A Collection Method for Isotopic Analysis of Gaseous Nitrous Acid

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A Collection Method for Isotopic Analysis of Gaseous Nitrous Acid (HONO) Jiajue Chai, and Meredith G. Hastings Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03561 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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Analytical Chemistry

A Collection Method for Isotopic Analysis of Gaseous Nitrous Acid (HONO) Jiajue Chai*,† and Meredith G. Hastings† †

Department of Earth, Environmental and Planetary Sciences and Institute at Brown for

Environment and Society, Brown University, Providence, RI 02912, United States

Corresponding Author *Phone: 401-863-6853; fax: 401-863-3839; e-mail: [email protected].

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ABSTRACT The sources and chemistry of gaseous nitrous acid (HONO) in the environment are of great interest. HONO is a major source of atmospheric hydroxyl radical (OH), which greatly impacts air quality and climate. HONO is also a major indoor pollutant that threatens human health. However, the large uncertainty of HONO sources and chemistry hinders an accurate prediction of the OH budget. Isotopic analysis of HONO may provide a tool for tracking the sources and chemistry of HONO. In this study, a modified annular denuder system (ADS) was developed to quantitatively capture HONO for offline nitrogen and oxygen isotopic analysis (δ15N and δ18O) using the denitrifier method. The ADS method was tested using laboratory generated HONO (400 ppbv-1 ppmv) and validated by parallel HONO collection with a standard, basic impinger (BI) method. The ADS system shows complete capture of HONO without isotopic fractionation. The uncertainty (1 sigma) based on repeated measurements across the entire analytical procedure is 0.6‰ for δ15N and 0.5‰ for δ18O. The ADS method was also tested in roadside collections of ambient HONO (0.4-1.3 ppbv) for isotopic analysis, and was found to be robust for low concentration collections over 3- and 12-hour collection times. In order to ensure ability to use this method in the laboratory and in the field, storage conditions for the collected HONO samples were tested and samples can be stored with consistent δ15N and δ18O for 60 days. This method enables future work to utilize the isotopic composition of HONO for studying HONO chemical formation pathways, as well as atmospheric sources and chemistry.


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Nitrous acid (HONO) is a key atmospheric species as it is an important nighttime sink of reactive nitrogen NOx (NO+NO2) and a daytime source of hydroxyl radical (OH), the major oxidant species of the atmosphere that determines the lifetime of many trace gases. Direct photolysis of HONO is the dominant source of OH in the early morning via R1, when other primary OH sources such as the photolysis of ozone and formaldehyde are still weak.1 HONO → OH + NO

R1

Recent field and modeling studies found HONO photolysis accounts for up to 30%-60% on average of the daily OH budget in the boundary layer2,3,4 with ambient HONO mixing ratios ranging from several pptv in rural areas up to a few ppbv in highly polluted regions. 5,6,7,8 OH is the major atmospheric oxidant which initiates the photochemical degradation of volatile organic compounds (VOCs) emitted by natural and anthropogenic sources. 9 This process, in the presence of NOx, is mainly responsible for air quality degradation via photochemical smog production (i.e., ozone, particulate matter and secondary aerosols) that threaten human10,11 and ecosystem health.12 OH can also oxidize SO2 to produce H2SO4, the primary source for sulfate aerosol, which has an indirect effect on climate via cloud formation. 13 On the other hand, the concentration of OH directly controls the lifetime of greenhouse gases such as methane. Therefore, HONO has an important connection to the global radiative budget and climate.

HONO is also connected to human health impacts, especially due to exposure to high concentrations indoors. In indoor environments, HONO can accumulate via emissions from gas stoves and space heaters.14 Indoor HONO has been found at significantly higher mixing ratios (up to 90 ppbv)15,16,17 than outdoor ambient HONO (up to a few ppbv).18 Indoor HONO has been found to impact infant asthma19,20 via oxidative damage and gene mutation. 21,22,23 Moreover, in a tobacco-smoking indoor environment, HONO can easily nitrosate Nicotine to form a variety of carcinogenic nitrosamines mutations and DNA strand breaks especially for infants.

26


24,25

that can induce


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In the atmosphere, nighttime HONO is predominantly produced via heterogeneous reduction of NO2 on wet surfaces (R2)27 and chemical models including this reaction can reproduce observed nighttime HONO concentrations. 2NO2 + H2O (surface) → HONO + HNO3

R2

By contrast, daytime HONO model predictions typically underestimate field observations by several times, indicating large missing sources of HONO. 28 , 29 A number of heterogeneous photochemical pathways for HONO production have been proposed as HONO missing sources, including NO2 conversion to HONO on photoactive surfaces such as soot,30 humic acid31 and organic surfaces;32 and photolysis of nitrate on surfaces, in acidic aqueous solutions and snow.33 Recent studies have indicated significant HONO emissions directly from biomass burning,34 vehicular exhaust, 35,36 and biogenic and nonbiogenic soil emissions.37,38 To date, there is no established HONO source inventory, because of limited numbers of measurements, high reactivity, and the tremendous spatial and temporal variability of HONO. In general, air quality models (such as Community Multiscale Air Quality Model) can only reproduce up to 60% of observed daytime mean HONO levels in the atmosphere by properly including HONO chemical formations and estimated direct emissions in the model, 39 , 40 which still leads to underestimation of concentrations of OH, ozone and secondary aerosols. Given the big gap, it is important to identify and constrain different sources of HONO. Stable isotope analysis offers a potential way of distinguishing and tracking different sources for a specific compound if the isotopic signature of each individual source is distinguishable (e.g., CH441 and CO242). Recent research on the isotopic analysis of gaseous NOx (NO + NO2) has been able to discriminate the ranges of different emission sources (see references 43 , 44 , 45 , and references therein). This method requires that NOx be collected in a scrubber solution in the form of nitrate (NO3-), from which the isotopic signatures of N (expressed in δ


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notation

a

) are determined using the denitrifier method.

measurements of the natural abundance of

15

N/14N and

18

46

Thus far, no direct

O/16O for HONO have been

reported. Wu et al. reported 15N relative exceedance of HONO emitted from soil samples spiked with

15

N labeled urea using high performance liquid chromatography mass

spectrometry (HPLC-MS). 47 Scharko et al. measured the HO15NO emitted from soil fertilized by 15N labeled ammonium sulfate using chemical ionization mass spectrometry (CIMS). 48 For environmental concentrations and isotope natural abundance, these methods have an insufficient detection limit and relatively large uncertainties.

In this study we develop a highly efficient HONO capture system for the purpose of analyzing the δ15N and δ18O of HONO at natural abundance via the denitrifier method. For the capture of HONO, we modify an annular denuder system (ADS) that has been widely used for collecting reactive gases for concentration measurements. 49 , 50 , 51 Fundamentally, gas stream flowing through an annular denuder forms a laminar flow that ensures the maximum sorption of absorbate. Traditional ADS for capturing HONO is comprised of three denuders, with the first denuder coated with sodium chloride (NaCl) solution to capture gaseous nitric acid (HNO3(g)), and the second and third denuders coated with sodium carbonate (Na2CO3) solution to capture HONO and correct for interferences from NO2.52,53,54 However, this setup was found to have interferences due to potential deposition of N-containing species, especially particulate matter and HNO3 evaporated from ammonium nitrate (NH4NO3) particles.55,56 We modified the setup by replacing the first denuder with a particulate filter and a nylasorb filter that remove particulate nitrate and gaseous HNO3, making the system feasible for future field applications. In this paper, we present a laboratory verified technique for consistently and efficiently capturing HONO and determining its isotopic composition δ15N and δ18O. Ultimately, this method will be applied for environmental studies, allowing for potential quantification of indoor and outdoor HONO sources, as well as chemical pathways of formation. a

δ is defined as relative isotopic enrichments of a sample with respect to a reference material. δ=(Rsample ⁄ Rstandard −1)×1000‰, where for δ15N, R=15N ⁄ 14N and Rstandard is atmospheric N2; for δ18O, R=18O ⁄ 16O and Rstandard is Vienna-Standard Mean Ocean Water (VSMOW).



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EXPERIMENTAL SECTION Laboratory Generation of HONO HONO is a highly reactive gaseous species and therefore no commercial HONO gas is available. We built a laboratory HONO generation system based on the work of Febo et al.,57 which is displayed in Figure S1 in supporting information (SI). The HONO system consists of a hydrochloric acid (HCl) diffusion system and a glass reactor containing two glass frits (coarse) that hold 1 g sodium nitrite (NaNO2) (Sigma-Aldrich Reagent Plus). The HCl diffuser contains a temperature controlled 1-L gas-tight glass vessel filled with HCl solution (12 M) and HCl permeable Teflon tubing (Sigma Aldrich brand; outside diameter, 1.9 mm; wall thickness, 0.21mm; length, 26 mm) immersed in the HCl solution. The vessel is placed into a thermostatic bath (Lindberg/blue M WB1120A-1) to keep the temperature consistent (±0.1°C). Helium carrier gas stream is humidified to a relative humidity (RH) of 40% ± 2%（measured by Omega HX93BD series), and this RH ensures the optimum conversion efficiency of HCl to HONO.57 The flow of each line is controlled via an MKS type 1179A mass flow controller. A total gas flow rate of 1.9 L/min was applied here. The humidified stream is continuously flowed through the HCl diffuser, and the downstream HONO reactor. HONO is produced via the aqueous phase reaction R3, followed by desorption and diffusion into gas phase (R4), and is finally swept out of the reactor. H+(aq) + NO2-(aq) → HONO(aq)

R3

HONO(aq) ⇆ HONO(gas)

R4

The reactor is placed on a hot plate (~60°C) with continuous magnetic stirring, in order to facilitate desorption of HONO from the bulk NaNO2 surface and minimize the heterogeneous decomposition of HONO.57 In this study we generated HONO at two different concentrations ~1 ppmv and ~400 ppbv (calculated by gas flow rate, collection time and collected HONO concentration based on ideal gas law) by varying the thermostatic bath temperature from 35°C to 45°C. The variation of this temperature changes the diffusion speed of HCl molecules from liquid to gas phase, thus altering the


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mass flow rate of HCl into the HONO reactor. Based on similar principles, the HONO concentration can also be regulated via the control of the length of the permeable tubing, or the concentration of HCl solution. Annular Denuder System (ADS) A detailed description of an annular denuder (URG Corp.) can be found in EPA method 4-2. 58 In short, an annular denuder consists of 2 concentric coaxial glass tubes fixed inside a Teflon coated stainless steel tube. Laminar flow created by the Teflon coated straightener (one end of the stainless steel tube) passes through the orifice between the glass tubes, where the target gaseous species is captured on the coated glass walls. The HONO collection system consists of three parts in order: 1) a particulate filter and a nylasorb filter to remove particulate nitrate and HNO3 respectively, and thus eliminate the major significant interference in the atmosphere;43 2) two annular denuders; and 3) a pumping system if used for field measurement (Figure 1). Each denuder, following a standard EPA method, was coated with a solution of 10 mL of Na2CO3 (1% w/v)+glycerol (1% v/v)+Methanol-H2O solution (1:1 volume ratio). Note that both methanol and glycerol are certified ACS plus with purity of ≥99.8% and ≥99.5% respectively. After coating, the denuders were dried using zero air and capped immediately. After each collection, the coating was extracted in 10 ml ultrapure water (18.2 MΩ) in two sequential 5 mL extractions.58 The original coating solution has a pH of ~11. The efficacy of the coating solution was also tested with lower pHs (9 and 8) by diluting the original coating solution with ultrapure water. The two annular denuder setup (A and B, Figure 1) works in a way to correct interference of NO2, which can also be absorbed to the annular denuders in a residue amount. Previous studies have shown that NO2 absorption tended to distribute evenly among all denuders in the ADS with the same Na2CO3 coating.52,54 The first denuder (labeled A) traps HONO and residue impurities of NO2 in the form of nitrite, and the second denuder (labeled B) only traps interference NO2 as nitrite (since all HONO has been already trapped by denuder A). Denuder B is used to correct the HONO concentration (i.e. nitrite concentration in denuder B subtracted from that in denuder A) and isotopic signature



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collected in denuder A (see equations below). We tested NO2 interference by flowing 100 ppbv NO2 through our ADS at room temperature, two different RH (20% and 40%) and a flow rate of 2 L/min. The extracted solution with a pH of ~10 was stored until further analysis. Concentrations of nitrite and nitrate in the solution were measured via colorimetric methods using a Westco nutrient analyzer (Smartchem 200, Westco Scientific Instruments, Inc.). Nitrate concentration was measured to ensure that oxidation of nitrite to nitrate had not occurred, and because the denitrifier method will convert both nitrite and nitrate for isotopic analysis.46 The reproducibility of the concentration measurement was ±0.3 µmol L-1 (1σ) when a sample was repeatedly measured (n=30). A detection limit of 0.07 µmol L-1 for nitrite and 0.1 µmol L-1 for nitrate was determined, and no detectable nitrite or nitrate was found in the blank coating solution. In addition to the ADS method, 10 mL of slightly basic solution with 0.01M NaOH in a chemically inert PFA (fluoropolymers) impinger (SKC Inc.) was used as a standard method to collect HONO exclusively under our experimental condition (denoted as basic impinger (BI)).57 The BI method is therefore used in this study for comparison against, and validation of, the new ADS system. It is worth noting that interference from NO2 should be minimal (NO2 is a byproduct in the HONO generator) for BI collection.59 During the collection, a constant stream of HONO in helium from the generator flows through the collection system. ADS and BI collections were performed alternatively, each with a time duration of 5-6 minutes. The average collection efficiency over the collection period for both methods, expressed in E1 were calculated based on experimental parameters including collection time (tADS and tBI), solution volume (VADS and VBI) and nitrite concentration collected by ADS ([NO2-]A and [NO2-]B) and BI ([NO2]BI). The ratio between HONO collection rate of ADS and that of BI (standard method for complete capture) gives the HONO collection efficiency of ADS: Efficiency = {([NO2-]A − [NO2-]B) × VADS ⁄ tADS } ÷ {[NO2-]BI × VBI ⁄ tBI } Isotopic Analysis


E1

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Nitrogen and oxygen isotopes of nitrite were analyzed using the bacterial denitrifier method developed by Sigman et al.46 and Casciotti et al., 60 and data were corrected following the scheme in reference 61. In brief, denitrifying bacteria (P. aureofaciens) that naturally lack the N2O reductase gene reduce nitrite (and nitrate, if present) in a liquid sample to N2O, a stable gaseous product. The N2O analyte was purified by helium purging, scrubbing of carbon dioxide and water, and cryo-trapping on an automated sample preparation and purification system,46,60 before it is directed into an Isotope Ratio Mass Spectrometer (ThermoFisher Delta V Plus) for isotopic analysis at m/z 44 (14N14N16O), 45 (14N15N16O), and 46 (14N14N18O). Three internationally recognized nitrate reference materials (aka standards) were run alongside samples. USGS34 and IAEA-N3 are used to correct the 45/44 ratio that determines the final δ15N, and USGS35 is used to correct the 46/44 that determines the final δ18O.62 Both the δ15N and δ18O are corrected for isobaric interferences at m/z 44, 45, and 46.61 The δ15N represents complete conversion from nitrate or nitrite to N2O and is additionally corrected for a blank associated with the bacteria. The δ18O correction process additionally includes kinetic isotopic fractionation and equilibrium isotopic exchange between water and sample (nitrate or nitrite) based on previous studies.60, 63 Note that the conversion of nitrite to N2O results in less fractionation than that for the nitrate to N2O for the reference materials, and this is accounted for in the correction scheme.63 Reproducibility is based upon the pooled standard deviation of IAEA N3 (δ15N =0.3‰ for n=30), USGS34 (δ15N = 0.4‰ for n=30), and USGS35 (δ18O=0.4‰ for n=30). The denitrifier method has been shown to be robust for isotopic analysis in our laboratory for samples with as little as 5 nmol N, 64 but 10-20 nmol is typically targeted.46,60 In experiments here, 20 nmol N was targeted for each sample injection, and the injection volumes were determined by nitrite concentration measured beforehand (see above).

Isotopic Correction of HONO for Two-Annular Denuder System Similar to concentration measurement, isotopic analysis was carried out for denuder A and B separately after ADS collection. To account for potential interference of NO2, the



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δ15N and δ18O are corrected based upon comparison of denuder A and B: δ15NHONO = (δ15NA – f×δ15NB) ⁄ (1-f)

E2

δ18OHONO = (δ18OA – f×δ18OB) ⁄ (1-f)

E3

δ15NHONO (δ18OHONO) is the isotopic composition for HONO alone; δ15NA (δ18OA) and δ15NB (δ18OB) are isotopic results determined for denuder A and B respectively; f is the fraction of residue NO2 calculated from the nitrite concentration of denuder B extract divided by the total nitrite consisting of HONO and residue NO2 on denuder A. Storage of Denuder-Extracted Nitrite We tested the effect of lapsed time between the ADS HONO collection and the extraction of the nitrite analyte to ensure stability of the concentration and isotopes of the collected HONO. Two 5-minute consecutive ADS collections of 400 ppbv HONO were performed in a row. One was extracted right after the collection while the other one was kept at room temperature and extracted two days after the collection. The concentration and isotopic signature for the two cases were compared. In addition, we also tested the stability of extracted HONO isotopic composition as nitrite in solution. Ultimately, the HONO collection system can be used to study HONO in the environment, and the storage of the solutions would likely be necessary. In addition, it has been shown that nitrite in solution can exchange oxygen atoms with water, changing the isotopic composition of the analyte of interest in this case.63 To assess the preservation of the isotopic composition of nitrite over time, samples were stored under different combinations of solution pH (7, 8.5 and 10), temperature (-20°C, 4°C and 20°C) and storage time (40 and 60 days). The δ15N and δ18O of nitrite for all solutions was measured using the denitrifier method. Preliminary Field Application of ADS System The method was deployed at a roadside sampling site (41.83°N; 71.40°W) located on Brown University campus in Providence, RI. The sampling site is a surface street located 0.8 km from a major highway (Interstate-95). The PTFE sampling inlet was attached on


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the side of a building 5 meters above the ground. The sampling setup was identical to the one tested in laboratory (Figure 1). Both daytime and nighttime HONO isotopic composition were studied, with an expectation of higher concentrations of HONO at night when loss is at minimum compared to daytime when HONO loss via photolysis is significant. A series of collections were made on two separate days — 21:00 09/02/2016 (local time) to 18:00 09/03/2016 at 3-hour resolution, and from 09:00 06/09/2017 to 09:00 06/10/2017 at 3- and 12-hour resolution. NOx concentration was monitored with a chemiluminescence NOx analyzer (Thermo Fisher Scientific Model 42i) on the first HONO sampling date. The NOx analyzer is appropriate for stationary measurements at 1 min resolution, and the analyzer was calibrated based on the procedure described in Ref. 44. After sampling, the same procedure was followed as with the laboratory experiments, i.e. denuder extraction, NO2- concentration measurement and determination of δ15N- and δ18O-HONO.

RESULTS AND DISCUSSION Validation of HONO Collection with ADS The laboratory HONO generating system condition was adjusted to produce a desired concentration, as described in the experimental section. In this study, we performed the capture of HONO at two different concentrations—0.4±0.03 ppmv and 1±0.04 ppmv by varying the thermostatic bath temperature from 35°C to 45°C. In each set (pair) of experiments, ADS capture of HONO from the generator was performed following the BI collection. The NaOH BI was used as a standard collection method (based on the acid base reaction R3) for comparison against, and validation of, the ADS system in terms of concentration and isotopic signature measurement.57 Results are presented in Table 1, including collection conditions (HONO concentration and ADS coating solution pH), collection rate for both methods and collection efficiency. Three different pH values for denuder coating solution were investigated. The original coating solution described in the experimental section has a pH of ~11, and this was used in the first 11 experiments. The average collection efficiency (expressed by E1) for all 11 experiments is 100±1% shown in Figure 2, suggesting a satisfactory efficiency of HONO collection. This agrees well



with the findings from Febo et al.57 (and references therein). With complete collection of HONO, it is very likely that isotopic fractionation upon collection of HONO can be ruled out. Additional experiments were conducted to test the impact of diluting the pH (to 9 and 8) of the original coating solution on HONO collection efficiency. The lower coating solution pH (close to neutral condition) was tested for the purpose of reducing the possible interference of acidic species such as SO2 for future atmospheric application, because SO2 can potentially increase the residue adsorption of NO2 on the denuder walls.65 However, the HONO collection efficiency (400 ppbv) at coating solution pH of 9 and 8 (experiment 12 and 13 in Table 1 and Figure 2) are 77.2% and 70.8% respectively, indicating incomplete collections of HONO from the gas stream and potential for isotopic fractionation. Therefore, a denuder coating solution with pH=11 is recommended to trap HONO for isotopic analysis. Under high SO2 concentrations, SO2 interference can be removed by adding an annular denuder coated with tetrachloromercurate (TCM) inline prior to the two carbonate coated denuders,65 or HONO concentration can be corrected for each SO2 concentration based on laboratory measurements.

Interference from Other N-Containing Species Using a similar carbonate denuder coating and two denuder system, Allegrini et al. found low reactivity for NO2 and PAN, with less than 2% and 5% of total NO2 and PAN, respectively, removed under various humidities at flow rates up to 12 L/min. Further, they demonstrated that NO2 absorbed on the first and second denuder in almost the same amount, allowing for determination of HONO concentration by difference.52 To verify this finding in our setup, NO2 interference was tested by flowing 100 ppbv NO2 through our ADS at room temperature, two different humidities and a flow rate of 2 L/min. At RH=20%, 0.4% of total NO2 was trapped in the ADS, with the difference between the two denuders of 1%; at RH=40%, 3% of total NO2 was trapped in the ADS, with the difference between the two denuders of 4%. This finding for limited NO2 interference is consistent with ref. 54. We expect the previous finding of a lack of PAN interference to hold true as well, and this will be tested in a future study. In addition, it has been shown that the glycerol in the denuder coating solution effectively prevents oxidation of nitrite to nitrate by ozone on the denuder walls.54 In both of our laboratory and field studies, the


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negligible concentration of nitrate measured in the samples indicates HONO was conserved as nitrite on the denuder walls.

Isotopic Analysis of Laboratory Collected HONO Isotopic analysis for δ15N and δ18O of nitrite were completed for solutions from each collection using both ADS and basic impinger collection methods. Because of the near 100% efficiency of collection via the BI, isotopic fractionation of the generated HONO in the collection process is expected to be negligible. Similar to the concentration measurements, the δ15N and δ18O of nitrite collected in the BI is used to validate the ADS methodology. Results are listed in Table 2. At 1 ppmv HONO, nitrite δ15N for BI and ADS are -5.9±0.2‰ (n=7) and -5.9±0.5‰ (n=7) respectively, while nitrite δ18O for BI and ADS are -12.3±0.5‰ (n=7) and -12.1±0.4‰ (n=7) respectively. At 400 ppbv HONO, nitrite δ15N for BI and ADS are -0.7±0.8‰ (n=4) and -0.6±0.6‰ (n=4) respectively, while nitrite δ18O for BI and ADS are -10.1±0.3‰ (n=4) and -9.7±0.5‰ (n=4) respectively (1σ). The errors here represent one standard deviation of reproducibility over repeated collections (n). For the total 11 experiments, uncertainty is assigned with the larger value for each method. Therefore, we report the uncertainty for repeated measurements (n=11) of δ15N- and δ18O-HONO via ADS method as 0.6‰ and 0.5‰ respectively. However, for comparison of the two methods, the standard deviations of repeated measurement from both ADS method and BI methods result in a greater propagated uncertainty of 1.0‰ (δ15N) and 0.8‰ (δ18O).

In order to test the consistency of ADS and BI methods for isotopic measurements, we plot the isotopic difference (∆δ15N and ∆δ18O) between laboratory HONO collected in ADS and that collected in BI for all experiments in Figures 3(a) and 3(b). In Figure 3(a), ∆δ15N ranges from -0.3‰ to +0.7‰, all falling within the expected propagated standard deviation (±1.0‰). In Figure 3(b), ∆δ18O ranges from -0.2‰ to +1.1‰, falling within the expected propagated standard deviation (±0.8‰) except one experiment with ∆δ18O=1.1‰. It is unclear why this outlier emerged, but one possible reason is the unexpected temperature disturbance of the HONO generation system that changed the oxygen equilibrium isotope effect between HONO and H2O molecule in the gas stream.



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Overall, the results show good agreement of isotope measurements between ADS and BI isotope collection methods. This suggests that the ADS method conserves the isotope signature of HONO without any detectable isotopic fractionation, and therefore δ15N and δ18O in the rinsed solution from ADS collection reflects that in the gas phase HONO stream from the reactor during the collection period. It can be noted, however, that nitrite δ15N and δ18O under both method conditions differs from that of the original NaNO2 salt (δ15N=1.4±0.1‰ and δ18O=4.9±0.5‰); both have lower δ15N and δ18O compared to that of the original NaNO2 salt. Moreover, depletion is greater for high concentrations of HONO (1 ppmv) than for low concentrations of HONO (400 ppbv), by ~5.0‰ and 2.2‰ for δ15N and δ18O respectively. This difference indicates that isotopic fractionation may occur to various extents under different laboratory HONO generation conditions. The quantitative interpretation for this difference is beyond the scope of this paper, however, we qualitatively explore a potential explanation for this isotope fractionation via laboratory HONO generation processes. First, acid-base reaction between HCl and NaNO2 (R3) is expected to deplete the heavy isotope in HONO relative to the original NaNO2. Because the aqueous phase reaction is diffusion limited which favors the lighter isotope reaction. Second, the evaporation of HONO from the aqueous phase into the gas phase (R4) could deplete the heavy isotope in the gas phase via kinetic isotope effect. Although the constant carrier gas flow can reduce the equilibrium isotope effect, the overall result should be depleted heavy isotope in the gas phase HONO. Third, heterogeneous dissociation of HONO on the reactor bed is another important step. The mechanism of HONO dissociation is not clear, however, one possible mechanism has been proposed to proceed via decomposition of HONO on a NaNO2-H2O surface (R5), forming NO and NO2, 57 which are two indicators for HONO dissociation. HONO(g) + HONO(aq) → NO + NO2 + H2O

R5

Based upon the nitrite concentration collected in denuder A (HONO + residue NO2) and B (residue NO2), at 1 ppmv HONO generation condition, residue NO2-nitrite account for 13% to 18% of the sum of nitrite in denuder A as shown in Table 2. This indicates the occurrence of the dissociation process (e.g. R5) of HONO. Furthermore, the observed


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δ15N-HONO (-5.9±0.5‰) is lower (i.e. depleted in

15

N) while δ15N-NO2 (3.9±0.1‰) is

higher than the original δ15N-NaNO2 (1.4±0.1‰) at 1 ppmv HONO generating condition (Table 2), fits with the dissociation process (e.g. R5) of HONO leaving the HONO(g) more depleted in

15

N. At 400 ppbv HONO generation condition, residue NO2-nitrite

concentrations in denuder B are all below detection limit of Westco measurement (0.07 µM), demonstrating nitrite in denuder A is 100% from HONO shown in Table 2, in agreement with the result that NO2 to HONO ratio is less than 2% via NOx analyzer concentration measurements under the same condition (Table 2). This suggests that HONO dissociation (R5) occurred at 1 ppmv HONO generation to a larger extent than that at 400 ppbv HONO generation. Therefore, it makes sense that the generated HONOδ15N and -δ18O are more depleted for the high HONO concentration (1 ppmv) than for the low HONO concentration (400 ppbv).

Note that the high concentration of HONO studied here is critical to test for breakthrough from the first denuder. Previous studies have shown highly efficient HONO capture in the first denuder under low HONO concentrations.52,53,54 For the purpose of isotopic analysis, 10-20 nmol/sample nitrite mass is typically targeted. The collection time will therefore vary with different HONO concentrations and collection flow rate to meet the minimum sample requirement. For example, at a typical sampling flow rate of 7 L/min with a target of 10 nmol/sample, a minimum collection time for HONO of 0.1 ppbv (remote atmosphere) to 100 ppbv (biomass burning plume) is 2 hours to 6 seconds, respectively.

Stability of ADS Collected Samples The variation of nitrite concentration between the two extraction timings (extraction right after the HONO collection and two days later) is 3.2%, within the 1σ precision of 5.0% calculated from results in Table 1. The variations of δ15N- and δ18O-HONO between the two cases are 0.4‰ and 0.5‰ respectively, falling well within the uncertainty of the ADS method. We therefore recommend the denuder can be stored for up to two days at room temperature before the extraction.



To ensure that collected samples maintain their original isotopic composition, several storage conditions were tested. This is particularly important in this case considering that the oxygen isotopic composition of nitrite in solution can change due to exchange of oxygen molecules with water. This exchange can easily occur at low and neutral pH, and leads to 18O enrichment in nitrite.63 Casciotti et al. carried out a systematic evaluation of storage conditions (temperature=-20, +4, and +22°C, pH=6, 8, 10, 12) for preserving oxygen isotopic signature of nitrite samples in isotope labeled fresh water and artificial seawater solutions. Based upon the experiments, it was recommended that the best storage conditions are pH=12 and 4°C (possibly 20°C) for 20 µmol/L nitrite and at pH 10 for higher concentrations of nitrite over a period of 8 weeks.63 The matrix of our solutions is different than that previously tested - the extracted nitrite solution from the denuder contains ~100 µM carbonate (CO32-) (comparable to concentrations of nitrite), and even higher concentrations of glycerol (C3H8O3). Therefore it is important to investigate the effect of different storage conditions on the stability of δ15N and δ18O for nitrite in the denuder extract solution. In total, three solution pH (pH=7, 8.5 and 10), three storage temperatures (-20°C, 4°C and 20°C) and 0-60 days storage time were investigated for a denuder extract solution with 16.6 µM nitrite after HONO collection. The original extract solution has a pH of ~10. For pH of 8.5 and 7, the solutions were adjusted by titrating with 8 M HCl. In general, we find that nitrite-δ15N is stable for all 7 conditions over the course of 60 days, with a maximum variation of ±0.2‰ relative to the initial nitrite-δ15N (Figure S2(a) in SI). Therefore, δ15N preservation is insensitive to storage pH and temperature. However, the stability of δ18O varied significantly with different storage conditions, especially pH. The more basic the solution, the less extent oxygen isotope exchange happens, consistent with the trends found by Casciotti et al.63 The best storage condition for stabilizing δ18O is pH = 10 and room temperature, under which the change of δ18O is smaller than 0.1‰ for at least 60 days. At pH 7 and three temperatures (4°C, 20°C and +20°C), nitrite-δ18O decreased from -10.4‰ to -20.0‰, -20.7‰ and -20.7‰, respectively over the course of 40 days. This likely indicates oxygen isotopic exchange. However, Casciotti et al. found that the abiotic isotopic exchange of nitrite with a series of 18O labeled waters near neutral pH and at room temperature resulted in δ18O of nitrite consistently ~14‰ higher than that of the coexisting water at equilibrium. 63 Given the


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δ18O of water in our solutions (Milli-Q) is -6‰, the dramatic

18

O depletion in nitrite in

this study cannot be explained by equilibrium isotope exchange between nitrite and water. Rather, we hypothesize oxygen isotopic exchange of carbonate with nitrite might play a major role. Although no study has determined such equilibrium isotopic effect for carbonate-nitrite isotopic exchange, a study on carbonate-water isotopic exchange revealed that δ18O of carbonate and bicarbonate are respectively 40‰ and 35‰ higher than that of the coexisting water at equilibrium at room temperature.66 Comparing the equilibrium

isotopic

effect

of

nitrite-water

exchange

(14‰)

carbonate/bicarbonate-water exchange (40‰ and 35‰), we can infer

with 18

that

of

O of nitrite is

depleted compared to that of carbonate and bicarbonate. In addition, there is no oxygen isotopic exchange between glycerol and water, 67 and therefore this will not affect the isotopic composition of our denuder extract solution. At pH 8, the preservation of nitrite-δ18O was greatly improved compared to pH 7. Nitriteδ18O increased slightly from -10.4‰ to -8.6‰ (-20°C), -9.1‰ (4°C) and -9.6‰ (+20°C) respectively during the period of 60 days (Figure S2(b) in SI). This likely indicates a slow oxygen isotopic exchange between nitrite and water. Nitrite-δ18O stored at +20°C outperformed that at 4°C and -20°C. Based on this, the pH 10 treatment was only tested at room temperature. Under this condition, nitrite-δ18O was completely preserved during the period of 60 days. Therefore pH=10 and room temperature storage is recommended for the denuder extract solution over a period of 2 months. Thorough tests of storage conditions are necessary if any changes to the sampling solution matrix are needed for use in future environmental studies to ensure that the oxygen isotopes of HONO are interpreted correctly.

Field Measurements of Ambient HONO Field tests of the ADS system for HONO isotopic analysis were carried out at a roadside sampling site on a local street in Providence, RI. The δ15N-HONO, δ18O-HONO and recovered average HONO mixing ratio in each collection period along with the environmental conditions are presented in Table 3. Each sample was collected for 3 hours, except for collection 8 with a sampling time of 12 hours. The recovered HONO



mixing ratio ranges from 0.8-1.3 ppbv and 0.4-1.0 ppbv for nighttime and daytime, respectively. The NO2 mixing ratio at the same site measured with a NOx analyzer during collections 1-3 averaged 11 ppbv, resulting in an average HONO/NO2 ratio of ~10%. This ratio agrees well with the results of Stutz et al. for HONO and NO2 in urban air measured by differential optical absorption spectroscopy (DOAS),68 considering the high relative humidity 79%-89% in collections 1-3. The nighttime δ15N-HONO for collections in September and June are similar to each other (-12.4±0.3‰ (1σ; #1-3) versus -11.9±0.6‰ (#7-8)) and different from the daytime collections (-8.0±0.2‰, #4-6). The δ18O-HONO varies considerably amongst the samples, and in particular for daytime compared to nighttime samples. Based upon studies of δ18O-HNO3,69,70 our expectation is that δ18O-HONO should reflect oxidant chemistry in the atmosphere and therefore should vary diurnally, seasonally, and with different HONO production mechanisms.18 The very high values observed for δ18O-HONO (e.g., 89.0‰) likely reflect isotopic interaction with ozone (δ18O-O3(bulk) ≈ 110‰, 71 especially in nighttime compared to daytime chemistry. Interpretation of the HONO isotopic data in terms of environmental factors (e.g., emission sources, formation mechanisms, oxidant chemistry and/or exchange) will be the subject of future publications. Here, our purpose is to show the robust application of the method at ambient concentrations (0.4-1.3 ppbv) and over relatively long collection times (3 and 12 hours) compared to the laboratory experiments.

CONCLUSION The HONO ADS collection system coupled with offline δ15N and δ18O isotope analysis developed in this study exhibits consistent, nearly 100% collection efficiency and reproducible isotope analysis with a reported 1σ uncertainty of ±0.6‰ (δ15N) and ±0.5‰ (δ18O), respectively. The two-denuder system allows for correction of potential interference from residue NO2. The ADS method was also validated in a field based roadside collection of HONO, and was shown to be robust for collection of low concentration HONO for isotopic analysis. In addition, optimal sample storage conditions (pH=10 and 20°C) were validated. Overall, this method offers a reliable way to obtain δ15N and δ18O of HONO under different conditions.


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This method can be deployed for both laboratory and field-based studies of HONO. However, when deploying this setup for field measurements, careful correction of residue NO2 may be needed under heavily polluted conditions (e.g. high SO2 concentration that leads to more NO2 absorption on denuder walls). In addition, measurement of nitrate concentration is also necessary to exclude the oxidation of nitrite on denuder walls. Field based sampling of HONO in various environments via this method may provide for a novel way to quantify HONO sources and chemistry, and will be the subject of future work.

AUTHOR INFORMATION Corresponding Author *Phone: 401-863-6853; fax: 401-863-3839; e-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for the helpful discussions with Jack Dibb, C. Franklin Goldsmith, David Miller, Wendell Walters and Yingdi Liu. We wish to thank the two anonymous reviewers for their insightful comments and suggestions. We also thank Ruby Ho, and David Murray for their technical support. This work was supported by funding from the National Oceanic and Atmospheric Administration (AC4 Award NA16OAR4310098) and the National Science Foundation (AGS 1351932).

Table 1. HONO collection rate using both BI method and ADS method. The efficiency is calculated by E1 in the text. Samples 1-7 were collections of 1 ppmv HONO using the original coating solution with pH of 11. Samples 8-11 were



collections of 400 ppbv HONO using the original coating solution with pH of 11. Samples 12 and 13 were collections of 400ppbv HONO using the coating solution with pH=9 and 8, respectively. BI (nmol NO2ADS(nmol Efficiency% Condition Sample# /min) NO2-/min) 1 ppmv 1 87.7 86.7 98.9 pH=11 2 79.4 80.7 101.7 3 82.4 83.2 100.9 4 85.5 84.9 99.2 5 87.7 86.5 98.7 6 79.4 78.8 99.3 7 82.4 84.5 102.5 400 ppbv 8 33.3 33.0 99.1 pH=11 9 36.9 37.5 101.5 10 37.1 37.2 100.4 11 37.0 36.8 99.5 pH=9 12 31.2 24.1 77.2 pH=8 13 39.5 27.9 70.8

Table 2. Isotope results (δ15N, δ18O (‰) of HONO) for both ADS and BI methods. Experimental conditions for each sample collection are in Table 1.a Sample # δ15N_ADS δ15N_BI δ18O_ADS δ18O_BI f(NO2) b 1 -6.2 -6.1 -12.1 -11.9 0.16 2 -5.2 -5.9 -11.6 -12.1 0.14 3 -6.3 -6.1 -11.8 -11.9 0.14 4 -5.5 -5.7 -12.4 -13.0 0.18 5 -5.3 -5.7 -12.7 -13.0 0.13 6 -6.2 -6.0 -11.8 -12.2 0.17 7 -6.3 -6.0 -11.9 -12.2 0.15 8 -0.3 -0.3 -9.4 -9.7 0.00 9 -1.5 -1.7 -9.3 -10.4 0.00 10 0.1 0.3 -10.4 -10.2 0.00 11 -0.8 -1.1 -9.8 -10.3 0.00 a 15 For original nitrite salt, δ N-NaNO2 = 1.4±0.1‰ (n=11); b f(NO2) is the concentration fraction of residual-NO2 versus HONO+residue-NO2 collected in denuder A. The average δ15N-NO2 and δ18O-NO2 for samples 1-7 are 3.9±0.1‰ and -9.3±0.7‰, respectively.


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Table 3. Roadside ambient HONO isotopic analysis using the ADS method. RH is relative humidity. Recovered δ15N δ18O Temperaturea RHa Collection Time of HONO ‰ ‰ (%) (°C) # collection (ppbv) 9/2/16 21:00 1 0.8 -12.3 26.6 18.8 79.0 9/3/16 00:00 9/3/16 00:00 2 1.0 -12.1 20.3 17.1 87.8 9/3/16 03:00 9/3/16 03:00 3 1.3 -12.7 31.8 16.4 89.0 9/3/16 06:00 6/9/17 09:00 4 0.4 -8.1 25.1 20.0 48.3 6/9/17 12:00 6/9/17 12:00 5 0.5 -7.6 83.9 24.1 38.5 6/9/17 15:00 6/9/17 15:00 6 1.0 -8.1 56.4 24.4 46.5 6/9/17 18:00 6/9/17 18:00 7 1.2 -12.3 56.3 23.3 48.0 6/9/17 21:00 6/9/17 21:00 8 0.8 -11.5 69.7 18.0 71.4 6/10/17 06:00 a temperature and RH represent averages over the collection times recorded in the website https://www.wunderground.com/history/.



Figure 1. Annular denuder system for HONO capture for isotopic analysis. Denuder A captures both HONO and residue NO2. Denuder B captures only NO2, which is used to correct concentrations and isotopes of HONO on Denuder A (see text).

Figure 2. HONO collection efficiency ADS compared with basic impinger collection. Efficiency is calculated via E1 in the text. Refer to Table 1 for experiment conditions. Error bars are 1σ precision of efficiencies of experiments 1-11.


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Experiment number

Experiment number

(a) (b) Figure 3. Reproducibility of isotopic composition (a) δ15N and (b) δ18O comparing ADS collected HONO with BI collected HONO. ∆δ15N (∆δ18O) on the y-axis is the difference between ADS δ15N (δ18O)-HONO and BI δ15N (δ18O)-HONO. Dashed lines are 1σ precision for repeated experiments (δ15N=±0.6‰ and δ18O=±0.5‰; n=11) via ADS method. Solid lines are propagated precision of both ADS and BI collection (δ15N=±1.0‰ and δ18O=±0.8‰).

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A Collection Method for Isotopic Analysis of Gaseous Nitrous Acid

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