Journal of Environmental Monitoring 

Baseline ambient gaseous ammonia concentrations in the Four 

Corners area and eastern Oklahoma, USA 

Mark E. Sather *a, Johnson Mathew b, Nghia Nguyen b, John Lay b, George Golod b, Robert Vet c, 

Joseph Cotie d, Terry Hertel d, Erik Aaboe d, Ryan Callison e, Jacque Adam e, Danielle Keese e, 

Jeremy Freise e

, April Hathcoat e, Brenda Sakizzie f, Michael King f, Chris Lee f, Sylvia Oliva g, 

George San Miguel g, Leon Crow h and Frank Geasland h

aAir Quality Analysis Section, U.S. Environmental Protection Agency (EPA) Region 6, 1445 Ross Avenue, 

Dallas, TX 75202, USA bHouston Laboratory, U.S. EPA Region 6, 10625 Fallstone Road, Houston, TX 77099, USA

c

Environment Canada, Air Quality Research Division, 4905 Dufferin Street, Toronto, Ontario, Canada 

M3H 5T4 dNew Mexico Environment Department, 1301 Siler Road Building B, Santa Fe, NM 87507, USA e

Cherokee Nation Environmental Programs, 208 E. Allen Road, Tahlequah, OK 74464, USA

f

Southern Ute Indian Tribe, Air Quality Program, P.O. Box 737, Ignacio, CO 81137, USA

gMesa Verde National Park, Natural Resources, Mesa Verde, CO 81330, USA

hQuapaw Tribe of Oklahoma, P.O. Box 765, Quapaw, OK 74363, USA

Received 12th May 2008 , Accepted 11th September 2008

First published on the web 25th September 2008

Ambient ammonia monitoring using Ogawa passive samplers was conducted in the Four Corners area and 

eastern Oklahoma, USA during 2007. The resulting data will be useful in the multipollutant management 

of ozone, nitrogen oxides, and visibility (atmospheric regional haze) in the Four Corners area, an area with 

growing oil/gas production and increasing coal-based power plant construction. The passive monitoring 

data also add new ambient ammonia concentration information for the U.S. and will be useful to scientists 

involved in present and future visibility modeling exercises. Three week integrated passive ammonia 

samples were taken at five sites in the Four Corners area and two sites in eastern Oklahoma from 

December, 2006 through December, 2007 (January, 2008 for two sites). Results show significantly higher 

regional background ammonia concentrations in eastern Oklahoma (1.8 parts per billion (ppb) arithmetic 

mean) compared to the Four Corners area (0.2 ppb arithmetic mean). Annual mean ammonia 

concentrations for all Four Corners area sites for the 2007 study ranged from 0.2 ppb to 1.5 ppb. Peak 

ambient ammonia concentrations occurred in the spring and summer in both areas. The passive samplers 

deployed at the Stilwell, Oklahoma site compared favorably with other passive samplers and a continuous 

ammonia monitoring instrument. 

1. Introduction 

In late 2005 the Four Corners Air Quality Task Force (FCAQTF) was formed to address air quality issues 

Journals Journal of Environmental Monitoring Advance Articles DOI: 10.1039/b807984f

J. Environ. Monit., 2008 DOI: 10.1039/b807984f Paper

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associated with growth in the oil/gas production and coal-fired power plant industries.1 The Four Corners 

area includes southwest Colorado, northwest New Mexico, southeast Utah, and northeast Arizona. One of 

the concerns of the FCAQTF is that ambient ozone concentrations in the area may not meet the new U.S. 8 

hour ozone standard of 0.075 parts per million (ppm).2 In order to mitigate increases in ambient ozone 

concentrations, certain controls are being considered for the ozone precursor nitrogen oxides, such as 

selective and non-selective catalytic reduction on oil and gas production equipment and power plants. The 

use of selective or non-selective catalytic reduction controls would, however, result in the release of more 

ammonia emissions to the atmosphere. Ammonia is known to be a precursor of fine particulate matter,3

which adversely affects public health, atmospheric visibility, and acid deposition patterns.4 Currently, it is 

not known whether or not increased ammonia emissions in the Four Corners area would be significant 

enough to cause a degradation in atmospheric visibility. To enable air quality managers to answer this 

question in the future, ambient monitoring of gaseous ammonia (NH3) was instituted to set a current 

baseline. Then in the future, after nitrogen oxide controls have been implemented along with any gaseous 

ammonia increases due to growth in other sources such as population and mobile vehicles, ambient 

monitoring of gaseous NH3 can take place again to compare to the current baseline. Thus, the states of 

New Mexico and Colorado in the Four Corners area requested to work with the U.S. Environmental 

Protection Agency (EPA) Region 6 in 2006 in order to obtain a current ambient ammonia monitoring 

baseline. To keep costs low Ogawa passive sampling devices (PSDs) were deployed for the baseline study, 

and the U.S. EPA Region 6 laboratory in Houston, Texas, provided the laboratory preparation and ion 

chromatographic (IC) analytical work. Simple to use PSDs, including the Ogawa PSD, have been 

successfully used in many published ambient ammonia studies around the world.5–15 The U.S. currently 

lacks a routine national Federal Reference Method (FRM) ambient monitoring network for ammonia, so 

the data from the 2007 study in the Four Corners area and eastern Oklahoma also add new ambient 

ammonia concentration information for the U.S., adding to previous ambient ammonia monitoring data 

collected in eastern, western, and southeastern U.S. states.14–19 

2. Experimental 

2.1 Study sites 

There were six passive ammonia sites that sampled for the entire year-long study. In addition, in response 

to the Quapaw Tribe's interest in obtaining ambient ammonia concentrations around a nearby mushroom 

processing facility, a seventh site participated in the study for the last eight 3 week integrated sampling 

periods. A map of all study sites from the three different areas, including annual mean ammonia 

concentrations for the six sites that operated year-long, is depicted in Fig. 1. A variety of site types was 

desired and included: regional background sites in both the Four Corners area (Mesa Verde National Park) 

and eastern Oklahoma (Stilwell); oil and gas development impacted sites in the Four Corners area (Navajo 

Lake and Southern Ute). The Southern Ute site was also impacted by mobile source emissions, being 

nearby a major highway; a coal-fired power plant impacted site in the Four Corners area (Substation); an 

urban site in the Four Corners area (Farmington Airport), and finally a nearby source oriented site situated 

to record higher ambient ammonia concentrations (Quapaw site in northeastern Oklahoma). 

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2.2 Field instrumentation 

For the passive ammonia sampling, Ogawa passive sampling devices (PSDs) were used, previously 

obtained from Ogawa & Co., Inc., Pompano Beach, Florida. The Ogawa PSD is composed of a solid 

cylindrical polymeric body (two centimetres by three centimetres long) with a diffusion barrier and two 

Fig. 1 Map of the 2007 passive ammonia 

monitoring sites and annual mean concentrations in 

the Four Corners area and eastern Oklahoma.

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stainless steel screens on each side. At most of the sites the ammonia PSDs were mounted on aluminium 

pole assemblies about 1.5 metres above ground level (agl). A few of the sites installed the aluminium pole 

assemblies on top of existing air monitoring sampling trailers (about 5 metres agl). Each PSD was placed 

under a rain shelter for wind face velocity and precipitation protection. Gaseous ammonia, upon natural 

diffusion into the PSD, was collected on filters coated with citric acid. The filters used in this study were 

purchased from Ogawa by the New Mexico Environment Department, the Cherokee Nation, and the U.S. 

EPA Region 6 laboratory in Houston. The detection limit of the ammonia PSD in our study, defined as two 

times the standard deviation of the blank values, was 0.18 ug m−3 for 3 week integrated deployments, 

which compares favorably with previous passive ammonia field studies.13,14,20 Every deployment involved 

duplicate sampling for both field samples and field transport blanks. The two field transport blanks were 

averaged and subtracted from each field sample. The two field samples were then averaged for the final 

ammonia concentration. All calculations were done in accordance with the Ogawa sampling protocol for 

ammonia21 and included corrections for local ambient temperature. 

Since 2004 the Cherokee Nation has operated a continuous ammonia monitor at their Stilwell, 

Oklahoma site. The continuous ammonia monitor is a Pranalytica Nitrolux 200 continuous trace level 

ammonia analyser, which measures ambient ammonia concentrations by ultrasonic acoustic spectroscopy. 

Weekly performance checks (e.g. for error or maintenance flags, filter or battery change, laser overheat) on 

the Nitrolux instrument are conducted by the Cherokee Nation, and once every year the instrument is sent 

to Pranalytica for checkout and any required repairs. For 2007 the Stilwell Nitrolux instrument was 

operated from January 1 through September 15, 2007. 

2.3 Laboratory procedures 

2.3.1 Sample preparation and handling. Since gaseous ammonia is ubiquitous in trace quantities in 

the laboratory environment (e.g. from human breath and in moisture from sweat glands), and since it was 

desired to credibly monitor low regional background ammonia concentrations, great care was taken in 

preparing and handling the passive ammonia PSDs. First, the ammonia PSDs were prepared in an 

ammonia-scrubbed down-draft clean hood. The PSDs were then placed into thick (at least 0.05 millimetres 

thick) Ziploc bags, and the Ziploc bags were then placed into SC475 68 millilitre (ml) plastic vials with 

tight screw caps (from Environmental Express in Mt. Pleasant, South Carolina). The plastic vials, along 

with log sheets and return mail labels, were then packed into thermal bags for either FedEx or UPS small 

box shipping. Onsite sample change-out training was conducted with all of the operators for all of the sites 

for consistency and to further help in minimizing sample contamination. Powder-free disposable gloves 

were employed by all laboratory chemists and monitoring site operators. 

2.3.2 Chemical analyses. After each three weeks of sampling the ammonia PSDs were shipped back 

to the EPA Region 6 Houston laboratory. The ammonia filters were removed from the PSDs under the 

down-draft clean hood. The filters were then extracted with 8 ml of deionized water and ultrasonicated for 

45 minutes. Sample solutions were analysed using a Metrohm MIC-2 advanced modular ion 

chromatograph (IC). The Metrohm IC was equipped with a guard column and MetroSep C2-250 (4 

millimetres (mm) by 250 mm) separation column for cations. An eluent mixture of 4 millimoles (mM) 

tartaric acid plus 1 mM 2,6-pyridine dicarboxylic acid (PDCA) at a flow rate of 1 ml min−1 was used to 

elute the ammonium ion (IC detection limit for the ammonium ion = 0.025 ppm). The IC was used in nonsuppressed conductivity detection mode with detector settings at a range of one millisiemen per centimetre 

(mS cm−1) and 100 uS cm−1 full scale. 

3. Results and discussion 

3.1 Precision and comparisons of the passive data 

The precision for the year-long study was reviewed by analysis of the absolute differences of 124 data 

pairs from all seven sites (Fig. 2), using the median absolute difference and 90th percentile of all of the 

absolute difference values per Sirois and Vet.22 The absolute difference between duplicate field samples 

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had a median value of 0.1 ppb with 90% having a 0.3 ppb or lower difference. Lower precision was 

measured at the higher ambient ammonia concentrations (all from the Quapaw site). For the 16 data pairs 

from the Quapaw site the absolute difference between duplicate field samples had a median value of 0.2 

ppb with 90% having a 0.75 ppb or lower difference. 

The Ogawa PSDs used in this study have been successfully validated and used in previous published 

studies.13,14,18 To support our confidence that the Ogawa PSDs were providing reliable indicators of 

ambient ammonia concentrations, we conducted two data comparisons. First, at the Stilwell site, the 

passive ammonia data were compared to the continuously running Nitrolux monitoring data. The Nitrolux 

monitor operated through September 15, 2007. As seen in Fig. 3, the passive ammonia data overall tracked 

well with the Nitrolux ammonia data, with the Nitrolux ammonia concentrations usually higher than the 

passive ammonia concentrations. The correlation coefficient, r, was 0.74. The difference in arithmetic 

means was 0.7 ppb, but was not statistically significant (2.6 ppb mean ammonia concentration for the 

Nitrolux versus a 1.9 ppb mean ammonia concentration for the passive samples). It is important to note 

that since the Nitrolux monitor was not calibrated on site (i.e. only calibrated by the manufacturer once a 

year), the data from the Nitrolux monitor could only be viewed as providing an estimate of ambient 

ammonia concentrations rather than providing more precise measurements of ambient ammonia 

concentrations. Second, also at the Stilwell site, the passive ammonia data were compared to 

supplementary passive ammonia data being collected through a short-term Environment Canada study. 

Both studies used the Ogawa passive sampling device (PSD), with the Environment Canada Ogawa PSDs 

slightly modified (i.e. using 25 millimetre diameter filters instead of 14.5 millimetre diameter filters, and 

having more holes in the diffusion barriers), and operating at 2 week intervals instead of 3 week intervals. 

The Environment Canada PSDs were loaded, unloaded and analysed at their own laboratory in Toronto, 

Canada. The Environment Canada ammonia study ran through March, 2007, so a three month comparison 

with the EPA Region 6 passive data was possible. As seen in Table 1 the six week and twelve week 

average concentrations of the EPA Region 6 passive ammonia data and the Environment Canada passive 

ammonia data compared within +7% at best and within −20% at worst (calculated as 100× [EPAEC]/EPA). 

Table 1 Comparison of EPA Region 6 and Environment Canada Ogawa passive sample data at the Stilwell 

site; mean ammonia concentrations; 2007 

Fig. 2 Precision of the Ogawa NH3 PSDs for the 

2007 Four Corners area and eastern Oklahoma 

passive ammonia study.

Time period EPA Region 6 sampler mean (ppb) Environment Canada sampler mean (ppb) 

1/2–2/13 (first 6 weeks) 1.5 1.4

2/13–3/27 (second 6 weeks) 2.0 2.4

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Additional information which contributed to confidence in the passive data was obtained from the 

Quapaw Tribe source-oriented site. This site not only provided important data for the tribe concerning 

ambient ammonia concentrations close to a local mushroom processing facility (i.e. about 0.75 miles NNE 

of the facility), but also checked the ability of the PSD to adequately record higher concentrations of 

ambient ammonia concentrations compared to the considerably lower ambient ammonia concentrations 

recorded at the Four Corners area and Stilwell sites. Fig. 4 shows the time series of all passive ammonia 

samples taken at the Quapaw site (first samples taken July 13–July 31, 2007). The first four sampling 

periods involved collocated samples taken at slightly different heights (3 metres agl versus 5 metres agl), 

and those samples tracked close with the platform ambient ammonia concentration average at 4.9 ppb and 

the trailer top ambient ammonia concentration average at 5.0 ppb. For the second four sampling periods, 

the trailer top pair of duplicate samples was moved about 0.25 miles closer to the mushroom processing 

plant. All samples tracked well with the samples closer to the mushroom processing facility recording the 

higher passive ammonia concentrations (6.4 ppb average at 0.5 miles from the facility versus a 4.8 ppb 

average at 0.75 miles from the facility). 

1/2–3/27 (entire 12 weeks) 1.7 1.9

Fig. 3 Comparison of 3 week integrated passive 

ammonia data with continuous hourly Nitrolux 

ammonia data at the Stilwell, Oklahoma site; data 

capture of the Nitrolux monitor > 70% for each time 

period; 12/12/06–9/11/07.

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3.2 Time series analyses 

A time series of the 3 week integrated passive ammonia samples was constructed for each site for the yearlong 2007 study period. For the five Four Corners area sites (Fig. 5) the time series traces of passive 

ammonia concentrations generally increased from the beginning winter months to the warmer summer 

months, reaching peak ammonia concentrations during the July 10–July 31, 2007 time period, before 

generally decreasing in concentrations during the fall months. There were two notable exceptions to this 

general trend. First, the Southern Ute site recorded an additional peak ammonia concentration during the 

spring (the April 17–May 8, 2007 time period). There is moderate local agricultural activity around the 

Southern Ute site, so field fertilization probably caused the spring peak. Second, the Substation site, 

impacted by two coal-fired power plants, recorded additional peak ammonia concentrations during the fall 

(the October 2–October 23, 2007 and the October 23–November 13, 2007 time periods). These fall 

concentrations at the Substation site were also the highest ambient ammonia concentrations recorded in the 

Four Corners area for the year-long study. 

The time series trace for the Stilwell, Oklahoma site is shown in Fig. 6. The Stilwell site recorded its 

peak ambient ammonia concentration during the spring (the March 27–April 17, 2007 time period), 

probably due to agricultural fertilization activity. Table 2 displays the summary statistics for the five Four 

Corners area sites and the Stilwell, Oklahoma site. Of note is that the Stilwell site, representing regional 

Fig. 4 Passive ammonia data time series for the 

Quapaw, Oklahoma site; 3 week integrated samples.

Fig. 5 Passive ammonia data time series for the Four 

Corners area sites; 3 week integrated samples; 

ending dates 1/3/08 for Southern Ute and 1/22/08 

for Mesa Verde NP.

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background ammonia concentrations for eastern Oklahoma (1.8 ppb arithmetic mean), was nine times 

higher than the regional background site for the Four Corners area (0.2 ppb arithmetic mean at Mesa Verde 

National Park). Agricultural ammonia emissions, including microbial decomposition of animal waste from 

confined animal feeding operations (CAFOs), are higher in the region around eastern Oklahoma compared 

to the Four Corners area, resulting in a greater county ammonia emissions density for the Stilwell site 

versus the Four Corners area.23 

Table 2 Summary statistics for Four Corners area and Stilwell, Oklahoma passive ammonia sites; 3 week 

integrated samples; December 12, 2006–December 26, 2007a

In the Four Corners area, the urban Farmington Airport site recorded the highest annual mean ammonia 

concentration (1.5 ppb), followed by the power plant impacted Substation site (1.1 ppb). The rural Navajo 

Lake site, located in an area with growing oil and gas production, recorded an annual mean ammonia 

concentration (0.2 ppb) identical to the regional background site at Mesa Verde National Park. The 

Southern Ute site, also located in an area with growing oil and gas production but with added mobile 

source impacts, recorded a higher annual mean of 0.8 ppb.

Site (type) 

Mean 

(ppb) Min. (ppb) 

Max. 

(ppb) (ppb) 

95% confidence interval 

around mean (ppb) 

Substation (power plant impacted) 1.1 0.2 3.5 0.9 0.7–1.6

Farmington Airport (urban) 1.5 0.5 2.9 0.5 1.2–1.7

Navajo Lake (rural, oil/gas 

impacted)

0.2 No 

detection

0.5 0.1 0.1–0.3

Southern Ute (oil/gas/mobile 

source impacted)

0.8 0.3 1.5 0.3 0.7–1.0

Mesa Verde NP (rural, regional 

background)

0.2 No 

detection

0.6 0.2 0.1–0.3

Stilwell (rural, regional 

background)

1.8 0.8 3.3 0.6 1.5–2.1

a

Ending dates December 31, 2007 for Substation, Farmington Airport and Navajo Lake, January 3, 2008 for Southern Ute, and January 22, 

2008 for Mesa Verde NP. 

Fig. 6 Passive ammonia data time series for the 

Stilwell, Oklahoma site; 3 week integrated samples.

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3.3 Examination of the passive data in a national and international context 

The Four Corners area and eastern Oklahoma passive ammonia data were reviewed in the context of other 

United States and International ammonia monitoring studies. Table 3 presents summaries of some yearlong ambient ammonia monitoring studies culled from a current literature review. The Four Corners area 

and eastern Oklahoma ranges of annual mean concentrations for ambient ammonia are lower compared to 

the International and Delaware, U.S. studies, but similar to the concentration ranges seen in the 

southeastern U.S. states. Peak ambient ammonia concentrations in the spring or summer or fall were seen 

at sites in all of the studies, with winter maxima occurrences rare. 

Table 3 Comparison of Four Corners area and eastern Oklahoma passive ammonia data with other USA 

and International studies 

4. Conclusion 

The 2007 passive ammonia monitoring study in the Four Corners area and eastern Oklahoma was 

important because it provided first time year-long baseline data of ambient ammonia concentrations in the 

Four Corners area and eastern Oklahoma in a simple and cost-effective manner, and it added new multisite ambient ammonia concentration information for additional areas in the United States. It is important 

for air quality managers, when assessing nitrogen oxide emission controls for ambient ozone mitigation, to 

also consider ambient ammonia and atmospheric visibility impacts. The passive ammonia data in this study 

had a median absolute difference between duplicate field samples of 0.1 ppb, and compared favorably to 

other collocated passive data and a collocated continuous instrument at the Stilwell, Oklahoma site. In a 

larger context, the Four Corners area/eastern Oklahoma 2007 ambient ammonia concentrations were lower 

compared to previous international and eastern U.S. studies, but similar to previous measurements taken in 

some southeastern U.S States. In the future, after the implementation of new nitrogen oxide controls on 

oil/gas and power plant sources in the Four Corners area along with any growth in other ammonia sources, 

Study area (reference #) Time period 

Range of annual mean 

concentrations (ppb) Peak season(s) 

Four Corners Area, USA, New 

Mexico, Colorado (this study)

12/2006–1/2008 0.2–1.5 Summer and fall (also 

one spring peak)

Eastern Oklahoma, USA (this 

study)

12/2006–12/2007 1.8 (one site) Spring

Delaware inland bays, USA14 4/2000–4/2001 <0.7–>8.6 Spring and summer

SE USA, Georgia, Alabama, 

Mississippi, Florida17

1/2004–12/2004 0.23–2.44 Spring and late 

summer/fall

Denmark heathland24 5/1995–5/1996 2.9 (one site) Spring

Scotland, UK 10 m from road 

edges25

4/2002–4/2003 0.2–4.5 Rural, semi-rural: late 

summer/fall 

Other sites with high 

background: winter

Austria and Bavaria5 10/1995–10/1996 

and 4/1998–4/1999

1.3–9 Rural: summer

36 regional sites in Asia, Africa 

and South America7

9/1999–8/2000 <1–22 (median values) Site to site variations

Netherlands8 9/2000–9/2001 1.4–25.7 Spring

Switzerland9 10/1999–9/2000 0.6–10.7 Summer

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multiple passive samplers are planned to be deployed again to determine if ambient ammonia 

concentrations have significantly changed from the 2007 baseline data. Similarly in eastern Oklahoma in 

the future, passive samplers could be deployed again to determine if ammonia concentrations, largely from 

regional agricultural emissions, have significantly changed. It is recommended that the future studies be 

multi-year in duration in order to properly assess inter-annual variability of the ambient ammonia 

concentrations. The 2007 (and future) passive ammonia data should prove useful to modelers in present 

and future atmospheric visibility modeling exercises of both the Four Corners area and eastern Oklahoma. 

The models should be more accurate with inclusion of the passive ambient ammonia data, and the models 

will help determine if any atmospheric changes in ammonia concentrations significantly affect atmospheric 

visibility. 

Acknowledgements 

The authors are grateful to all the field operators involved in day to day careful ambient data collection. 

The authors thank Mr. Till Stoeckenius of ENVIRON Corporation for review of the draft manuscript, and 

thank Maria Martinez, David Neleigh, Rick McMillin and Dr Melvin Ritter for their support of passive 

monitoring studies. The authors also thank Melody Lister and Jeff Danielson for the excellent GIS map 

production. 

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