# 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 Li 1 

### , Florian M. Schwandner 1, 4 

### , H. James Sewell 2 

### , Angela Zivkovich 2 

### , Mark Tigges 3 

### , Suresh Raja 1, 5


### , Stephen Holcomb 3,6 

### , John V. Molenar 3 

### , Lincoln Sherman 3 

### , Cassie Archuleta 3


### , Taehyoung Lee 1,7 

### , 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 Continuous measurements of the atmospheric trace gases ammonia (NH 3 

### ) and nitric acid (HNO 3 

### ) and of 1 fine particle (PM 2.5 

### ) ammonium (NH 4 + 

### ), nitrate (NO 3 - 

### ) and sulfate (SO 4 2- 

### ) were conducted using a 2 denuder/filter system from December 2006 to December 2011 at Boulder, Wyoming, a region of active 3 gas production. The average five year concentrations of NH 3 

### , HNO 3 

### , NH 4 + 

### , NO 3 - 

### and SO 4 2- 

### were 0.17, 4 0.19, 0.26, 0.32, and 0.48 μg/m 3 

### , respectively. Significant seasonal patterns were observed. The 5 concentration of NH 3 

### was higher in the summer than in other seasons, consistent with increased NH 3 

### 6 emissions and a shift in the ammonium nitrate (NH 4 

### NO 3 

### ) equilibrium toward the gas phase at higher 7 temperatures. High HNO 3 

### concentrations were observed both in the summer and the winter. Elevated 8 wintertime HNO 3 

### production appeared to be due to active local photochemistry in a shallow boundary 9 layer over a reflective, snow-covered surface. PM 2.5 

### NH 4 + 

### and SO 4 2- 

### concentrations peaked in summer 10 while NO 3 - 

### concentrations peaked in winter. Cold winter temperatures drive the NH 3 

### - HNO 3 

### - NH 4 

### NO 3 

### 11 equilibrium toward particulate NH 4 

### NO 3 

### . A lack of NH 3 

### , however, frequently results in substantial 12 residual gas phase HNO 3 

### even under cold winter conditions. 13

### Keywords: Ammonium; Nitrate; Gas/particle partitioning; Denuder; Wyoming 14 15 1. Introduction 16 Ammonia (NH 3 

### ) is the most abundant alkaline gas in the troposphere and is important for its ability to 17 neutralize acidic components such as sulfuric acid (H 2 

### SO 4 

### ) and nitric acid (HNO 3 

### ) which form, 18 respectively, by oxidation of emissions of sulfur dioxide (SO 2 

### ) and nitrogen oxides (NO X 

### ). NH 3 

### is also an 19 important contributor to nitrogen deposition (Beem et al., 2010; Benedict et al., 2013), which can cause 20 eutrophication, soil acidification and biotic community changes (Cowell and Apsimon, 1998; Groot 21 Koerkamp et al., 1998). It is widely recognized that important sources of NH 3 

### include agricultural 22 activities such as fertilization and livestock emissions (Li et al., 2006; Misselbrook et al., 2000; Sutton et 23 al., 2000). Reactions of HNO 3 

### and H 2 

### SO 4 

### with ambient NH 3 

### generally form submicron ammonium nitrate 24 (NH 4 

### NO 3 

### ) and ammoniated sulfate (NH 4 

### HSO 4 

### , (NH 4 

### ) 2 

### SO 4 

### , or other forms) particles which can adversely 25 impact human respiratory health and impair atmospheric visibility through visible light scattering. In the 26 United States (U.S.) a set of Class 1 areas (including National Parks and Wilderness Areas) has been 27 identified for protection from visibility impairment through the Regional Haze Rule. Because oil and gas 28 production regions of the western U.S. are often located near these visibility-protected areas, close 29 attention is paid to their emissions of fine particle precursors. In order to reduce NO x 

### emissions from 30 natural gas drilling and production activities, for example, selective catalytic reduction (SCR) can be 31 installed on drill rigs. While SCR can yield large reductions of NO x 

### emissions; however, there is a risk of 32 increased NH 3 

### emission into the atmosphere. 33 Western Wyoming is one region of active recent gas development where several air quality concerns have 34 been raised (McMurray et al., 2013). Emissions of NO x 

### have been of concern both because of possible 35 impacts on regional haze and, especially, due to documented impacts on severe winter ozone episodes 36 (Schnell et al., 2009). SCR implementation in the region has been active in recent years as one effort to 37 limit winter ozone episodes. While these winter ozone episodes are believed to be local in nature, NO x 

### 38 emission impacts on regional haze may be more widespread. Unfortunately, few measurements exist in 39 the region of NH 3 

### and haze impact assessments are generally forced to rely on an assumed background 40 NH 3 

### concentration level. While a number of recent studies have considered the role NH 3 

### plays in the 41 formation of fine particles across the United States in both urban and rural areas (Bari et al., 2003; 42 Benedict et al., 2013; Edgerton et al., 2007; Gong et al., 2011; Heald et al., 2012; Lee et al., 2008; Nowak 43 et al., 2010; Sather et al., 2008), knowledge of atmospheric concentrations of NH 3 

### , and their seasonal 44 variability is still rather limited, especially in the interior western United States. Here we present five 45 years of observations of concentrations of gaseous NH 3 

### and HNO 3 

### and fine particle concentrations of 46

### NH 4 + 

### , SO 4 2- 

### and NO 3 - 

### from Boulder, Wyoming, a site in the heart of an active gas production region. 47 These measurements provide the longest term record of NH 3 

### concentration measurements in this part of 48 the U.S. and provide new insight into typical NH 3 

### concentrations in the region, their seasonal variability, 49 and the gas-particle partitioning of the NH 3 

### - NH 4 + 

### - HNO 3 

### - NO 3 - 

### - SO 4 2- 

### system that is one important 50 contributor to regional haze. 51
2. Experimental methods 52 Measurements were made southwest of Boulder, Wyoming (42o43'7.9"N, 109o45'10.4"W) in the 53 northwestern part of the Pinedale anticline area. Two visibility-protected areas, Bridger Wilderness Area 54 and Fitzpatrick Wilderness Area, are located within 100 km. The Boulder area and nearby natural gas 55 fields are situated on a high plateau between the Wind River Range to the east and the Wyoming Range to 56 the west. Strong surface-based inversions, with inversion pools intersecting topography levels down to 57 50m above ground (Schnell et al., 2009), are common in the region, especially during wintertime. The 58 population density in Boulder, Wyoming is sparse with only 8.9 people per square km. The Jonah Gas 59 Field and the Pinedale Anticline Gas Field, together representing one of the largest gas production regions 60 in the U.S., are close to the sampling site with several active gas wells located approximately 3 km west 61 of the sampling site. In 2008, there were more than 500 permitted wells in the Jonah Gas Field and an 62 additional 3100 wells are expected to be drilled in this field over the next 75 years. Total production in 63 this region in 2011 was nearly 171 billion cubic feet of natural gas and 1.5 million barrels of oil 64 (http://www.encana.com/pdf/communities/usa/JonahField-FactSheet.pdf ). NO x 

### from the gas extraction 65 operations and transportation emissions are the largest contributor to local NO x 

### emissions (Fig. S1a). For 66 NH 3 

### emissions, there are not many large sources in this immediate area. However, the Snake River Valley 67 to the west of the measurement site is a large area of intense agricultural activity with elevated NH 3 

### 68 emissions and concentrations (Clarisse et al., 2009) (Fig. S1b). Installation of more SCR systems in the 69 Jonah-Pinedale region could elevate local NH 3 

### concentrations, contributing to more particle formation 70 and visibility degradation. 71 Concentrations of gaseous NH 3 

### and HNO 3 

### and PM 2.5 

### (particles with an aerodynamic diameter less than 72 2.5 micrometers) inorganic ions (NH 4 + 

### , SO 4 2- 

### , NO 3 - 

### , K + 

### , Mg 2+ 

### and Ca 2+ 

### ) were sampled using a URG 73 denuder/filter system (University Research Glassware, Inc., Model 3000CA), which was installed at 1.5 74 meter height, followed by laboratory extraction and analysis by ion chromatography. The URG sampling 75 system has been widely used because of its good performance in sampling acidic gaseous and volatile 76 aerosols (Bari et al., 2003; Beem et al., 2010; Edgerton et al., 2007; Lee et al., 2004; Lin et al., 2006). Air 77 was drawn through a Teflon-coated PM 2.5 

### cyclone followed by two 242mm annular denuders connected in 78 series, a 47-mm filter pack containing a nylon filter (Nylasorb, pore size1μm, Pall Corporation) and 79

### another annular denuder (from Dec. 2006 through July 11 th 

### 2008, samples were collected used a backup 80 coated filter rather than a 3 rd 

### denuder). Air flow was maintained at a constant mass flow rate by means of 81 a mass flow controlled pump (URG Inc.). The total flow rate through the system was nominally 10L/min 82 at ambient conditions. Actual sample volumes were determined using a sample pressure-compensated 83 dry gas meter. The first denuder was coated with sodium chloride (NaCl) to collect gaseous HNO 3 

### and the 84 second was coated with phosphorus acid (H 3 

### PO 4 

### ) to collect gaseous NH 3 

### . The last denuder (or coated 85 filter) was phosphorus acid-coated to collect any NH 3 

### re-volatilized from NH 4 + 

### salt particles collected on 86 the filter. Nylon filters have been shown to retain volatilized HNO 3 

### , but loss of NH 4 + 

### can be significant 87 (Yu et al., 2006). The sample trains were prepared in the lab at Colorado State University (CSU), and then 88 shipped weekly and installed by a local site operator. Samples were typically collected twice a week (one 89 4 day sample and one 3 day sample). After sampling, the sample train was shipped back to the lab at CSU. 90 The denuders were extracted with 10 ml deionized water, and the extracts refrigerated before analysis. 91 Nylon filters were ultrasonically extracted for 55 minutes in 6ml of high purity deionized water. Ion 92 chromatography using a Dionex dual channel system was used to analyze the denuder and filter extracts. 93 Cations (Na + 

### , NH 4 + 

### , K + 

### , Mg 2+ 

### and Ca 2+ 

### ) in the samples were separated with a methanesulfonic acid eluent 94 on a Dionex CG12A guard column and CS12A separation column followed by a CSRS ULTRA II 95 suppressor and detected by a Dionex conductivity detector. Anions (Cl - 

### , NO 3 - 

### , SO 4 2- 

### ) in the samples were 96 separated with a carbonate/bicarbonate eluent on a Dionex AG14A guard column and AS14A separation 97 column followed by an ASRS ULTRA II suppressor and detected using a Dionex conductivity detector. 98 99 Meteorological data, including temperature, relative humidity and wind speed, were obtained from a co100 located weather station (2 meter height) operated by Air Resource Specialists, Inc. 101 102 Sample recovery was high, although there were occasional periods where samples could not be collected 103 on the normal schedule (e.g, from bad weather affecting sampler shipment or operator access). Field and 104 laboratory blanks were collected throughout the study and used to determine the method detection limit 105 (MDL) and to blank-correct results. The MDLs for NH 3 

### , HNO 3 

### , NH 4 + 

### , SO 4 2- 

### , NO 3 - 

### , K + 

### , Mg 2+ 

### and Ca 2+ 

### 106 were determined as 0.012, 0.012, 0.002, 0.017, 0.001, 0.005, 0.007 and 0.023 μg/m 3 

### , respectively. 107 Replicate extract analyses yielded measurement precisions of 5.4%, 3. 8%, 3.5%, 0.8%, 2.1%, 4.9%, 108 7.6%, and 6.2% (relative standard deviation) for NH 3 

### , HNO 3 

### , NH 4 + 

### , SO 4 2- 

### , NO 3 - 

### , K + 

### , Mg 2+ 

### and Ca 2+


### , 109 respectively. 110 111
3. Results and discussion 112

### 113 From December 2006 through December 2011, 505 samples were collected. The summary of the annual 114 and seasonal mean and standard deviation for all the trace gas concentrations, particulate species 115 concentrations and meteorological parameter values are summarized in Table S1 in the supplement. 116 Because of the high latitude of this continental sampling site and the monthly average temperatures, the 117 following months were defined as representing specific seasons, for the purpose of discussing the 118 analytical results below: April and May were defined as spring; June, July and August as summer; 119 September and October as fall; and November through March as winter. 120 121 Fig. 1 shows time series of the concentrations of gaseous NH 3 

### and HNO 3 

### and PM 2.5 

### NH 4 + 

### and NO 3 - 

### across 122 the five year measurement period. NH 3 

### concentrations peak in summer while NO 3 - 

### concentrations peak in 123 winter. HNO 3 

### exhibits a distinct bimodal seasonal concentration pattern with summer and winter maxima. 124 As shown in Fig. 2, NH 4 + 

### , SO 4 2- 

### and NO 3 - 

### were the three most abundant inorganic ions in PM 2.5 

### in all 125 seasons. The concentration of NH 4 + 

### varied least across seasons. During the warm season SO 4 2- 

### was the 126 most abundant inorganic anion in PM 2.5 

### , while during winter the concentration of NO 3 - 

### was highest. More 127 details concerning the trends of each of the trace gas and particulate species will be presented below. 128 129 3.1 Concentrations of major gas and fine particle species, their seasonal patterns, and gas-particle 130 partitioning 131 Gaseous NH 3 

### exhibited a clear seasonal concentration pattern, ranging from an average concentration of 132 0.04 μg/m 3 

### in winter to 0.39 μg/m 3 

### in summer (Fig. 3a). The maximum quarterly NH 3 

### average 133 concentration was 0.47 μg/m 3 

### in summer 2008, 15 times higher than the winter 2008-2009 average of 134 0.03 μg/m 3 

### . The significantly higher summer concentration reflects a strong influence of temperature. 135 Previous studies have reported similar phenomena (Edgerton et al., 2007; Gupta et al., 2003; Meng et al., 136 2011; Plessow et al., 2005; Walker et al., 2004). Higher levels of NH 3 

### in the summer are consistent with 137 the positive influence of higher temperatures on NH 3 

### emissions (e.g., from natural soils, agricultural 138 operations, and fires) and the decomposition of particulate NH 4 

### NO 3 

### into gaseous NH 3 

### and HNO 3 

### . 139 140 One potential local source of NH 3 

### is increased use of SCR for high-efficiency NO x 

### control on drill rigs in 141 the region. While use of SCR increased during the period of observation at Boulder, however, there is no 142 clear increase in local NH 3 

### concentrations over the study. The annual mean concentration of NH 3 

### did not 143 significantly increase during the study period. From 2007 to 2011, the annual NH 3 

### average concentrations 144 in each year were 0.14, 0.20, 0.23, 0.15 and 0.13 μg/m 3 

### , respectively, suggesting that SCR emissions did 145 not noticeably influence local concentrations of ambient NH 3 

### . 146

### 147 Table S2 shows cross-correlation coefficients for measured trace gases, particle ions and meteorological 148 parameters. Some correlation was found between NH 3 

### and K + 

### (r 2 

### =0.16). As illustrated by satellite fire149 detect images (Fig. S2), there were more wild fires present around Boulder, Wyoming in 2007 and 2008; 150 the correlation coefficients (r 2 

### ) between NH 3 

### and K + 

### in those two years were 0.33 and 0.40. As a marker 151 of biomass burning, the correlation between NH 3 

### and K + 

### may suggest a positive influence of fire 152 emissions on NH 3 

### concentrations (Anderson et al., 2003; Hegg et al., 1988; McMeeking et al., 2009; 153 Sutton et al., 1995). 154 155 A background NH 3 

### mixing ratio of 1 ppbv is often assumed when estimating impacts of NO x 

### emissions 156 on visibility and regional haze in western regions of the U.S. where ambient NH 3 

### concentration data are 157 sparse or unavailable. Such estimates might be made, for example, through plume dispersion simulations 158 using CALPUFF or other EPA-preferred models. The 5-year Boulder data records provide a better basis 159 for choosing a representative background NH 3 

### concentration for the Pinedale region. Figure 4 reveals 160 that seasonal mean NH 3 

### mixing ratios ranged between a maximum of 0.85 ppbv (in summer 2008) and 161 0.03 ppbv (in winter 2010). The average for the full 5-year study period was 0.30 ppbv, less than one162 third of the typically assumed background level. Even if PM 2.5 

### NH 4 + 

### (much of which certainly reacted 163 with sulfuric and nitric acids upwind of the measurement region) and gaseous NH 3 

### are combined, the 164 average mixing ratio (0.63 ppbv) remains well below 1 ppbv. Assumption of a 1 ppbv NH 3 

### background 165 concentration in model simulations, therefore, will lead to an overprediction of visibility impacts 166 associated with local NO x 

### emissions. 167 168 The five year average HNO 3 

### mixing ratio was observed to be 0.03 ppbv, indicating typically low 169 concentrations of HNO 3 

### occur in this area. Seasonal mean HNO 3 

### mixing ratios (Fig. 4) ranged between 170 0.13 ppbv (in summer 2007) and 0.01 ppbv (in spring 2008). As illustrated in Fig. 3b, HNO 3 

### 171 concentrations display a distinct bimodal seasonal pattern, with higher average concentrations in the 172 summer (0.23 μg/m 3 

### ) and in mid-winter (January/February average = 0.26 μg/m 3 

### ) than in other seasons. 173 One also can see in Fig. 4 that variability in observed HNO 3 

### concentrations is quite high in January and 174 February. Previous studies have generally shown that HNO 3 

### peaks in summer with lower concentrations 175 during winter (Adon et al., 2010; Gupta et al., 2003; Lee et al., 1999; Plessow et al., 2005). Increased 176 concentrations of HNO 3 

### are expected in the summer because of more intense and longer lasting 177 photochemical activity associated with higher sun angles and longer days. Higher summer temperatures 178 also promote dissociation of NH 4 

### NO 3 

### back to gaseous NH 3 

### and HNO 3 

### , as discussed above (Seinfeld and 179 Pandis, 2006). The high winter concentrations observed at Boulder, by contrast, are quite unusual. The 180

### peak wintertime HNO 3 

### concentration climbed as high as 1.40 μg/m 3 

### for a single sample collected from 181 February 22 th



### to 25 th 

### in 2008. A closer look at the HNO 3 

### timeline in Figure 1 reveals frequent winter 182 periods of elevated HNO 3 

### concentrations. Other measurements at Boulder reveal that this area is 183 frequently subject to periods of elevated winter ozone (Schnell et al., 2009) that occur during sunny 184 winter periods when snow covers the ground. Strong nocturnal and morning temperature inversions that 185 set up under these conditions trap local emissions of NO x 

### and volatile organic compounds, associated 186 largely with local energy production activities, in a shallow mixing layer while daytime photochemical 187 activity is enhanced by strong reflectance from the bright snow surface. The photochemical reactions that 188 generate ozone concentrations well in excess of the U.S. National Ambient Air Quality Standard can also 189 lead to substantial oxidation of the locally emitted NO X 

### to HNO 3 

### . While cold winter conditions favor 190 reaction of HNO 3 

### with NH 3 

### to form fine particle NH 4 

### NO 3 

### (as evidenced by the winter NH 4 

### NO 3 

### spikes in 191 Figure 1), the Boulder observations reveal that all gaseous NH 3 

### has often been consumed during these 192 episodes leaving a substantial fraction of the HNO 3 

### “trapped” in the gas phase. 193

194 Ambient NH 4 + 

### concentrations at Boulder exhibited little seasonal pattern (Fig. 3c). The annual mean 195 concentrations for 2007 to 2011 were also similar to each other at 0.29, 0.28, 0.23, 0.22 and 0.27 μg/m 3


### , 196 respectively. Formation of fine particle NH 4 + 

### is influenced by a variety of factors, including the 197 availability of gaseous NH 3 

### and the availability of acidic sulfate aerosol and gaseous HNO 3 

### . Increases in 198 NH 3 

### and SO 4 2- 

### at Boulder during warmer months of the year will tend to increase NH 4 + 

### concentrations as 199 well. Formation of fine particle NH 4 

### NO 3 

### , however, is favored in winter. As previously discussed, the 200 formation of NH 4 

### NO 3 

### is thermodynamically favored by high relative humidity and low temperatures. 201 During the winter in Boulder, the average temperature was -7.8 oC and average relative humidity was 202 72.5%. These offsetting seasonal patterns appear to result in an overall NH 4 + 

### concentration pattern that 203 shows little seasonality at Boulder. 204 205 The annual average concentrations of PM 2.5 

### NO 3 - 

### measured at Boulder were 0.30, 0.36, 0.29, 0.27 and 206 0.37 μg/m 3 

### in 2007 through 2011, respectively. The NO 3 - 

### concentrations exhibited a strong seasonality, 207 with maximum values in winter and minimum values in summer (Fig. 3d). Because NH 4 

### NO 3 

### formation is 208 not favored under the warm, dry conditions of summer, the mean summer NO 3 - 

### concentration was only 209 0.11 μg/m 3 

### . In winter, it increased to 0.60 μg/m 3 

### , as NH 4 

### NO 3 

### formation was more strongly favored. As 210 indicated in Fig. 3d, considerable variability was also observed in winter NO 3 - 

### concentrations, similar to 211 the pattern discussed above for HNO 3 

### , with maximum observed concentrations exceeding 2.0 μg/m 3 

### in 212 December, January, February, and March. 213

### 214 SO 4 2- 

### shows a seasonal cycle with maximum values in the warm season (Fig. 3e). This seasonal pattern is 215 typical of SO 4 2- 

### , due to enhanced photochemical activity and higher concentrations of hydroxyl radical, 216 which can oxidize SO 2 

### to SO 4 2- 

### (Behera and Sharma, 2010). In-cloud oxidation of SO 2 

### to SO 4 2- 

### can also 217 be enhanced in summer when hydrogen peroxide concentrations are typically higher. Annual average 218 concentrations of SO 4 2- 

### at Boulder in 2007 through 2011 were 0.54, 0.53, 0.47, 0.38, and 0.47 μg/m 3



### . 219 220 In addition to anthropogenic emissions of nitrogen and sulfur species, wild and prescribed fires also 221 contribute significantly to fine particle concentrations in the western U.S. (Jaffe et al., 2008; Malm et al., 222 2004). Water soluble potassium ion concentrations, one marker of biomass burning (Andreae, 1983; Duan 223 et al., 2004), were elevated in summer (Fig. 3f). The average concentration of K + 

### in the summer was 0.03 224 μg/m 3 

### , which was nearly three times higher than the value in the winter (See Table S1). Not surprisingly, 225 summer K + 

### concentrations varied substantially; interannual variability in fire occurrence and the influence 226 of emissions from fires that do occur on air quality at Boulder are expected. Across the sampling period, 227 a number of wild fires occurred upwind of the site in summer. 228 229 Given the seasonally varying partitioning of NH 3 

### / NH 4 + 

### and HNO 3 

### / NO 3 - 

### between the gas and particle 230 phases, it is useful to also examine seasonal changes in total reduced inorganic nitrogen in the -3 231 oxidation state [N(-III)=NH 3 

### +NH 4 + 

### ] and in total oxidized inorganic nitrogen in the +5 oxidation state 232 [N(+V)=HNO 3 

### +NO 3 - 

### ]. As illustrated in Fig. 3g, N(-III) concentrations show a peak in the summer 233 months, consistent with higher NH 3 

### emissions and greater SO 4 2- 

### concentrations at this time of year. N(+V) 234 concentrations, by contrast, peak in the winter months. This winter peak reflects the formation of HNO 3 

### 235 in a shallow boundary layer during winter ozone episodes as well as winter formation of fine particle 236 NH 4 

### NO 3 

### , with its longer atmospheric residence time than HNO 3 

### . 237 238 To investigate the seasonal phase changes of NH 3 

### /NH 4 + 

### and HNO 3 

### /NO 3 - 

### , we define the ammonia gas 239 fraction (F NH3 

### = the NH 3 

### gas concentration divided by the sum of the NH 3 

### gas and fine particle NH 4 + 

### 240 concentrations) and the nitric acid gas fraction (F HNO3 

### = the HNO 3 

### gas concentration divided by the sum 241 of the HNO 3 

### gas and fine particle NO 3 - 

### concentrations), where all concentrations are expressed in molar 242 units. The monthly average partitioning for the reduced and oxidized inorganic nitrogen forms is plotted 243 in Fig. 5. There was a gradual transition from the cooler months, when the particle phase was favored, to 244 the warmer months, when the gas phase was favored, for both species. A maximum monthly average in 245 the gas phase fraction of NH 3 

### occurred in July (0.64). This was more than 10 times higher than the 246 minimum monthly average of 0.06 which occurred in January. Similarly, the HNO 3 

### gas fraction (F HNO3 

### ) 247

### was found to be highest in summer (0.73 in July) and lowest in winter (0.24 in January). The high 248 summer level of F NH3 

### reflects greater NH 3 

### emissions and the thermodynamic tendency for NH 4 

### NO 3 

### to 249 dissociate to NH 3 

### and HNO 3 

### at high temperature. The higher summer value of F HNO3 

### also reflects the 250 tendency for NH 4 

### NO 3 

### to dissociate at higher temperatures. The still appreciable winter F HNO3 

### level, 251 which is not typical of previous results (Bari et al., 2003; Gupta et al., 2003; Sharma et al., 2007), reflects 252 the continued photochemical production of HNO 3 

### at levels which exceed the amount of NH 3 

### available to 253 participate in NH 4 

### NO 3 

### formation. 254 255 Shifts in the equilibrium partitioning among gaseous NH 3 

### and HNO 3 

### and particulate NH 4 

### NO 3 

### depend on 256 relative humidity (RH), temperature (T) and the concentrations of NH 3 

### and HNO 3 

### . Ambient relative 257 humidity at the Boulder measurement site was usually less than the deliquescence relative humidity (DRH) 258 of NH 4 

### NO 3 

### so that we can simplify matters and consider here formation of solid NH 4 

### NO 3 

### . Under this 259 condition, this reaction’s equilibrium constant (K p 

### ) is the expected product of the NH 3 

### and HNO 3 

### 260 concentrations and is given by the empirical formula (R1): 261 262 (R1) 263
264 where K p 

### is in units of ppbv 2 

### and T is measured ambient temperature in Kelvin (Stelson and Seinfeld, 265 2007). The measured, apparent reaction constant (K m 

### ) can be described as follows (R2): 266 267 (R2) 268 269 where [NH 3 

### ] is the gaseous NH 3 

### mixing ratio (ppbv) and [HNO 3 

### ] is the gaseous HNO 3 

### mixing ratio 270 (ppbv). NH 4 

### NO 3 

### formation is favored when K m 

### exceeds K p 

### . Fig. 6 shows the variation of both the 271 theoretical equilibrium constant (shown as a solid line) and measured constant values (for each sample) 272 with temperature (1000/T) across all seasons. This presentation of the data clearly illustrates that NH 4 

### NO 3 

### 273 formation is only favored during wintertime; even then, it is not favored during all sample periods. At 274 warmer times of the year, the product of NH 3 

### and HNO 3 

### concentrations is insufficient to yield NH 4 

### NO 3 

### 275 formation at seasonal temperatures. 276 277 3.2 Comparison with other measurements 278 279

### Fig. 7 presents a comparison of observations from this study with other observations from the Clean Air 280 Status and Trends Network (CASTNET; http://www.epa.gov/castnet/), the Interagency Monitoring of 281 Protected Visual Environments (IMPROVE; http://vista.cira.colostate.edu/improve), the National 282 Atmospheric Deposition Program Ammonia Monitoring Network (AMoN; 283 http://nadp.sws.uiuc.edu/amon/) and seven sets of ambient composition measurements completed by our 284 lab at CSU in the western U.S. (Table S3). Concentrations of particle and gas phase species observed at 285 Boulder were, overall, among the lower average concentrations measured across this set of western sites. 286 Comparing mean values of NH 3 

### and HNO 3 

### and fine particle NH 4 + 

### , NO 3 - 

### , and SO 4 2- 

### at Boulder with the 287 other rural locations, the concentrations measured at Boulder were generally lower than those observed at 288 sites further east such as Santee Sioux, South Dakota, Konza Prairie, Kansas, Cherokee Nation, 289 Oklahoma, and Palo Duro, Texas. They were also significantly lower than concentrations measured 290 closer to more populated areas, such as those at Sequoia National Park, California, Joshua Tree National 291 Park, California, and Loveland, Colorado. Boulder, Wyoming NH 3 

### concentrations were substantially 292 lower than NH 3 

### concentrations measured at sites more strongly impacted by regional agriculture/animal 293 feeding operations, such as Brush, Colorado. Overall concentrations were fairly similar, however, 294 between Boulder and other remote sites in central and western Colorado and in western Wyoming, 295 suggesting some regional representativeness of the concentrations measured in Boulder (aside from the 296 winter ozone episodes). Although the Boulder measurement site is only approximately 65 km from the 297 Snake River Plain Valley, an area of intense agricultural activity with high NH 3 

### emissions (Clarisse et al., 298 2009), the low NH 3 

### concentrations observed at Boulder suggest that the Wyoming (Palisades) Mountain 299 Range blocks at least the most direct transport of these emissions while other local NH 3 

### emissions are 300 limited in their contributions to ambient NH 3 

### concentrations. 301 302 3.3 Interspecies correlations, the measured ion charge balance, and the importance of organic acids 303 304 Fig. 8 illustrates the correlation between fine particle NH 4 + 

### and SO 4 2- 

### in different seasons. Significant 305 correlations were found in all seasons except in winter. The highest correlation (r 2 

### = 0.84) was in the fall 306 and the lowest was in the winter (r 2 

### =0.15). The weak correlation in winter results from substantial 307 NH 4 

### NO 3 

### formation during this time period. If one plots the excess NH 4 + 

### (the amount beyond that needed 308 to fully neutralize fine particle SO 4 2- 

### ), one finds it to be strongly correlated with fine particle NO 3 - 

### during 309 winter (r 2 

### =0.76; slope of 0.81), modestly correlated in fall (r 2 

### =0.31; slope of 1.01), and showing almost no 310 correlation in spring and summer (see Fig. S3). 311 312

### Overall, on the basis of the seasonal variation of comparisons between NH 4 + 

### and SO 4 2- 

### and excess NH 4 + 

### 313 and NO 3 - 

### , one can conclude that most fine particle NH 4 + 

### in summer exists as (NH 4 

### ) 2 

### SO 4 

### while both 314 (NH 4 

### ) 2 

### SO 4 

### and NH 4 

### NO 3 

### are found in fine particles in winter. An excess of NH 4 + 

### in summer when NO 3 - 

### 315 concentrations are low, however, suggests that other unmeasured anionic species might also be important 316 components of the fine particles. This pattern also appears in some fall and spring samples. This issue 317 can be further evaluated by considering the overall ionic charge balance of measured fine particle anion 318 (NO 2 - 

### , NO 3 - 

### , SO 4 2- 

### ) and cation (NH 4 + 

### , Na + 

### , K + 

### , Mg 2+, 

### Ca 2+ 

### ) concentrations. Fig. 9 presents the seasonal 319 variation of the ionic charge balance. During spring and winter, the charge balance is very close to 1:1. 320 During fall and, especially, summer, however, the charge balance generally indicates a deficiency of 321 anions. Previous studies (Barsanti et al., 2009; Trebs et al., 2005) have reported that organic acids such as 322 oxalic acid can be important contributors to the charge balance of fine mode aerosols. The warm season 323 anion deficit observed here is consistent with higher organic acid concentrations during summer, 324 coinciding with periods of enhanced photochemical production of secondary organic aerosols and 325 increased biomass burning. Future measurements of summertime Boulder fine particle concentrations 326 will include analysis of oxalate. 327 328 4 Conclusion 329 330 A five-year study of concentrations of gaseous NH 3 

### and HNO 3 

### and of fine particle inorganic ions was 331 conducted in an active gas production region in Boulder, Wyoming. The five-year annual mean 332 concentrations of NH 3 

### , HNO 3 

### , NH 4 + 

### , NO 3 - 

### and SO 4 2- 

### were 0.17, 0.19, 0.26, 0.32, and 0.48 μg/m 3


### , 333 respectively. NH 3 

### exhibited a strong seasonal variation, with higher concentrations during the summer 334 and lower concentrations during the winter. The low annual average NH 3 

### mixing ratio of 0.30 ppb 335 suggests that the default value of 1 ppb often used in regional assessments of visibility impacts from NO x 

### 336 source emissions is higher than necessary. 337 338 Observed NH 3 

### concentrations correlated well with ambient temperature indicating the important influence 339 of temperature on emissions and, likely, the greater long distance transport of those emissions during 340 warmer times of year when mixing layers deepen. By contrast, higher concentrations of particulate NO 3 - 

### 341 were observed in the winter when lower temperatures favor formation of NH 4 

### NO 3 

### . HNO 3 

### concentrations 342 showed an unusual bimodal seasonal variation with higher levels both in summer (an expected result of 343 active photochemical oxidation and a tendency for NH 4 

### NO 3 

### to decompose at higher temperatures) and in 344 winter. The unusual winter HNO 3 

### peak appears to be the result of active photochemical processing of 345 local NO x 

### emissions in a shallow boundary layer during periods of snow cover and a lack of NH 3 

### to fully 346

### tie up HNO 3 

### through fine particle NH 4 

### NO 3 

### formation. Examination of the equilibrium thermodynamics of 347 NH 4 

### NO 3 

### formation, seasonal local temperatures, and available concentrations of gaseous NH 3 

### and HNO 3 

### , 348 indicates that NH 4 

### NO 3 

### should be expected primarily in winter, as observed. 349 350 Acknowledgements 351 The authors thank Amy Sullivan, Xi (Doris) Chen, Katie (Beem) Benedict, Arsineh Hecobian, Yele Sun 352 and Ali (Bote) Willey at Colorado State University for their helpful suggestions and support of the 353 measurement program. The authors also thank Jennifer Frazier of the Wyoming Department of 354 Environmental Quality/Air Quality Division and Ted J. Porwoll of the U.S. Forest Service for carrying 355 out on-site operations. 356 357 References 358 Adon, M., Galy-Lacaux, C., Yoboué, V., Delon, C., Lacaux, J.P., Castera, P., Gardrat, E., Pienaar, J., Al 359 Ourabi, H., Laouali, D., Diop, B., Sigha-Nkamdjou, L., Akpo, A., Tathy, J.P., Lavenu, F., Mougin, E., 360 2010. Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in 361 Africa using passive samplers. Atmospheric Chemistry and Physics 10, 7467-7487. 362 Anderson, N., Strader, R., Davidson, C., 2003. Airborne reduced nitrogen: ammonia emissions from 363 agriculture and other sources. Environment International 29, 277-286. 364 Andreae, M.O., 1983. Soot Carbon and Excess Fine Potassium: Long-Range Transport of Combustion365 Derived Aerosols. Science 220, 1148-1151. 366 Bari, A., Ferraro, V., Wilson, L.R., Luttinger, D., Husain, L., 2003. Measurements of gaseous HONO, 367 HNO 3 

### , SO 2 

### , HCl, NH 3 

### , particulate sulfate and PM 2.5 

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### , HNO 3 

### , NH 3 

### and SO 2 

### 397 and related particulate matter at a rural site in Rampur, India. Atmospheric Environment 37, 4837-4846. 398 Heald, C.L., J. L. Collett, J., Lee, T., Benedict, K.B., Schwandner, F.M., Li, Y., Clarisse, L., Hurtmans, 399 D.R., Van Damme, M., Clerbaux, C., Coheur, P.F., Philip, S., Martin, R.V., Pye, H.O.T., 2012. 400 Atmospheric ammonia and particulate inorganic nitrogen over the United States. Atmospheric Chemistry 401 and Physics 12, 10295-10312. 402 Hegg, D.A., Radke, L.F., Hobbs, P.V., Riggan, P.J., 1988. Ammonia emissions from biomass burning. 403 Geophysical Research Letters 15, 335-337. 404 Jaffe, D., Hafner, W., Chand, D., Westerling, A., Spracklen, D., 2008. Interannual Variations in PM2.5 405 due to Wildfires in the Western United States. Environmental Science & Technology 42, 2812-2818. 406 Lee, H.S., Kang, C.-M., Kang, B.-W., Kim, H.-K., 1999. Seasonal variations of acidic air pollutants in 407 Seoul, South Korea. Atmospheric Environment 33, 3143-3152. 408 Lee, T., Kreidenweis, S.M., Collett, J.L., 2004. Aerosol Ion Characteristics During the Big Bend Regional 409 Aerosol and Visibility Observational Study. Journal of the Air & Waste Management Association 54, 410 585-592. 411 Lee, T., Yu, X., Ayres, B., Kreidenweis, S., Malm, W., Collett, J.L., 2008. Observations of fine and 412 coarse particle nitrate at several rural locations in the United States. Atmospheric Environment 42, 2720- 413 2732. 414 Li, Y., Schwab, J.J., Demerjian, K.L., 2006. Measurements of ambient ammonia using a tunable diode 415 laser absorption spectrometer: Characteristics of ambient ammonia emissions in an urban area of New 416 York City. Journal of Geophysical Research: Atmospheres 111, D10S02. 417

### Lin, Y., Cheng, M., Ting, W., Yeh, C., 2006. Characteristics of gaseous HNO 2 

### , HNO 3 

### , NH 3 

### and 418 particulate ammonium nitrate in an urban city of Central Taiwan. Atmospheric Environment 40, 4725- 419 4733. 420 Malm, W.C., Schichtel, B.A., Pitchford, M.L., Ashbaugh, L.L., Eldred, R.A., 2004. Spatial and monthly 421 trends in speciated fine particle concentration in the United States. Journal of Geophysical Research: 422 Atmospheres 109, D03306. 423 McMeeking, G.R., Kreidenweis, S.M., Baker, S., Carrico, C.M., Chow, J.C., Collett, J.L., Hao, W.M., 424 Holden, A.S., Kirchstetter, T.W., Malm, W.C., Moosmüller, H., Sullivan, A.P., Wold, C.E., 2009. 425 Emissions of trace gases and aerosols during the open combustion of biomass in the laboratory. Journal of 426 Geophysical Research: Atmospheres 114, D19210.McMurray, J., Roberts, D., Fenn, M., Geiser, L., Jovan, 427 S., 2013. Using Epiphytic Lichens to Monitor Nitrogen Deposition Near Natural Gas Drilling Operations 428 in the Wind River Range, WY, USA. Water, Air, & Soil Pollution 224, 1-14. 429 Meng, Z.Y., Lin, W.L., Jiang, X.M., Yan, P., Wang, Y., Zhang, Y.M., Jia, X.F., Yu, X.L., 2011. 430 Characteristics of atmospheric ammonia over Beijing, China. Atmospheric Chemistry and Physics 11, 431 6139-6151. 432 Misselbrook, T.H., Van Der Weerden, T.J., Pain, B.F., Jarvis, S.C., Chambers, B.J., Smith, K.A., Phillips, 433 V.R., Demmers, T.G.M., 2000. Ammonia emission factors for UK agriculture. Atmospheric Environment 434 34, 871-880. 435 Nowak, J.B., Neuman, J.A., Bahreini, R., Brock, C.A., Middlebrook, A.M., Wollny, A.G., Holloway, J.S., 436 Peischl, J., Ryerson, T.B., Fehsenfeld, F.C., 2010. Airborne observations of ammonia and ammonium 437 nitrate formation over Houston, Texas. Journal of Geophysical Research: Atmospheres 115, D22304. 438 Plessow, K., Spindler, G., Zimmermann, F., Matschullat, J., 2005. Seasonal variations and interactions of 439 N-containing gases and particles over a coniferous forest, Saxony, Germany. Atmospheric Environment 440 39, 6995-7007. 441 Sather, M.E., Mathew, J., Nguyen, N., Lay, J., Golod, G., Vet, R., Cotie, J., Hertel, T., Aaboe, E., 442 Callison, R., Adam, J., Keese, D., Freise, J., Hathcoat, A., Sakizzie, B., King, M., Lee, C., Oliva, S., San 443 Miguel, G., Crow, L., Geasland, F., 2008. Baseline ambient gaseous ammonia concentrations in the Four 444 Corners area and eastern Oklahoma, USA. Journal of Environmental Monitoring 10, 1319-1325. 445 Schnell, R.C., Oltmans, S.J., Neely, R.R., Endres, M.S., Molenar, J.V., White, A.B., 2009. Rapid 446 photochemical production of ozone at high concentrations in a rural site during winter. Nature Geoscience 447 2, 120-122. 448 Seinfeld, J.H., Pandis, S.N., 2006. Atmospheric chemistry and physics: from air pollution to climate 449 change. Wiley. 450 Sharma, M., Kishore, S., Tripathi, S.N., Behera, S.N., 2007. Role of atmospheric ammonia in the 451 formation of inorganic secondary particulate matter: A study at Kanpur, India. Journal of Atmospheric 452 Chemistry 58, 1-17. 453 Stelson, A.W., Seinfeld, J.H., 2007. Relative humidity and temperature dependence of the ammonium 454 nitrate dissociation constant. Atmospheric Environment 41, Supplement, 126-135. 455

### Sutton, M.A., Dragosits, U., Tang, Y.S., Fowler, D., 2000. Ammonia emissions from non-agricultural 456 sources in the UK. Atmospheric Environment 34, 855-869. 457 Sutton, M.A., Place, C.J., Eager, M., Fowler, D., Smith, R.I., 1995. Assessment of the magnitude of 458 ammonia emissions in the United Kingdom. Atmospheric Environment 29, 1393-1411. 459 Trebs, I., Metzger, S., Meixner, F.X., Helas, G., Hoffer, A., Rudich, Y., Falkovich, A.H., Moura, M.A.L., 460 da Silva, R.S., Jr., Artaxo, P., Slanina, J., Andreae, M.O., 2005. The The NH 4 + 

### -NO 3 − 

### -Cl − 

### -SO 4 2− 

### -H 2 

### O 461 aerosol system and its gas phase precursors at a pasture site in the Amazon Basin: How relevant are 462 mineral cations and soluble organic acids? Journal of Geophysical Research: Atmospheres 110, 463 D07303.Walker, J.T., Whitall, D.R., Robarge, W., Paerl, H.W., 2004. Ambient ammonia and ammonium 464 aerosol across a region of variable ammonia emission density. Atmospheric Environment 38, 1235-1246. 465 Yu, X., Lee, T., Ayres, B., Kreidenweis, S., Malm, W., Collett, J.L., 2006. Loss of fine particle 466 ammonium from denuded nylon filters. Atmospheric Environment 40, 4797-4807. 4673.02.01.00.01.61.20.80.40.01/1/077/1/071/1/087/1/081/1/097/1/091/1/107/1/101/1/117/1/111/1/12Date2.01.51.00.50.00.200.150.100.050.00HNO3(g)NO3-(p)NH3(g)NH4+(p)

Concentration (μg/m 3 )2007200820092010(a)(b)2011(c)SO42-K+ K + Concentration (μg/m 3 )Fig. 1. Temporal variations of concentrations of (a) HNO3and NO3-, (b) NH3and NH4+and (c)SO42-andK+from 2007 through 2011 at Boulder,Wyoming.1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0Concentration (μg/m3)

Spring Summer Fall Winter

Ca 2+ Mg 2+ K + NH 4 + SO 4 2- NO 3 -



### Fig. 2. Average mass concentrations of the chemical species in PM 2.5 

### by season across the 5 year sampling period.1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 HNO 3 (b) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0Concentration ( μg/m3) NH 3 (a)

2.0

1.5

1.0

0.5

0.0Concentration ( μg/m3) NH 4 + (c) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 NO 3 - (d)

2.5
2.0
1.5
1.0
0.5
0.0Concentration ( μg/m3) SO 4 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
0.0JanFebMarAprMayJunJulAugSepOctNovDec N(+V) (h) 2.0

1.5

1.0

0.5

0.0Concentration ( μgN/m3)JanFebMarAprMayJunJulAugSepOctNovDec N(-III) (g)



### Fig. 3. The monthly variation of (a) NH 3 

### , (b) HNO 3 

### , (c) NH 4 + 

### , (d) NO 3 - 

### , (e) SO 4 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/m 3 

### ; in panels (g) and (h) the concentrations are expressed as μgN/m 3



### .1.5

1.2

0.9

0.6

0.3

0.0NH3and HNO3Concentration ( ppbv )WinterSpringSummerFallWinterSpringSummerFallWinterSpringSummerFallWinterSpringSummerFallWinterSpringSummerFallWinter

NH 3 HNO 3 2007 2008 2009 2010 2011 2006



### Fig. 4. The seasonal variations of NH 3 

### and HNO 3 

### 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

0.0mol/molJanFebMarAprMayJunJulAugSepOctNovDec 

## F HNOx 

## F NHx



### Fig. 5. Monthly variation of the ammonia conversion ratio (F NH3 

### ) and nitric acid conversion ratio (F HNO3 

### ). See text for details.10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1[NH3][HNO3] [ppbv2]

4.0 3.9 3.8 3.7 3.6 3.5 3.4 1000/T(K -1 ) Theoretical equilibrium constant Winter Summer Fall Spring



### Fig. 6. Comparison of the measured [NH 3 

### (gas)][HNO 3 

### (gas)] product with the theoretical equilibrium constant for NH 4 

### NO 3 

### 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

NO3 SO4 NH4

HNO3 NH3 NO 3 - SO 4 2- NH 4 + HNO 3 NH 3 10) Palo Duro,TX

NO3 SO4 NH4 HNO3 NH3 NO 3 - SO 4 2- NH 4 + HNO 3 NH 3 11) Brush, CO 9) Cherokee Nation, OK NO3 SO4 NH4

HNO3 NH3 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 NO3 SO4 NH4

HNO3 NH3 16) Joshua Tree National Park,CA

6 4 2 0 NO3 SO4 NH4

HNO3 NH3 NO 3 - SO 4 2- NH 4 + HNO 3 NH 3 15) Gore Pass,CO

NO3 SO4 NH4 HNO3 NH3 NO 3 - SO 4 2- NH 4 + HNO 3 NH 3 14) Chiricahua National Monument,AZ

NO3 SO4 NH4 HNO3 NH3 NO 3 - SO 4 2- NH 4 + HNO 3 NH 3 12) Loveland, CO 6
4
2
0 NO3 SO4 NH4

HNO3 NH3 6) Boulder,WY

NO3 SO4 NH4 HNO3 NH3 5) NOAA Climate Station, WY

NO3 SO4 NH4

HNO3 NH3 3) Grand Targhee, WY

NO3 SO4 NH4 HNO3 NH3 4) Driggs, ID

NO3 SO4 NH4

HNO3 NH3 NO 3 - SO 4 2- NH 4 + HNO 3 NH 3 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 NH 4 + 

### and HNO 3 

### at Craters of the Moon National Monument, Idaho. Concentrations are in μg/m 3 

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

50

40

30

20

10

0NH4+(nmol/m3)

30 25 20 15 10 5 0 Spring y= 1.88x R 2 =0.58 2:1 60

50

40

30

20

10

0 30 25 20 15 10 5 0 Summer y= 2.37x R 2 =0.77 2:1

60

50

40

30

20

10

0NH4+(nmol/m3)

30 25 20 15 10 5 0 SO 4 2- [nmol/m 3 ] Fall y= 2.28x R 2 =0.83 2:1 60

50

40

30

20

10

0 30 25 20 15 10 5 0 SO 4 2- [nmol/m 3 ] Winter y= 3.27x R 2 =0.15 2:1



### Fig. 8. Seasonal relationships of NH 4 + 

### versus SO 4 2- 

### concentrations.70

60

50

40

30

20

10

0[NO2-]+[NO3-]+2*[SO42-][103mol/m3]

70 60 50 40 30 20 10 0 1:1 (a)Spring
y = 0.95 x R 2 = 0.62 70

60

50

40

30

20

10

0 70 60 50 40 30 20 10 0 1:1 (b)Summer
y = 0.71 x R 2 =0.72

70

60

50

40

30

20

10

0[NO2-]+[NO3-]+2*[SO42-][103mol/m3]

70 60 50 40 30 20 10 0 [10 3 mol/m 3 ] [NH 4 + ]+[Na + ]+[K + ]+2*[Mg 2+ ]+2*[Ca 2+ ] 1:1 (c)Fall
y = 0.79 x R 2 = 0.80 70

60

50

40

30

20

10

0 70 60 50 40 30 20 10 0 [10 3 mol/m 3 ] [NH 4 + ]+[Na + ]+[K + ]+2*[Mg 2+ ]+2*[Ca 2+ ] 1:1 (d)Winter
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.

### • 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

# 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 NH 3

μg/m 3 HNO 3 μg/m 3 NO 3 - μg/m 3 SO 4 2- μg/m 3 NH 4 + μg/m 3 K + μg/m 3 Mg 2+ μg/m 3 Ca 2+ μg/m 3 T

oC RH % WS m/s Mean 0.14 0.13 0.18 0.65 0.23 0.01 0.02 0.04 4.80 56.76 5.24 Spring (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

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

Mean 0.18 0.18 0.12 0.51 0.22 0.02 0.01 0.05 7.17 54.94 4.12 Autumn (N=87) SD 0.14 0.11 0.07 0.23 0.10 0.01 0.00 a 0.03 5.37 14.07 1.09

Mean 0.04 0.19 0.60 0.36 0.28 0.01 0.00 b 0.02 -7.76 72.54 3.85 Winter (N=212) SD 0.06 0.18 0.60 0.23 0.17 0.01 0.00 c 0.03 5.45 8.67 1.76

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

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

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

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

### Table S2. Correlation coefficients (r) between concentrations of trace gases, particulate species

and meteorological parameters based on all the data.

Species NH 3 HNO 3 NO 3 - SO 4 2- NH 4 + K + Ca 2+ RH WS T NH 3 1 0.23 -0.30 0.32 0.06 0.40 0.34 -0.53 0.01 0.72 HNO 3 1 0.43 0.16 0.47 0.29 0.10 -0.19 -0.31 0.12 NO 3 - 1 -0.08 0.63 -0.03 -0.19 0.44 -0.38 -0.54 SO 4 2- 1 0.45 0.35 0.46 -0.33 -0.04 0.33 NH 4 + 1 0.28 0.14 0.07 -0.36 -0.11 K + 1 0.32 -0.46 -0.05 0.41 Ca 2+ 1 -0.46 0.04 0.41



### Table S3. Site information, data sources and sampling period for Fig. 5. Site State HNO 3 NH 4 + NO 3 - SO 4 2- NH 3 Source Period Source Period Source Period Chiricahua NM AZ CASTNET a 01/02/2007~01/03/2012 IMPROVE b 01/03/2007~12/31/2010 AMON c 03/15/2011~01/31/2012 Joshua Tree NP d 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 NM e 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 05/04/2011~09/21/2011Center 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) NO x 

### and b) NH 3 

### from the National Emissions Inventory 2008 (http://www.epa.gov/ttnchie1/net/2008inventory.html). The sampling site is indicated by the (+) signs.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 (r 2 

### ) between concentrations of NH 3 

### and K + 

### was 0.33, 0.40, 0.02, 0.16 and 0.03.16

14

12
10
8

6

4

2
0Excess NH4+(nmol/m3)

16 14 12 10 8 6 4 2 0 Spring y= 1.02x R 2 =0.07 1:1 16

14

12
10
8

6

4

2
0 16 14 12 10 8 6 4 2 0 Summer y= 1.44x R 2 =0.02 1:1

16

14

12
10
8

6

4

2
0Excess NH4+(nmol/m3)

16 14 12 10 8 6 4 2 0 NO 3 - [nmol/m 3 ] Fall y= 1.01x R 2 =0.31 1:1 60

50

40

30

20

10

0 60 50 40 30 20 10 0 NO 3 - [nmol/m 3 ] Winter y= 0.81x R 2 =0.76 1:1



### Fig. S3. Seasonal relationships of excess NH 4 + 

### versus NO 3 - 

### concentrations.