Collection of Ammonia for High Time-Resolved Nitrogen Isotopic

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Collection of Ammonia for High Time-Resolved Nitrogen Isotopic Characterization Utilizing an Acid-Coated Honeycomb Denuder Wendell W. Walters, and Meredith G. Hastings Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01007 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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Analytical Chemistry

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Collection of Ammonia for High Time-Resolved Nitrogen Isotopic Characterization Utilizing an Acid-

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Coated Honeycomb Denuder

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Wendell W. Walters*†,‡ and Meredith G. Hastings†,‡

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†

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Providence, RI 02912.

6

‡

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02912

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

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

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9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

*Corresponding author: [email protected]

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ACS Paragon Plus Environment

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.

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Nitrogen stable isotope analysis (δ15N) of ammonia (NH3) has shown potential to be a useful tool for

29

characterizing emission sources and sink processes. However, to properly evaluate NH3 emission sources

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and sink processes under ambient conditions it is necessary to collect and characterize the chemical

31

speciation between NH3 and particulate ammonium (p-NH4+), together referred to as NHx. Current NH3

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collection methods have not been verified for their ability to accurately characterize δ15N-NH3 and/or

33

provide necessary chemical speciation (i.e., δ15N-NH3 and δ15N-NH4+). Here, we report on the suitability

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of an established collection device that can provide NHx speciation, an acid-coated (2% citric acid (w/v) +

35

1% glycerol (w/v) in 80:20 methanol to water solution) honeycomb denuder (HCD) with a downstream

36

filter pack housed in the ChemComb Speciation CartridgeTM (CCSC), for characterizing δ15N-NH3 under

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a variety of laboratory-controlled conditions and field collections. The collection method was tested under

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varying NH3 concentration, relative humidity, temperature, and collection time at a flow rate of 10 liters

39

per minute (LPM). The acid-coated HCD collection device and subsequent chemical processing for δ15N-

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NH3 analysis is found to have excellent accuracy and precision of ±1.6‰ (2σ), with an operative capacity

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of ~400 µg of collected NH3 for concentrations ≤207 ppbv. This work presents the first laboratory

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verified method for δ15N-NH3 analysis and will be useful in future air quality studies.

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53

Introduction

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Ammonia (NH3) is the primary alkaline molecule in the atmosphere and plays a key role in numerous

55

atmospheric processes that have important implications for human health and climate via new particle

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nucleation1,2. Understanding NH3 emission sources and sink processes, especially in urban areas, is a

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challenging task due to the coexistence of many locally produced sources including emissions from local

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traffic3–6, fuel combustion, industrial processes, humans7–11, and transport of agricultural NH312–14.

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Analysis of the stable isotope composition of trace gases is an established tool for quantifying emission

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sources,15 as various surface sources and sink processes often exhibit characteristic isotopic compositions

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(“fingerprints”).16 This tool allows for an understanding of emission sources as well as means to track the

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chemical and physical processes responsible for removal of trace gases at a process level. Previous

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researchers have used this technique to evaluate contributions of various sources and sink processes of

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numerous relatively-long lived trace gases including methane (CH4),17,18 carbon dioxide (CO2),19,20 carbon

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monoxide (CO),21,22 and nitrous oxide (N2O).23,24 Applying this tool to relatively-short lived NH3 might

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also enable an evaluation of its emission sources and sink processes, providing distinct information

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amidst large spatial and temporal variabilities. Previous measurements of the nitrogen (N) stable isotope

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composition of NH3 (δ15N-NH3)1 emission sources have reported large NH3 source δ15N variability and

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suggest considerable overlap between various emission sources (Fig. 1)25–31. However, some of these

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observations also include NH3 process driven δ15N effects, such as NH3 volatilization resulting in a

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gradual increase in δ15N. Thus, there is a need to better diagnose δ15N values associated with emissions

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sources, while also constraining chemical or physical process effects.

73 74

1

δ15N(‰) = [(15Rsample)/(15Rreference) – 1] × 1000, where 15R is the ratio of 15N/14N and air N2 is the N isotopic reference.

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75 76 77

Fig. 1. Previously reported δ15N-NH3 values of NH3 source emissions and NH3 volatilization (e.g.

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fertilizer and animal waste)25–31.

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techniques (legend), and many sources and process driven effects (NH3 volatilization) show large

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variations in δ15N-NH3. Coal-fired power plants are indicated as CFPP.

Prior measurements have used a wide variety of NH3 collection

81 82

Numerous collection techniques have been used to collect NH3 as NH4+ for off-line N isotopic analysis

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including wet scrubbers25–27, passive diffusion samplers31,34, an acidic absorbing solution contained within

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an evacuated gas sampling bulb28, and active sampling using acid impregnated filters29,30 (Fig. 1). While

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these methods have been designed to accurately reflect NH3 concentrations, they have not been

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laboratory-verified for their suitability for δ15N-NH3 analysis; therefore, the δ15N accuracy and/or

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precision of these collection devices are uncertain. The potential for collection methods to impact δ15N-

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NH3 was highlighted in Skinner et al., (2006), in which a comparison between several active and passive

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NH3 collection devices including gas-scrubbing bubbler, moss bag, shuttle sampler, and diffusion tube

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revealed significantly different δ15N-NH3 values and variances even when sampling the same emission 4 DRAFT


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source (field fumigation site). This result, and the contrasting reported environmental values, questions

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the accuracy of previously reported δ15N-NH3 values.

93 94

An additional complication when sampling ambient air for δ15N-NH3 analysis is that NH3 exists in

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thermodynamic equilibrium with nitric acid (HNO3), hydrochloric acid (HCl), and their neutralized

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condensed phases including ammonium nitrate (NH4NO3) and ammonium chloride (NH4Cl)35–40. NH3

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will also neutralize sulfuric acid (H2SO4), but due to the low vapor pressure of its product ammonium

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sulfate ((NH4)2SO4), it resides nearly completely in the condensed phase40. The coexistence of NH3 and

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NH4+, together referred as NHx, makes it possible that N isotopic equilibrium between NH3 and NH4+

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could occur25,29, which will scramble the 14N and 15N isotopes between NH3 and NH4+ based on statistical

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mechanics41:

102

+ ⇌

+

(R1)

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The theoretical equilibrium constant (K) or isotopic fractionation factor (α) for R1 is 1.034 ± 0.002 at

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298.1 K41, indicating that isotopic equilibrium will favor the partitioning of 15N into p-NH4+, resulting in

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higher δ15N values in p-NH4+ than in NH3 (Eq. 1).

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. = . =

= 1.034 ± 0.002

(Eq. 1)

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Additionally, unidirectional neutralization reactions involving NH3 and H2SO4 have been suggested to

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result in a kinetic isotope effect leading to the initial δ15N of p-NH4+ to be -28‰ relative to ambient NH3

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based on relative diffusion rates of 14NH3 and 15NH342. Therefore, speciated δ15N measurements of NHx

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could provide valuable information about NH3 gas to particle conversion and new particle formation,

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indicating whether p-NH4+ is formed under either kinetic or equilibrium controlled processes. Due to the

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potential for δ15N of ambient NH3 to be altered during particle formation, it is critical that under ambient

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air sampling conditions the collection technique must allow for the speciation of NHx to evaluate the

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impact of NH3 neutralization reactions/thermodynamic equilibrium on δ15N-NH3. The denuder-filter

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combination is an established NH3 sampling technique that will effectively speciate NHx43–49. In this

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system, NH3 is first removed from the sampled air stream on an acid-coated glass denuder followed by

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collection of p-NH4+ on a downstream filter pack. These methods have been in use for decades using

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several denuder geometries including a simple boroscillate tube43, honeycomb denuder44,45, and annular

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denuder46,50 geometries to measure NHx concentrations12,48,51–53.

120 121

Here we present a new application of the denuder-filter sampling technique for characterizing δ15N-NH3.

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We have extensively tested a commercially available honeycomb denuder coated with an acid solution

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and housed in a ChemComb Speciation CartridgeTM, which has been well-characterized for its ability to

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speciate gas and particulate matter components44,45, for its suitability for δ15N-NH3 analysis under a

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variety of simulated environmental conditions and field collections. This work is a crucial step in

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developing a record of reliable δ15N-NH3 measurements. Ultimately, a field deployable method suitable

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for δ15N-NH3 analysis and capable of achieving high time-resolved measurements will help make

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considerable progress on evaluating spatial and temporal variability in environmental NH3 emission

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sources and sink processes.

130 131

Experimental Section

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NH3 Collection Using an Acid-Coated Honeycomb Denuder. NH3 is collected on a honeycomb

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denuder (HCD) coated with a 2% citric acid (w/v) 1% glycerol (w/v) in an 80:20 methanol to ultra-high

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purity water (18.2 MΩ, Milli-Q (MQ)) solution and housed in a ChemComb Speciation CartridgeTM

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(CCSC). We note that a range of acid coating solutions may be used to capture NH3 including phosphoric

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acid and oxalic acid50; however, we have limited our tests to citric acid due to its low [NH4+] blanks

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compared to other acids43, making it more suitable for environmental research and the need for high-time 6 DRAFT


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resolved measurements. The employed sampling system has been extensively described and characterized

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in previous works44,45. Briefly, the CCSC contains a polytetrafluoroethylene (PTFE) coated inlet to limit

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NH3 loss (previous work has found this inlet to have an NH3 transmission efficiency greater than 97.3%

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utilizing this PTFE coating54) and a PTFE coated stainless steel PM2.5 impactor that removes particulate

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matter larger than 2.5 µm and facilitates an evenly distributed flow through the 212 hexagonal channels of

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the HCD at a flow rate of 10 liters per minute (LPM). The indented circular reservoir of the impactor

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plate was evenly covered with PTFE grease (Chemours Krytox GPL 207) to prevent particle bounce and

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to limit NH3 absorption. Within the CCSC, a borosilicate transition piece allows laminar flow to be

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achieved followed by two acid-coated HCDs in series that are separated by a PTFE spacer. The first

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“capture” HCD is used to remove NH3 from the sampled airstream, and the second HCD is used as

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control to check for possible NH3 breakthrough in our control tests. After the HCD, there is an available

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filter pack within the CCSC to collect particulate matter on a filter, though this was not used during our

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control tests of NH3 collection. An image of the sampling apparatus is provided in the Supporting

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Information (Fig. S1).

152 153

NH3 Collection Experiments. The laboratory NH3 collection set-up is depicted in Fig. 2. Briefly, a 20.7

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(±1%) ppmv flow of NH3 in N2 tank (Praxair) was controlled using a mass-flow controller. This tank was

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diluted to achieve variable [NH3] using the laboratory N2 compressed gas line that was further purified

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using Molecular Sieve 5Å and Silica Gel. NH3 was not detectable in the purified N2 gas line. The N2

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compressed gas line was divided using a 3-way union fitting made from PTFE with one of the lines

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passing through a gas-washing bottle containing MQ water and then through a water trap, allowing for

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separate N2 lines containing “wet” N2 and “dry” N2. The flow rates of wet N2 and dry N2 were controlled

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using separate rotameters. Altering the relative flow rate of wet N2 to dry N2 allowed for testing various

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levels of relative humidity (RH). The flow of wet N2 and dry N2 were combined using a PTFE 3-way

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union fitting. The total N2 flow was then combined with the flow from the NH3 tank using another 3-way 7 DRAFT



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PTFE union fitting that was also connected to the inlet of the CCSC. The outlet of the CCSC was

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connected to a rotameter to ensure that “in-flow” was approximately equal to the “out-flow”, and

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temperature(±0.5°C) and RH(±3%) were monitored in the excess out-flow (Elitech GSP-6). Due to the

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reactiveness or “stickiness” of NH3 with surfaces, all tubing in our setup was composed of polyvinylidene

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fluoride (PVDF), which has been shown to be chemically inert to NH355.

168 169

Fig. 2. Schematic diagram of the experimental setup for the evaluation of NH3 collection using an acid-

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coated honeycomb denuder housed in the ChemComb Speciation CartridgeTM. The sample lines were

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made of polyvinylidene fluoride, and the three-way union connectors were made from PTFE to limit the

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absorption of NH3.

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The sampling cartridge is designed for an optimal flow rate of 10 LPM44, so all experiments were

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conducted at this flow rate. Variable “target” NH3 concentrations ([NH3]) were tested including 31.3,

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107, 207, and 2070 ppbv; however, we note the possibility that the [NH3] in the gas tank may not be as

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accurate as reported, and that NH3 loss in the sampling line may lower the actual flowed [NH3]. The

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targeted [NH3] encompass a range somewhat typical of what is expected for a range of environmental

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sampling conditions, including near emission sources and ambient air. Ambient conditions will likely

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have [NH3] lower than 31.3 ppbv (e.g. near single ppbv levels)51, but this was the lowest concentration we

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could achieve given a mass flow controller minimum flow rate of 0.015 LPM. However, this should not

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limit our evaluation of the applicability of this collection technique because denuders are limited by high

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concentrations of the gases to be denuded44, such that lower concentrations should be denuded as

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efficiently as the higher tested concentrations. NH3 flow tests were also conducted over a variety of

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temperatures by placing the sampling cartridge and inlet in an ice bath and heating the sample line and

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cartridge using heat tape to test the NH3 collection efficiency under a variety of environmentally-relevant

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temperatures. After NH3 collection, the HCD and single-channel denuder tube were extracted using 30

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mL of MQ water. NH4+ blanks in the extraction solution were always found to be less than 0.3 µM-NH4+

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(n = 12), near our analytical detection limit (see below).

190 191

Due to NH3 reactiveness with surfaces, we also evaluated the CCSC as a source of NH3 loss (e.g. PTFE

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coated inlet and PTFE coated impactor plate) and a potential source for δ15N-NH3 fractionation. This was

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tested by collecting NH3 using a perfluoroalkoxy (PFA) gas-scrubbing impinger containing 30 mL of a

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2% citric acid solution connected in series with a second PFA impinger to check for breakthrough. These

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control tests were conducted at an [NH3] of 2070 ppbv, a flow rate of 1 LPM, and for a collection time of

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5 minutes. An impinger with an acidic scrubbing solution is an established efficient collector of NH3 (e.g.

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U.S. EPA Method 17), but does not adequately provide the speciation of NHx that suits our needs for

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δ15N-NH3 determination in ambient air. After NH3 collection the solution contained within the gas

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scrubbing impinger were transferred to a 50-mL centrifuge tube and stored in a refrigerator until future

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concentration and isotope analysis.

201 202

[NH4+] Concentration and δ15N Isotopic Analysis. All collected NH4+ samples were analyzed for their

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concentration using colorimetric analysis based on the indophenol blue method56 that was automated

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using a discrete UV-Vis analyzer (Westco SmartChem 2.0). Reproducibility calculated from replicate

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measurements was ±0.5 µM-NH4+. The determination of δ15N-NH4+ follows previously described

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methods57. Briefly, samples are diluted to at least 10 µM-NH4+ using MQ water to a volume of 10 mL in

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50 mL vials that are acid-washed, triple-rinsed with MQ water, and ashed at 500 °C for 4 hours. The

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samples are then oxidized to nitrite (NO2-) using hypobromite (BrO-) in an alkaline solution, which was

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synthesized as previously described in Zhang et al., (2007). Since accurate isotopic measurements are

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sensitive to the conversion of NH4+ to NO2- (i.e. incomplete conversion would lead to undesirable δ15N

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fractionation)57, reaction conditions including volume of the oxidation solution and reaction time were

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optimized for our sample matrix. [NO2-] oxidation yields were measured using a standard colorimetric

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absorption technique (e.g. US EPA Method 353.2) automated using a discrete UV-Vis analyzer (Westco

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SmartChem 2.0). Reproducibility based upon replicate measurements of samples and quality control

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standards was found to be ±0.3 µM-NO2-. Next, 0.4 mL of 0.4 M sodium arsenite (NaAsO2) was added to

216

the samples to remove remaining BrO-.

217 218

Once the NH4+ samples are converted to NO2-, they can be routinely analyzed for their N isotopic

219

composition using previously established chemical methods that convert NO2- to N2O57,58. Briefly, 20

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nmol of NO2- samples are transferred to 20 mL vials that were acid-washed, triple-rinsed with MQ water,

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and ashed at 500 °C for 4 hours. Vials are crimp capped with PTFE/butyl septa and flushed with helium

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(He) for 10 minutes. NO2- is subsequently reduced to N2O using 2 mL of 1M sodium azide buffered in

223

30% acetic acid solution58. Subsequently, the solutions are neutralized using 6M NaOH with a 0.1%

224

phenolphthalein solution to indicate when the solutions reached pH > 8.2, which is done to limit the

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possibility of toxic hydrazoic acid (HN3) from escaping into the laboratory. Samples are then analyzed

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for their δ15N-N2O composition using an automated N2O extraction system coupled to a continuous flow

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Isotope Ratio Mass Spectrometer for m/z 44, 45, and 46 measurements59. In each sample batch,

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unknowns are calibrated with respect to two internationally recognized NH4+ isotopic reference materials, 10 DRAFT


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IAEA-N2 and USGS25, with δ15N values of 20.3‰, and -30.3‰, respectively60,61, that are run between

230

approximately every 10 unknowns. The isotopic standards undergo the exact same chemical processing

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as the unknowns and are used to correct for isotopic fractionation resulting from the chemical conversion

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of NH4+ to N2O. For each batch sample analysis, replicates of two NO2- standards with known isotope

233

values (RSIL-N7373 and RSIL-N10219 with δ15N = -79.6‰ and 2.8‰, respectively)62 are run as a

234

quality control to monitor the conversion of NO2- to N2O and system stability. Corrections to determine

235

δ15N-NH4+ are performed by accounting for isobaric influences, blank effects, and calibrating the

236

unknowns to the internationally recognized δ15N-NH4+ standards.

237 238

Results and Discussion

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The evaluation of the performance of a collection method for the isotopic analysis of a reactive gas should

240

involve a check of the following parameters (adapted from Perrino et al., 1990):

241

1. Matrix suitable for isotopic analysis. The matrix of the collection reagent must be suitable for

242

the processing associated with converting the collected reaction product to an appropriate form

243

for isotopic analysis.

244 245 246

2. Collection efficiency. The collection method must be shown to be a quantitative sink for the target analyte to limit an isotopic bias. 3. Selectivity of the collection method. The collection method should only retain the target analyte

247

and no other compounds should yield the same reaction product. This is expected to be a trivial

248

requirement for NH3 since it is the primary inorganic alkaline gas50.

249

4. Known operative capacity. The operative capacity (defined as the amount of target analyte that

250

can be collected while maintaining a collection efficiency >95%)63 must be known for the

251

intended sample collection duration.

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252 253 254

5. Stability of reaction product. The collected reaction product should not be released or compromised once collected. 6. Comparison to other collection devices. Generally, there are no internationally recognized

255

isotopic standards for reactive gases. Therefore, the potential bias of the collection device,

256

primarily inlet loss, needs to be evaluated. For example, by comparison with other collection

257

devices in which this bias is expected to be low.

258 259

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7. Field applicability. Field applicability of the collection method should be demonstrated for the intended sample collection times with collection performed in replicates.

260 261

Matrix effect on NH4+ oxidation. The limiting step in δ15N-NH4+ analysis utilizing methods described in

262

Zhang et al., 2007 is the oxidation of NH4+ to NO2-. Thus, we evaluated the impact of our sample matrix,

263

2% citric acid + 1% glyercol coating solution extracted in 30 mL of MQ, on the conversion of NH4+ to

264

NO2- for a variety of reaction parameters including variable BrO- amounts and reaction times. Solutions

265

containing 10 mL of 10 μM-NH4+ were created using either the sample matrix or MQ water and were

266

oxidized using either 1 or 2 mL of the alkaline BrO- solution. Triplicates were performed for each unique

267

matrix and BrO- volume combination. Fig. 3 displays the NO2- oxidation yields that were measured as a

268

function of time ranging from 0.2 to 31.5 hours. Quantitative NH4+ to NO2- oxidation is found in our

269

sample matrix for both 1 and 2 mL additions of the alkaline BrO- solution, and complete oxidation is

270

achieved in as early as 20 minutes (Fig. 3). Additionally, quantitative conversion was found to be

271

maintained for up to 31.5 hours, indicating [NO2-] stability in the oxidation solution (Fig. 3). Therefore, it

272

is our recommendation that 1 mL of the alkaline BrO- solution is enough to ensure complete NH4+

273

oxidation to NO2- for a 20-minute reaction time in a 10-mL solution containing 10-µM of NH4+. Since

274

incomplete NH4+ to NO2- oxidation would result in an undesirable isotope effect, it is also our

275

recommendation that oxidation yields should be checked for each sample. Once converted to NO2-,

276

samples can be readily reduced to N2O using azide in an acetic acid buffer as previously described57,58. 12 DRAFT


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We find δ15N-NH4+ for isotopic standards in the sample matrix to have a standard deviation of 0.6‰ and

278

0.7‰ (1σ) for IAEAN2 (n = 29) and USGS25 (n = 27), respectively, resulting in an overall pooled

279

standard deviation of 0.7‰. Overall, these results demonstrate the sample matrix to be suitable for δ15N-

280

NH4+ analysis.

281 282

Fig. 3. Effect of reaction time, BrO- volume addition, and sample matrix on the oxidation of NH4+ to

283

NO2- and stability of the NO2- reaction product. Quantitative oxidation is achieved within 20 minutes for

284

both 1 and 2 mL additions of BrO-, and the NO2- product is stable for at least 31.5 hours.

285 286 287

NH3 Collection Performance. The collection efficiency of the 2% citric acid + 1% glycerol coated HCD

288

for NH3 capture for varying sampling conditions is displayed in Fig. 4. Collection efficiency (E; Eq. 2)

289

and total recovery (Q; Eq. 3) were calculated according to:

290

! = 100"1 # $ ⁄$ &

(Eq. 2)

291

$'(' = $ + $

(Eq. 3) 13

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Page 14 of 27

292 293

Where 1 and 2 refer to the capture and breakthrough denuder, respectively. Overall, NH3 collection

294

efficiency was found to be higher than the 95% threshold63 under most sampling conditions for a

295

collection amount up to 388.8 µg of NH3, except for the flow tests at an extremely elevated [NH3] of 2070

296

ppbv (Fig. 4). At this elevated [NH3], collection efficiency was observed to quickly diminish after an NH3

297

collection amount of 14.8 µg. This result is consistent with Koutrakis et al. 1993, that found the acid

298

coated HCD to be best suited for an [NH3] lower than 200 ppbv. Our results indicate excellent NH3

299

collection efficiency between 1.3 to 388.8 µg-NH3 for a collection time up to 24 hours (Table S1). Since

300

we found collection efficiency to deteriorate for NH3 collection amounts greater than 388.8 µg of NH3

301

(Fig. 4), we suggest this amount as the operative capacity of the collection system.

302 303

The measured δ15N-NH3 for the NH3 capture tests are also shown in Fig. 4. When collection efficiency is

304

found to be >95%, we find reproducible δ15N-NH3 values with a 2σ of ±1.6‰ (n = 75). Interestingly, the

305

measured 2σ is very close reproducibility of our NH4+ isotopic standards of 1.4‰ (2σ), suggesting that

306

the limiting factor in our analysis is the off-line processing for δ15N analysis rather than NH3 collection.

307

When collection efficiency is less than 95%, we observe an undesirable δ15N-NH3 isotope effect (Fig. 4);

308

thus, it is critical to stay within the operative capacity of the tested NH3 collection device. Based on

309

ANOVA single factor analysis, we find no statistical differences in δ15N-NH3 for any of the simulated

310

conditions when collection efficiency is >95% (p-value is 0.97), suggesting that the tested collection

311

technique is suitable for a variety of environmental conditions (i.e. temperature and relative humidity)

312

except for elevated [NH3] (e.g. >2000 ppbv). We note that we do not know the absolute δ15N-NH3 value

313

of our NH3 tank, but comparison with other collection devices can indicate any potential sampling

314

artifacts that might bias our δ15N-NH3 values such as NH3 loss on the sampling cartridge inlet (see

315

below).

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Page 15 of 27 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


316 317 318

Fig. 4. Summary of NH3 capture data utilizing a 2% citric acid + 1% glycerol coated honeycomb denuder

319

(HCD) and a gas scrubbing impinger containing a 2% citric acid solution (GSI) including (a) NH3

320

collection efficiency (%) as a function of amount of collected ammonia (µg) from laboratory flow tests

321

for a range of [NH3] and (b) measured δ15N-NH3 as a function of collection efficiency for the laboratory

322

flow tests. Collection efficiency of 95% (minimum target) is indicated as the solid black line. When NH3

323

collection efficiency was found to be >95%, the measured δ15N-NH3 was found to be (x̄±2σ) -2.5±1.6‰

324

(n=75) and -2.4±2.0‰ (n=5) for the HCD and GSI, respectively.

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Page 16 of 27

325 326

Stability of Collected NH4+. Previous work utilizing a 1% citric acid coating solution found significant

327

NH3 desorption as high as 41% after 12 hours, which could severely limit its use for atmospheric

328

sampling for extended periods of time50. We tested for this possibility in our tested sampling system by

329

loading approximately the same amount of NH3 on several denuders and immediately extracted the NH4+

330

reaction product as well as waited 24 and 72 hours before extraction. This test was conducted in

331

triplicates for each extraction time, and the extracted [NH4+], δ15N-NH3, and calculated desorption loss are

332

displayed in Table 1. Based on the average of the three replicates for each test condition, we estimate a

333

desorption of 1.8% and 3.6% for 24-hr and 72-hr, respectively. However, we find no statistical

334

differences between QTOT, E(%), or δ15N-NH3 between the immediately extracted samples and those that

335

were extracted after 24 and 72 hr. This suggests that NH3 desorption in our sampling system and coating

336

solution is minimal for a period up to 72 hours. Both our flow tests and Koutrakis et al., 1993 found an

337

NH3 collection efficiency of >99.5% for 24-hour flow times, further suggesting that the 2% citric acid +

338

1% glycerol coating solution on a HCD should be sufficient for extending sampling periods (up to at least

339

24 hours). While we can only speculate on differences in NH3 desorption between our work and Perrino

340

and Gherardi, 1999, we can point out that we have tested a HCD coated with a 2% citric acid + 1%

341

glycerol solution while Perrino and Gherardi, 1999 performed their testing using an annular denuder

342

coated with a 1% citric acid solution.

343 344 345 346

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Page 17 of 27 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


347

Table 1. Summary of the results of NH3 desorption following capture on a 2% citric acid +1% glyercol

348

coated HCD. Values are displayed as averages (±1σ) for triplicates and indicate the amount of collected

349

NH3 (QTOT), collection efficiency (E), δ15N-NH3, and calculated percentage of desorbed NH3.

350

Elapsed QTOT (µg) E(%) Time (hr) 11.2 ± 1.2 98.5 ± 0.1 0 11.0 ± 0.8 97.8 ± 0.3 24 10.8 ± 0.7 98.0 ± 0.7 72 * calculated as 100"1 # $)*)"+ ,- . /-& $)*)"0 /-& )

δ15NNH3(‰) -2.9 ± 0.5 -2.7 ± 0.6 -2.3 ± 0.3

Desorbed amount (%)* -1.8 3.6

351 352

Comparison to other NH3 Collection Devices. Collection efficiencies and δ15N-NH3 for the gas-

353

scrubbing impinger (GSI) are also displayed in Fig. 4. Overall, collection efficiency was always found to

354

be greater than 99.2% and δ15N-NH3 were found to be -2.4±2‰ (±2σ), respectively, for 5 trials (Table

355

S2). The measured δ15N-NH3 are found to be in statistical agreement with the measured δ15N-NH3 from

356

the acid-coated HCD housed in a CCSC (-2.7±1.6‰) when collection efficiency was >95% (Fig 2). This

357

comparison indicates that similar δ15N-NH3 values were achieved for the various NH3 collection devices,

358

suggesting that the CCSC housing is not a significant source of NH3 loss and does not impart a δ15N-NH3

359

bias compared to other collection devices in which inlet loss is assumed to be negligible.

360 361

Field Applicability. Ambient urban air in Providence, RI, USA was sampled in replicates (side-by-side

362

collections) for varying times including 1 (n=3), 24 (n=5), and 48 (n=4) hours by flow-controlling an air

363

sampling pump attached to the outlet of the CCSC to 10 LPM (Table S3). Collection efficiency was

364

found to be >98% for each sampled time interval (Fig. 4), further indicating minimal NH3 desorption for

365

sampling periods up to 48 hours utilizing our collection approach. The collected NH3 amount ranged

366

from 1.7 to 66.9 µg of NH3, which is well-within the tested operative capacity (~400 µg of NH3) of the

367

collection device (Fig. 4). Average reproducibility, calculated as the average of absolute differences 17 DRAFT



Page 18 of 27

368

between the collected replicate samples for NH3 and δ15N-NH3, is 0.010 ± 0.008 µmol/m3 and 0.9±0.7‰,

369

respectively. Additionally, the average percent error between replicate concentration measurements is

370

found to be 16.9 ± 9.3%. The observed average reproducibility in δ15N-NH3 is close to our recommended

371

2σ error for this method of ±1.6‰ based on our laboratory control tests. The interpretation of the

372

measured δ15N-NH3 is beyond the scope of this work; however, these samples show that the tested

373

method allows for high-time resolution (on order of an hour) and reliable δ15N-NH3 measurements for

374

sampling periods up to 48 hours. We note that the collection time resolution of this method is dependent

375

upon achieving a minimum needed for isotopic analysis. For this study, 20 nmol samples were processed,

376

therefore requiring a minimum of 0.34 µg of NH3 for field collections.

377 378

Conclusion

379

We have critically evaluated a 2% citric acid + 1% glycerol coated HCD for its suitability for δ15N-NH3

380

analysis that has included a careful consideration of reagent blanks, matrix effects, collection efficiency,

381

selectivity, operative capacity, reaction product stability, and isotopic bias. Our results indicate the tested

382

method to be appropriate for δ15N-NH3 determination with a precision of ±1.6‰ (2σ; n = 75), for an

383

operative capacity (collection efficiency > 95%) of ~400 µg of collected NH3. Our field tests indicate

384

that the collection method is suitable for δ15N-NH3 characterization of ambient air with δ15N-NH3

385

resolution on the order of an hour. The high time resolution and excellent δ15N-NH3 precision of this

386

method will be applied in future work to improve our understanding of NH3 emission sources, sink

387

processes, and diurnal patterns of NH3. Additionally, the denuder-filter pack combo collection technique

388

has the potential to be extended for isotopic analysis of other gaseous compounds such as sulfur dioxide

389

(SO2), nitric acid (HNO3), and nitrogen dioxide (NO2) using an appropriate HCD coating for quantitative

390

gas collection. Utilizing the acid-coated HCD for NH3 collection, a carbonate-coated HCD for acidic gas

391

collection (e.g. HNO3 and SO2), and an appropriate filter for PM2.5 collection could enable the

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392

simultaneous determination of δ15N-NH3 and δ15N-NH4+(p) and a new tool for investigating the

393

atmospheric dynamics of NHx.

394 395 396 397

Acknowledgements. W.W.W. acknowledges support from an Atmospheric and Geospace Sciences

398

National Science Foundation Postdoctoral Fellow (Grant # 1624618) during the course of this study. This

399

research was also supported by funding to M.G.H. from the National Oceanic and Atmospheric

400

Administration (AC4 Award NA16OAR4310098) and the National Science Foundation (AGS 1351932).

401

We thank Ruby Ho and Nadia Colombi for laboratory assistance.

402 403

Supporting Information. Sampling apparatus image (Fig. S1). Summary of honeycomb denuder

404

control tests (Table S1). Summary of gas scrubbing impinger control tests (Table S2). Summary of

405

ambient air replicate samples (Table S3).

406 407

References

408 409 410 411 412 413 414 415 416 417 418 419

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570


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571 572 573

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Collection of Ammonia for High Time-Resolved Nitrogen Isotopic

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