel. Laboratory analyses 

will be performed under the direction of Dr. Jeff Collett at the CSU Atmospheric Science Department, and ARS 

will process and report the data. (ARS and CSU performed the same roles in the ammonia monitoring in 
AECOM Environment

June 2009 www.aecom.com

Page 3 of 6

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station

Wyoming that is described in Attachment A.) AECOM and Dr. Tombach will analyze and interpret the results 

that are provided to reviewing agencies by SRP. 

SUMMARY OF PROJECT STEPS 

1. Review Monitoring Program Concepts with EPA and the Federal Land Managers (FLMs) 

Items to be addressed in a conference call or meeting include the following: 

• Location of the monitoring site 

• instrumentation and detection limits 

• frequency and duration of sampling 

• laboratory procedures 

• quality assurance 

• reporting of concentrations below the lower quantifiable limit (LQL) 

2. Select Actual Monitoring Site 

Because of their familiarity with the area, SRP and FLM personnel may be able to offer suggestions for a 

location for the site that meets the criteria mentioned earlier. Final site selection will be carried out by 

representatives of SRP, AECOM, and ARS during a 3-day survey field trip. Photographic and mapping 

documentation for the preferred site will be provided to the reviewing agencies, as noted below. 

3. Develop Monitoring Protocol and QAPP 

ARS, with contributions from AECOM and SRP, will produce two documents: 

(1) A Monitoring Protocol that describes the elements of the monitoring program in 

detail, with a description of the site and photographs. 

(2) A Quality Assurance Project Plan (QAPP) that is compliant with EPA guidance. 

These documents will be provided to EPA and FLMs for their comments. 

4. Prepare Site, Install Equipment, and Train Site Operators 

ARS will prepare the selected site for monitoring, with support from SRP for electric power connections, and 

will install the equipment, verify instrument performance, and calibrate the instruments. Day-to-day operation 

of the monitoring site will be performed by either ARS or SRP personnel. 

5. Review Early Monitoring Results 

The initial ammonia measurements will be reviewed by AECOM and Dr. Tombach to determine whether the 

measurements will provide the required information. The adequacy of the LQL for 3-day samples will be 

reviewed and causes for any abnormally high concentrations will be identified if possible. If adjustments to the 

monitoring program are needed, they will be made at this time and EPA and the FMs will be advised of any 

changes. 
AECOM Environment

June 2009 www.aecom.com

Page 4 of 6

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station

6. Report Monitoring Program Results 

ARS will provide a report with validated monitoring data and documentation of reasons for data flags. Based 

on analysis of this data, AECOM will recommend ammonia concentration values to use for CALPUFF 

modeling of NGS emissions. 

More details on the monitoring program are provided in Attachment B, in excerpts of an ARS document. 

SCHEDULE 

For formal monitoring to begin later this year, the table below identifies tentative dates of key milestones. 

Before 15 July 2009 Discuss the conceptual plan with EPA and the FLMs 

15 July Begin site selection 

15 August Finalize arrangements for selected site

15 September Begin initial monitoring to verify proper operation of the sample 

collection, analysis, and data processing system.

Winter 2009 Begin formal monitoring. 

sometime after winter 2010 End monitoring.
AECOM Environment

June 2009 www.aecom.com

Page 5 of 6

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station

Figure 1: Class I Areas in the Vicinity of the Navajo Generating Station (NGS) 
AECOM Environment

June 2009 www.aecom.com

Page 6 of 6

Conceptual Plan for Ammonia Monitoring in the 

Vicinity of the Navajo Generating Station

Figure 2: Ammonia Sources in the Vicinity of the Navajo Generating Station (NGS) 
Attachment A 

Air Resource Specialists Inc. 

Ammonia Monitoring Program 

in 

Wyoming 
NH3 Monitoring in the Upper Green River Basin, 

Wyoming 

Extended Abstract #70 

John V. Molenar1

H. James Sewell2

Jeffrey Collett3

Cassie Archuleta1

Mark Tigges1 

Florian M. Schwandner3

Suresh Raja3

1

 Air Resource Specialists, Inc., 1901 Sharp Point Drive, Suite E, Fort Collins, CO 

80525 

2

 Shell Exploration & Production Company, 4582 S. Ulster Street Parkway, Suite 500, 

Denver, CO 80237 

3

 Department of Atmospheric Science, Colorado State University, Fort Collins, 

CO 80523-1371 

Principal Contact: John V. Molenar, Vice President, Air Resource Specialists, Inc., 

1901 Sharp Point Drive, Suite E, Ft. Collins, CO 80525, phone: 970-484-7941, Fax: 970-

484-3423, email: jmolenar@air-resource.com 

INTRODUCTION

A long term ammonia air monitoring study was initiated in December, 2006 at Boulder, 

Wyoming, by Shell Exploration & Production Company. The monitoring site is located 

in the Upper Green River Basin of western Wyoming southwest of the Bridger 

Wilderness, a Class I area with an IMPROVE monitoring site. This region is 

experiencing rapid development of natural gas resources with possible consequences of 

air quality and visibility impacts in the Bridger Wilderness. Only very limited short-term 

ammonia measurements were previously available for this region. Thus, the primary 

objective of this study is to characterize the local airborne nitrogen budget and, 

specifically, ammonia concentrations and concentrations of related gases and particles in 

the basin for at least one year. Gaseous and particulate samples were collected twice per 

week (integrated 3-day and 4-day samples) beginning December 15, 2006 and will 

continue through May, 2008. Samples were collected using coated annular denuders and 

stacked filters in a University Research Glassworks (URG) sampler. The Colorado State 

University Atmospheric Science Department provided laboratory-prepared sample media 

and laboratory analysis for gas and particle concentrations. Standard operating 

procedures, technical instructions, and a QAPP for instrument installation, operation and 

maintenance, field sampling, filter handling, and laboratory analyses were developed and 

submitted to Wyoming Department of Environmental Quality – Air Quality Division. 
MONITORING AND LABORATORY PROCEDURES 

Table 1 lists the instrumentation used and parameters monitored for the Boulder station 

ammonia study. Scheduled denuder and filter changes were performed twice per week, 

providing three- to four-day integrated samples for the duration of the study period. 

Ammonia concentrations and the concentrations of related gases and particles were 

measured using coated annular denuders and stacked filters in a University Research 

Glassworks (URG) sampler. The Colorado State University Atmospheric Science 

Department provided laboratory-prepared sample media and laboratory analysis for gas 

and particle concentrations. The two-channel sampler operated continuously, the air flow 

being controlled by a programmable pump with a mass flow sensor, and subsequent 

volumetric gas flow meters. The sampling method requires that metered air is drawn 

through a PM2.5 size selective cyclone inlet, then through a series of annular denuders and 

filters. The denuders are coated with an acid-based substrate to collect ammonia gas. A 

second denuder is coated with NaCl to collect gaseous nitric acid. A subsequent stack of 

two filters is utilized to collect particles. A nylon filter collects particles, and a second, 

acid-coated filter collects any ammonia volatilized from collected, semi-volatile 

ammonium nitrate particles. The nylon filter retains any nitric acid volatilized from 

particulate ammonium nitrate. Following field exposure, the denuders, filters, and field 

data logs are sent to the CSU lab for chemical extraction and analysis. Total sampler air 

volume from the data sheet is then used to calculate concentrations. 

Table 1: Instrumentation and Monitored Parameters Boulder, Wyoming Air Quality 

Station 

Component Instrumentation Height Frequency Parameters 

GASEOUS URG denuders 1.5m 

3-day and 4-

day integrated 

samples 

Ammonia, NH3

Nitric Acid, 

HNO3

PARTICULATE URG stacked 

filters 1.5m 

3-day and 4-

day integrated 

samples 

Ammonium, 

NH4

+

Nitrate, NO3

-

Sulfate, SO4

2- 

RESULTS 

Figure 1 shows a timeline of gaseous and particulate concentrations from December, 

2006 – January, 2008. Figure 2 plots the monthly mean, standard deviation, minimum, 

and maximum NH3 concentrations. NH3 concentrations are below 1 ppbv for most of the 

monitoring period, peaking in August, 2007 at 1.55 ppbv. Elevated NH3 concentrations 

here coincide with warmer summer months. A shift in ammonium nitrate equilibrium 

toward the gas phase might be responsible for some of this increase, although an increase 

in total reduced nitrogen (ammonium + ammonia) during the summer suggests that 

changes in emissions and or transport patterns are likely also important contributors. 
Figure 1: Timeline of measured concentrations. 

0.0

0.5

1.0

1.5

2.0

12/1512/2212/29

1/5

1/121/191/26

2/22/9

2/162/23

3/23/9

3/163/233/30

4/6

4/134/204/27

5/4

5/115/185/25

6/16/8

6/156/226/29

7/6

7/137/207/27

8/3

8/108/178/248/31

9/7

9/149/219/2810/5

10/1210/1910/26

11/211/9

11/1611/2311/30

12/7

12/1412/2112/28

1/4

1/111/18

date 2006-2008

NH3 (g) NH4 + (p)

HNO3 (g) NO3 - (p)

SO4 2- (p)

g m-3 ion / u concentrat

Figure 2: 2007 monthly and annual NH3 concentration data 

Three events stand out: high particulate NO3

-

 in January, 2007; high particulate NO3

-

 in 

December, 2007 – January, 2008; and high particulate SO4

-2 in May, 2007. The winter 

nitrate events are interesting as all reduced nitrogen is present as particle phase 

ammonium while considerable nitric acid remains in the gas phase. Sufficient increases 

in ammonia emissions during this period could have substantially increased PM2.5

concentrations by further ammonium nitrate formation. 

Figure 3 displays a timeline of the gas/particle partitioning of the measured species for 

the same time period. Particulate sulfate (red) dominates throughout most of the year. 

Reduced nitrogen (green bars) shows an increased partitioning into the gas phase 
(ammonium to ammonia). Increased particulate nitrate in late winter is consistent with 

thermodynamic expectations: ammonium nitrate formation is favored at lower 

temperatures and higher relative humidities. 

Figure 3: Timeline of gas-particle partitioning. Concentrations in μg/m3, top bar 

stack for particles, bottom bar stack for gases. 

gas-particle partitioning

-2

-1

0

1

2

3

12/1512/29

1/121/26

2/9

2/23

3/9

3/23

4/6

4/20

5/4

5/18

6/1

6/156/297/137/278/108/24

9/7

9/2110/5

10/19

11/2

11/1611/3012/1412/28

1/111/25

start date

concentration / ug m-3

NH4 + (p) NO3 - (p)

SO4 2- (p) NH3 (g)

HNO3 (g)

PM

GAS

RESIDENCE TIME ANALYSIS 

Ammonia weighted back trajectories were used to identify the geographic source areas 

most likely to contribute to the highest measured ammonia days. Back trajectory analyses 

use interpolated measured or modeled meteorological fields to estimate the most likely 

central path over geographical areas that provided air to a receptor at a given time. The 

method essentially follows a parcel of air backward in hourly steps for a specified length 

of time. Back trajectories account for the impact of wind direction and wind speed on 

delivery of emissions to the receptor, but do not account for chemical transformation, 

dispersion and deposition of emissions.

Trajectories were generated using the Hybrid-Single Particle Lagrangian Integrated 

Trajectory (HYSPLIT) model developed by the National Oceanic and Atmospheric 

Administration’s (NOAA) Air Resources Laboratory (ARL). Detailed information 

regarding the trajectory model and these data sets can be found on NOAA’s Web site 

(http://www.arl.noaa.gov/ready/hysplit4.html). 

Three back trajectories were generated per day, including end times of 0400, 1200 and 

2000 MST and end heights of 100 m. Each hourly point along a 72-hour back trajectory 

paths was weighted with measured ammonia concentration corresponding to the end date 

of each trajectory. The ammonia values associated with each hourly point were then 

summed and normalized into 1/4 degree horizontal grid cells of latitude and longitude. 

Figure 4 presents a map of the ammonia weighted residence time for 2007. One path of 

influence follows the Snake River from Idaho to the Columbia River. This is a significant 
agriculture region. Another distinct path is from the South-Southwest along I-15 to 

Nevada then along the Colorado River. In addition to the major urban areas of Salt Lake 

City and Las Vegas, this pathway includes the agriculture regions of Star Valley north of 

Salt Lake City, the Wasatch front in Utah, and the Colorado River Basin. 

Figure 4: NH3 weighted residence time analysis for 2007. 

SUMMARY 

A 15 month study in the Upper Green River Basin of Wyoming measuring ammonia and 

nitric acid gasses and ammonium, nitrate, and sulfate particles has been completed. The 

results of the study show that in the Upper Green River Basin of Wyoming 2007 

ammonia concentrations are (1) quite variable throughout the year, (2) are below or near 

detectable limits from December through late February, (3) peak in August at 1.55 ppbv, 

and (4) have a yearly mean value of 0.24 ppbv. NH3 weighted Residence time analysis 

indicates that much of the NH3 present is transported into the region from agriculture and 

urban areas to the West and Southwest. 
Attachment B 

Air Resource Specialists Inc. 

Proposed Work Scope for Ammonia and Related Gas Sampling 

in the Vicinity of the Navajo Generating Station 
AECOM Environment

June 2009 www.aecom.com

Page 2 of 3 Attachment B to Conceptual Plan for Ammonia Monitoring in 

the Vicinity of the Navajo Generating Station

Proposed ARS Work Scope for Ammonia and Related Gas 

Sampling in the Vicinity of the Navajo Generating Station 

June 18, 2009 

Overall Objective

The program’s overall objective will be to determine the background concentrations of ammonia and related 

gases and aerosols in northern Arizona and southern Utah for use in air quality models related to proposed 

actions anticipated by the Navajo Generating Station. This would be accomplished by establishing a 

regional monitoring site in remote areas between the Navajo Generating Station and nearby Class I areas. 

Due to the homogeneity of the area, we anticipate that monitoring at one site should be sufficient, and 

further discussions in this regard can be held with the Federal Land Managers. 

Scope of Work and Assumptions 

1. Equipment Procurement: ARS proposes to purchase and configure URG 3000C dual-channel annular 

denuder and stacked filter samplers and support systems, including environmental enclosures, mounts, 

sample shipping cases, air temperature and barometric pressure sensors, and all other supporting 

components. It is assumed that the system will operate on 120 VAC line power. 

2. Site Selection: ARS will join AECOM and Salt River Project (SRP) representatives to plan for and select 

a monitoring site. It is anticipated that the siting will be complete by late Summer 2009. 

3. Site Preparation and Installation: It is assumed that SRP will arrange for and perform any necessary 

power installations and directly pay for utility service throughout the project. ARS will travel to the site; 

install, calibrate, and verify instrument performance; and train the site operators. It is anticipated that 

installation will occur in late Summer/early Fall 2009. 

4. Quality Assurance Project Plan (QAPP): A detailed QAPP of the monitoring project, compliant with EPA 

guidance, will be prepared by ARS in cooperation with AECOM. The QAPP will be distributed to 

appropriate state agencies and federal land managers for their review, comment, and ultimate approval. 

5. Program Operation 

• Duration: Winter 2009-2010 (exact duration to be discussed with EPA and FLMs) 

• Sampling Strategy: It is necessary to achieve 0.15 ppb or better resolution on all samples. Ideally 

24-hour samples will be taken on the EPA national 3-day schedule to allow direct comparison with 

other monitoring programs (i.e., IMPROVE); however, to achieve the required resolution, it may be 

necessary to increase the sample duration to 48 or 72 hours (centered in the EPA 3-day schedule). 

The final sampling strategy will be determined prior to program initiation. Regardless of the final 

strategy, the program scope assumes one (1) sample every three (3) days at each site(s). 

• Laboratory Analysis: Colorado State University (Dr. Jeff Collett) will prepare, ship, and receive all 

sample media and perform the laboratory analysis to yield the per sample concentration of NH3, 

HNO3 , NH4+, NO3-, and SO4--. This phase also includes all laboratory QA and field QA (i.e., lab 

standards, field blanks, shipping blanks, etc.), as well as all expendable media and shipping costs to 

and from the site. 

• Calibration, Maintenance, and Site Operator Support: ARS will perform quarterly calibrations and 

maintenance of the site. A repair and replacement parts contingency is included in this phase. 

CSU will travel to the site once to perform onsite quality assurance verification tests. ARS will 
AECOM Environment

June 2009 www.aecom.com

Page 3 of 3 Attachment B to Conceptual Plan for Ammonia Monitoring in 

the Vicinity of the Navajo Generating Station

provide on-demand telephone site operator support throughout the project to address sample 

change issues, perform QA checks and diagnostics, and perform minor repairs. 

• Site Operators: A site operator will change sample media every three (3) days. 

• Data Reporting: All laboratory results will be quality assured and verified. Monthly spot reports and 

quarterly data reports will be prepared and delivered and will include operational timelines, analyte 

concentration tables and plots, quality assurance issues (i.e., blanks concentrations and 

corrections, detectable limits, etc.) and other narratives and products that will quantify the overall 

accuracy of the program. The reports will be accompanied by digital files of all tabular and graphic 

products.