Accepted Manuscript

Observations of ammonia, nitric acid, and fine particles in a rural gas production

region

Yi Li, Florian M. Schwandner, H. James Sewell, Angela Zivkovich, Mark Tigges,

Suresh Raja, Stephen Holcomb, John V. Molenar, Lincoln Sherman, Cassie

Archuleta, Taehyoung Lee, Jeffrey L. Collett

PII: S1352-2310(13)00760-7

DOI: 10.1016/j.atmosenv.2013.10.007

Reference: AEA 12499

To appear in: Atmospheric Environment

Received Date: 7 April 2013

Revised Date: 26 September 2013

Accepted Date: 2 October 2013

Please cite this article as: Li, Y., Schwandner, F.M., Sewell, H.J., Zivkovich, A., Tigges, M., Raja, S.,

Holcomb, S., Molenar, J.V., Sherman, L., Archuleta, C., Lee, T., Collett Jr., , J.L., Observations of

ammonia, nitric acid, and fine particles in a rural gas production region, Atmospheric Environment

(2013), doi: 10.1016/j.atmosenv.2013.10.007.

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Observations of ammonia, nitric acid, and fine particles in a rural gas production region 

Yi Li1, Florian M. Schwandner1, 4, H. James Sewell2, Angela Zivkovich2, Mark Tigges3, Suresh Raja1, 5, 

Stephen Holcomb3,6, John V. Molenar3

, Lincoln Sherman3, Cassie Archuleta3, Taehyoung Lee1,7, Jeffrey 

L. Collett, Jr.1,* 

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

2 Shell Exploration and Production Co., Denver, CO 80237, USA. 

3 Air Resource Specialists, Inc., Fort Collins, CO, 80525, USA. 

4 Current Address: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, 

Pasadena, California 91109, USA. 

5 Current Address: Providence Engineering and Environmental Group, Irving, TX 75038, USA. 

6 Current Address: NiSource, Merrillville, IN 46410, USA. 

7 Current Address: Department of Environmental Science, Hankuk University of Foreign Studies, Yongin, 

449-791, Republic of Korea 

* Corresponding author, collett@atmos.colostate.edu (J.L. Collett) 

Abstract 

1 Continuous measurements of the atmospheric trace gases ammonia (NH3) and nitric acid (HNO3) and of 

fine particle (PM2.5) ammonium (NH4

+

), nitrate (NO3

-

) and sulfate (SO4

2- 2 ) were conducted using a 

3 denuder/filter system from December 2006 to December 2011 at Boulder, Wyoming, a region of active 

gas production. The average five year concentrations of NH3, HNO3, NH4

+

, NO3

-

 and SO4

2- 4 were 0.17, 

0.19, 0.26, 0.32, and 0.48 μg/m35 , respectively. Significant seasonal patterns were observed. The 

6 concentration of NH3 was higher in the summer than in other seasons, consistent with increased NH3

7 emissions and a shift in the ammonium nitrate (NH4NO3) equilibrium toward the gas phase at higher 

8 temperatures. High HNO3 concentrations were observed both in the summer and the winter. Elevated 

9 wintertime HNO3 production appeared to be due to active local photochemistry in a shallow boundary 

layer over a reflective, snow-covered surface. PM2.5 NH4

+

 and SO4

2- 10 concentrations peaked in summer 

while NO3

-

11 concentrations peaked in winter. Cold winter temperatures drive the NH3- HNO3- NH4NO3

12 equilibrium toward particulate NH4NO3. A lack of NH3, however, frequently results in substantial 

13 residual gas phase HNO3 even under cold winter conditions. 
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14 Keywords: Ammonium; Nitrate; Gas/particle partitioning; Denuder; Wyoming 

15 

16 1. Introduction 

17 Ammonia (NH3) is the most abundant alkaline gas in the troposphere and is important for its ability to 

18 neutralize acidic components such as sulfuric acid (H2SO4) and nitric acid (HNO3) which form, 

19 respectively, by oxidation of emissions of sulfur dioxide (SO2) and nitrogen oxides (NOX). NH3 is also an 

20 important contributor to nitrogen deposition (Beem et al., 2010; Benedict et al., 2013), which can cause 

21 eutrophication, soil acidification and biotic community changes (Cowell and Apsimon, 1998; Groot 

22 Koerkamp et al., 1998). It is widely recognized that important sources of NH3 include agricultural 

23 activities such as fertilization and livestock emissions (Li et al., 2006; Misselbrook et al., 2000; Sutton et 

24 al., 2000). Reactions of HNO3 and H2SO4 with ambient NH3 generally form submicron ammonium nitrate 

25 (NH4NO3) and ammoniated sulfate (NH4HSO4, (NH4)2SO4, or other forms) particles which can adversely 

26 impact human respiratory health and impair atmospheric visibility through visible light scattering. In the 

27 United States (U.S.) a set of Class 1 areas (including National Parks and Wilderness Areas) has been 

28 identified for protection from visibility impairment through the Regional Haze Rule. Because oil and gas 

29 production regions of the western U.S. are often located near these visibility-protected areas, close 

30 attention is paid to their emissions of fine particle precursors. In order to reduce NOx emissions from 

31 natural gas drilling and production activities, for example, selective catalytic reduction (SCR) can be 

32 installed on drill rigs. While SCR can yield large reductions of NOx emissions; however, there is a risk of 

33 increased NH3 emission into the atmosphere. 

34 Western Wyoming is one region of active recent gas development where several air quality concerns have 

35 been raised (McMurray et al., 2013). Emissions of NOx have been of concern both because of possible 

36 impacts on regional haze and, especially, due to documented impacts on severe winter ozone episodes 

37 (Schnell et al., 2009). SCR implementation in the region has been active in recent years as one effort to 

38 limit winter ozone episodes. While these winter ozone episodes are believed to be local in nature, NOx

39 emission impacts on regional haze may be more widespread. Unfortunately, few measurements exist in 

40 the region of NH3 and haze impact assessments are generally forced to rely on an assumed background 

41 NH3 concentration level. While a number of recent studies have considered the role NH3 plays in the 

42 formation of fine particles across the United States in both urban and rural areas (Bari et al., 2003; 

43 Benedict et al., 2013; Edgerton et al., 2007; Gong et al., 2011; Heald et al., 2012; Lee et al., 2008; Nowak 

44 et al., 2010; Sather et al., 2008), knowledge of atmospheric concentrations of NH3, and their seasonal 

45 variability is still rather limited, especially in the interior western United States. Here we present five 

46 years of observations of concentrations of gaseous NH3 and HNO3 and fine particle concentrations of 
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NH4

+

, SO4

2- and NO3- 47 from Boulder, Wyoming, a site in the heart of an active gas production region. 

48 These measurements provide the longest term record of NH3 concentration measurements in this part of 

49 the U.S. and provide new insight into typical NH3 concentrations in the region, their seasonal variability, 

and the gas-particle partitioning of the NH3- NH4

+

- HNO3- NO3

-

- SO4

2- 50 system that is one important 

51 contributor to regional haze. 

52 2. Experimental methods 

53 Measurements were made southwest of Boulder, Wyoming (42o43'7.9"N, 109o45'10.4"W) in the 

54 northwestern part of the Pinedale anticline area. Two visibility-protected areas, Bridger Wilderness Area 

55 and Fitzpatrick Wilderness Area, are located within 100 km. The Boulder area and nearby natural gas 

56 fields are situated on a high plateau between the Wind River Range to the east and the Wyoming Range to 

57 the west. Strong surface-based inversions, with inversion pools intersecting topography levels down to 

58 50m above ground (Schnell et al., 2009), are common in the region, especially during wintertime. The 

59 population density in Boulder, Wyoming is sparse with only 8.9 people per square km. The Jonah Gas 

60 Field and the Pinedale Anticline Gas Field, together representing one of the largest gas production regions 

61 in the U.S., are close to the sampling site with several active gas wells located approximately 3 km west 

62 of the sampling site. In 2008, there were more than 500 permitted wells in the Jonah Gas Field and an 

63 additional 3100 wells are expected to be drilled in this field over the next 75 years. Total production in 

64 this region in 2011 was nearly 171 billion cubic feet of natural gas and 1.5 million barrels of oil 

65 (http://www.encana.com/pdf/communities/usa/JonahField-FactSheet.pdf). NOx from the gas extraction 

66 operations and transportation emissions are the largest contributor to local NOx emissions (Fig. S1a). For 

67 NH3 emissions, there are not many large sources in this immediate area. However, the Snake River Valley 

68 to the west of the measurement site is a large area of intense agricultural activity with elevated NH3

69 emissions and concentrations (Clarisse et al., 2009) (Fig. S1b). Installation of more SCR systems in the 

70 Jonah-Pinedale region could elevate local NH3 concentrations, contributing to more particle formation 

71 and visibility degradation. 

72 Concentrations of gaseous NH3 and HNO3 and PM2.5 (particles with an aerodynamic diameter less than 

2.5 micrometers) inorganic ions (NH4

+

, SO4

2-, NO3-

, K+, Mg2+ and Ca2+ 73 ) were sampled using a URG 

74 denuder/filter system (University Research Glassware, Inc., Model 3000CA), which was installed at 1.5 

75 meter height, followed by laboratory extraction and analysis by ion chromatography. The URG sampling 

76 system has been widely used because of its good performance in sampling acidic gaseous and volatile 

77 aerosols (Bari et al., 2003; Beem et al., 2010; Edgerton et al., 2007; Lee et al., 2004; Lin et al., 2006). Air 

78 was drawn through a Teflon-coated PM2.5 cyclone followed by two 242mm annular denuders connected in 

79 series, a 47-mm filter pack containing a nylon filter (Nylasorb, pore size1μm, Pall Corporation) and 
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another annular denuder (from Dec. 2006 through July 11th 80 2008, samples were collected used a backup 

coated filter rather than a 3rd 81 denuder). Air flow was maintained at a constant mass flow rate by means of 

82 a mass flow controlled pump (URG Inc.). The total flow rate through the system was nominally 10L/min 

83 at ambient conditions. Actual sample volumes were determined using a sample pressure-compensated 

84 dry gas meter. The first denuder was coated with sodium chloride (NaCl) to collect gaseous HNO3 and the 

85 second was coated with phosphorus acid (H3PO4) to collect gaseous NH3. The last denuder (or coated 

filter) was phosphorus acid-coated to collect any NH3 re-volatilized from NH4

+

86 salt particles collected on 

the filter. Nylon filters have been shown to retain volatilized HNO3, but loss of NH4

+

87 can be significant 

88 (Yu et al., 2006). The sample trains were prepared in the lab at Colorado State University (CSU), and then 

89 shipped weekly and installed by a local site operator. Samples were typically collected twice a week (one 

90 4 day sample and one 3 day sample). After sampling, the sample train was shipped back to the lab at CSU. 

91 The denuders were extracted with 10 ml deionized water, and the extracts refrigerated before analysis. 

92 Nylon filters were ultrasonically extracted for 55 minutes in 6ml of high purity deionized water. Ion 

93 chromatography using a Dionex dual channel system was used to analyze the denuder and filter extracts. 

Cations (Na+, NH4

+

, K+, Mg2+ and Ca2+ 94 ) in the samples were separated with a methanesulfonic acid eluent 

95 on a Dionex CG12A guard column and CS12A separation column followed by a CSRS ULTRA II 

suppressor and detected by a Dionex conductivity detector. Anions (Cl-

, NO3

-

, SO4

2- 96 ) in the samples were 

97 separated with a carbonate/bicarbonate eluent on a Dionex AG14A guard column and AS14A separation 

98 column followed by an ASRS ULTRA II suppressor and detected using a Dionex conductivity detector. 

99 

100 Meteorological data, including temperature, relative humidity and wind speed, were obtained from a co101 located weather station (2 meter height) operated by Air Resource Specialists, Inc. 

102 

103 Sample recovery was high, although there were occasional periods where samples could not be collected 

104 on the normal schedule (e.g, from bad weather affecting sampler shipment or operator access). Field and 

105 laboratory blanks were collected throughout the study and used to determine the method detection limit 

(MDL) and to blank-correct results. The MDLs for NH3, HNO3, NH4

+

, SO4

2-, NO3-

, K+, Mg2+ and Ca2+ 106 

were determined as 0.012, 0.012, 0.002, 0.017, 0.001, 0.005, 0.007 and 0.023 μg/m3

107 , respectively. 

108 Replicate extract analyses yielded measurement precisions of 5.4%, 3. 8%, 3.5%, 0.8%, 2.1%, 4.9%, 

7.6%, and 6.2% (relative standard deviation) for NH3, HNO3, NH4

+

, SO4

2-, NO3-

, K+, Mg2+ and Ca2+ 109 , 

110 respectively. 

111 

112 3. Results and discussion 
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113 

114 From December 2006 through December 2011, 505 samples were collected. The summary of the annual 

115 and seasonal mean and standard deviation for all the trace gas concentrations, particulate species 

116 concentrations and meteorological parameter values are summarized in Table S1 in the supplement. 

117 Because of the high latitude of this continental sampling site and the monthly average temperatures, the 

118 following months were defined as representing specific seasons, for the purpose of discussing the 

119 analytical results below: April and May were defined as spring; June, July and August as summer; 

120 September and October as fall; and November through March as winter. 

121 

Fig. 1 shows time series of the concentrations of gaseous NH3 and HNO3 and PM2.5 NH4

+

 and NO3

-

122 across 

the five year measurement period. NH3 concentrations peak in summer while NO3

-

123 concentrations peak in 

124 winter. HNO3 exhibits a distinct bimodal seasonal concentration pattern with summer and winter maxima. 

As shown in Fig. 2, NH4

+

, SO4

2- and NO3-

 were the three most abundant inorganic ions in PM2.5 125 in all 

seasons. The concentration of NH4

+ varied least across seasons. During the warm season SO42- 126 was the 

most abundant inorganic anion in PM2.5, while during winter the concentration of NO3

-

127 was highest. More 

128 details concerning the trends of each of the trace gas and particulate species will be presented below. 

129 

130 3.1 Concentrations of major gas and fine particle species, their seasonal patterns, and gas-particle 

131 partitioning 

132 Gaseous NH3 exhibited a clear seasonal concentration pattern, ranging from an average concentration of 

0.04 μg/m3 in winter to 0.39 μg/m3133 in summer (Fig. 3a). The maximum quarterly NH3 average 

concentration was 0.47 μg/m3134 in summer 2008, 15 times higher than the winter 2008-2009 average of 

0.03 μg/m3135 . The significantly higher summer concentration reflects a strong influence of temperature. 

136 Previous studies have reported similar phenomena (Edgerton et al., 2007; Gupta et al., 2003; Meng et al., 

137 2011; Plessow et al., 2005; Walker et al., 2004). Higher levels of NH3 in the summer are consistent with 

138 the positive influence of higher temperatures on NH3 emissions (e.g., from natural soils, agricultural 

139 operations, and fires) and the decomposition of particulate NH4NO3 into gaseous NH3 and HNO3. 

140 

141 One potential local source of NH3 is increased use of SCR for high-efficiency NOx control on drill rigs in 

142 the region. While use of SCR increased during the period of observation at Boulder, however, there is no 

143 clear increase in local NH3 concentrations over the study. The annual mean concentration of NH3 did not 

144 significantly increase during the study period. From 2007 to 2011, the annual NH3 average concentrations 

in each year were 0.14, 0.20, 0.23, 0.15 and 0.13 μg/m3

145 , respectively, suggesting that SCR emissions did 

146 not noticeably influence local concentrations of ambient NH3. 
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147 

148 Table S2 shows cross-correlation coefficients for measured trace gases, particle ions and meteorological 

parameters. Some correlation was found between NH3 and K+ (r2149 =0.16). As illustrated by satellite fire150 detect images (Fig. S2), there were more wild fires present around Boulder, Wyoming in 2007 and 2008; 

the correlation coefficients (r2) between NH3 and K+151 in those two years were 0.33 and 0.40. As a marker 

of biomass burning, the correlation between NH3 and K+

152 may suggest a positive influence of fire 

153 emissions on NH3 concentrations (Anderson et al., 2003; Hegg et al., 1988; McMeeking et al., 2009; 

154 Sutton et al., 1995). 

155 

156 A background NH3 mixing ratio of 1 ppbv is often assumed when estimating impacts of NOx emissions 

157 on visibility and regional haze in western regions of the U.S. where ambient NH3 concentration data are 

158 sparse or unavailable. Such estimates might be made, for example, through plume dispersion simulations 

159 using CALPUFF or other EPA-preferred models. The 5-year Boulder data records provide a better basis 

160 for choosing a representative background NH3 concentration for the Pinedale region. Figure 4 reveals 

161 that seasonal mean NH3 mixing ratios ranged between a maximum of 0.85 ppbv (in summer 2008) and 

162 0.03 ppbv (in winter 2010). The average for the full 5-year study period was 0.30 ppbv, less than onethird of the typically assumed background level. Even if PM2.5 NH4

+

163 (much of which certainly reacted 

164 with sulfuric and nitric acids upwind of the measurement region) and gaseous NH3 are combined, the 

165 average mixing ratio (0.63 ppbv) remains well below 1 ppbv. Assumption of a 1 ppbv NH3 background 

166 concentration in model simulations, therefore, will lead to an overprediction of visibility impacts 

167 associated with local NOx emissions. 

168 

169 The five year average HNO3 mixing ratio was observed to be 0.03 ppbv, indicating typically low 

170 concentrations of HNO3 occur in this area. Seasonal mean HNO3 mixing ratios (Fig. 4) ranged between 

171 0.13 ppbv (in summer 2007) and 0.01 ppbv (in spring 2008). As illustrated in Fig. 3b, HNO3

172 concentrations display a distinct bimodal seasonal pattern, with higher average concentrations in the 

summer (0.23 μg/m3

) and in mid-winter (January/February average = 0.26 μg/m3173 ) than in other seasons. 

174 One also can see in Fig. 4 that variability in observed HNO3 concentrations is quite high in January and 

175 February. Previous studies have generally shown that HNO3 peaks in summer with lower concentrations 

176 during winter (Adon et al., 2010; Gupta et al., 2003; Lee et al., 1999; Plessow et al., 2005). Increased 

177 concentrations of HNO3 are expected in the summer because of more intense and longer lasting 

178 photochemical activity associated with higher sun angles and longer days. Higher summer temperatures 

179 also promote dissociation of NH4NO3 back to gaseous NH3 and HNO3, as discussed above (Seinfeld and 

180 Pandis, 2006). The high winter concentrations observed at Boulder, by contrast, are quite unusual. The 
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peak wintertime HNO3 concentration climbed as high as 1.40 μg/m3

181 for a single sample collected from 

February 22th to 25th 182 in 2008. A closer look at the HNO3 timeline in Figure 1 reveals frequent winter 

183 periods of elevated HNO3 concentrations. Other measurements at Boulder reveal that this area is 

184 frequently subject to periods of elevated winter ozone (Schnell et al., 2009) that occur during sunny 

185 winter periods when snow covers the ground. Strong nocturnal and morning temperature inversions that 

186 set up under these conditions trap local emissions of NOx and volatile organic compounds, associated 

187 largely with local energy production activities, in a shallow mixing layer while daytime photochemical 

188 activity is enhanced by strong reflectance from the bright snow surface. The photochemical reactions that 

189 generate ozone concentrations well in excess of the U.S. National Ambient Air Quality Standard can also 

190 lead to substantial oxidation of the locally emitted NOX to HNO3. While cold winter conditions favor 

191 reaction of HNO3 with NH3 to form fine particle NH4NO3 (as evidenced by the winter NH4NO3 spikes in 

192 Figure 1), the Boulder observations reveal that all gaseous NH3 has often been consumed during these 

193 episodes leaving a substantial fraction of the HNO3 “trapped” in the gas phase. 

194 

Ambient NH4

+

195 concentrations at Boulder exhibited little seasonal pattern (Fig. 3c). The annual mean 

concentrations for 2007 to 2011 were also similar to each other at 0.29, 0.28, 0.23, 0.22 and 0.27 μg/m3196 , 

respectively. Formation of fine particle NH4

+

197 is influenced by a variety of factors, including the 

198 availability of gaseous NH3 and the availability of acidic sulfate aerosol and gaseous HNO3. Increases in 

NH3 and SO4

2- at Boulder during warmer months of the year will tend to increase NH4+

199 concentrations as 

200 well. Formation of fine particle NH4NO3, however, is favored in winter. As previously discussed, the 

201 formation of NH4NO3 is thermodynamically favored by high relative humidity and low temperatures. 

202 During the winter in Boulder, the average temperature was -7.8 oC and average relative humidity was 

72.5%. These offsetting seasonal patterns appear to result in an overall NH4

+

203 concentration pattern that 

204 shows little seasonality at Boulder. 

205 

The annual average concentrations of PM2.5 NO3

-

206 measured at Boulder were 0.30, 0.36, 0.29, 0.27 and 

0.37 μg/m3

in 2007 through 2011, respectively. The NO3

-

207 concentrations exhibited a strong seasonality, 

208 with maximum values in winter and minimum values in summer (Fig. 3d). Because NH4NO3 formation is 

not favored under the warm, dry conditions of summer, the mean summer NO3

- 209 concentration was only 

0.11 μg/m3. In winter, it increased to 0.60 μg/m3210 , as NH4NO3 formation was more strongly favored. As 

indicated in Fig. 3d, considerable variability was also observed in winter NO3

-

211 concentrations, similar to 

the pattern discussed above for HNO3, with maximum observed concentrations exceeding 2.0 μg/m3

212 in 

213 December, January, February, and March. 
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214 

SO4

2- 215 shows a seasonal cycle with maximum values in the warm season (Fig. 3e). This seasonal pattern is 

typical of SO4

2- 216 , due to enhanced photochemical activity and higher concentrations of hydroxyl radical, 

which can oxidize SO2 to SO4

2- (Behera and Sharma, 2010). In-cloud oxidation of SO2 to SO42- 217 can also 

218 be enhanced in summer when hydrogen peroxide concentrations are typically higher. Annual average 

concentrations of SO4

2- at Boulder in 2007 through 2011 were 0.54, 0.53, 0.47, 0.38, and 0.47 μg/m3

219 . 

220 

221 In addition to anthropogenic emissions of nitrogen and sulfur species, wild and prescribed fires also 

222 contribute significantly to fine particle concentrations in the western U.S. (Jaffe et al., 2008; Malm et al., 

223 2004). Water soluble potassium ion concentrations, one marker of biomass burning (Andreae, 1983; Duan 

et al., 2004), were elevated in summer (Fig. 3f). The average concentration of K+224 in the summer was 0.03 

μg/m3225 , which was nearly three times higher than the value in the winter (See Table S1). Not surprisingly, 

summer K+226 concentrations varied substantially; interannual variability in fire occurrence and the influence 

227 of emissions from fires that do occur on air quality at Boulder are expected. Across the sampling period, 

228 a number of wild fires occurred upwind of the site in summer. 

229 

Given the seasonally varying partitioning of NH3/ NH4

+

 and HNO3/ NO3

-

230 between the gas and particle 

231 phases, it is useful to also examine seasonal changes in total reduced inorganic nitrogen in the -3 

oxidation state [N(-III)=NH3+NH4

+

232 ] and in total oxidized inorganic nitrogen in the +5 oxidation state 

[N(+V)=HNO3+NO3

-

233 ]. As illustrated in Fig. 3g, N(-III) concentrations show a peak in the summer 

months, consistent with higher NH3 emissions and greater SO4

2- 234 concentrations at this time of year. N(+V) 

235 concentrations, by contrast, peak in the winter months. This winter peak reflects the formation of HNO3

236 in a shallow boundary layer during winter ozone episodes as well as winter formation of fine particle 

237 NH4NO3, with its longer atmospheric residence time than HNO3. 

238 

To investigate the seasonal phase changes of NH3/NH4

+

 and HNO3/NO3

-

239 , we define the ammonia gas 

fraction (FNH3 = the NH3 gas concentration divided by the sum of the NH3 gas and fine particle NH4

+

240 

241 concentrations) and the nitric acid gas fraction (FHNO3 = the HNO3 gas concentration divided by the sum 

of the HNO3 gas and fine particle NO3

-

242 concentrations), where all concentrations are expressed in molar 

243 units. The monthly average partitioning for the reduced and oxidized inorganic nitrogen forms is plotted 

244 in Fig. 5. There was a gradual transition from the cooler months, when the particle phase was favored, to 

245 the warmer months, when the gas phase was favored, for both species. A maximum monthly average in 

246 the gas phase fraction of NH3 occurred in July (0.64). This was more than 10 times higher than the 

247 minimum monthly average of 0.06 which occurred in January. Similarly, the HNO3 gas fraction (FHNO3)
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248 was found to be highest in summer (0.73 in July) and lowest in winter (0.24 in January). The high 

249 summer level of FNH3 reflects greater NH3 emissions and the thermodynamic tendency for NH4NO3 to 

250 dissociate to NH3 and HNO3 at high temperature. The higher summer value of FHNO3 also reflects the 

251 tendency for NH4NO3 to dissociate at higher temperatures. The still appreciable winter FHNO3 level, 

252 which is not typical of previous results (Bari et al., 2003; Gupta et al., 2003; Sharma et al., 2007), reflects 

253 the continued photochemical production of HNO3 at levels which exceed the amount of NH3 available to 

254 participate in NH4NO3 formation. 

255 

256 Shifts in the equilibrium partitioning among gaseous NH3 and HNO3 and particulate NH4NO3 depend on 

257 relative humidity (RH), temperature (T) and the concentrations of NH3 and HNO3. Ambient relative 

258 humidity at the Boulder measurement site was usually less than the deliquescence relative humidity (DRH) 

259 of NH4NO3 so that we can simplify matters and consider here formation of solid NH4NO3. Under this 

260 condition, this reaction’s equilibrium constant (Kp) is the expected product of the NH3 and HNO3

261 concentrations and is given by the empirical formula (R1): 

262 

263 (R1) 

264 

where Kp is in units of ppbv2265 and T is measured ambient temperature in Kelvin (Stelson and Seinfeld, 

266 2007). The measured, apparent reaction constant (Km) can be described as follows (R2): 

267 

268 (R2) 

269 

270 where [NH3] is the gaseous NH3 mixing ratio (ppbv) and [HNO3] is the gaseous HNO3 mixing ratio 

271 (ppbv). NH4NO3 formation is favored when Km exceeds Kp. Fig. 6 shows the variation of both the 

272 theoretical equilibrium constant (shown as a solid line) and measured constant values (for each sample) 

273 with temperature (1000/T) across all seasons. This presentation of the data clearly illustrates that NH4NO3

274 formation is only favored during wintertime; even then, it is not favored during all sample periods. At 

275 warmer times of the year, the product of NH3 and HNO3 concentrations is insufficient to yield NH4NO3

276 formation at seasonal temperatures. 

277 

278 3.2 Comparison with other measurements 

279 
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280 Fig. 7 presents a comparison of observations from this study with other observations from the Clean Air 

281 Status and Trends Network (CASTNET; http://www.epa.gov/castnet/), the Interagency Monitoring of 

282 Protected Visual Environments (IMPROVE; http://vista.cira.colostate.edu/improve), the National 

283 Atmospheric Deposition Program Ammonia Monitoring Network (AMoN; 

284 http://nadp.sws.uiuc.edu/amon/) and seven sets of ambient composition measurements completed by our 

285 lab at CSU in the western U.S. (Table S3). Concentrations of particle and gas phase species observed at 

286 Boulder were, overall, among the lower average concentrations measured across this set of western sites. 

Comparing mean values of NH3 and HNO3 and fine particle NH4

+

, NO3

-

, and SO4

2- 287 at Boulder with the 

288 other rural locations, the concentrations measured at Boulder were generally lower than those observed at 

289 sites further east such as Santee Sioux, South Dakota, Konza Prairie, Kansas, Cherokee Nation, 

290 Oklahoma, and Palo Duro, Texas. They were also significantly lower than concentrations measured 

291 closer to more populated areas, such as those at Sequoia National Park, California, Joshua Tree National 

292 Park, California, and Loveland, Colorado. Boulder, Wyoming NH3 concentrations were substantially 

293 lower than NH3 concentrations measured at sites more strongly impacted by regional agriculture/animal 

294 feeding operations, such as Brush, Colorado. Overall concentrations were fairly similar, however, 

295 between Boulder and other remote sites in central and western Colorado and in western Wyoming, 

296 suggesting some regional representativeness of the concentrations measured in Boulder (aside from the 

297 winter ozone episodes). Although the Boulder measurement site is only approximately 65 km from the 

298 Snake River Plain Valley, an area of intense agricultural activity with high NH3 emissions (Clarisse et al., 

299 2009), the low NH3 concentrations observed at Boulder suggest that the Wyoming (Palisades) Mountain 

300 Range blocks at least the most direct transport of these emissions while other local NH3 emissions are 

301 limited in their contributions to ambient NH3 concentrations. 

302 

303 3.3 Interspecies correlations, the measured ion charge balance, and the importance of organic acids 

304 

Fig. 8 illustrates the correlation between fine particle NH4

+ and SO42- 305 in different seasons. Significant 

correlations were found in all seasons except in winter. The highest correlation (r2

306 = 0.84) was in the fall 

and the lowest was in the winter (r2

307 =0.15). The weak correlation in winter results from substantial 

NH4NO3 formation during this time period. If one plots the excess NH4

+

308 (the amount beyond that needed 

to fully neutralize fine particle SO4

2-), one finds it to be strongly correlated with fine particle NO3-

309 during 

winter (r2=0.76; slope of 0.81), modestly correlated in fall (r2310 =0.31; slope of 1.01), and showing almost no 

311 correlation in spring and summer (see Fig. S3). 

312 
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Overall, on the basis of the seasonal variation of comparisons between NH4

+

 and SO4

2- and excess NH4+

313 

and NO3

-

, one can conclude that most fine particle NH4

+

314 in summer exists as (NH4)2SO4 while both 

(NH4)2SO4 and NH4NO3 are found in fine particles in winter. An excess of NH4

+

 in summer when NO3

-

315 

316 concentrations are low, however, suggests that other unmeasured anionic species might also be important 

317 components of the fine particles. This pattern also appears in some fall and spring samples. This issue 

318 can be further evaluated by considering the overall ionic charge balance of measured fine particle anion 

(NO2

-

, NO3

-

, SO4

2-) and cation (NH4+

, Na+, K+, Mg2+, Ca2+ 319 ) concentrations. Fig. 9 presents the seasonal 

320 variation of the ionic charge balance. During spring and winter, the charge balance is very close to 1:1. 

321 During fall and, especially, summer, however, the charge balance generally indicates a deficiency of 

322 anions. Previous studies (Barsanti et al., 2009; Trebs et al., 2005) have reported that organic acids such as 

323 oxalic acid can be important contributors to the charge balance of fine mode aerosols. The warm season 

324 anion deficit observed here is consistent with higher organic acid concentrations during summer, 

325 coinciding with periods of enhanced photochemical production of secondary organic aerosols and 

326 increased biomass burning. Future measurements of summertime Boulder fine particle concentrations 

327 will include analysis of oxalate. 

328 

329 4 Conclusion 

330 

331 A five-year study of concentrations of gaseous NH3 and HNO3 and of fine particle inorganic ions was 

332 conducted in an active gas production region in Boulder, Wyoming. The five-year annual mean 

concentrations of NH3, HNO3, NH4

+

, NO3

-

 and SO4

2- were 0.17, 0.19, 0.26, 0.32, and 0.48 μg/m3

333 , 

334 respectively. NH3 exhibited a strong seasonal variation, with higher concentrations during the summer 

335 and lower concentrations during the winter. The low annual average NH3 mixing ratio of 0.30 ppb 

336 suggests that the default value of 1 ppb often used in regional assessments of visibility impacts from NOx

337 source emissions is higher than necessary. 

338 

339 Observed NH3 concentrations correlated well with ambient temperature indicating the important influence 

340 of temperature on emissions and, likely, the greater long distance transport of those emissions during 

warmer times of year when mixing layers deepen. By contrast, higher concentrations of particulate NO3

- 341 

342 were observed in the winter when lower temperatures favor formation of NH4NO3. HNO3 concentrations 

343 showed an unusual bimodal seasonal variation with higher levels both in summer (an expected result of 

344 active photochemical oxidation and a tendency for NH4NO3 to decompose at higher temperatures) and in 

345 winter. The unusual winter HNO3 peak appears to be the result of active photochemical processing of 

346 local NOx emissions in a shallow boundary layer during periods of snow cover and a lack of NH3 to fully 
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tie up HNO3 347 through fine particle NH4NO3 formation. Examination of the equilibrium thermodynamics of 

348 NH4NO3 formation, seasonal local temperatures, and available concentrations of gaseous NH3 and HNO3, 

349 indicates that NH4NO3 should be expected primarily in winter, as observed. 

350 

351 Acknowledgements 

352 The authors thank Amy Sullivan, Xi (Doris) Chen, Katie (Beem) Benedict, Arsineh Hecobian, Yele Sun 

353 and Ali (Bote) Willey at Colorado State University for their helpful suggestions and support of the 

354 measurement program. The authors also thank Jennifer Frazier of the Wyoming Department of 

355 Environmental Quality/Air Quality Division and Ted J. Porwoll of the U.S. Forest Service for carrying 

356 out on-site operations. 

357 

358 References 

359 Adon, M., Galy-Lacaux, C., Yoboué, V., Delon, C., Lacaux, J.P., Castera, P., Gardrat, E., Pienaar, J., Al 

360 Ourabi, H., Laouali, D., Diop, B., Sigha-Nkamdjou, L., Akpo, A., Tathy, J.P., Lavenu, F., Mougin, E., 

361 2010. Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in 

362 Africa using passive samplers. Atmospheric Chemistry and Physics 10, 7467-7487. 

363 Anderson, N., Strader, R., Davidson, C., 2003. Airborne reduced nitrogen: ammonia emissions from 

364 agriculture and other sources. Environment International 29, 277-286. 

365 Andreae, M.O., 1983. Soot Carbon and Excess Fine Potassium: Long-Range Transport of Combustion366 Derived Aerosols. Science 220, 1148-1151. 

367 Bari, A., Ferraro, V., Wilson, L.R., Luttinger, D., Husain, L., 2003. Measurements of gaseous HONO, 

368 HNO3, SO2, HCl, NH3, particulate sulfate and PM2.5 in New York, NY. Atmospheric Environment 37, 

369 2825-2835. 

370 Barsanti, K.C., McMurry, P.H., Smith, J.N., 2009. The potential contribution of organic salts to new 

371 particle growth. Atmospheric Chemistry and Physics 9, 2949-2957. 

372 Beem, K.B., Raja, S., Schwandner, F.M., Taylor, C., Lee, T., Sullivan, A.P., Carrico, C.M., McMeeking, 

373 G.R., Day, D., Levin, E., Hand, J., Kreidenweis, S.M., Schichtel, B., Malm, W.C., Collett Jr, J.L., 2010. 

374 Deposition of reactive nitrogen during the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) 

375 study. Environmental Pollution 158, 862-872. 

376 Behera, S.N., Sharma, M., 2010. Investigating the potential role of ammonia in ion chemistry of fine 

377 particulate matter formation for an urban environment. The Science of the total environment 408, 3569-

378 3575. 

379 Benedict, K.B., Day, D., Schwandner, F.M., Kreidenweis, S.M., Schichtel, B., Malm, W.C., Collett Jr, 

380 J.L., 2013. Observations of atmospheric reactive nitrogen species in Rocky Mountain National Park and 

381 across northern Colorado. Atmospheric Environment 64, 66-76. 
MANUSCRIPT ACCEPTED

ACCEPTED MANUSCRIPT

382 Clarisse, L., Clerbaux, C., Dentener, F., Hurtmans, D., Coheur, P.-F., 2009. Global ammonia distribution 

383 derived from infrared satellite observations. Nature Geoscience 2, 479-483. 

384 Cowell, D.A., Apsimon, H.M., 1998. Cost-effective strategies for the abatement of ammonia emissions 

385 from European agriculture. Atmospheric Environment 32, 573-580. 

386 Duan, F., Liu, X., Yu, T., Cachier, H., 2004. Identification and estimate of biomass burning contribution 

387 to the urban aerosol organic carbon concentrations in Beijing. Atmospheric Environment 38, 1275-1282. 

388 Edgerton, E.S., Saylor, R.D., Hartsell, B.E., Jansen, J.J., Alan Hansen, D., 2007. Ammonia and 

389 ammonium measurements from the southeastern United States. Atmospheric Environment 41, 3339-3351. 

390 Gong, L., Lewicki, R., Griffin, R.J., Flynn, J.H., Lefer, B.L., Tittel, F.K., 2011. Atmospheric ammonia 

391 measurements in Houston, TX using an external-cavity quantum cascade laser-based sensor. Atmospheric 

392 Chemistry and Physics Discuss. 11, 16335-16368. 

393 Groot Koerkamp, P.W.G., Metz, J.H.M., Uenk, G.H., Phillips, V.R., Holden, M.R., Sneath, R.W., Short, 

394 J.L., White, R.P.P., Hartung, J., Seedorf, J., Schröder, M., Linkert, K.H., Pedersen, S., Takai, H., Johnsen, 

395 J.O., Wathes, C.M., 1998. Concentrations and Emissions of Ammonia in Livestock Buildings in Northern 

396 Europe. Journal of Agricultural Engineering Research 70, 79-95. 

397 Gupta, A., Kumar, R., Kumari, K.M., Srivastava, S.S., 2003. Measurement of NO2, HNO3, NH3 and SO2

398 and related particulate matter at a rural site in Rampur, India. Atmospheric Environment 37, 4837-4846. 

399 Heald, C.L., J. L. Collett, J., Lee, T., Benedict, K.B., Schwandner, F.M., Li, Y., Clarisse, L., Hurtmans, 

400 D.R., Van Damme, M., Clerbaux, C., Coheur, P.F., Philip, S., Martin, R.V., Pye, H.O.T., 2012. 

401 Atmospheric ammonia and particulate inorganic nitrogen over the United States. Atmospheric Chemistry 

402 and Physics 12, 10295-10312. 

403 Hegg, D.A., Radke, L.F., Hobbs, P.V., Riggan, P.J., 1988. Ammonia emissions from biomass burning. 

404 Geophysical Research Letters 15, 335-337. 

405 Jaffe, D., Hafner, W., Chand, D., Westerling, A., Spracklen, D., 2008. Interannual Variations in PM2.5 

406 due to Wildfires in the Western United States. Environmental Science & Technology 42, 2812-2818. 

407 Lee, H.S., Kang, C.-M., Kang, B.-W., Kim, H.-K., 1999. Seasonal variations of acidic air pollutants in 

408 Seoul, South Korea. Atmospheric Environment 33, 3143-3152. 

409 Lee, T., Kreidenweis, S.M., Collett, J.L., 2004. Aerosol Ion Characteristics During the Big Bend Regional 

410 Aerosol and Visibility Observational Study. Journal of the Air & Waste Management Association 54, 

411 585-592. 

412 Lee, T., Yu, X., Ayres, B., Kreidenweis, S., Malm, W., Collett, J.L., 2008. Observations of fine and 

413 coarse particle nitrate at several rural locations in the United States. Atmospheric Environment 42, 2720-

414 2732. 

415 Li, Y., Schwab, J.J., Demerjian, K.L., 2006. Measurements of ambient ammonia using a tunable diode 

416 laser absorption spectrometer: Characteristics of ambient ammonia emissions in an urban area of New 

417 York City. Journal of Geophysical Research: Atmospheres 111, D10S02. 
MANUSCRIPT ACCEPTED

ACCEPTED MANUSCRIPT

418 Lin, Y., Cheng, M., Ting, W., Yeh, C., 2006. Characteristics of gaseous HNO2, HNO3, NH3 and 

419 particulate ammonium nitrate in an urban city of Central Taiwan. Atmospheric Environment 40, 4725-

420 4733. 

421 Malm, W.C., Schichtel, B.A., Pitchford, M.L., Ashbaugh, L.L., Eldred, R.A., 2004. Spatial and monthly 

422 trends in speciated fine particle concentration in the United States. Journal of Geophysical Research: 

423 Atmospheres 109, D03306. 

424 McMeeking, G.R., Kreidenweis, S.M., Baker, S., Carrico, C.M., Chow, J.C., Collett, J.L., Hao, W.M., 

425 Holden, A.S., Kirchstetter, T.W., Malm, W.C., Moosmüller, H., Sullivan, A.P., Wold, C.E., 2009. 

426 Emissions of trace gases and aerosols during the open combustion of biomass in the laboratory. Journal of 

427 Geophysical Research: Atmospheres 114, D19210.McMurray, J., Roberts, D., Fenn, M., Geiser, L., Jovan, 

428 S., 2013. Using Epiphytic Lichens to Monitor Nitrogen Deposition Near Natural Gas Drilling Operations 

429 in the Wind River Range, WY, USA. Water, Air, & Soil Pollution 224, 1-14. 

430 Meng, Z.Y., Lin, W.L., Jiang, X.M., Yan, P., Wang, Y., Zhang, Y.M., Jia, X.F., Yu, X.L., 2011. 

431 Characteristics of atmospheric ammonia over Beijing, China. Atmospheric Chemistry and Physics 11, 

432 6139-6151. 

433 Misselbrook, T.H., Van Der Weerden, T.J., Pain, B.F., Jarvis, S.C., Chambers, B.J., Smith, K.A., Phillips, 

434 V.R., Demmers, T.G.M., 2000. Ammonia emission factors for UK agriculture. Atmospheric Environment 

435 34, 871-880. 

436 Nowak, J.B., Neuman, J.A., Bahreini, R., Brock, C.A., Middlebrook, A.M., Wollny, A.G., Holloway, J.S., 

437 Peischl, J., Ryerson, T.B., Fehsenfeld, F.C., 2010. Airborne observations of ammonia and ammonium 

438 nitrate formation over Houston, Texas. Journal of Geophysical Research: Atmospheres 115, D22304. 

439 Plessow, K., Spindler, G., Zimmermann, F., Matschullat, J., 2005. Seasonal variations and interactions of 

440 N-containing gases and particles over a coniferous forest, Saxony, Germany. Atmospheric Environment 

441 39, 6995-7007. 

442 Sather, M.E., Mathew, J., Nguyen, N., Lay, J., Golod, G., Vet, R., Cotie, J., Hertel, T., Aaboe, E., 

443 Callison, R., Adam, J., Keese, D., Freise, J., Hathcoat, A., Sakizzie, B., King, M., Lee, C., Oliva, S., San 

444 Miguel, G., Crow, L., Geasland, F., 2008. Baseline ambient gaseous ammonia concentrations in the Four 

445 Corners area and eastern Oklahoma, USA. Journal of Environmental Monitoring 10, 1319-1325. 

446 Schnell, R.C., Oltmans, S.J., Neely, R.R., Endres, M.S., Molenar, J.V., White, A.B., 2009. Rapid 

447 photochemical production of ozone at high concentrations in a rural site during winter. Nature Geoscience 

448 2, 120-122. 

449 Seinfeld, J.H., Pandis, S.N., 2006. Atmospheric chemistry and physics: from air pollution to climate 

450 change. Wiley. 

451 Sharma, M., Kishore, S., Tripathi, S.N., Behera, S.N., 2007. Role of atmospheric ammonia in the 

452 formation of inorganic secondary particulate matter: A study at Kanpur, India. Journal of Atmospheric 

453 Chemistry 58, 1-17. 

454 Stelson, A.W., Seinfeld, J.H., 2007. Relative humidity and temperature dependence of the ammonium 

455 nitrate dissociation constant. Atmospheric Environment 41, Supplement, 126-135. 
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456 Sutton, M.A., Dragosits, U., Tang, Y.S., Fowler, D., 2000. Ammonia emissions from non-agricultural 

457 sources in the UK. Atmospheric Environment 34, 855-869. 

458 Sutton, M.A., Place, C.J., Eager, M., Fowler, D., Smith, R.I., 1995. Assessment of the magnitude of 

459 ammonia emissions in the United Kingdom. Atmospheric Environment 29, 1393-1411. 

460 Trebs, I., Metzger, S., Meixner, F.X., Helas, G., Hoffer, A., Rudich, Y., Falkovich, A.H., Moura, M.A.L., 

da Silva, R.S., Jr., Artaxo, P., Slanina, J., Andreae, M.O., 2005. The The NH4

+

-NO3

−

-Cl−-SO4

2− 461 -H2O 

462 aerosol system and its gas phase precursors at a pasture site in the Amazon Basin: How relevant are 

463 mineral cations and soluble organic acids? Journal of Geophysical Research: Atmospheres 110, 

464 D07303.Walker, J.T., Whitall, D.R., Robarge, W., Paerl, H.W., 2004. Ambient ammonia and ammonium 

465 aerosol across a region of variable ammonia emission density. Atmospheric Environment 38, 1235-1246. 

466 Yu, X., Lee, T., Ayres, B., Kreidenweis, S., Malm, W., Collett, J.L., 2006. Loss of fine particle 

467 ammonium from denuded nylon filters. Atmospheric Environment 40, 4797-4807. 
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3.02.01.00.01.61.20.80.40.0

1/1/07 7/1/07 1/1/08 7/1/08 1/1/09 7/1/09 1/1/10 7/1/10 1/1/11 7/1/11 

Date

2.01.51.00.50.0

HNO3 (g) NO3- (p)

 NH3 (g) NH4+ (p)

Concentration (μg/m

3

)

2007 2008 2009 2010

(a)

(b)

2011

(c) SO42- K+

Fig. 1. Temporal variations of concentrations of (a) HNO3 and NO3-, (b) NH3 and NH4+ and (c) SO42- and K+ from 2007 thro

Wyoming. 
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1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Concentration (μg/m

3

)

Spring Summer Fall Winter

 Ca2+ Mg2+ K+ NH4

+

 SO4

2-

 NO3

-

Fig. 2. Average mass concentrations of the chemical species in PM2.5 by season across the 5 year 

sampling period. 
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1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

HNO3

1.6 (b)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Concentration ( μg/m

3 )

NH3

(a)

2.0

1.5

1.0

0.5

0.0

Concentration ( μg/m

3 )

NH4

(c) +

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

NO3

(d) -

2.5

2.0

1.5

1.0

0.5

0.0

Concentration ( μg/m

3 )

SO4

2- (e) 0.25

0.20

0.15

0.10

0.05

0.00

K

(f) +

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Jan

FebMar

Apr

May

Jun

Jul

AugSep

Oct

NovDec

N(+V) 2.0 (h)

1.5

1.0

0.5

0.0

Concentration ( μgN/m

3 )

Jan

FebMar

Apr

May

Jun

Jul

AugSep

Oct

NovDec

N(-III) (g)

Fig. 3. The monthly variation of (a) NH3, (b) HNO3, (c) NH4

+

, (d) NO3

-

, (e) SO4

2-, (f) K+

, (g) N(-

III) and (h) N(+V) concentrations from 2007 to 2011. The grey shading represents minimum and 

maximum concentrations and the y-error bars represent standard deviations of average 

concentrations. For panels (a) to (f) the concentrations are expressed in μg/m3

; in panels (g) and 

(h) the concentrations are expressed as μgN/m3

.
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1.5

1.2

0.9

0.6

0.3

0.0

NH3 and HNO3 Concentration ( ppbv )

WinterSpring

Summer

Fall 

WinterSpring

Summer

Fall 

WinterSpring

Summer

Fall 

WinterSpring

Summer

Fall 

WinterSpring

Summer

Fall 

Winter

 NH3

 HNO3

2006 2007 2008 2009 2010 2011

Fig. 4. The seasonal variations of NH3 and HNO3 mixing ratios from 2007 to 2011. The plotted 

points are the seasonal mean values and the Y-error bars represent standard deviations. 

1.0

0.8

0.6

0.4

0.2

0.0

mol/mol

Jan 

Feb

Mar

Apr

May

Jun

Jul

AugSep

Oct

NovDec

FHNOx

FNHx

Fig. 5. Monthly variation of the ammonia conversion ratio (FNH3) and nitric acid conversion ratio 

(FHNO3). See text for details. 
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10-5

10-4

10-3

10-2

10-1

100

101

[NH3][HNO3] [ppbv

2

]

3.4 3.5 3.6 3.7 3.8 3.9 4.0

1000/T(K-1)

 Theoretical equilibrium constant

 Winter

 Summer

 Fall

 Spring

Fig. 6. Comparison of the measured [NH3 (gas)][HNO3(gas)] product with the theoretical 

equilibrium constant for NH4NO3 as a function of temperature across the different seasons. 

 2) Craters of the Moon 

 National Monument,ID 6

4

2

0

 1) Mount Rainier 

 National Park,WA

CA

OR

WA

NV

ID

UT

AZ

TX

NM

WY

ND

SD

CO

NE

KS

MT

OK

 8) Konza Prairie, KS

NO3SO4NH4HNO3NH3 NO3

- SO42- NH4+ HNO3 NH3

 10) Palo Duro,TX

NO3SO4NH4HNO3NH3 NO3

- SO42- NH4+ HNO3 NH3

 11) Brush, CO

9) Cherokee Nation, OK

NO3SO4NH4HNO3NH3

 7) Santee, SD

 

6

4

2

0

 18)Yosemite

 National Park,CA

6

4

2

0

 17) Sequoia 

 National Park,CA

6

4

2

0

NO3SO4NH4HNO3NH3

 16) Joshua Tree 

 National Park,CA

6

4

2

0

NO3SO4NH4HNO3NH3 NO3

- SO42- NH4+ HNO3 NH3

 15) Gore Pass,CO

 

NO3SO4NH4HNO3NH3 NO3

-

SO4

2-NH4+

HNO3 NH3

 14) Chiricahua 

National Monument,AZ

NO3SO4NH4HNO3NH3 NO3

- SO42- NH4+ HNO3 NH3

 12) Loveland, CO

6

4

2

0

NO3SO4NH4HNO3NH3

 6) Boulder,WY

NO3SO4NH4HNO3NH3

 5) NOAA 

 Climate Station, WY

 

NO3SO4NH4HNO3NH3

 3) Grand Targhee, WY

 

NO3SO4NH4HNO3NH3

 4) Driggs, ID

NO3SO4NH4HNO3NH3 NO3

-

SO4

2-NH4+

HNO3 NH3

 13) Rocky Mountain 

 National Park,CO

Fig. 7. Comparison of average levels of gases and aerosol species concentrations for this study 

and other sampling locations in the western U.S. There was no measurement of NH4

+

 and HNO3 

at Craters of the Moon National Monument, Idaho. Concentrations are in μg/m3. More 

information about the comparison data can be found in Table S3. 
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60

50

40

30

20

10

0

 NH4+ (nmol/m

3

)

0 5 10 15 20 25 30

Spring

y= 1.88x

R

2

=0.58

2:1 60

50

40

30

20

10

0

0 5 10 15 20 25 30

Summer

y= 2.37x

R

2

=0.77

2:1

60

50

40

30

20

10

0

 NH4+ (nmol/m

3

)

0 5 10 15 20 25 30

SO4

2- [nmol/m3

]

Fall

y= 2.28x

R

2

=0.83

2:1

60

50

40

30

20

10

0

0 5 10 15 20 25 30

SO4

2- [nmol/m3

]

Winter

y= 3.27x

R

2

=0.15

2:1

Fig. 8. Seasonal relationships of NH4

+

 versus SO4

2- concentrations. 
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70

60

50

40

30

20

10

0

[NO2

-]+[NO3

-]+2*[SO42-

]

[10

3 mol/m

3

]

0 10 20 30 40 50 60 70

(a)Spring 1:1

y= 0.95 x

R

2

= 0.62

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70

(b)Summer 1:1

y= 0.71 x

R

2

=0.72

70

60

50

40

30

20

10

0

[NO2

-]+[NO3

-]+2*[SO42-

]

[10

3 mol/m

3

]

0 10 20 30 40 50 60 70

[103

 mol/m3]

[NH4

+

]+[Na+]+[K+]+2*[Mg2+]+2*[Ca2+]

(c)Fall 1:1

y= 0.79 x

R

2

= 0.80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70

[103 mol/m3]

[NH4

+

]+[Na+]+[K+]+2*[Mg2+]+2*[Ca2+]

(d)Winter 1:1

y=1.01 x

R

2

= 0.81

1/1/2007

1/1/2008

1/1/2009

1/1/2010

1/1/2011

1/1/2012

Fig. 9. Seasonal charge balance, where the different colors represent the various sampling periods. 
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• Five years of measurements reveal low ammonia concentrations in western 

Wyoming 

• Ammonia and nitric acid concentrations show a strong seasonal variation 

• Ammonia concentrations peak in summer 

• Nitric acid concentrations peak in summer and winter 

 
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Supporting Material for 

Observations of ammonia, nitric acid, and fine particles in a rural gas production region

This file includes Table S1-S3 and Fig. S1-S2. 

Table S1. Seasonal and yearly averages and standard deviations of gases, aerosol species and 

meteorological parameters. 

Season NH3 

μg/m3

HNO3 

μg/m3

NO3

- 

μg/m3

SO4

2- 

μg/m3

NH4

+ 

μg/m3

K

+ 

μg/m3

Mg2+ 

μg/m3

Ca2+ 

μg/m3

T 

oC 

RH 

% 

WS 

m/s 

Spring Mean 0.14 0.13 0.18 0.65 0.23 0.01 0.02 0.04 4.80 56.76 5.24 

(N=80) SD 0.12 0.12 0.09 0.33 0.12 0.01 0.03 0.04 4.54 11.10 1.30 

Summer Mean 0.38 0.23 0.11 0.54 0.26 0.03 0.01 0.04 15.67 45.59 4.31 

(N=126) SD 0.23 0.11 0.08 0.23 0.15 0.03 0.01 0.03 3.68 13.10 0.66 

Autumn Mean 0.18 0.18 0.12 0.51 0.22 0.02 0.01 0.05 7.17 54.94 4.12 

(N=87) SD 0.14 0.11 0.07 0.23 0.10 0.01 0.00a 0.03 5.37 14.07 1.09 

Mean 0.04 0.19 0.60 0.36 0.28 0.01 0.00b Winter 0.02 -7.76 72.54 3.85 

(N=212) SD 0.06 0.18 0.60 0.23 0.17 0.01 0.00c

 0.03 5.45 8.67 1.76 

2007 Mean 0.14 0.22 0.30 0.54 0.29 0.02 0.01 0.03 3.82 57.82 4.17 

(N=105) SD 0.18 0.17 0.28 0.32 0.12 0.03 0.02 0.03 11.43 17.06 1.16 

2008 Mean 0.20 0.20 0.36 0.53 0.28 0.01 0.01 0.03 2.33 59.00 4.63 

(N=104) SD 0.27 0.19 0.47 0.31 0.20 0.02 0.02 0.04 10.69 16.91 1.68 

2009 Mean 0.23 0.17 0.29 0.47 0.23 0.02 0.00 0.03 3.04 63.27 4.14 

(N=101) SD 0.21 0.10 0.43 0.23 0.12 0.01 0.01 0.04 10.10 13.96 1.22 

2010 Mean 0.15 0.17 0.27 0.38 0.22 0.01 0.01 0.03 3.97 59.66 4.25 

(N=97) SD 0.15 0.12 0.41 0.20 0.12 0.01 0.01 0.02 10.58 16.25 1.53 

2011 Mean 0.13 0.18 0.37 0.47 0.27 0.02 0.01 0.04 2.46 61.2 4.0 

(N=98) SD 0.15 0.13 0.56 0.26 0.14 0.01 0.01 0.04 10.64 13.81 1.50 

a

 The actual value is 0.003. 

b

 The actual value is 0.003. 

c

 The actual value is 0.004. 

N,T, RH and WS represent the number of samples, temperature, relative humidity and wind speed, respectively. 
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Table S2. Correlation coefficients (r) between concentrations of trace gases, particulate species 

and meteorological parameters based on all the data. 

Species NH3 HNO3 NO3

-

 SO4

2- NH4+

 K+ Ca2+ RH WS T 

NH3 1 0.23 -0.30 0.32 0.06 0.40 0.34 -0.53 0.01 0.72 

HNO3 1 0.43 0.16 0.47 0.29 0.10 -0.19 -0.31 0.12 

NO3

-

 1 -0.08 0.63 -0.03 -0.19 0.44 -0.38 -0.54 

SO4

2- 1 0.45 0.35 0.46 -0.33 -0.04 0.33 

NH4

+

 1 0.28 0.14 0.07 -0.36 -0.11 

K

+

 1 0.32 -0.46 -0.05 0.41 

Ca2+ 1 -0.46 0.04 0.41 

Table S3. Site information, data sources and sampling period for Fig. 5. 

Site State HNO3 NH4

+ NO3-

 SO4

2- NH3

 Source Period Source Period Source Period 

Chiricahua 

NM AZ CASTNETa

 01/02/2007~01/03/2012 IMPROVEb 01/03/2007~12/31/2010 AMONc 03/15/2011~01/31/2012 

Joshua 

Tree NPd CA CASTNET 01/02/2007~01/03/2012 IMPROVE 01/06/2007~12/31/2010 AMON 03/01/2011~01/31/2012 

Sequoia 

NP CA CASTNET 01/02/2007~01/03/2012 IMPROVE 01/03/2007~12/31/2010 AMON 03/22/2011~01/31/2012 

Yosemite 

NP CA CASTNET 01/02/2007~01/03/2012 IMPROVE 01/03/2007~12/31/2010 AMON 03/15/2011~01/31/2012 

Cherokee 

Nation KS CASTNET 01/02/2007~01/03/2012 IMPROVE 01/03/2007~12/31/2010 AMON 10/30/2007~01/31/2012 

Mount 

Rainier 

NP 

WA CASTNET 01/02/2007~01/03/2012 IMPROVE 01/03/2007~12/31/2010 AMON 03/17/2011~01/31/2012 

Craters of 

the Moon 

NMe

ID N/A N/A IMPROVE 01/03/2007~12/31/2010 AMON 06/07/2010~01/31/2012 

Palo Duro TX CASTNET 04/24/2007~01/03/2012 CASTNET 04/24/2007~01/03/2012 AMON 10/30/2007~01/31/2012 

Konza 

Prairie KS CASTNET 01/02/2007~01/03/2012 CASTNET 01/02/2007~01/03/2012 AMON 03/01/2011~01/31/2012 

Santee SD CASTNET 01/02/2007~01/03/2012 CASTNET 01/02/2007~01/03/2012 AMON 07/05/2011~02/15/2012 

Gore Pass CO CSU(Beem et 

al., 2010) 

03/15/2006~04/28/2006 

& 

07/06/2006~08/11/2006 

CSU 

03/15/2006~04/28/2006 

& 

07/06/2006~08/11/2006 

CSU 

03/15/2006~04/28/2006 

& 

07/06/2006~08/11/2006 

Rock 

Mountain 

NP 

CO CSU(Benedict 

et al., 2013) 12/01/2008~12/01/2009 CSU 12/01/2008~12/01/2009 CSU 12/01/2008~12/01/2009 

Boulder CO CSU 12/01/2006~12/31/2011 CSU 12/01/2006~12/31/2011 CSU 12/01/2006~12/31/2011 

Driggs ID CSU(Benedict 

et al., in prep) 04/06/2011~09/21/2011 CSU 04/06/2011~09/21/2011 CSU 04/06/2011~09/21/2011 

Targhee 

Base WY CSU(Benedict 

et al., in prep) 04/21/2011~09/21/2011 CSU 04/21/2011~09/21/2011 CSU 04/21/2011~09/21/2011 

NOAA 

Climate WY CSU(Benedict et al., in prep) 05/04/2011~09/21/2011 CSU 05/04/2011~09/21/2011 CSU 05/04/2011~09/21/2011 
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Center 

Brush CO CSU(Benedict 

et al., 2013) 12/11/2008~12/03/2009 CSU 12/11/2008~12/03/2009 CSU 12/11/2008~12/03/2009 

Loveland CO CSU(Benedict 

et al., 2013) 12/11/2008~12/03/2009 CSU 12/11/2008~12/03/2009 CSU 12/11/2008~12/03/2009 

a

 CASTNET (The Clean Air Status and Trends Network, http://epa.gov/castnet/). 

b

 IMPROVE (Interagency Monitoring of Protected Visual Environments, http://vista.cira.colostate.edu/improve/). 

c

 AMON (The Ammonia Monitoring Network, http://nadp.sws.uiuc.edu/amon/). 

d NP means National Park. 

e NM means National Monument. 

f CSU means Colorado State University. 

Fig. S1. Annual emissions (in tons) by county for a) NOx and b) NH3 from the National 

Emissions Inventory 2008 (http://www.epa.gov/ttnchie1/net/2008inventory.html). The sampling 

site is indicated by the (+) signs.
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a) b)

c) d)

e)

Fig. S2. Satellite images of wild fires (http://earthdata.nasa.gov/data/nrt-data/firms/active-firedata) observed in the vicinity of the measurement site (shown as red star) in (a) 2007, (b) 2008, (c) 

2009, (d) 2010 and (f) 2011. From 2007 to 2011, the squared correlation coefficient (r2

) between 

concentrations of NH3 and K+

 was 0.33, 0.40, 0.02, 0.16 and 0.03. 
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16

14

12

10

8

6

4

2

0

Excess NH4+ (nmol/m

3

)

0 2 4 6 8 10 12 14 16

Spring

y= 1.02x

R

2

=0.07

1:1 16

14

12

10

8

6

4

2

0

0 2 4 6 8 10 12 14 16

Summer

y= 1.44x

R

2

=0.02

1:1

16

14

12

10

8

6

4

2

0

Excess NH4+ (nmol/m

3

)

0 2 4 6 8 10 12 14 16

NO3

-

 [nmol/m3]

Fall

y= 1.01x

R

2

=0.31

1:1 60

50

40

30

20

10

0

0 10 20 30 40 50 60

NO3

-

 [nmol/m3]

Winter

y= 0.81x

R

2

=0.76

1:1

Fig. S3. Seasonal relationships of excess NH4

+

 versus NO3

-

 concentrations.