Selective Collection of Particulate Ammonium for Nitrogen Isotopic

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Selective Collection of Particulate Ammonium for Nitrogen Isotopic Characterization Using a Denuder-Filter Pack Sampling Device Wendell W. Walters, Danielle E Blum, and Meredith G. Hastings Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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Analytical Chemistry

Selective Collection of Particulate Ammonium for Nitrogen Isotopic Characterization Using a Denuder-Filter Pack Sampling Device Wendell W. Walters*1,2, Danielle E. Blum2,3, and Meredith G. Hastings1,2 1Department

of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook Street,

Providence, RI 02912. 2Institute

at Brown for Environment and Society, Brown University 85 Waterman Street, Providence, RI

02912 3Department

of Chemistry, Brown University, 324 Brook Street, Providence, RI 02912

*Corresponding author: [email protected]

ACS Paragon Plus Environment

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract. Nitrogen stable isotope analysis (δ15N) of particulate ammonium (NH4+) may provide additional constraints on this critical component of fine particulate matter; however, no previous collection method has been verified for its ability to accurately and precisely characterize δ15N(NH4+). This is a critical point due to the difficulty of quantitative NH4+ collection and possible sampling artifacts. Here, we report on δ15N(NH4+) precision using an established denuder-filter pack combination with two filter configurations including (1) a Nylon filter plus an acid impregnated cellulose filter and (2) an acid impregnated glass fiber filter for NH4+ collection in both laboratory-controlled environments and ambient air samples. Laboratory NH4+ were generated from the nebulization of ammonium salt solutions and collected using a filter pack sampling train for off-line concentration and isotopic measurement. Quantitative collection of NH4+ was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δ15N(NH4+) precision of ±0.9‰ (1σ; n=24) and ±0.6‰ (n=9) for the nylon plus citric acid impregnated cellulose filter and for the citric acid impregnated glass fiber filter, respectively. Field sample reproducibility was assessed from 24-hour collected side-by-side samples and indicated δ15N(NH4+) to be reproducible within 1.1‰, consistent with the laboratory findings. This work represents the first established method for speciated NH4+ collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.


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1.

Introduction Inorganic ions represent a major component of fine particulate matter (PM2.5) that can contribute up

to 70% of their total masses.1 However, the inorganic contribution to PM2.5 is often difficult to predict, because it is formed via complex chemistry and interplay amongst gaseous precursors from a variety of sources. Important precursor emissions include sulfur dioxide (SO2), nitrogen oxides (NOx = NO + NO2), and ammonia (NH3) that can react to form particulate sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+), and be transported far away from their emission sources.2 Previous regulation efforts (i.e. Clean Air Acts) have reduced anthropogenic emissions of SO2 and NOx from fossil-fuel combustion sources, but NH3 is yet to be regulated in many regions around the world from both fossil-fuel and agricultural emission sources3. Today, NH3 and its secondary product NH4+ dominate total nitrogen that is deposited in the United States via wet and dry deposition4, which has important implications for human health and the environment. Therefore, a fundamental understanding of NHx (NH3 + NH4+) sources, and NH3-NH4+ gas-to-particle conversion mechanisms and its influences on new particle formation and coagulation is critical.5 Nitrogen stable isotope analysis (δ15N) of NHx may provide additional insights into emission sources and transformation mechanisms, as they often exhibit characteristic isotopic compositions or (“fingerprints”).6 This tool may allow for distinction of emission sources as well as an understanding of the chemical and physical processes responsible for NHx removal at a process level. The partitioning of NHx between the gas, solid, and aqueous phases is driven by thermodynamic equilibrium that depends on the relative humidity, temperature, and particle chemical composition. This equilibrium is typically achieved on the order of minutes7, suggesting that isotopic equilibrium between NH3(g) and the solid or aqueous phases of NH4+ is also likely rapidly achieved in the atmosphere (R1-R3)8,9: 15

𝑁𝐻3(𝑔) +

14

14

+ 𝑁𝐻4(𝑠) ⇌ 𝑁𝐻3(𝑔) +

15

𝑁𝐻3(𝑔) +

14

14

15

+ 𝑁𝐻4(𝑠)

+ 𝑁𝐻4(𝑎𝑞) ⇌ 𝑁𝐻3(𝑔) +

15

+ 𝑁𝐻4(𝑎𝑞)


(R1)

(R2)

3


15

𝑁𝐻3(𝑔) +

14

14

𝑁𝐻3(𝑎𝑞)⇌ 𝑁𝐻3(𝑔) + 15𝑁𝐻3(𝑎𝑞)

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(R3)

Recent ab initio calculations predict nitrogen equilibrium constants (K) or isotopic fractionation factors (α) of 1.031(±0.004), 1.034(±0.004), and 1.004(±0.003) at 25 °C for R1, R2, and R3, respectively10. This suggests that the 15N isotope will preferentially partition into the condensed phase of NHx with an isotopic enrichment factor (ε(‰) = 1000(α-1)) ranging from 4 to 34‰ relative to NH3(g) depending on the conversion mechanism, potentially scrambling original source δ15N(NH3) signatures as NH3 is converted to NH4+. Additionally, previous works have suggested that unidirectional NH3(g) neutralization reactions involving sulfuric acid (H2SO4(g)) may induce a kinetic isotope effect of approximately -28‰ based on diffusion rates of 15NH3 relative to 14NH311. These isotope effects (e.g. equilibrium and kinetic) offers the possibility to use nitrogen isotope separation factors between aerosol NH4+ and NH3(g) ((Δ15δNNH4+/NH3 = δ15N(NH4+) – δ15N(NH3)) to track gas-to-particle phase conversion mechanisms during nucleation and condensation events10. Thus, to fully utilize this isotopic tool requires a method that enables the speciated collection of NHx for separate, accurate, and precise δ15N analysis of NH3(g) and NH4+. Most previous δ15N studies involving NHx have collected a single phase of NHx for subsequent isotopic analysis. This includes collection of NH4+ on a particulate filter11 followed by collection of NH3(g) using a passive acid coated filter12,13. These approaches make it difficult to use δ15N as a way to evaluate NH3 emission source without knowing the partitioning of NHx phases and its associated isotope fractionation.10 Studies that have attempted speciated NHx for δ15N analysis have been rare, and even then, questions remain as to whether quantitative NHx speciation was achieved. These studies have commonly speciated NHx utilizing a filter-pack combination involving a particle filter for NH4+ collection followed by an acid coated cellulose filter for NH3(g) collection.14,15 Problems with this approach, however, arise due to the potential for positive and negative sampling artifacts.16 Positive artifacts may result from the uptake of gases by collected particles or by the filter medium17, while negative artifacts may result from volatilization of collected particles, which is particularly relevant for semi-volatile NH4+ 16. These artifacts also likely alter the δ15N values of the phase components and their interpretations.


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The denuder-filter combination is a well-established sampling technique that can effectively collect and speciate NHx16,18. In this system, NH3(g) is first removed from the sampled air-stream on an acid coated glass denuder followed by collection of NH4+ on a filter pack. Utilizing such a system, we have recently reported on a laboratory- and field-tested collection method for δ15N(NH3) analysis utilizing an acid coated honeycomb denuder19; however, to date, there have been no NH4+ collection methods that have laboratoryverified their suitability for speciated δ15N(NH4+) analysis. This is a critical point because to quantitatively utilize this tool requires that the isotopic measurements accurately and precisely reflect the environment in which NH4+ was collected. Atmospheric NH4+ is notoriously difficult to accurately characterize for concentration because of its volatile nature16, which will lead to an undesirable δ15N(NH4+) fractionation effect and influence interpretation of the atmospheric dynamics involving NHx. Thus, it is critical that sampling devices are rigorously tested for their suitability for isotopic characterization including identification of sampling artifacts, biases, and collection limits before their field application19. We report on a laboratory and field-tested speciated NH4+ collection device that is suitable for ambient air collection and isotopic analysis. The device is a ChemComb Speciation Cartridge™20,21, which contains a PM2.5 impactor, two honeycomb denuders for the removal of acidic and basic gases, followed by a 4-stage filter pack for the collection of PM2.5. Here, we have tested this method’s suitability for δ15N(NH4+) characterization via NH4+ collection under a variety of laboratory and field conditions, and subsequent extraction and off-line analysis for δ15N(NH4+) using established laboratory techniques. This work is an important step in establishing robust sampling techniques for accurate and precise δ15N(NHx) characterization and will be useful in furthering our understanding of NHx dynamics in the atmosphere.


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2.

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Experimental Section

NH4+ Collection. Collection of NH4+ was conducted using a series of filters in two different configurations. The first configuration included a Nylon filter (Cole-Parmer, 0.8 μm pore, 47 mm diameter) followed by two cellulose filters (Whatman, 8 μm pore, 47 mm diameter) impregnated with 5% (w/v) citric acid (Fischer Chemical >99.5%) in ultra-high purity water (18.2 MΩ, Milli-Q (MQ)). This approach enables a cost-effective way for PM2.5 monitoring (i.e. see large networks such as the Interagency Monitoring of PROtected Visual Environments (IMPROVE))22 due to its quantitative capture of semi-volatile NO3-, but NH4+ volatilization can be significant ranging between 1 to 65% based on previous field observations16. In the first configuration, the Nylon filter is used to collect PM2.5, the downstream 5% (w/v) citric acid coated cellulose filter is used to collect any volatilized NH4+, and the final 5% (w/v) citric acid filter is used as a control to check for breakthrough, potentially allowing for simultaneous monitoring of reactive species and speciated δ15N(NHx) measurements. The second filter configuration included two glass-fiber filters (Whatman, 1.5 μm pore, 47 mm diameter) impregnated with 5% (w/v) citric acid. This configuration may be suitable for NHx monitoring and speciated δ15N(NHx) measurement but cannot provide the simultaneous reactive species monitoring that incorporation of the Nylon filter allows for due to large potential for NO3- volatilization off the acid-coated glass fiber filter. In this set-up, the first acid-coated glass fiber filter is used for the collection of NH4+, and the second filter is used to check for breakthrough. The series of filters were housed in a PTFE four-stage filter pack contained within the ChemComb Speciation Cartridge™ (CCSC). Use of a CCSC for inorganic gas and particle speciation has been extensively described and characterized in previous works20,21. Briefly, gases are removed on coated denuders and particles are removed using a filter pack. Due to its much smaller diffusion rate compared to gases, PM2.5 cannot migrate to the denuder walls during their residence time, allowing for speciation between gaseous and particulate components based on physical separation17.


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Nylon filters were rinsed and sonicated with MQ water for 30 minutes three times and dried using ultrapure nitrogen gas in a vacuum desiccator prior to use. The cellulose and glass fiber filters were not washed prior to use because of disintegration issues and were directly impregnated with the 5% citric acid solution. A range of acid coating solutions may be used to impregnate filters for NHx capture including phosphoric acid18. We have limited our tests to citric acid due to its low NH4+ blank compared to other acids24, but expect other commonly used acid solutions for NHx capture to be viable as well, provided that they are checked for “blank” NH4+ artifacts. Immediately after NH4+ collection, filters were extracted in 20 mL of MQ water, sonicated for 60 minutes, and filtered through 0.22 μm syringe filters to remove disintegrated filter pieces and possible microbial contaminants (particularly for the ambient air collected samples). Following this, the filters were removed, and the extraction solutions were frozen until subsequent analysis. Freezing solutions has been shown to be an adequate technique to preserve NH4+25. Previous stability tests have indicated minimal loss of NH4+ on acid coated filters when stored in a refrigerator for up to 2 weeks with loss of 7 to 9% observed for up to 5 weeks26. Therefore, extraction of NH4+ or proper preservation initiatives (e.g. freezing samples) should be conducted soon after collection to minimize any potential for losses.

Laboratory Collection Experiments. A 3-jet collision nebulizer (CH Technologies) was used to generate NH4+ particles in the laboratory using two types of salt solutions that included 25 mM ammonium nitrate (NH4NO3; Acros >99%) and 12.5 mM ammonium sulfate ((NH4)2SO4; Fischer Chemical >99.7%), representing the most common types of NH4+ aerosols as well as a range of volatility. Briefly, an ultra-pure nitrogen tank (PurityPlus Gas) flow-controlled at 4.0 standard liters per minute (SLPM) was connected to the nebulizer. The generated NH4+ was diluted with ultra-pure nitrogen that was flow-controlled at 6.0 SLPM. NHx was not detectable in the ultra-pure nitrogen. The flow of the NH4+ line and the nitrogen dilution line were combined using a 3-way union that was also connected to the inlet of the CCSC. Temperature and relative humidity were monitored (Elitech GS6) in the outflow of


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the CCSC and were typically 21.0 ± 1.0 ºC and 45 ± 10%, respectively. All tubing and unions used to connect the nebulized NH4+ to the CCSC were composed of PTFE. A schematic of the experimental setup is provided in the Supporting Information (Figure S1). All experiments were conducted at the CCSC’s optimal flow rate of 10 SLPM20. In these experiments, upstream denuders were not utilized in the sample collections. For the first filter configuration (Nylon and citric acid impregnated cellulose filters), triplicate trials were conducted at 10, 20, and 30 minutes for both the NH4NO3 and (NH4)2SO4 solutions to test the loading capacity of the sampling system. Additionally, NH4+ desorption tests were conducted by collecting NH4+ for 10 minutes and then flowing ultra-pure nitrogen through the CCSC at a rate of 10 SLPM for 24 hours. Due to the potential for NH4+ volatilization off the Nylon filter, the limit of the collection system is likely determined by the 5% citric acid coated filter. Thus, the NH3(g) (e.g. volatilized product of NH4+) collection limit of the acid impregnated filter was also tested in a similar manner as depicted in Figure S1 and exactly as previously described19. For this test, a 20.1 ppmv tank of NH3 (Praxair) diluted in N2 was flowed at a rate of 0.015 LPM and further diluted with a humidified ultra-pure N2 to a final flow rate of 10 SLPM that was connected to the inlet of the CCSC. Temperature and relative humidity for these experiments were typically 21.0 ± 1.0 ºC and 40 ± 10%, respectively. The filter pack for these experiments only contained two acid impregnated cellulose filters and variable flow times were tested ranging from 1 to 24 hours. For the second filter configuration (acid impregnated glass fiber filter) triplicate trials were conducted at 5, 10, and 15 minutes using only the nebulized NH4NO3 solution (Figure S1).

Field Applicability. Utilizing the CCSC, NH4+ samples from ambient air were collected over 24-hour periods in Providence, RI, USA (41.8240 ºN, 71.4128 ºW). A weather-proof enclosure for the CCSC with a built-in weather station (ProWeather Station TP3000WC) was installed on the rooftop of a building on the campus of Brown University, and air was sampled at a flow rate of 10 SLPM using a mass-flow controlled (Dakota Mass Flow Controller 6AGC1AL55-10AB2; precision ±1‰) vacuum pump (Welch


8

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2546B-01). Ambient NH3(g) was first denuded using two 2% citric coated honeycomb denuders, with the first denuder used for NH3 capture and the second used to check for breakthrough. Ambient NH4+ was collected downstream of the denuders using the two laboratory-tested filter configurations including (1) a series of Nylon and two 5% (w/v) citric acid coated filters and (2) a series of two 5% (w/v) citric acid impregnated glass fiber filters. Replicate (side-by-side) samples were collected throughout the year to investigate NH4+ concentration and δ15N reproducibility under a variety of field conditions (i.e. temperature and relative humidity). The filters were extracted and processed as previously described.

Concentration and δ15N Isotopic Analysis. The NH4+ concentrations ([NH4+]) in all extraction solutions were analyzed using colorimetric analysis based on the indophenol blue method27 that was automated using a discrete UV-Vis analyzer (Westco SmartChem 2.0). In addition, concentration measurements were made for [NO2-], [NO3-], and [SO42-] in the Nylon and glass fiber filter extraction solutions from the laboratory nebulization experiments. [NO2-] and [NO3-] was measured using a standard colorimetric technique (e.g. EPA Method 353.2) involving the cadmium reduction of NO3- to NO2- that undergoes diazotization with sulfanilamide dihydrochloride followed by detection of absorbance at 520 nm that was automated using a discrete UV-Vis analyzer (Westco SmartChem 2.0). [SO42-] was measured using ion chromatography (Dionex IonPac AS22 with a 3.5 mM carbonate and 1.0 mM bicarbonate elution solution). Standard lab protocols were followed that included calibration to anion standards and blank and quality control standard measurements that were made every 5 to 7 samples. Reproducibility calculated from replicate measurements of quality control standards were ±2%, ±1%, ±1%, and ±1% for [NH4+], [NO2-] [NO3-], and [SO42-], respectively. Lower detection limits were approximately 0.3, 0.2 0.5, and 2.0 μM for [NH4+], [NO2-], [NO3-], and [SO42-], respectively. The determination of δ15N(NH4+) is based on a coupled off-line wet-chemistry technique involving hypobromite (BrO-) oxidation and acetic acid/sodium azide reduction28. Samples are diluted to at least 10 μM-NH4+ using MQ water to a volume of 10 mL in 50 mL vials. The samples are then


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Page 10 of 27

oxidized to nitrite (NO2-) using BrO- in an alkaline solution, which was synthesized as previously described28. After 30 minutes the reaction was stopped by 0.4 mL addition of 0.4 M sodium arsenite (NaAsO2) to remove remaining BrO-. [NO2-] oxidation yields were measured using a standard colorimetric absorption technique (e.g. US EPA Method 353.2) automated using a discrete UV-Vis analyzer (Westco SmartChem 2.0) to confirm quantitative conversion of NH4+ to NO2-. After conversion, analysis of δ15N was conducted using a previously established chemical method that reduces sample (aqueous) NO2- to gaseous nitrous oxide (N2O)28,29. In brief, 20 nmol of NO2- samples are transferred to 20 mL vials that are crimcapped with PTFE/butyl septa and flushed with helium (He) for 10 minutes. NO2- is subsequently reduced to N2O using 1 mL of 1M sodium azide buffered in 30% acetic acid solution29. After at least a 30-minute reaction time, the solutions are neutralized using 6M NaOH. Samples are then analyzed for their δ15N(N2O) composition using an automated N2O extraction system coupled to a continuous flow Isotope Ratio Mass Spectrometer for m/z 44, 45, and 46 measurements30. In each batch analysis, unknowns are calibrated with respect to two internationally recognized NH4+ isotopic reference materials, IAEA-N2 and USGS25, with δ15N values of 20.3‰, and -30.3‰, respectively31,32, which are run between approximately every 10 unknowns. The isotopic standards undergo the exact same chemical processing as the unknowns and are used to correct for isotopic fractionation and blank effects resulting from the chemical conversion of NH4+ to N2O and had a pooled standard deviation (1σ) of ±0.5‰ (n =18; IAEA-N2) and ±0.6‰ (n=18; USGS25). For each batch sample analysis, two NO2standards with known isotope values (RSIL-N7373 and RSIL-N10219 with δ15N = -79.6‰ and 2.8‰, respectively)33 are run as a quality control to monitor the conversion of NO2- to N2O and system stability. These reference materials had a standard deviation of ±0.5‰ (n=5) and ±0.4‰ (n=5), respectively. We note that potential interferences for our isotopic measurement include both nitrite (NO2-) and reduced organic nitrogen. Our concentration analysis has indicated that [NO2-] were generally within our detection limit (0.2 μM) and always less than 3% of the measured [NH4+] in our collected denuders and filters. If [NO2-] were significant, this interferent can be readily removed by treatment with sulfanilic acid


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as previously described28. The chemical method used for NH4+ oxidation to NO2- will potentially also oxidize reduced organic nitrogen components to NO2-, representing a potential interferent to the measured δ15N(NH4+) particularly for the ambient air samples28. Additionally, presence of reduced organic compounds could interfere with our [NH4+] measured via the indophenol blue method. However, this interferent is expected to be minor in our sampling conditions (urban atmosphere), as NHx concentrations are generally much larger (~10×) than reduced organic components34. In environments with an expected relatively elevated reduced organic nitrogen compounds (e.g. rural locations), analytical techniques to separate inorganic NH4+ and reduced organic components may be needed to reduce the potential for this interferent but is beyond the scope of this work.

3. Results and Discussion. Filter Blanks. Five blanks were measured for the Nylon, citric acid impregnated cellulose filters, and citric acid impregnated glass fiber filters. The Nylon filters were found to have an undetectable amount of NH4+, while the citric acid-impregnated cellulose filters and glass fiber filters were found to have a minor blank of approximately 2.2 ±0.5 and 3.4 ±0.2 μM-[NH4+] (n=5; Table S1), respectively. The acid impregnated cellulose and glass fiber filter would represent approximately 45±10 and 68±5 nmol of NH4+ per filter (extracted in 20 mL), respectively. An appropriate concentration correction was made to account for the blanks observed on the acid impregnated filters. The δ15N(NH4+) of the acid impregnated cellulose and glass-fiber filters had consistent value of (-10.2 ±1.1‰; n =3) and (-1.0 ±1.1‰; n =3), respectively, and a “blank” δ15N(NH4+) correction was made based off mass-balance as previously described (Eq. 1)35:

15

𝛿

𝑁(𝑁𝐻4+ )𝑠𝑎𝑚𝑝𝑙𝑒

=

𝛿15𝑁(𝑁𝐻4+ )𝑡𝑜𝑡𝑎𝑙[𝑁𝐻4+ ]𝑡𝑜𝑡𝑎𝑙 ― (𝛿15𝑁(𝑁𝐻4+ )𝑏𝑙𝑎𝑛𝑘[𝑁𝐻4+ ]𝑏𝑙𝑎𝑛𝑘)

[𝑁𝐻4+ ]𝑡𝑜𝑡𝑎𝑙 ― [𝑁𝐻4+ ]𝑏𝑙𝑎𝑛𝑘


(Eq. 1)

11


Page 12 of 27

The NH4+ blank was fairly negligible in our laboratory experiments, averaging 1.5% of the total collected NH4+ (Table S2-S5), and accounting for the filter NH4+ blank resulted in a δ15N correction of no more than 0.4‰. The relative amount of NH4+ blank was higher in the ambient collected samples that averaged 16.5±4.0% (n=8) and 13.4±10.0%(n=6) for the total NH4+ collected on the Nylon + 5% citric acid impregnated cellulose filter and the 5% citric acid impregnated glass fiber filter configurations, respectively (Table S6), and accounting for the NH4+ filter blank resulted in a δ15N correction of no more than 3.2‰.

Nylon and Citric Acid Impregnated Cellulose Nebulization Experiments. For the first tested filter configuration utilizing a Nylon filter and two subsequent citric acid impregnated cellulose filters, the NH4+ collection fraction (CF) for individual filters were calculated as followed (Eq. 2):

𝐶𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑 [𝑁𝐻4+ ](𝑁𝑦𝑙𝑜𝑛 𝑜𝑟 𝑐𝑖𝑡𝑟𝑖𝑐 1 𝑜𝑟 𝑐𝑖𝑡𝑟𝑖𝑐 2)

𝐶𝐹 = ∑𝐶𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑 [𝑁𝐻 + ] 4

(Eq. 2)

(𝑁𝑦𝑙𝑜𝑛 𝑜𝑟 𝑐𝑖𝑡𝑟𝑖𝑐 1 𝑜𝑟 𝑐𝑖𝑡𝑟𝑖𝑐 2)

For trials conducted for 10 to 30 minutes, the Nylon CF was 0.85 ±0.04 and 0.87 ±0.04 for generated NH4NO3 and (NH4)2SO4 particles, respectively (Figures 1A and 1B). For the 24-hour desorption tests, the observed NH4+ volatilization increased as the Nylon collection fraction decreased to 0.70 ±0.06 and 0.83 ±0.02, for NH4NO3 and (NH4)2SO4 particles, respectively. Overall, significant NH4+ volatilization is found for both NH4NO3 and (NH4)2SO4 particles, with greater volatilization (i.e. lower CF) observed for the NH4NO3 particles than observed for (NH4)2SO4 particles, as expected, reflecting the low vapor pressure and higher stability of (NH4)2SO4. Collection efficiency (CE) of the NH4+ filter sampling train was calculated according to the following (Eq. 3): 𝐶𝐸(%) = 100(1 ― 𝑄2 𝑄1)


(Eq. 3)

12

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where Q1 represents the sum of NH4+ collected on the nylon filter and the first 5% citric acid impregnated cellulose filter and Q2 represents the NH4+ collected on the second 5% citric acid impregnated cellulose filter (i.e. NH4+ breakthrough), respectively. While NH4+ volatilization off the Nylon filter was significant, the first 5% citric acid impregnated cellulose filter quantitatively captured the volatilized NH4+, as evidenced by a collection efficiency (Eq. 3) of 0.998 ±0.002 for the Nylon filter plus the first 5% citric acid impregnated cellulose for both the NH4NO3 and (NH4)2SO4 particles (Figures 1A and 1B). No significant decrease in collection efficiency was found, even for the 24-hour desorption experiments, when using a Nylon plus backup acid impregnated cellulose filter. The linear relationship between the collected NH4+ (Nylon and first 5% citric acid impregnated cellulose filter) and NO3- (Nylon filter) and NH4+ and SO42- was found to have a slope of 1.013 ±0.015 and 1.99 ±0.02, respectively, within error of their stochiometric relationships (Figures 1C and 1D), indicating conservation of mass for the collected particles. We note that because upstream denuders were not utilized in these experiments, it is possible that the observed NH4+ collected on the acid coated filter could be related to volatized NH3(g) within the sample line and during nebulization. Thus, the observed volatilization should represent an upper limit of the volatilization off the nylon filter in these laboratory experiments. However, the main takeaway is that NH4+ and its volatilized product (i.e. NH3) is quantitatively collected using a Nylon and a backup acid coated cellulose filter. The volatilization of NH4+ had a significant impact on the measured δ15N(NH4+) (Figures 1E and 1F). The NH4+ retained on the Nylon filters had a δ15N(NH4+) of -0.5 ±1.4‰ and 0.6 ±0.8‰, while the volatilized NH4+ captured using a 5% citric acid impregnated cellulose filter had a δ15N(NH4+) value of 29.0 ±4.0‰ and -34.1 ±1.0‰ for the NH4NO3 and (NH4)2SO4 particles, respectively. The isotopic enrichment factors between the retained (Nylon filter) and volatilized (5% citric acid impregnated cellulose filter) (e.g. δ15N(NH4+)retained – δ15N(NH4+)volatilized) was 28.6 ±2.7‰ and 34.7 ±1.2‰ for NH4NO3 and (NH4)2SO4 particles, respectively. These values are near the theoretical calculated equilibrium enrichment values involving NH4+(aq) or NH4+(s) with NH3(g) of 34 ±4‰ and 31 ±4‰ at 298 K10. The


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mass-weighted δ15N(NH4+)Bulk (Eq. 4) collected on the Nylon filter and the backup acid-coated filter had a value of -5.6 ±0.8‰ and -3.6 ±0.6‰, for NH4NO3 and (NH4)2SO4, respectively, which was calculated as (Eq. 4): 𝛿15𝑁(𝑁𝐻4+ )𝐵𝑢𝑙𝑘 = 𝑓𝑁𝑦𝑙𝑜𝑛(𝛿15𝑁(𝑁𝐻4+ )𝑁𝑦𝑙𝑜𝑛) + 𝑓𝐶𝑖𝑡𝑟𝑖𝑐(𝛿15𝑁(𝑁𝐻4+ )𝐶𝑖𝑡𝑟𝑖𝑐)

(Eq. 4)

were f represents the fraction of NH4+ collected on the Nylon and 5% citric acid impregnated cellulose filter, respectively. Aliquots of the nebulized NH4NO3 and (NH4)2SO4 solutions were taken before, during, and after the laboratory tests and had a measured δ15N(NH4+) of -2.1 ±0.1‰ (n=3) and -5.2 ±0.3‰ (n=3), respectively. The solution values are statistically significantly different from the δ15N(NH4+)Bulk measured from the generated and collected NH4+ (p95%)36 of at least 336.0 μg of NH3 at a flow rate at 10 SLPM. The measured δ15N(NH3) had an average value of -2.1 ±1.0‰ (n=12) (Figure 3). A significant δ15N(NH3) outlier was identified based on a Grubb’s test (p99% of NH3(g) was collected and retained on a single citric acid coated honeycomb denuder, as the collected [NH4+] amount on the breakthrough denuder was always within our detection limits. This is consistent with our previous findings as the collected NH3(g) never exceeded 1.2 μmol, which is well within the reported operative capacity of the citric acid honeycomb denuder of ~23.5 μmol19. NH3(g) concentration and δ15N(NH3) was found to be reproducible within (calculated as the absolute difference between the two collected side-by-side samples) 0.010 μmol/m3 and 1.8‰, respectively, consistent with our previous results19. The percent error in the concentration of NH3(g) was found to be within 13.1%, which was calculated according to Eq. 5: % 𝐸𝑟𝑟𝑜𝑟 =

|𝐴 ― 𝐵| 𝑎𝑣𝑒𝑟𝑎𝑔𝑒(𝐴,𝐵)

× 100

(Eq. 5)

where A and B refer to the two side-by-side collected samples. We note that for two of the ambient replicate trials (1B & 2B; Table 1), δ15N(NH3) was not measured for both collected samples.

Ambient NH4+ was collected in a downstream filter pack using both tested filter configurations. The first configuration utilized a nylon filter plus 5% citric acid coated cellulose filter. In this configuration, a large fraction of NH4+ volatilization off the Nylon filter was observed that averaged 75 ±13% (n=8) (Table S6). This NH4+ volatilization is much higher than observed in our laboratory tests (Figure 1), which may be related to diurnal changes in temperature and relative humidity over the 24-hour ambient collections as well as a much more complex ambient particle composition comprising of a mixture of organic and inorganic components. We also note that our airstream was denuded prior to particle collection, which would facilitate NH4+ volatilization compared to our laboratory collections in which no denuders were utilized. Our observed NH4+ volatilization also tends to be higher than previous


19


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reported in denuded Nylon filter collections with NH4+ loss up to 65%16. However, we note that our Nylon filters had a smaller pore size (0.8 μm) compared to this previous study (1.0 μm), which would have increased the pressure drop across the filter, increasing the potential for NH4+ volatilization. While significant, the volatilized NH4+ was nearly quantitatively captured using a single citric acid impregnated cellulose filter, consistent with our laboratory findings, as ambient NH4+ collection efficiency (Eq. 3) was found to be >97% (Table S5), as the [NH4+] measured on the breakthrough citric acid cellulose filter was found to range 1.5 to 2.8 μM, near the blank [NH4+] of 2.4 ±0.8 μM. The NH4+ concentration and δ15N(NH4+) were reproducible within 0.006 (n=3) μmol/m3 (with an average percent error of 8.5 ±8.2%; Eq. 5) and 0.8 ±0.2‰ (Table 1), respectively.

The isotopic separation factor between NH4+ and NH3

(Δδ15N) was found to range between 12.1 and 19.6‰ (Table 1). The δ15N(NHx) was found to range between -12.7 and -7.0‰, which was calculated according to the following (Eq. 6): 𝛿15𝑁(𝑁𝐻𝑥) = 𝑓𝑁𝐻3𝛿15𝑁(𝑁𝐻3) + (1 ― 𝑓𝑁𝐻3)𝛿15𝑁(𝑁𝐻4+ )

(Eq. 6)

where fNH3 refers to the fraction of NHx that exists in the gaseous phase (NH3) and is calculated according to: [𝑁𝐻3]

𝑓𝑁𝐻3 = [𝑁𝐻 ] + [𝑝 ― 𝑁𝐻 + ] 3

4

(Eq. 7)

In only one replicate collection was δ15N(NH3) measured for both side-by-side samples for this filter configuration. Despite this small sample size, both Δδ15N and δ15N(NHx) was found to be reproducible within 0.9 and 0.6‰, respectively, which was consistent with the reproducibility measured for the acid impregnated glass fiber filter configuration (Table 1). Ambient replicate collection of NH4+ was also conducting using the citric acid impregnated glass fiber filter. Overall, near quantitative collection of NH4+ was achieved using a single citric acid impregnated glass fiber filter with a collection efficiency >98% (Table S6). The NH4+ concentration and δ15N(NH4+) was found to be reproducible within 0.006 μmol/m3 (with an average percent error of 10.4


20

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±2.1%) and 0.9 ±0.1‰, respectively. Δδ15N and δ15N(NHx) was found to be reproducible within 1.6‰ and 1.4‰, respectively (Table 1). A direct comparison between the two filter configurations for the collection of ambient NH4+ was also made (Table 1). Overall, the two filter configurations agreed well with one another as average [NH4+], δ15N(NH4+), Δδ15N, and δ15N(NHx) reproducibility was found to be within 0.02 μmol/m3 (with a percent error within 13.3%), 1.1‰, 2.2‰, and 1.1‰, respectively. Interpretation of the measured δ15N(NH3), δ15N(NH4+), Δδ15N, and δ15N(NHx) is beyond the scope of this work. We note that the measured Δδ15N was found to range from 10.7 to 31.4‰, with the higher end values near the predicted isotopic equilibrium between NH3(g) and the condensed NH4+ phases of 31 ±4‰ (R1) and 34 ±4‰ (R2)10. However, lower Δδ15N values were also observed, which may reflect incomplete NH3 and NH4+ isotopic equilibrium or interactions with NH3 and aqueous interfaces (R3)19. Since the isotopic offset between gaseous and particle phase of NHx is not consistent, this suggests that δ15N(NHx) cannot be predicted based off the measurement of a single phase, indicating that simultaneous speciated NHx collection for isotopic analysis is essential. Overall, our ambient replicate collections suggest that both filter configurations tend to reproducible similar NH4+ concentrations and δ15N(NH4+) values and both are suitable for ambient NH4+ collection for off-line concentration and isotopic analysis.


21


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Table 1. Summary of 24-hour ambient air side-by-side replicate collections (A and B) conducted in Providence, RI, USA including average temperature (Temp(°C)), average relative humidity (RH%), [NH4+], f(NH3), δ15N(NH3), δ15N(NH4+), Δδ15N, and δ15N(NHx). Two denuder filter pack configurations were tested including: (1) citric acid denuder and nylon + citric acid cellulose filters (CD-N-CC) and (2) citric acid denuder and citric acid impregnated glass fiber filter (CD-CGFF). The absolute difference between the two replicate samples is shown by the values in parentheses.

Collection Configuration CD-N-CC 1A 1B

Temp (°C)

RH (%)

[NH3(g)] μmol/m3

[NH4+] μmol/m3

f(NH3)a

δ15N(NH3) (‰)

δ15N(NH4+) (‰)

Δδ15N (‰)

δ15N(NHx) (‰)b

20.7

86.3

0.067 0.061 (0.005)

0.017 0.017 (0.00)

0.80 0.79 (0.01)

-14.4 --

5.2 6.3 (0.9)

19.6 --

-10.5 --

2A 2B

23.4

87.1

0.081 0.071 (0.010)

0.020 0.014 (0.006)

0.81 0.83 (0.02)

-10.8 --

8.6 7.9 (0.6)

19.4 --

-7.0 --

3A 3B

11.0

52.4

0.047 0.046 (0.001)

0.017 0.016 (0.001)

0.73 0.74 (0.01)

-17.1 -17.3 (0.2)

-0.1 -1.1 (1.0)

17.0 16.1 (0.9)

-12.5 -13.1 (0.6)

8.8

80

0.046 0.041 (0.005)

0.025 0.023 (0.002)

0.64 0.64 (0.00)

-24.5 -22.6 (1.8)

6.9 7.7 (0.8)

31.4 30.4 (1.0)

-13.1 -11.7 (1.4)

12.6

93.7

0.042 0.045 (0.003)

0.045 0.051 (0.006)

0.47 0.47 (0.00)

-16.9 -17.6 (0.7)

10.0 10.9 (0.9)

26.9 28.5 (1.6)

-2.7 -2.5 (0.2)

11.0

52.4

0.046 0.048 (0.002)

0.016 0.014 (0.002)

0.74 0.77 (0.03)

-17.1 -18.2 (1.1)

-0.1 1.0 (1.1)

17.0 19.2 (2.2)

-12.7 -13.8 (1.1)

12.0

73.4

0.033 0.031 (0.002)

0.008 0.007 (0.001)

0.79 0.81 (0.02)

-13.9 -12.8 (1.1)

-1.6 -2.1 (0.5)

12.3 10.7 (1.6)

-11.3 -10.7 (0.6)

CD-CGFF 4A 4B 5A 5B Comparison CD-N-CC 6A CD-CGFF 6B CD-N-CC 7A CD-CGFF 7B a

calculated according to Eq. 6

b

calculated according to Eq. 7


22

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4. Conclusion We have critically evaluated a method for NH4+ collection via two separate filter trains that includes (1) a Nylon filter plus a 5% citric acid impregnated cellulose filter and (2) a citric acid impregnated glass fiber filter for δ15N(NH4+) analysis. Our results indicate significant NH4+ volatilization off the Nylon filter that is quantitatively collected on the backup 5% citric acid impregnated cellulose filter in both laboratory and ambient environment collections, indicating the need to analyze the NH4+ collected on both filters. However, NH4+ was found to be quantitatively collected using a single acid impregnated glass fiber filter in both laboratory and field tests. The laboratory results indicate a δ15N(NH4+) precision of ±0.9‰ (1σ; n = 24) and ±0.6‰ (n = 9) for the nylon plus acid impregnated cellulose filters and the acid impregnated glass fiber filter, respectively. This δ15N(NH4+) precision is confirmed in field replicate collections with reproducibility found to be within 1.1‰ and with [NH4+] typically within 13.3%. We note that we have limited our tests to the use of a citric acid, which we believe should represent a limiting case, as stronger acids such as phosphoric acid should perform as well and if not better for retaining NH4+. These results have important consequences for previously reported δ15N(NHx) values. Most previous studies have speciated NHx for δ15N analysis via collection of NH4+ on a particulate filter followed by collection of NH3(g) on an acid coated filter. However, our results demonstrate that significant NH4+ volatilization off a particulate filter can occur, which has a large isotopic fractionation that was measured to range from 27.6 ±2.5‰ to 33.1 ±0.8‰, consistent with theoretical estimates of NH4+(s) or NH4+(aq) isotopic exchange with NH3(g). Thus, there is potential for a positive δ15N bias in previous δ15N(NH4+) measurements from aerosol filters and a negative δ15N bias in downstream acid filters utilized for speciated NH3(g) collection for δ15N(NH3) measurement. The tested NH4+ collection method (denuder + filter train) represents the first method that has been demonstrated to properly provide NHx speciation for robust simultaneous measurement of δ15N(NH3), δ15N(NH4+), Δδ15N, and δ15N(NHx) as demonstrated by field replicate collections.


23


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Due to the simplicity and cost-effectiveness of collection of NH4+ using a single acid impregnated glass fiber filter, this should be the recommended approach if NHx monitoring and δ15N(NHx) measurement is the primary goal. While the collection of NH4+ using both a Nylon and an acid impregnated cellulose filter may complicate the system, it may be the best approach for δ15N measurement of speciated NHx along with simultaneous inorganic gaseous/particulate concentration measurements. Utilizing a basic (e.g. sodium carbonate) and acidic (e.g. citric acid) coated denuder prior to collection of PM2.5, will selectively remove the acidic (e.g. HNO3, HCl) and basic (NH3) gases, respectively. The PM2.5 that is not removed via the denuders will be collected on the Nylon filter that has been shown to quantitatively capture semi-volatile particulate nitrate and incorporation of a back-up acid coated cellulose filter will quantitatively capture the volatized NH4+. Quantitative collection of inorganic PM2.5 and inorganic gases (denuded downstream) is desirable, as it will enable inorganic thermodynamic modeling involving the NH4+-NO3--SO42- system such as ISORROPIA, with new isotopic constraints (δ15N(NH3) & δ15N(NH4+)) to evaluate this thermodynamic system.

Acknowledgements. W.W.W. acknowledges support from an Atmospheric and Geospace Sciences National Science Foundation Postdoctoral Fellow (Grant # 1624618) during the course of this study. This research was also supported by funding to M.G.H. from the National Science Foundation (AGS 1351932). The authors also acknowledge support from an Institute at Brown for Environment and Society Internal Seed Grant (GR300123). We thank Ruby Ho and Joseph Orchardo for laboratory assistance.

Supporting Information. Laboratory collection schematic. Filter NH4+ blank summary. NH4NO3 nebulization and collection on Nylon and acid cellulose filter summary. (NH4)2SO4 nebulization and


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collection of Nylon and acid cellulose filter summary. NH4NO3 nebulization and collection on acid glass fiber filter. NH3(g) collection on acid coated filter summary. Ambient replicate collection summary.

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TOC Artwork.


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Selective Collection of Particulate Ammonium for Nitrogen Isotopic

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