1

A Pilot Monitoring Study of Atmospheric NHx at Selected 

IMPROVE sites

Extended Abstract # 19

Xi Chen1, Derek Day2, Bret Schichtel2, William Malm2, Jose Mojica3, Chuck McDade3, Eva 

Hardison4

, Sonia M. Kreidenweis1, and Jeffrey L. Collett, Jr.1,*

1 Atmospheric Science Department, Colorado State University, Fort Collins, CO 80523 USA

2 NPS/CIRA, Colorado State University, Fort Collins, CO 80523 USA

3 Crocker Nuclear Laboratory, University of California, Davis, CA 95616 USA

4 

* Corresponding author. Tel.: +1970 491 8697; fax: +1970 491 8483.

Environmental Chemistry Department, RTI International, RTP, NC 27709 USA

E-mail address: collett@atmos.colostate.edu (J.L. Collett Jr).

INTRODUCTION

 Among all reactive nitrogen-bearing species, ammonia (NH3 ) is the most abundant basic 

gas in the atmosphere. The primary sources for atmospheric NH3 are agricultural emissions such 

as domestic animals, nitrogen fertilizer application and biomass burning.

1,2 Due to its acid 

neutralizing capacity, NH3 can react with acidic species (e.g., H2SO4, HNO3

 

and HCl) to form 

secondary inorganic particles. 

 Visibility is one of most desirable elements in Class I areas like national parks. The 

Interagency Monitoring of Protected Visual Environments (IMPROVE) program has been 

measuring visibility degrading aerosols since 1988 in federally protected areas.

3,4 Most 

IMPROVE sites have particle measurements of PM2.5 mass, elements by XRF, ne gative ions and 

EC/OC, but no direct cation monitoring.

3 Despite the importance of gaseous ammonia in forming 

atmospheric aerosols and contributing to nitrogen deposition and visibility degradation,

ammonia/ammonium is rarely regularly measured by existing air quality monitoring networks. 

 The objective of this study is in validating whether IMPROVE samplers could be 

modified to incorporate acid-impregnated filters to monitor ammonia/ammonium with 

acceptable accuracy and precision, which is eventually adopted to address ammonia partitioning 

between gas and particle phase, monitor impacts from particle formed from ammonia 

neutralization on visibility in national parks, and determine sources and contributions of 

ammonia to excess nitrogen deposition in Rocky Mountain ecosystems.

EXPERIMENTAL DESCRIPTIONS

 Samplers for NHx (NH3 and NH4

+

) study were adapted from regular IMPROVE sampler

module B which is installed for monitoring anion species by nylon filters. The IMPROVE 

mod ule B sampler was originally coupled with a carbonate-coated denuder to remove nitric acid 

as the nylon filters used for IMPROVE particle collection tend to retain gaseous acidic species,5

but since the purpose of this study is to measure ammonia/ammonium, the nitric acid denuder is 

not included in the modified version. 37mm cellulose fiber filters coated with phosphorous acid 
2

(225-18A, SKC Inc., Eight Four, PA) were used for sampling NHx. The acid-coated cellulose 

filters used are designed to collect both gas phase NH3 and PM2.5 particle phase NH4

+ ; NHx is 

adopted to refer to a collective of collected NH3 and N H4

+

 Acid coated cellulose filters were loaded in or unloaded from sampler cartridges in a 

laminar flow hood or a nitrogen-atmosphere rigid glovebox with ammonia scrubbing to ensure

low background influence to accurately interpret the obtained sampling results. Both sampled 

cellulose and nylon filters were extracted for ammonium concentration measurements. Collected 

filters were extracted in DI water using sonicator for 45 min. Ammonia/ammonium

concentrations were analyzed using ion chromatography with Dionex CS12A separation column.

. 

 Through a pilot study initiated in June 2010, total NHx is being measured on a 1-in-3 day 

sampling basis at 9 IMPROVE sites, mostly in the Rocky Mountain region of the western U.S.

Table 1 lists details of site locations and elevations., IMPROVE NHx cartridges for each week 

were kept in individual zip-seal bags with a pieces of phosphorous acid-soaked paper towel 

included in shipping/storage as NH3 

Table 1. IMPROVE NHx sampling site locations and estimated NH

scrubber. Sample cartridge assembly and sample loadings 

were completed at Colorado State University (CSU) and cartridges were shipped directly to site 

operators; while collected sample filters were returned to Research Triangle Institute (RTI) for 

unloading and subsequent analysis.

3 /NH4

+

Site

dry deposition fluxes.

Site code Latitude Longitude Elevation 

(m)

NH3 Dep

(kgN/ha/yr)

dry NH4

+

Dep

 (kgN/ha/yr)

dry

Bandelier, NM BANDS 35.7797 -106.2664 1988

Bondville, IL BONDS 40.0520 -88.3733 263 1.72±0.75 0.21±0.07

Cedar Bluff, KS CEBLS 38.7701 -99.7634 665

Chiricahua, AZ CHIRS 32.0094 -109.3890 1554 0.42±0.46 0.17±0.10

Glacier, MT GLACS 48.5105 -113.9966 975 0.27±0.21 0.01±0.01

Mesa Verde, CO MEVES 37.1984 -108.4907 2172 0.09±0.09 0.09±0.04

Rocky Mountain, CO ROMOS 40.2783 -105.5457 2760 0.16±0.15 0.08±0.02

Wind Cave, SD WICAS 43.5576 -103.4838 1296 0.23±0.19 0.11±0.02

Yellowstone, WY YELLS 44.5653 -110.4002 2425 0.09±0.03 0.05±0.00

 

RESULTS AND DISCUSSION

 Collocated sampling tests at CSU for two weeks revealed excellent agreement between 

total NHx measurements made using the NHx IMPROVE module and a conventional URG 

annular denuder/filter-pack sample, meaning that the adapted IMPROVE sampling modules 

provide reliable measurements at a much lower cost, which are significant concerns and 

improvements for monitoring of atmospheric pollutants.

 After NHx sampling began in June 2010, an unidentified prominent peak was observed to 

elute after the ammonium peak in IC analysis. The mysterious peak was subsequently 

determined as methylamine after comparing with standards and independent validation by timeof-flight mass spectrometry. The source of the appearance of methylamine was successfully 

assessed and identified, which was discovered to be a chemical artifact associated with reaction 
3

of collected ammonia/ammonium on filters with the sampling holder in use. The material used 

for the filter holder was Polyoxymethylene (POM), which was susceptible to acid degradation. 

These findings led to uncovering an unforeseen incompatibility of the material used for standard 

deployment of IMPROVE network filters with the acid-coated filters for ammonia/ammonium 

collection. Acid etching of IMPROVE POM cartridges was reflected as degradation in releasing 

formaldehyde. Methylamine as a chemical artifact in this case was subsequently formed from 

released formaldehyde and collected ammonia/ammonium on filters. Temperatures, duration of 

filter storage inside cartridges, and degree of acid etching on POM cartridges appeared to 

determine extent of methylamine formation and NH4

+

 NHx field blanks collected from April 2011 to January 2012 did not suggest much 

difference from site to site with average concentration of 0.030±0.007 μg/m

loss. After further compatibility tests, 

Polypropylene was proven to successfully meet all aspects required for NHx sampling; new 

cartridges were manufactured and IMPROVE NHx sampling resumed starting from late April 

2011.

3

 Dry deposition of NH

. Collocated 

samplers were installed at Rocky Mountain National Park (ROMOS) and Bondville, IL 

(BONDS). Precision of collected NHx suggest that collocated samplers yield excellent 

agreement. Monthly average concentrations for IMPROVE NHx sites from April to January 

2012 are shown in Figure 1. Overall NHx observed for the sample period at YELLS was lowest 

in concentration, while CEBLS and BONDS were the highest. NHx measurements showed a 

pattern of increasing NHx concentrations in late spring/early summer (June) and a decrease in 

winter, starting in September for most of the sites. This pattern is consistent with expected 

seasonal patterns in emissions of ammonia. Both BONDS and CEBLS, however, still exhibit 

quite abundant winter NHx, which may reflect continued agricultural emissions trapped within a 

shallower winter boundary layer.

3 /NH4

+ was estimated by adopting SO2 /particulate dry deposition 

velocities calculated by the Multi-Layer Model used by CASTNET, which had sites co-located 

with most of the pilot IMPROVE NHx samplers (except sites BANDS and CEBLS). As 

CASTNET only reports dry deposition velocities for SO2 , HNO3 and fine particles, estimations

of NH3 deposition velocities were estimated based on those of SO2 and a scaling factor. Given 

that solubility is the dominant contributor to determine deposition velocities, SO2 and NH3 , with 

similar solubility as reflected in Henry’s or effective Henry’s law coefficient, are most likely to 

track each other as of depos ition velocities under various conditions such as surface wetness. A 

ratio of deposition velocities of NH3 /SO2 was estimated from modeling b y Zhang et al.6 under 

various land use types and wetness to be 1.15±0.14. Dry deposition fluxes were determined 

from NH3 /NH4

+ concentrations along with dry deposition velocities on a monthly basis. Average 

fluxes determined from 04/2011 to 11/2011 are tabulated in table 1. NH4

+ concentrations derived 

in this study, from cation analysis of the regular IMPROVE ion module filter, represent a lower 

bound on true values due to volatilization loss of NH4

+ from nylon filters. Since NH3 is obtained 

as (NHx – NH4

+ ), estimated NH3 concentrations should be considered upper bounds to the true 

values. Because of its much higher deposition velocity, dry NH3 is observed to be a much larger 

contributor to reactive nitrogen deposition than dry NH4

+ throughout the study region. Dry 

NH3 /NH4

+ deposition flux at BONDS is observed much higher than other sites and possibly 

exceeding critical loads for nitrogen deposition, which was estimated at around 1.5 kgN/ha/yr of 

nitrogen wet deposition.7, 8
4

Figure 1. IMPROVE NHx concentrations from April 2011 to January 2012.

SUMMARY

 IMPROVE samplers were adapted to collect total NHx (gas phase ammonia and particle 

phase ammonium) to monitor particle impacts on visibility, and determine sources and 

contributions of ammonia to excess nitrogen deposition in Rocky Mountain ecosystems. Selected 

sampling site include 9 IMPROVE sites: Rocky Mountain, CO; Mesa Verde, CO; Yellowstone, 

WY; Glacier, MT; Bandelier, NM; Bondville, IL; Cedar Bluff, KS; Wind Cave, S D; Chiricahua, 

AZ. Phosphorous acid coated cellulose filters were evaluated to determine their ability to 

accurately monitor total NHx. Tests in Fort Collins, CO revealed excellent agreement between 

total NHx measurements made using the new IMPROVE module and a conventional URG 

annular denuder/filter-pack sample. The IMPROVE NHx sampling module can provide reliable 

measurements of total NHx at a much lower cost than conventional, multi-component samplers, 

facilitating its widespread deployment. Initial NHx filter cartridges were machined from 

Polyoxymethylene (POM), which proved susceptible to acid degradation and resulted in artifact 

methylamine formation during field deployment in 2010. Methylamine was formed from 

released formaldehyde and collected ammonia/ammonium on filters. After further testing, the 

filter holders were remanufactured from polypropylene. This eliminated the artifact amine 

formation and the pilot network was restarted in spring 2011 with the new design. NHx 

measurements conducted from April to December 2011 showed a pattern of increasing NHx 

concentrations in late spring/early summer (June) and a decrease in winter, starting in September 
5

for most of the sites. This pattern is consistent with expected seasonal patterns in agricultural 

emissions of ammonia. Both Bondville and Cedar Bluff, however, still exhibit quite abundant 

winter NHx, which may reflect continued agricultural emissions trapped within a shallower 

winter boundary layer. Dry deposition of NH3 /NH4

+ was estimated by adopting SO2

ACKNOWLEDGMENTS

/particulate 

dry deposition velocities calculated by the Multi-Layer Mode l used by CASTNET, which had 

sites co-located with most of the pilot IMPROVE NHx samplers.

This work was sponsored by the National Park Service through a cooperative agreement with 

Colorado State University (Task #J2350093004).

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