Seasonal ambient ammonia and ammonium concentrations in a pilot IMPROVE NHx monitoring network in the western United States

Atmospheric Environment 91 (2014) 118e126

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Seasonal ambient ammonia and ammonium concentrations in a pilot IMPROVE NHx monitoring network in the western United States Xi Chen a,1, Derek Day b, Bret Schichtel b, William Malm b, Ashleigh K. Matzoll c, Jose Mojica c, Charles E. McDade c, Eva D. Hardison d, David L. Hardison d, Steven Walters d, 2, Mark Van De Water c, Jeffrey L. Collett Jr. a, * a

Atmospheric Science Department, Colorado State University, Fort Collins, CO 80523, USA NPS/CIRA, Colorado State University, Fort Collins, CO 80523, USA Crocker Nuclear Laboratory, University of California, Davis, CA 95616, USA d Environmental Chemistry Department, RTI International, RTP, NC 27709, USA b c

h i g h l i g h t s
A pilot sub-network within IMPROVE was initiated for NHx (NH3 and NHþ 4 ).
Concentrations of ambient NHx in the Rocky Mountain region are investigated.
NHx was collected onto acid impregnated filters.
Temporal and spatial patterns of NHx concentrations are observed.
The role of wildfires as ammonia/ammonium sources is explored.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 November 2013 Received in revised form 24 March 2014 Accepted 27 March 2014 Available online 27 March 2014

Ammonia and ammonium are important atmospheric trace constituents that affect particulate matter concentrations and contribute to reactive nitrogen deposition. We refer to and measure the sum of ammonia and ammonium as NHx. To better understand concentrations of NHx in remote areas, the Interagency Monitoring of Protected Visual Environments (IMPROVE) fine particulate matter (<2.5 mm) sampler was modified to measure NHx at a subset of locations in the routine IMPROVE network. To sample NHx, an additional IMPROVE PM2.5 sampler was installed. Samples were collected on phosphorous acid impregnated cellulose filters held in polypropylene filter holders. While the standard IMPROVE filter holder is made of Delrin (polyoxymethylene, POM), reactions between collected NHx and formaldehyde released by phosphorous acid degradation of the POM holder produced substantial amounts of artifact methylamine, especially during warm sampling periods. This artifact did not occur with the new polypropylene holder design, and no methylamine was measured above the method detection limit of 0.003 mg/m3. Samples collected using the new IMPROVE NHx sampling system were evaluated against samples collected with a collocated URG annular denuder/filter-pack module for 6 weeks; the observed bias was 7%. The NHx monitors were deployed at a total of nine sites in the U.S. Rocky Mountain and Great Plains regions and to the east, and at Bondville, Illinois, from April 2011 to August 2012. Collocated samplers at Rocky Mountain National Park, Colorado, and Bondville, Illinois, demonstrated excellent measurement precision. The data revealed a pattern of increasing NHx concentrations in late spring/early summer (June) and a decrease in winter, starting in September for most of the sites. This pattern is consistent with expected seasonal patterns in agricultural emissions of ammonia. Sites closer to agricultural sources at Bondville and Cedar Bluff (Kansas), however, still exhibit quite abundant winter NHx, which may reflect continued local agricultural emissions trapped within a

Keywords: Ammonia Ammonium NHx IMPROVE Methylamine

* Corresponding author. E-mail address: [email protected] (J.L. Collett). 1 Now at National Risk Management Research Laboratory, Environmental Protection Agency, Research Triangle Park, NC 27711, USA. 2 Now at Division of Air Quality, North Carolina Department of Environmental and Natural Resources, Raleigh, NC 27699, USA. http://dx.doi.org/10.1016/j.atmosenv.2014.03.058 1352-2310/Ó 2014 Elsevier Ltd. All rights reserved.

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shallower winter boundary layer. A probable impact of wildfires on NHx concentrations was observed for Bandelier NM, Chiricahua NM, and Yellowstone NP during summer/fall 2011. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Among all reactive nitrogen-bearing species, ammonia (NH3) is the most abundant basic gas in the atmosphere. The primary sources for atmospheric NH3 are agricultural emissions such as livestock and nitrogen fertilizer application (Bouwman et al., 1997; Galloway et al., 2004). Biomass burning can also be an important source of ammonia emissions (Akagi et al., 2011; Bouwman et al., 1997; Goode et al., 2000; Yokelson et al., 1997), although the strength of this source remains poorly quantified. Due to its acid neutralizing capacity, NH3 can react with acidic species (e.g., H2SO4, HNO3, HCl, and oxalic acid) to form secondary, particle phase nitrogen-containing compounds. The degree of neutralization by NH3 typically determines the acidity of the ambient sulfate aerosol, which is important to aerosol hygroscopicity and visibility, human health effects, and the earth’s radiation budget. The deposition of NH3 and particulate ammonium in natural or semi-natural ecosystems may cause eutrophication, soil acidification and loss of biodiversity (Bergström and Jansson, 2006; Bouwman et al., 2002; Stevens et al., 2004). Horvath (1992) shows that the light extinction coefficient for 0.5 mm diameter (NH4)2SO4 is approximately 1.5 times that for H2SO4 over the entire visible light range, and that NH4NO3 formed by reaction of gaseous ammonia and gaseous nitric acid scatters visible light even more efficiently, which degrades visibility. Good visibility is one of the most desirable features in many national parks and other class I areas. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program has been measuring light scattering aerosols since 1988 in many of these federally protected areas (Malm et al., 1994, 2004). IMPROVE sites feature routine measurements of PM2.5 mass, elements by XRF, inorganic anions, and organic and elemental carbon, but no direct cation monitoring (Malm et al., 1994). Despite the importance of gaseous ammonia in forming atmospheric aerosols and contributing to nitrogen deposition and visibility degradation, ammonia and ammonium are not regularly measured with the time resolution and accuracy needed to fully address these issues. Measuring ambient ammonia and ammonium concentrations with the desired accuracy and low background contamination can be challenging. NH4NO3 is a semi-volatile substance and is readily volatilized from substrates such as Teflon filters. Nylon filters will retain nitrate volatilized as nitric acid, but any ammonium volatilized as ammonia is generally lost (Babich et al., 2000; Yu et al., 2005, 2006). When accurate measurement of particulate ammonium is required, volatilized ammonium is often recaptured by a back-up, acid-coated denuder or filter (Lee et al., 2008; Yu et al., 2005, 2006). Ubiquitous sources of NH3 in a lab, e.g. human perspiration and breath, can easily contaminate exposed sampling media during preparation for field deployment; additional precautions are needed to prevent such contamination (Cheng and Tsai, 1997; Sakurai et al., 2005). Within the IMPROVE network, PM2.5 samples analyzed for ionic composition are collected on nylon filters. Hence, ammonium measurements, which are not routinely performed, likely underrepresent true ambient concentrations when ammonium nitrate is an important component of local aerosols. PM2.5 ammonium is measured in the EPA Chemical Speciation Trend Network (CSN) and Clean Air Status and Trends Network

(CASTNet), but these observations suffer from a negative bias due to ammonium nitrate volatilization as illustrated by the testing of Yu et al. (2005), and CSN does not currently measure gaseous ammonia. Ammonia is measured in the National Atmospheric Deposition Program (NADP) Ammonia Monitoring Network (AMON) network using passive diffusion devices, but at bi-weekly duration which limits its utility for source apportionment, modeling, and visibility studies. Also, AMON does not measure particulate ammonium. The South Eastern Aerosol Research and Characterization (SEARCH) Network is successfully measuring gas/ particle ammonia/ammonium by using denuder/filter assembly samplers. However, the network is limited only to the southeastern region of the U.S. and its operation is relatively resource and labor intensive. To address the limitations to current monitoring approaches and coverage, we investigated simple modifications to the IMPROVE sampling system to measure NHx. A single acid-coated filter sampling approach was used to simultaneously capture ammonia plus ammonium to provide an accurate and precise measurement of NHx at low cost. An overview of spatial and seasonal NHx concentration variability is provided across nine IMPROVE sites in the Rocky Mountains, Great Plains and Midwestern United States. 2. Experimental description Samplers for NHx measurements were adapted from the IMPROVE PM2.5 module B sampler that collects PM2.5 samples on a Nylon filter. Multiple sampler and filter medium configurations were tested and are described in the supporting information. The final configuration, which yielded accurate and precise measurements of NHx, was deployed in a pilot network at select IMPROVE sites and is further described below. The modified IMPROVE sampler module used the standard 6-ft aluminum inlet with a PM2.5 cyclone. The Nylon filter was replaced with a phosphorous acid impregnated cellulose fiber filter. The IMPROVE filter holder cassette is normally made from Delrin (polyoxymethylene, POM). However, it was found that phosphorous acid could degrade the POM, releasing formaldehyde which then reacted with collected NHx on the filter to form artifact methylamine, especially during hot sampling periods. The filter holder material was changed to polypropylene, which eliminated the artifact (see the Supporting Information). The ion chromatograms were closely monitored for signs of methylamine (MDL of methylamine is 0.003 mg/m3) or any other sampling artifact. The filter cartridges were also carefully monitored for signs of acid etching. No distinguishable acid etching or deformity development were discovered on cartridges made from Polypropylene even when used repeatedly up to 6 months, indicating the compatibility of Polypropylene for this NHx monitoring approach. 2.1. Filter preparation and sample analysis 37 mm cellulose fiber filters (225-18A, SKC Inc., Eight Four, PA) were used for sampling NHx (gaseous NH3 plus PM2.5 NHþ 4 ). The filter impregnating solution was 3% phosphorous acid, which was prepared from 15 g H3PO3 (99%, SigmaeAldrich Corp. St. Louis, MO) dissolved in 50 ml deionized water (DI, Barnstead Ultrapure) and 450 ml methanol (ACS grade, Fisher Scientific Inc., Pittsburgh, PA).

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Prior to impregnating, the filters were washed once with the acid impregnating solution followed by at least 3 washings with DI water to remove any soluble impurities associated with the filters. Filters were washed in a PTFE container in an ultrasonic bath for 30 min each time. Washed filters were impregnated using phosphorous acid solution in a sonicator for 30 min after which they were placed in half of a clean Petri dish for drying. The adapted drying desiccator was directly connected to ultra-pure nitrogen gas to minimize exposure to laboratory ammonia. Dried cellulose filters were kept in sealed Petri dishes in sealed plastic bags (Ziploc) in the refrigerator until use in local method testing or shipment to field sites. In order to maintain low ammonia background, a paper towel soaked with the phosphorous acid solution was inserted in the storage bag as an ammonia scrubber. All filter preparation was completed at Colorado State University (CSU) in Fort Collins, Colorado. During method evaluation, test filters were extracted and analyzed by ion chromatography at CSU. Filters deployed at network sampling sites were shipped directly from the field to the IMPROVE ion analysis lab at RTI International (RTI) from the field for analysis. Acid impregnated cellulose filters were loaded and unloaded from sampler cartridges in a laminar flow hood (CSU) with ammonia scrubbing by spraying 15% citric acid on the pre-filter or in a nitrogen-atmosphere rigid glovebox (RTI) to reduce potential background contamination. 2.2. NHx filter analysis The exposed cellulose filters were extracted using DI water and 45 min of sonication according to the standard IMPROVE protocol. Ammonia/ammonium concentrations at CSU were analyzed using ion chromatography with a Dionex CS12A separation column and cation self-regenerating suppressor (CSRS); separations were conducted using 20 mM methanesulfonic acid (MSA) as eluent at a flow rate of 0.5 ml/min. The phosphorous acid impregnated cellulose filter extracts were analyzed at RTI on a Dionex Model ICS-3000 ion chromatograph equipped with a 2-mm IonPac CS18 cation separator column, a CSRS, and a 25-mL fixed- volume sample loop. The samples were eluted using an MSA eluent gradient at a flow rate of 0.3 ml/min. The MSA concentration was programmed to start at 0.5 mM MSA with a gradient to 1.0 mM at 25 min, 6.0 mM at 27.4 min and 8.0 mM at 33 min, followed by a step back to 0.5 mM. The total run time was 40 min at an elevated column temperature of 35  C. Analyses performed using these conditions resulted in baseline separation of the ammonium and methylamine peaks.

calculated as the sum of NHx from the primary NH3 denuder (gas phase NH3), nylon filter (particle phase NHþ 4 with some volatilization loss), and back-up NH3 denuder (volatilized ammonium). 3 Fig. 1a shows collected NHx (reported as NHþ 4 in mg/m throughout the entire paper) using the IMPROVE NHx module and the URG sampler. The two systems agree well. The pooled relative standard deviation of all IMPROVE NHx-URG sample pairs was 5%. A least squares linear regression fit of IMPROVE NHx vs. URG NHx suggests a slight low bias (7%) for the IMPROVE measurement and small positive intercept (0.25 mg/m3). This comparison indicates that the NHx measurement from the IMPROVE module is accurate to within better than 10% of the reference URG measurement. A comparison of collocated IMPROVE NHx module sampling at two network sites, Rocky Mountain National Park, Colorado, and Bondville, Illinois, demonstrates the excellent precision of the method (see Fig. 2). The squared correlation coefficient exceeds 0.97 at both sites and the slope is not distinguishable from 1.0. The PM2.5 ammonium concentration from the IMPROVE Module B Nylon filter was also measured during system testing and during pilot NHx network operation. The comparison of the IMPROVE to URG PM2.5 ammonium in Fort Collins is presented in Fig. 1b. As shown, there is high correlation with r2 ¼ 0.89 but NHþ 4 collected by IMPROVE nylon filters is systematically low by about 10%, evidence of expected loss of NHþ 4 from volatilization of ammonium nitrate collected on the IMPROVE nylon filter. Due to the variability of temperature and aerosol composition across space and time, these results from Fort Collins are not necessarily representative for other locations or time periods. The measured IMPROVE module B ammonium concentration is viewed as a lower bound on the true ammonium concentration in the ambient aerosol collected. Ammonium analysis of the collocated IMPROVE module B nylon filter provides one option to place a lower bound on the NHx fraction comprised by ammonium. Ammonium can also be independently estimated by assuming that all measured sulfate and nitrate at the IMPROVE sites are in the form of ammonium sulfate and ammonium nitrate. This provides a reasonable upper bound estimate on the ammonium concentrations in many cases. At some locations sulfate may only be partially neutralized by ammonium, while Lee et al. (2008) have shown the importance of supermicron particle nitrate and sulfate salts with non-ammonium cations at IMPROVE sites influenced by reacted sea salt or soil dust. At these locations the preceding approach would overestimate PM2.5 ammonium. At some locations and seasons, ammonium can exist in salt form with organic acids (e.g. oxalate as observed by Malm et al. (2005)); this ammonium would not be accounted for in the “upper bound” estimate derived from nitrate and sulfate concentrations.

2.3. Method evaluation 3. NHx pilot monitoring study Before deploying the adapted IMPROVE modules in the field to collect NHx, comparisons were conducted in MarcheApril 2010 at Fort Collins, CO, to examine method precision and accuracy. Additional testing was performed in JulyeAugust 2012 to confirm the spring 2010 results. The NHx sampling module accuracy was evaluated by comparison against a collocated URG annular denuder/filter-pack sampler, which has been extensively used and well-documented for sampling of ammonia and ammonium (Lee et al., 2008; Yu et al., 2005, 2006). Both the IMPROVE NHx and URG samplers utilized a 2.5 mm size cut cyclone inlet and samples were 24 h in duration. The URG sampling train consisted of, in order, a nitric acid denuder, primary ammonia denuder, nylon (Pall Nylasorb) filter, and a back-up ammonia denuder to capture any PM2.5 ammonium volatilized as ammonia. The URG denuder coating and drying procedures have been described in detail elsewhere (Yu et al., 2005). Total NHx collected by the URG sampling system was

A pilot study was conducted from April 21, 2011eAugust 31, 2012 to test the new NHx system within the routine IMPROVE monitoring network and to measure NHx concentrations at nine IMPROVE sites over a full year plus an additional spring/summer season when ammonia emissions are expected to peak. As shown in Table 1, six of the pilot network sites are located in the Rocky Mountain region, from Chiricahua National Monument (NM), Arizona, in the south to Glacier National Park (NP), Montana, in the north, a region with large relative increases in wet deposited ammonium over the past 20 years (Lehmann et al., 2007). To the east, NHx modules were installed at Wind Cave, South Dakota, and at Cedar Bluff, Kansas, which is located near intensive agricultural activities. A NHx monitor was also installed at the Bondville, Illinois, IMPROVE site, a more polluted site where several ammonia/ ammonium measurement campaigns have previously been conducted (e.g., Blanchard and Tanenbaum, 2005; Heald et al., 2012;

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Fig. 1. IMPROVE NHx sampler comparison with URG denuder/filter pack sampler. a) total NHx collected using IMPROVE acid-impregnated cellulose filters; b) IMPROVE nylon filter comparison with URG collected particulate NHþ 4.

Fig. 2. NHx collected by collocated acid-impregnated cellulose filter samplers at Rocky Mountain National Park (ROMOS) and Bondville (BONDS).

Lee et al., 2008; Sweet et al., 2005) and where high ammonia concentrations have been previously measured. Seven of the nine sites were collocated with or located nearby an NADP wet deposition and CASTNet monitoring sites. Bandelier had no collocated CASTNet site and Cedar Bluff had neither an NADP nor CASTNet site collocated. NADP observations provide measurements of ammonium wet deposition while the CASTNet observations can be used to estimate dry deposition of some inorganic sulfur and nitrogen compounds. Table 1 indicates whether IMPROVE NHx sites are collocated with either NADP or CASTNet monitoring sites. NHx monitoring was conducted using the same 1-in-3 day sampling period and 24-h duration used in the routine IMPROVE network sampling. However, to minimize the potential for contamination, the NHx acid-impregnated filters were mailed directly from CSU to the field and directly from the field to RTI for extraction and analysis. This differs from the protocol for routine IMPROVE filters, which are prepared at the Crocker Nuclear Laboratory at the University of California at Davis (UCD), shipped to the

field, shipped back to UCD, and then either analyzed locally or reshipped to an analytical lab (e.g., nylon filters to RTI for ions). To evaluate the extent of any contamination during shipping and handling, field blanks were collected every week. These were mounted in an unused position in the IMPROVE NHx module filter cassette holder and were prepared, shipped, extracted, and analyzed in the same way as actual sample filters. 3.1. Results and discussion NHx field blanks collected from April 2011 to August 2012 had very low NHx concentrations, with an average of only 0.026  0.006 mg/m3 (method detection limit of 0.018 mg/m3), indicating that there was minimal contamination from filter handling and shipping. There was also little site-to-site variation in blank values. Monthly average concentrations for all IMPROVE NHx sites from April 2011 to August 2012 are shown in Fig. 3. Overall NHx concentrations observed for this period were lowest at

Table 1 IMPROVE NHx sampling site information and locations. Site name

Site code

Latitude (oN)

Longitude (oW)

Elevation (m)

NADP

CASTNeT

Bandelier National Monument, NM Bondville, IL Cedar Bluff, KS Chiricahua National Monument, AZ Glacier National Park, MT Mesa Verde National Park, CO Rocky Mountain National Park, CO Wind Cave, SD Yellowstone National Park, WY

BANDS BONDS CEBLS CHIRS GLACS MEVES ROMOS WICAS YELLS

35.7797 40.0520 38.7701 32.0094 48.5105 37.1984 40.2783 43.5576 44.5653

106.2664 88.3733 99.7634 109.3890 113.9966 108.4907 105.5457 103.4838 110.4002

1988 263 665 1554 975 2172 2760 1296 2425

YES YES NO YES YES YES YES YES YES

NO YES NO YES YES YES YES YES YES

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Fig. 3. Monthly NHx average concentrations at IMPROVE NHx sites from April 2011 to August 2012 (Error bars indicate standard deviation of each month; * in the figures denotes no data available for that particular month; Shading of GLACS NHx concentration bars indicating possible influence from horse stable in the close vicinity of IMPROVE samplers).

Yellowstone NP and highest at Cedar Bluffs and Bondville. Across the sites, the NHx concentrations were generally higher at sites east of the Rocky Mountains. These sites are closer to agricultural activities where ammonia emissions are higher (an ammonia emission inventory map of the continental US is included in the Supporting Information). Monthly average NHx concentrations at Yellowstone NP, Rocky Mountain NP, and Mesa Verde NP for the entire sampling period did not exceed 1 mg/m3; while concentrations at Glacier NP, Wind Cave, and Bandelier NM were below 2 mg/ m3. The Glacier NP monitoring site was located near a horse stable used in the summer, which may have contributed to elevated summer NHx concentrations at that monitoring site. Monthly average NHx concentrations at Bondville and Cedar Bluff were generally well above 1 mg/m3 and reached up to 4 mg/m3. High NHx concentrations were also observed during summer 2011 at Chiricahua NM, with a July average NHx concentration of 4 mg/m3. Such high concentrations were not observed at Chiricahua NM during summer 2012 when the average July NHx concentration was 2 mg/ m3. Bondville and Cedar Bluff are both located in regions of intensive agriculture, so it is not surprising the NHx concentrations are highest at these locations. Chiricahua NM is, by contrast, in a fairly remote region without much local agriculture. As discussed below, it appears that the high summer 2011 NHx concentrations measured there are at least partly due to nearby wildfires. More detailed time series of measured NHx, NHþ 4 (measured on the IMPROVE module B filter) and estimated NHþ 4est (by assuming ammonium is present at a level to form ammonium sulfate and

ammonium nitrate by pairing with IMPROVE module B sulfate and nitrate concentrations) are presented in Fig. 4a and b. Annual averages are shown in Fig. 4c. As outlined above, the NHþ 4 measured using IMPROVE Module B represents a lower bound and NHþ 4est represents an estimated upper bound to the actual PM2.5 NHþ 4 concentration. As expected, the NHx concentration consistently equals or exceeds the upper bound NHþ 4est concentration which consistently equals or exceeds the lower bound measured NHþ 4 concentration. At most sites, NHx concentrations increase in late spring/early summer (June) and then decrease into winter, typically starting in September. This pattern is consistent with increases in ammonia emissions and concentrations during warmer times of the year observed elsewhere in North America (Yao and Zhang, 2013; Benedict et al., 2013; Li et al., 2013). For some high elevation sites, significant pollutant transport from lower altitude source regions is rare in winter as a shallower mixed layer tends to trap ground-based emissions near the surface. Benedict et al. (2013) show and discuss this strong pattern for ammonia, ammonium, and other species, for example, at Rocky Mountain NP. Interestingly, both Bondville and Cedar Bluff exhibit a different seasonal pattern for NHx. While summer NHx is highest at these locations, they also exhibit quite abundant winter NHx, which may reflect continued agricultural emissions (including emissions from fall/winter fertilization) trapped within a shallower winter boundary layer. The lower bound estimate for particulate NHþ 4 did not show much of a seasonal pattern across the year. During winter at most

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þ þ þ Fig. 4. (a) Time series of NHx, NHþ 4 and NH4est concentrations for sites exhibiting summer NHx peaks; (b) Time series of NHx, NH4 and NH4est concentrations for sites exhibiting þ þ both summer and winter NHx peaks; (c) Annual NHx, NHþ and NH average concentrations (calculated for duration June 2011 to May 2012). (NHþ 4 4est 4 and NH4est refer to ammonium measured from the IMPROVE module B nylon filter and estimated from IMPROVE module B sulfate and nitrate concentrations assuming ammonium is present as ammonium sulfate and ammonium nitrate, respectively.)

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sites, the NHþ 4 and NHx concentrations are rather similar, suggesting that most of the NHx is in particulate form. This is not unexpected, since ammonium nitrate formation is favored at low temperature and reduced ammonia emissions limit the amount of excess ammonia, beyond that needed to neutralize fine particle nitrate and sulfate, in the atmosphere. This issue has been examined in some detail, for example, by Li et al. (2013) for a site in western Wyoming. During summer as NHx rises, the lower bound NHþ 4 estimate falls well below measured NHx. Again, this is not unexpected, since ammonia emissions increase during this warmer time of year and ammonium nitrate is favored to partition back to the gas phase as ammonia and nitric acid when temperatures climb. It is also possible, however, that the negative bias in the lower bound NHþ 4 estimates is higher in summer when high afternoon temperatures are more likely to lead to volatilization of ammonium nitrate collected on the Module B nylon filter during the cooler initial part of the sampling period (IMPROVE sample collection runs from midnight to midnight). Some insight into this possibility can be gained by examining the “upper bound” NHþ 4 (as denoted as NHþ 4est ) estimate. Upper bound ammonium estimated from measured sulfate and nitrate concentrations closely tracked the measured NHþ 4 from nylon filters but with an average upscaling factor of 1.37  0.21. Moreover, during warmer seasons the magnitude of the up-scaling for upper bound to lower bound ammonium was indeed slightly higher than that during cooler seasons, consistent with increased volatilization of ammonium nitrate under warmer summer conditions. 3.2. Fire impact on NHx Biomass burnings events, including wild and prescribed fires, can significantly contribute to NH3 emissions when the burning fuel has high nitrogen content and smoldering combustion prevails, but its contributions remain poorly quantified (Akagi et al., 2011; Yokelson et al., 1997; Goode et al., 2000; Lee et al., 2008). Fig. 5 shows measured NHx and NHþ 4 concentrations along with elemental and organic carbon (EC and OC) concentrations and Kþ

concentrations for three sites which might have been impacted by wildfires during summer/fall 2011. Potassium ion concentrations were measured in the cation analysis along with ammonium in the IMPROVE module B nylon filter extracts. þ At Chiricahua NM, NHx, NHþ 4 , EC, OC, and K concentrations were all elevated on June 5 and June 11, 2011, with OC surging up to approximately 32 mg/m3. Two large fires burned during the summer of 2011 in this region of Arizona: the Horseshoe II and Wallow fires. The Horseshoe II fire, which burned in an area within 50 km from the Chiricahua NM IMPROVE sampling site, began on May 8, 2011 and was contained on June 25, 2011. The Wallow fire, the biggest 2011 fire recorded in Arizona (2180 km2 burn area), was active from May 29, 2011 to July 8, 2011. In this remote area, such high OC concentrations are typically a clear indicator of fire impact. The simultaneous increase in Kþ confirms a biomass burning source. The fact that the Kþ concentrations were only modestly elevated while quite abundant NHx was observed might suggest a strong influence from smoldering combustion on the high OC samples collected at Chiricahua NM. Lee et al. (2010) showed that potassium emissions are much stronger in the flaming phase of fires. Flaming combustion converts C, H, N and S in the burning fuel into highly oxidized gases CO2, H2O, NOx and SO2, respectively. Smoldering combustion, which involves processes of glowing and pyrolysis, produces more CO, CH4 and NH3. Several investigators demonstrated that ammonia emissions are favored under smoldering conditions vs. greater NOx emissions from flaming fires (Akagi et al., 2011; Goode et al., 2000; McMeeking et al., 2009; Yokelson et al., 1997). Ratios of EC/TC (total carbon as EC þ OC) during these fire episodes were well below 0.2, which agrees well with those reported by McMeeking et al. (2009) and references therein for smoldering fire conditions. Measured OC concentrations showed another less pronounced peak after June 11, 2011, while concentrations of other species, except NHx, dropped back to nearbackground levels. NHx concentrations measured at Chiricahua NM were continuously above 2 mg/m3 into August 2011. This prolonged summer period of high NHx, with additional high NHx concentrations also measured at Chiricahua NM in JulyeAugust 2012,

Fig. 5. Observed impact on NHx from fire incidents at CHIRS, BAND and YELLS 2011.

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suggests a possible contribution of sources other than wildfires to NHx concentrations in the region. This possibility will be explored in a future source contribution modeling analysis. The Bandelier NM, New Mexico site also appeared to be impacted by smoke in summer 2011. There were several missing samples due to either power outage or filter clogging caused by heavy smoke. Two fire-related NHx episodes were observed based on elevated concentrations of NHx along with EC, OC, and Kþ (Fig. 5). The first episode covered the period from early June until mid-June; the second episode was observed beginning on June 26 and continuing impact was observed until early August 2011. This timeline closely coincides with the period of the Las Conchas Fire. The Las Conchas Fire was the largest wildfire in New Mexico state history, burning more than 600 km2. The fire started in the Santa Fe National Forest on June 26, 2011, and was fully contained by August 3, 2011. Similar to Chiricahua NM, slightly elevated Kþ and pronounced NHx concentrations at Bandelier NM are suggestive of impacts from smoldering fire emissions. Ratios of EC/TC were also well below 0.2, supporting the hypothesis of smolderingdominated fire emissions (McMeeking et al., 2009). MODIS satellite fire incidents maps were retrieved from the NASA Fire Information for Resource Management System (FIRMS) (http:// earthdata.nasa.gov/data/near-real-time-data/firms). Fire incident maps from FIRMS confirmed the Bandelier and Chiricahua sites being in quite close vicinity of the particular fire incidents described above. However, as at Chiricahua NM, NHx concentrations above 1 mg/m3 continue to be observed when OC is low, suggestive of significant non-fire sources of ammonia. One possibility is that applied fire retardants, which can contain substantial amounts of ammonium-based fertilizer, may continue to release ammonia after a fire subsides. At Yellowstone NP, there were several episodes of elevated OC from late summer to fall 2011. However, based on fire incidents reported by MODIS, no incidents were detected around Yellowstone NP sampling site for the aforementioned period. One possibility is that the OC originated from more distant regional fires whose smoke was transported to Yellowstone. The opposite pattern observed (compare to Bandelier NM and Chiricahua NM) as high OC/EC/Kþ but only slightly elevated NHx/NHþ 4 suggests that the possible regional smoke emissions had little NHx or that the NHx was lost from the smoke during transport due to its high deposition velocity compared to particulate matter. 3.3. Atmospheric methylamine Methylamine, with molecular formula CH3NH2, one of the most common and abundant amines in the atmosphere, has both anthropogenic and natural sources. Methylamine sources include animal feedlots where anaerobic biodegradation of N-containing organic matter occurs; the food industry such as fish processing; metabolism of marine organisms; and biomass burning (Ge et al., 2011; Schade and Crutzen, 1995). Concentrations of gas phase methylamine have been detected as high as 5.3 mg/m3 inside livestock facilities (Kallinger and Niessner, 1999). Methylamine, like ammonia, is basic in nature, and it is subject to acidebase reactions and to photochemical degradation (Atkinson et al., 1977; Carl and Crowley, 1998; Ge et al., 2011). The lifetime of methylamine with respect to OH radical attack is on the order of hours (Ge et al., 2011); hence ambient concentrations of methylamine away from agriculture activities have seldom been found to be significant, usually in the magnitude of ng/m3 (Ge et al., 2011). That is consistent with our observations here. Methylamine concentrations in the NHx samples did not exceed their detection limit of 0.003 mg/m3 at any time or location during the sampling period with methylamine monitored.

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4. Summary and conclusions A pilot NHx sub-network within the IMPROVE network was initiated in 2010 to investigate concentrations of ammonia and ammonium in the Rocky Mountain and Great Plains regions of the western United States. Observations from the effort can also help define ammonia partitioning between the gas and particle phase, better constrain fine particle impacts on visibility, and determine sources and contributions of ammonia to excess nitrogen deposition in Rocky Mountain ecosystems. In this project, studies were carried out to develop and optimize an accurate, practical, and reliable sampler configuration which could be easily deployed within the IMPROVE monitoring network while providing reliable measurements of NHx at relatively low cost. Sample media preparation, shipping, and handling procedures were developed and examined in order to minimize background contamination. These included careful procedures for impregnated filter preparation and extraction in the laboratory, as well as the inclusion of an ammonia scrubber in sample shipments. The tested method yielded good accuracy, high precision and low detection limits. A seasonal pattern featuring an increase of NHx concentrations in late spring/early summer, followed by a decrease starting in the fall and dropping to lowest values in winter, was observed for all remote western sites in the Rocky Mountain region that were included in this monitoring effort. Such a pattern is consistent with enhanced ammonia emissions from agricultural and natural sources as temperatures rise. A similar seasonal NHx concentration pattern was observed at sites located further east in agricultural regions (Bondville and Cedar Bluff), except that these sites also experienced a secondary winter concentration maximum. Winter fertilization and continued agricultural activities could contribute to winter ammonia emissions in these regions, and, combined with a shallower winter boundary layer, offer a possible explanation for the high winter NHx observed at these two sites. Elevated NHx episodes which were accompanied by co-elevated EC and OC concentrations are postulated to be associated with several recorded fire incidents at Bandelier NM, Chiricahua NM and Yellowstone NP. The observed high NHx concentrations during periods of wildfire impact suggest an important and often underemphasized role of fires as ammonia/ammonium sources. The continuation of high NHx concentrations in these regions long after smoke particles (as represented by OC concentrations) subsided, suggest a need to examine whether the application of large amounts of ammonium-based fertilizers as major components of fire retardants are an important post-fire source of ammonia emissions. An absence of observable (>0.003 mg/m3) methylamine concentrations in the NHx samples suggests that it is not an important contributor to gas or particle phase reduced nitrogen in the study region, at least on a mass basis. This is an interesting finding, considering the diverse monitoring sites influenced by agricultural activities and smoke from wildfires. This may reflect low emissions (compared to ammonia) and/or rapid loss due to photochemical degradation during transport from sources to the monitoring sites. Initial network observations of substantial methylamine (see Supplement) were traced to an artifact produced by reaction of collected NHx with formaldehyde produced from acid degradation of the standard IMPROVE Delrin filter holder, emphasizing the need for careful validation of sampling methods under field-relevant conditions. Caution should be used when considering other observations of amines taken using Delrin filter holders, even when acid impregnated filter media are not involved, especially under sampling conditions with significant existing acidic species in the atmosphere such as in the vicinity of volcanic eruptions. The absence of significant mass concentrations of methylamine in the

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network should not be taken to mean that smaller quantities of amines are not important as contributors to new particle formation (Smith et al., 2010). Acknowledgments This work was sponsored by the National Park Service through a cooperative agreement with Colorado State University (Task #J2350093004). We are grateful to the IMPROVE site operators whose participation helped make this project possible. We thank Tammy Thompson for preparing the ammonia emission inventory map. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2014.03.058. References Akagi, S.K., Yokelson, R.J., Wiedinmyer, C., Alvarado, M.J., Reid, J.S., Karl, T., Crounse, J.D., Wennberg, P.O., 2011. Emission factors for open and domestic biomass burning for use in atmospheric models. Atmospheric Chemistry and Physics 11, 4039e4072. Atkinson, R., Perry, R.A., Pitts Jr., J.N., 1977. Rate constants for the reactions of the OH radical with CH3SH and CH3NH2 over the temperature range 299e426 K. Journal of Chemical Physics 66, 1578e1581. Babich, P., Davey, M., Allen, G., Koutrakis, P., 2000. Method comparisons for particulate nitrate, elemental carbon, and PM2.5 mass in seven US cities. Journal of Air and Waste Management Association 50, 1095e1105. Benedict, K.B., Day, D., Schwandner, F.M., Kreidenweis, S.M., Schichtel, B.A., Malm, W.C., Collett, J.L., 2013. Observations of atmospheric reactive nitrogen species in Rocky Mountain National Park and across northern Colorado. Atmospheric Environment 64, 66e76. Bergström, A.-K., Jansson, M., 2006. Atmospheric nitrogen deposition has caused nitrogen enrichment and eutrophication of lakes in the northern hemisphere. Global Change Biology 12, 635e643. Blanchard, C.L., Tanenbaum, S., 2005. Draft final Technical Memorandum: Analysis of Data from the Midwest Ammonia Monitoring Project. Envair, Albany, CA, USA. Bouwman, A.F., Lee, D.S., Asman, W.A.H., Dentener, F.J., Van Der Hoek, K.W., Olivier, J.G.J., 1997. A global high-resolution emission inventory for ammonia. Global Biogeochemical Cycles 11, 561e587. Bouwman, A.F., Van Vuuren, D.P., Derwent, R.G., Posch, M., 2002. A global analysis of acidification and eutrophication of terrestrial ecosystems. Water, Air, and Soil Pollution 141, 349e382. Carl, S.A., Crowley, J.N., 1998. Sequential two(blue) photon absorption by NO2 in the presence of H2 as a source of OH in pulsed photolysis kinetic studies: rate constants for reaction of OH with CH3NH2, (CH3)2NH, (CH3)3N, and C2H5NH2 at 295 K. Journal of Physical Chemistry A 102, 8131e8141. Cheng, Y.-H., Tsai, C.-J., 1997. Evaporation loss of ammonium nitrate particles during filter sampling. Journal of Aerosol Science 28, 1553e1567. Galloway, J.N., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger, S.P., Asner, G.P., Cleveland, C.C., Green, P.A., Holland, E.A., Karl, D.M., Michaels, A.F., Porter, J.H., Townsend, A.R., Vöosmarty, C.J., 2004. Nitrogen cycles: past, present and future. Biogeochemistry 70, 153e226. Ge, X., Wexler, A.S., Clegg, S.L., 2011. Atmospheric amines e Part I. A review. Atmospheric Environment 45, 524e546. Goode, J.G., Yokelson, R.J., Ward, D.E., Susott, R.A., Babbitt, R.E., Davies, M.A., Hao, W.M., 2000. Measurements of excess O3, CO2, CO, CH4, C2H4, C2H2, HCN, NO, NH3, HCOOH, CH3COOH, HCHO, and CH3OH in 1997 Alaskan biomass burning plumes by airborne Fourier transform infrared spectroscopy (AFTIR). Journal of Geophysical Research 105, 22147e22166. Heald, C.L., Collett Jr., J.L., Lee, T., Benedict, K.B., Schwandner, F.M., Li, Y., Clarisse, L., Hurtmans, D.R., Van Damme, M., Clerbaux, C., Coheur, P.-F., Philip, S.,

Martin, R.V., Pye, H.O.T., 2012. Atmospheric ammonia and particulate inorganic nitrogen over the United States. Atmospheric Chemistry and Physics 12, 10295e 10312. Horvath, H., 1992. Effects on visibility, weather and climate. In: Radojevic, M., Harrison, R.M. (Eds.), Atmospheric Acidity: Sources, Consequences and Abatement. Elsevier Applied Science, London (Chapter 13). Kallinger, G., Niessner, R., 1999. Laboratory investigation of annular denuders as sampling system for the determination of aliphatic primary and secondary amines in stackgas. Mikrochimica Acta 130, 309e316. Lee, T., Yu, X.-Y., Ayres, B., Kreidenweis, S.M., Malm, W.C., Collett Jr., J.L., 2008. Observation of fine and coarse particle nitrate at several rural locations in the United States. Atmospheric Environment 42, 2720e2732. Lee, T., Sullivan, A.P., Mack, L., Jimenez, J.L., Kreidenweis, S.M., Onasch, T.B., Worsnop, D.B., Malm, W., Wold, C.E., Hao, W.M., Collett Jr., J.L., 2010. Chemical smoke marker emissions during flaming and smoldering phases of laboratory open burning of wildland fuels. Aerosol Science and Technology 44 (9), iev. http://dx.doi.org/10.1080/02786826.2010.499884. Lehmann, C.M.B., Bowersox, V.C., Larson, R.S., Larson, S.M., 2007. Monitoring longterm trends in sulfate and ammonium in US precipitations: results from the National Atmospheric Deposition program/National Trends Network. Water, Air and Soil Pollution: Focus 7, 59e66. 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., 2013. Observations of ammonia, nitric acid, and fine particles in a rural gas production region. Atmospheric Environment. http://dx.doi.org/10.1016/j.atmosenv.2013.10.007. Malm, W.C., Sisler, J.F., Huffman, D., Eldred, R.A., Cahill, T.A., 1994. Spatial and seasonal trends in particle concentration and optical extinction in the United States. Journal of Geophysical Research-Atmospheres 99 (D1), 1347e1370. Malm, W.C., Schichtel, B.A., Pitchford, M.L., Ashbaugh, L.L., Eldred, R.A., 2004. Spatial and monthly trends in speciated fine particle concentration in the United States. Journal of Geophysical Research e Atmospheres 109, D03306. http:// dx.doi.org/10.1029/2003JD003739. Malm, W.C., Day, D.E., Carrico, C., Kreidenweis, S.M., Collett Jr., J.L., McMeeking, G., Lee, T., Carrillo, J., 2005. Intercomparison and closure calculations using measurements of aerosol species and optical properties during the Yosemite Aerosol Characterization Study. Journal of Geophysical Research 110, D14302. http://dx.doi.org/10.1029/2004JD005494. McMeeking, G.R., Kreidenweis, S.M., Baker, S., Carrico, C.M., Chow, J.C., Collett Jr., J.L., Hao, W.M., Holden, A.S., Kirchstetter, T.W., Malm, W.C., Moosmueller, H., Sullivan, A.P., Wold, C.E., 2009. Emissions of trace gases and aerosols during the open combustion of biomass in the laboratory. Journal of Geophysical Research 114, D19210. http://dx.doi.org/10.1029/2009JD011836. Sakurai, T., Fujita, S.-I., Hayami, H., Furuhashi, N., 2005. A study of atmospheric ammonia by means of modeling analysis in the Kanto region of Japan. Atmospheric Environment 39, 203e210. Schade, G.W., Crutzen, P.J., 1995. Emission of aliphatic amines from animal husbandry and their reactions: potential source of N2O and HCN. Journal of Atmospheric Chemistry 22, 319e346. Smith, J.N., Barsanti, K.C., Friedli, H.R., Ehn, M., Kulmala, M., Collins, D.R., Scheckman, J.H., Williams, B.J., McMurry, P.H., 2010. Observations of aminium salts in atmospheric nanoparticles and possible climatic implications. Proceedings of National Academy of Sciences 107, 6634e6639. Stevens, C.J., Dise, N.B., Mountford, J.O., Gowing, D.J., 2004. Impact of nitrogen deposition on the species richness of grasslands. Science 303, 1876e1879. Sweet, C., Caughey, M., Gay, D., 2005. Midwest Ammonia Monitoring Project: Summary for October 2003 through November 2004. Illinois State Water Survey, Champaign, IL, USA, p. 47. Yao, X.H., Zhang, L., 2013. Analysis of passive-sampler monitored atmospheric ammonia at 74 sites across southern Ontario, Canada. Biogeosciences 10, 7913e 7925. Yokelson, R.J., Ward, D.E., Susott, R.A., Reardon, J., Griffith, D.W.T., 1997. Emissions from smoldering combustion of biomass measured by open-path Fourier transform infrared spectroscopy. Journal of Geophysical Research 102, 18865e 18877. Yu, X.-Y., Lee, T., Ayres, B., Kreidenweis, S.M., Collett Jr., J.L., 2005. Particulate nitrate measurement using nylon filters. Journal of the Air & Waste Management Association 55, 1100e1110. Yu, X.-Y., Lee, T., Ayres, B.R., Kreidenweis, S.M., Collett Jr., J.L., 2006. Loss of fine particle ammonium from denuded nylon filters. Atmospheric Environment 40, 4797e4807.