Name: automated, high-resolution mobile collection system for the nitrogen isotopic analysis of nox
Uploaded: 2016-12-20T15:00:00
Duration: 7 min 14 s
Description: Watch this Scientific Journal Video about Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx at JoVE.com

PKW would like to thank the Voss Environmental Fellowship at Brown University for funding and support. This research was funded by a National Science Foundation CAREER Award (AGS-1351932) to MGH. The authors thank Ruby Ho for her support of this project. The authors would also like to thank Rebecca Ryals; Jim Tang and his research group at the Marine Biological Laboratory, specifically Elizabeth de la Reguera, for access to the LMA-3d and Li-COR 7000 analyzers; Barbara Morin at the Rhode Island Department of Environmental Management; Roy Heaton and Paul Theroux at the Rhode Island Department of Health; Adam McGovern at the Warren Alpert Medical School; and Charlie Vickers and Tom Keifer for collection system machining. We also thank the editorial staff and two anonymous reviewers for their excellent suggested edits that improved the manuscript.

<ol>
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W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> New Technologies for Use in Acid Deposition Networks, ASTM STD. 1052. , 133-149 (1990).</li><li>Sigman, D. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Bacterial+Method+for+the+Nitrogen+Isotopic+Analysis+of+Nitrate+in+Seawater+and+Freshwater.">A Bacterial Method for the Nitrogen Isotopic Analysis of Nitrate in Seawater and Freshwater.</a> Anal. Chem. 73 (17), 4145-4153 (2001).</li><li>Casciotti, K. L., Sigman, D. M., Hastings, M. G., B&#246;hlke, J. K., Hilkert, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+the+oxygen+isotopic+composition+of+nitrate+in+seawater+and+freshwater+using+the+denitrifier+method.">Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method.</a> Anal Chem. 74 (19), 4905-4912 (2002).</li><li>B&#246;hkle, J. K., Mroczkowski, S. J., Coplen, T. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+isotopes+in+nitrate:+new+reference+materials+for+18O:17O:16O+measurements+and+observations+on+nitrate-water+equilibrium.">Oxygen isotopes in nitrate: new reference materials for 18O:17O:16O measurements and observations on nitrate-water equilibrium.</a> Rapid Commun. Mass Spectrom. 17 (16), 1835-1846 (2003).</li><li>Kaiser, J., Hastings, M. G., Houlton, B. Z., R&#246;ckmann, T., Sigman, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tripe+oxygen+isotope+analysis+of+nitrate+using+the+denitrifier+method+and+thermal+decomposition+of+N2O.">Tripe oxygen isotope analysis of nitrate using the denitrifier method and thermal decomposition of N2O.</a> Anal Chem. 79 (2), 599-607 (2007).</li><li>Sun, K., Tao, L., Miller, D. J., Khan, M. A., Zondlo, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On-Road+Ammonia+Emissions+Characterized+by+Mobile,+Open-Path+Measurements.">On-Road Ammonia Emissions Characterized by Mobile, Open-Path Measurements.</a> Environ. Sci. Technol. 48 (7), 3943-3950 (2014).</li><li>Dahal, B., Hastings, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Technical+considerations+for+the+use+of+passive+samplers+to+quantify+the+isotopic+composition+of+NOx+and+NO2+using+the+denitrifier+method.">Technical considerations for the use of passive samplers to quantify the isotopic composition of NOx and NO2 using the denitrifier method.</a> Atmospheric Environment. , (2016).</li></ol>

The authors declare that they have no competing financial interests.

The protocol above details the steps involved, from the field collection of air samples in solution to the laboratory processing of these solutions, to yield concentration and isotopic results. The critical steps in this protocol include comparing NOx analyzer measurements, minimizing the time before the reduction of solutions. and maintaining stable flow rates. If directly comparing solutions with in situ measurements of NOx concentrations, it is very important that an NOx analyzer is calibrated for ranges relevant for the chosen environment and that short-term variability in NOx concentrations be understood in the context of the longer time collections for the solutions. The accurate determination of solution NO3- concentrations is also important, both for calculation of the airborne NOx concentrations and for determining accurate injection volumes for the isotopic denitrifier method. The time period of solution stability before sample reduction is important to ensure consistent isotope ratios. As a result of the oxidizing potential of the solution, it is possible to oxidize in solution other reactive nitrogen species, most notably NH3, as it can be in high enough concentrations in certain areas to potentially affect the concentration of NO3- in solution. The oxidation of NH4+ to NO3- is expected to take longer than the oxidation of NOx to NO3-, so it had been recommended to reduce the samples (and thus stop the reaction) within 1 day of sample collection. Given that field conditions may result in the requirement of longer solution storage times, the stability of solutions was tested by examining solutions with and without added ammonium. With and without the addition of ammonium chloride, concentration and isotope values were stable within the 1&#963; uncertainty range (1.5&#8240;) for up to one week (Figure 3). At two weeks after sampling, solutions with or without added NH4+ were not stable, in that NO3- concentration decreases were observed in some cases and blank corrections were no longer robust. Although it was expected that NO3- might increase over time due to NH4+ oxidation, decreases in concentration were actually observed in some cases, suggesting that even after two weeks, NH4+ interference is not causing the instability. As such, solutions should be reduced within one week, particularly if sampling is done in an environment with high NH3 concentrations (e.g., &#62;200 ppbv). Finally, it is also critical to record the flow rate during field collections. The flow rate measured at the inlet was found to vary considerably and is difficult to control, even with a critical orifice in the system, since it can be influenced by the clogging of the hydrophobic filters and/or the frit. It is recommended to record the flow rate periodically (e.g., at 5 min intervals) throughout collections periods, such that the volume of air collected over time for each sample can be accurately determined (see step 5).
There are several alternatives or possible modifications of the protocols presented. For example, an important advantage of the denitrifier method is the low sample size requirement12,13. However, other isotopic methods may be used. Similarly, we use colorimetric determination of concentration, but other methods can yield accurate NO3- concentration results.
Collection efficiency in the field, as detailed in Figure 2, is 92 &#177; 10%. This is critical to ensure that there is no fractionation during the collection process. With collection efficiency less than 100%, fractionation in the collection process can occur, biasing the resulting isotopic ratios measured. The efficacy of this new collection method across a range of conditions in urban-influenced air has been shown. Table 1 outlines the multiple tests that were done under ambient-air, near-roadside, and smog-chamber sampling conditions to determine the reproducibility of the method. All isotope ratio differences between systems are &#60;1.57&#8240;. This demonstrates the reproducibility of this method over a range of different sampling conditions. The field-based method has a precision and reproducibility significantly better than the ~12&#8240; isotopic ratio variations observed in the environment (Figure 4).
The most significant limitation of the method is the NO3- blank or background associated with the KMnO4 solution. A variety of KMnO4 types have been tested (e.g., crystals, powders, and stock solutions)4, and all contained NO3- before being exposed to NOx in the air. As a result, it is necessary to collect enough NOx as NO3- in solution to achieve a concentration above the blank. Further studies are currently underway to quantify the level at which the sample should exceed the blank concentration for the most accurate results. Under very low ambient NOx concentrations, it may be necessary to modify the collection conditions to maximize the sample concentration. For instance, the flow rate could be increased to collect more air in a shorter timeframe or the solution volume could be reduced to increase the air-to-solution volume and to concentrate the air collection. In any case, the solution must remain above the frit in the collection vessel to maintain the bubbling of air through the solution.
This method of NOx collection for isotopic analysis is unique among existing methods (e.g., passive samplers6,17 and sulfuric acid and hydrogen peroxide solution8) in that it has been laboratory- and field-verified with respect to field applicability, reproducibility, sample solution stability, and efficiency of collection under a range of field conditions. This novel method is unique in its capabilities to actively collect NOx in field environments for isotopic analysis at ambient concentrations at a 30-120 min time resolution. It collects NOx at near 100% efficiency and has been demonstrated repeatedly to be reproducible within the range of uncertainty of the method. Sample solutions collected in the field remain stable for up to 1 week before needing to be reduced. The method can collect samples over a range of concentrations and isotope ratios, and it is shown to be reproducible from collection to collection. This technique can be used for sampling under a variety of different conditions, including on-road, using the mobile laboratory approach outlined in the protocol. The interpretation of the spatiotemporal variability in vehicular emissions of NOx is the subject of a separate manuscript, in preparation (Miller, D.J., et al. 2016.&#160; J. Geophys. Atmos. Submitted).
Future sampling includes the application of this method to other types of NOx emissions (e.g., microbially produced emissions in soils and biomass fires). Isotopes are a potential way to track NOx sources, but only if different source signatures can be quantified and understood. Our new method makes it possible to quantify the isotopic composition of NOx from a variety of NOx emission sources and to directly test whether the impacts of emissions in the environment can be directly and quantitatively tracked.

An erratum was issued for: <a href="https://www.jove.com/video/54962/automated-high-resolution-mobile-collection-system-for-nitrogen">Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx</a>. An author name was updated.
One of the authors&#39; names was corrected from:
Mary O&#39;Conner
to:
Mary O&#39;Connor

An erratum was issued for: <a href="https://www.jove.com/video/54962/automated-high-resolution-mobile-collection-system-for-nitrogen">Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx</a>. An author name was updated.

Erratum: Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx

Atmospheric nitrogen oxides (NOx = NO + NO2) are important species in the global reactive nitrogen cycle1,2. NOx in the atmosphere is highly reactive and directly contributes to the oxidizing capacity of the atmosphere through its interactions with ozone (O3) and hydroxyl radical (OH). NOx is removed from the atmosphere on the scale of hours to days in the lower troposphere via oxidation to nitric acid (HNO3) or nitrate (NO3-), both of which are highly soluble and can be dry deposited on surfaces in gaseous and particulate aerosol forms or wet deposited by precipitation (e.g., acid rain)2. NOx is emitted from a variety of sources, including fossil fuel combustion, biomass burning, microbial processes in soils, and lightning. Source apportionment is crucial for understanding the impacts of individual sources, but the variety of sources, their variability in space and time, and the relatively short lifetimes of NOx and HNO3 make concentration analyses alone an inadequate metric. Stable isotopes may be useful as a way to better track the spatial patterns and temporal trends of sources and the chemistry of NOx and NO3- in the environment and to add new constraints on atmospheric models3. To date, the isotopic signatures associated with different NOx sources remain highly uncertain, particularly because of large uncertainties associated with previous methods4.
Previous studies represent a number of different active and passive collection methods and yield large ranges in reported isotopic values, even for the same emission source. Fibiger et al. found that previously used methods often varied greatly in terms of their efficiency in capturing NOx, with changes in conditions greatly influencing field collection (e.g., temperature, humidity, flow rates, age of solution)4. The inefficient uptake of previous NO and NO2 capture methods could lead to fractionations. For example, higher rates of oxidation for 14N relative to 15N could yield low biases in &#948;15N-NOx that are not representative of atmospheric values. In addition to methodological issues4,17, a variety of different types of air sampling may also contribute to differences in the reported ranges for isotope values associated with the same source. For example, isotopic signatures associated with vehicle emissions of NOx have been suggested based on collections at near-road sites5, in traffic tunnels6, and directly from the tailpipes of vehicles7,8. Furthermore, previous methods have time resolutions of 24 h at best, and significant changes in ambient NOx concentrations are observed on hourly (or shorter) timescales9, potentially limiting the application of isotopic detection for different sources. Many of the NOx collection methods require very strong oxidizing solutions capable of oxidizing NOx, but also other collected reactive nitrogen species (e.g., ammonium), to nitrate over time, potentially contributing an isotopic measurement interference. Some previous methods are also limited to collecting NO2 in solution, which provides only a limited understanding of NOx isotopes, as it does not collect NO (the primary emission). Thus, there is a need to capture NOx from different emissions sources using a consistent, validated method to better constrain whether the variability in isotopes of NOx (and NO3-) in the environment can be used to directly track sources and chemistry.
This paper reports on a field-based NOx collection technique for isotopic analysis with the requisite time resolution, collection efficiency (100%), and reproducibility (&#8804;1.5&#8240;) for application in multiple field environments. The method, originally described by Fibiger et al.4, is further validated through the demonstration of its collection efficiency under changing NOx and meteorological conditions in the field, the test of solution stability and ammonia interferences, and the substantiation of its reproducibility in urban environments. Spatial and temporal differences in isotopic values are investigated using a single laboratory- and field-verified method that can capture NOx in solution at high efficiency. This paper demonstrates the application of the method for near-road, on-road, and ambient urban air collections at time resolutions of 30 to 120 min.
In brief, NOx (NO and NO2) is collected from the atmosphere in a highly oxidizing solution as NO3-. At the same time, ambient NOx, NO2, and CO2 concentrations and other relevant data, such as GPS location and time of collection, are recorded. After a sample is collected, the solution is processed in the laboratory, which involves reducing the solution to stop the reaction, then neutralizing the solution pH for subsequent NO3- concentration and isotopic analyses. The NO3- concentration is determined here by an automated spectrophotometric (i.e., colorimetric) process. The nitrogen isotopic composition is determined using the denitrifier method, which quantitatively converts the NO3- in solution to gaseous N2O that is subsequently measured on an isotope ratio mass spectrometer. Laboratory and field blanks are also collected and measured as part of the collections to ensure the sample integrity. Below is a detailed step-by-step protocol.

<table border="0" cellpadding="0" cellspacing="0" width="686">
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			<td height="105" style="height:105px;width:247px;">Gas Washing Bottle</td>
			<td style="width:112px;"></td>
			<td style="width:136px;"></td>
			<td style="width:191px;">Custom Design used, numerous companies sell other gas washing bottles. The bottle needs to have a frit inside it.</td>
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		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Syringe Pump</td>
			<td style="width:112px;">Kloehn</td>
			<td style="width:136px;">Kloehn Versa Pump 6, 55 Series</td>
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			<td height="84" style="height:84px;width:247px;">PTFE Isolation Valves</td>
			<td style="width:112px;">Parker</td>
			<td style="width:136px;">002-00170-900</td>
			<td style="width:191px;">Both 2 three way and double two-way normally closed, electronically actuated valves</td>
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			<td height="63" style="height:63px;width:247px;">Gas Handling Teflon Tubing</td>
			<td style="width:112px;">McMaster Carr</td>
			<td style="width:136px;">5033K31</td>
			<td style="width:191px;">Quarter inch outer diameter, eigth inch inner diameter, PTFE tubing</td>
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			<td height="63" style="height:63px;width:247px;">Liquid Handling Tygon Tubing</td>
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			<td style="width:191px;">Quarter inch outer diameter, eigth inch inner diameter, PTFE tubing</td>
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			<td height="42" style="height:42px;width:247px;">Compression gas fittings and ball valves (assorted)</td>
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			<td height="21" style="height:21px;width:247px;">Flow calibrator</td>
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			<td height="21" style="height:21px;width:247px;">Compression PFA fittings</td>
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			<td height="42" style="height:42px;width:247px;">Data Acquisition Board</td>
			<td style="width:112px;">National Instruments</td>
			<td style="width:136px;">NI USB-6001</td>
			<td style="width:191px;">Used for valve switching</td>
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			<td height="21" style="height:21px;width:247px;">Solid State Relay</td>
			<td style="width:112px;">Crydom</td>
			<td style="width:136px;">DC60S5</td>
			<td style="width:191px;">Used for valve switching</td>
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			<td height="42" style="height:42px;width:247px;">Single Stage Filter Assembly</td>
			<td style="width:112px;">Savillex</td>
			<td style="width:136px;">401-21-25-50-21-2</td>
			<td style="width:191px;">Use 25 mm and 47 mm diameter holders</td>
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			<td height="63" style="height:63px;width:247px;">Nylon Membrane Filter</td>
			<td style="width:112px;">Pall Corporation</td>
			<td style="width:136px;">66509</td>
			<td style="width:191px;">1 micrometer filter, both 25 mm and 47 mm diameter filters</td>
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			<td style="width:112px;">Millipore</td>
			<td style="width:136px;">LCWP04700</td>
			<td style="width:191px;">10.0 micrometer, 25 mm and 47 mm diameter filters</td>
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			<td height="63" style="height:63px;width:247px;">Particle Membrane Filters</td>
			<td style="width:112px;">Millipore</td>
			<td style="width:136px;">FALP04700</td>
			<td style="width:191px;">1 micrometer filter, both 25 mm and 47 mm diameter filters</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Mini Diaphragm Pump</td>
			<td style="width:112px;">KNF</td>
			<td style="width:136px;">UN 816.1.2 KTP</td>
			<td style="width:191px;">Used for stationary lab</td>
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			<td height="21" style="height:21px;width:247px;">Mini Diaphragm Pump</td>
			<td style="width:112px;">KNF</td>
			<td style="width:136px;">PJ 26078-811</td>
			<td style="width:191px;">Used for mobile lab</td>
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			<td height="42" style="height:42px;width:247px;">Potassium Permanganate 1N Solution</td>
			<td style="width:112px;">Fischer Scientific</td>
			<td style="width:136px;">SP282-1</td>
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			<td style="width:112px;">Fischer Scientific</td>
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			<td height="21" style="height:21px;width:247px;">Ohaus Benchtop scale</td>
			<td style="width:112px;">Pioneer</td>
			<td style="width:136px;">EX224N</td>
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			<td height="42" style="height:42px;width:247px;">4 oz. Amber Glass Bottles</td>
			<td style="width:112px;">Qorpak Bottles</td>
			<td style="width:136px;">GLC-01926</td>
			<td style="width:191px;">(60 mL and WM 125 mL bottles)</td>
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			<td height="105" style="height:105px;width:247px;">Amber HDPE Bottles</td>
			<td style="width:112px;">Fischerbrand</td>
			<td style="width:136px;">300751</td>
			<td style="width:191px;">Part number given for 125 mL narrow mouth bottlesTwo varieties (125 wide mouth and narrow mouth of some volume)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Precleaned EPA Amber Wide-mouth Bottle, 500 mL</td>
			<td style="width:112px;">Cole Parmer</td>
			<td style="width:136px;">EW-99540-55</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Hydrogen Peroxide 30%</td>
			<td style="width:112px;">Fischer Chemical</td>
			<td style="width:136px;">H325-500</td>
			<td style="width:191px;">Corrosive</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Centrifuge 5810 R</td>
			<td style="width:112px;">Eppendorf</td>
			<td style="width:136px;">5821020010</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">50 mL Polypropylene Conical Tube</td>
			<td style="width:112px;">Falcon</td>
			<td style="width:136px;">14-432-22</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">12N Hydrochloric acid</td>
			<td style="width:112px;">Fischer Scientific</td>
			<td style="width:136px;">A114SI212</td>
			<td style="width:191px;">Corrosive</td>
		</tr>
		<tr height="231">
			<td height="231" style="height:231px;width:247px;">Colorimetric Nutrient Analyzer</td>
			<td style="width:112px;">Westco Scientific Instruments</td>
			<td style="width:136px;">SmartChem 170</td>
			<td style="width:191px;">In purchasing the Colorimetric Nutrient Analyzer, this comes with buffers, cleaning solutions, rinse solutions, and solutions for running the instrument, including the solutions to be able to activate the cadmium column in the instrument for nitrate analysis.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Automatic Titrator</td>
			<td style="width:112px;">Hanna Instruments</td>
			<td style="width:136px;">HI 901</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="80">
			<td height="80" style="height:80px;width:247px;">20 mL Clr Headspace Vial</td>
			<td style="width:112px;">Microleter, a WHEATON Company</td>
			<td style="width:136px;">W225283</td>
			<td style="width:191px;">Information listed&#160; is for 20 mL vials. 50 mL vials can also be purchased from the vendor listed.</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:247px;">Septa, 20 mm Gray Butyl Stopper</td>
			<td style="width:112px;">Microleter, a WHEATON Company</td>
			<td style="width:136px;">20-0025</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:247px;">Seal, 20 mm Standard Aluminium</td>
			<td style="width:112px;">Microleter, a WHEATON Company</td>
			<td style="width:136px;">20-0000AS</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">25G x 1 1/2 BD PrecisionGlide Needle</td>
			<td style="width:112px;">BD</td>
			<td align="right" style="width:136px;">305127</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">26G x 1/2 BD PrecisionGlide Needle</td>
			<td style="width:112px;">BD</td>
			<td align="right" style="width:136px;">305111</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:247px;">Helium</td>
			<td style="width:112px;"></td>
			<td style="width:136px;">05078-536</td>
			<td style="width:191px;">Can order from many different soures</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Crimper/Uncrimper</td>
			<td style="width:112px;">WHEATON</td>
			<td style="width:136px;">61010-1</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Isopropanol</td>
			<td style="width:112px;">Fischer Chemical</td>
			<td style="width:136px;">A459-1</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Syringes of varying size for mass spec injection</td>
			<td style="width:112px;">BD</td>
			<td style="width:136px;">Varies based on size</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Antifoam B Emulsion</td>
			<td style="width:112px;">Sigma-Aldrich</td>
			<td style="width:136px;">A5757-500ML</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="105">
			<td height="105" style="height:105px;width:247px;">IRMS</td>
			<td style="width:112px;">ThermoFischer Scientific</td>
			<td style="width:136px;">IQLAAMGAATFADEMBHW</td>
			<td style="width:191px;">The actual isotope ratio mass spectrometer is listed here. Our set up also includes a gas bench and an autosampler.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Gass Bench II</td>
			<td style="width:112px;">ThermoFischer Scientific</td>
			<td style="width:136px;">IQLAAEGAATFAETMAGD</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">TriPlus RSH&#8482; Autosampler</td>
			<td style="width:112px;">ThermoFischer Scientific</td>
			<td style="width:136px;">1R77010-0200</td>
			<td style="width:191px;">Choose product for headspace injection</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">42i NOx Concentration Analyzer</td>
			<td style="width:112px;">ThermoFischer Scientific</td>
			<td style="width:136px;">101350-00</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">NOx Box</td>
			<td style="width:112px;">Drummond Technologies</td>
			<td style="width:136px;">LMA-3D/LNC-3D</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">CO2 analyzer</td>
			<td style="width:112px;">Licor 7000</td>
			<td align="right" style="width:136px;">7000</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">GPS</td>
			<td style="width:112px;">Garmin</td>
			<td style="width:136px;">010-00321-31</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Model 146i Dynamic Gas Calibrator</td>
			<td style="width:112px;">ThermoFischer Scientific</td>
			<td style="width:136px;">102482-00</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:247px;">Model 111 Zero Air Supply</td>
			<td style="width:112px;">ThermoFischer Scientific</td>
			<td align="right" style="width:136px;">7734</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:247px;">50.2 ppm NO in N2 Gas standard</td>
			<td style="width:112px;">Praxis Air</td>
			<td style="width:136px;">Will vary with each tank of standard air purchased</td>
			<td style="width:191px;"></td>
		</tr>
	</tbody>
</table>

automated, high-resolution mobile collection system for the nitrogen isotopic analysis of nox

Nitrogen oxides (NOx = NO + NO2) are a family of atmospheric trace gases that have great impact on the environment. NOx concentrations directly influence the oxidizing capacity of the atmosphere through interactions with ozone and hydroxyl radicals. The main sink of NOx is the formation and deposition of nitric acid, a component of acid rain and a bioavailable nutrient. NOx is emitted from a mixture of natural and anthropogenic sources, which vary in space and time. The collocation of multiple sources and the short lifetime of NOx make it challenging to quantitatively constrain the influence of different emission sources and their impacts on the environment. Nitrogen isotopes of NOx have been suggested to vary amongst different sources, representing a potentially powerful tool to understand the sources and transport of NOx. However, previous methods of collecting atmospheric NOx integrate over long (week to month) time spans and are not validated for the efficient collection of NOx in relevant, diverse field conditions. We report on a new, highly efficient field-based system that collects atmospheric NOx for isotope analysis at a time resolution between 30 min and 2 hr. This method collects gaseous NOx in solution as nitrate with 100% efficiency under a variety of conditions. Protocols are presented for collecting air in urban settings under both stationary and mobile conditions. We detail the advantages and limitations of the method and demonstrate its application in the field. Data from several deployments are shown to 1) evaluate field-based collection efficiency by comparisons with in situ NOx concentration measurements, 2) test the stability of stored solutions before processing, 3) quantify in situ reproducibility in a variety of urban settings, and 4) demonstrate the range of N isotopes of NOx detected in ambient urban air and on heavily traveled roadways.

In the original method development work by Fibiger et al., the NOx collection method was rigorously tested in the laboratory under a variety of conditions4. Here, the focus is on updates to the method and field applications under a variety of environmental conditions. Results are reported on (1) field collection efficiency, (2) sample solution stability in terms of time before sample reduction and sensitivity to high concentrations of ammonium (NH4+) in solution, and (3) reproducibility in the field. The versatility of the method is demonstrated in its application for ambient air, near-road, and on-road measurements.
The average concentrations collected in solution were compared with those from 1 min NOx concentrations from the chemiluminescence NOx analyzer over a two-day diurnal study in ambient urban air in Providence, RI. Figure 2 details the collection efficiency during a period when concentrations varied over a large range, from ~2.5-18 ppbv NOx. Figure 2A displays a direct comparison of median NOx concentrations from the NOx analyzer compared with concentrations calculated from the solution and flow measurements, indicating that, on average, solution concentrations are 92% of median in situ concentrations. This falls within the expected uncertainty range of &#177;10%, but the difference likely reflects varying concentrations during the collection periods (Figure 2B). Based upon examination of the percentiles of the distribution of the 1-min NOx concentration data, the solution-based NOx concentrations are within the distribution for every collection interval (Figure 2B).
It has been recommended to complete the reduction of the samples collected in the field within 1 day after collection is complete (i.e., complete all of step 3). This target was suggested to reduce the potential for interference from the collection of other soluble nitrogen species, such as NH3, that could be converted to nitrate in the highly oxidizing KMnO4/NaOH solution over time. To test this more specifically, samples were collected in May and July 2015 in Providence, RI on the campus of Brown University, at a loading dock that is near a regularly traveled local road where diesel delivery trucks are regularly running in idling mode to unload. Samples were collected, and then aliquots of the samples were separated and reduced at different times (1 day, 4-7 days, and 13-15 days) after the sample collection (Figure 3A). Samples in Figure 3B were also collected during May and July but were prepared by adding 5 ml of 10 mM ammonium chloride to 450 ml of solution. This yielded a concentration of 111 &#181;M NH4+ in solution, corresponding to collecting 220 ppbv of NH3 in the air, if only NH3 was collected. These concentrations are the maximum expected during on-road measurements near vehicle NH3 sources16. With or without the added NH4+, samples reduced within 7 days after collection had consistent isotope ratios when compared to the first reduction (within 1 day of collection), all falling within the expected uncertainty range of &#177;1.5% (Figure 3A and 3B). Note that the &#177;1.5% uncertainty is representative of isotopic determinations of repeated collections of tank NOx4. The uncertainty associated with repeated measures of isotopic reference materials alone is typically 0.3%. After two weeks, however, samples with or without added NH4+ were not necessarily stable. While in some cases the isotope values still appear to be consistent (e.g., Figure 3A), samples exhibited small NO3- concentration increases (&#60;1 &#181;M) when compared with the first reduction and, in some cases, decreases in NO3- concentrations. With the added NH4+, it would have been expected that NO3- concentration would increase over time above the expected uncertainty range (~0.8 &#181;M) for concentration measurements, suggesting that even after two weeks, the NH4+ was not the source of interference. Further experiments are needed to better understand the source of this instability, though it is noted that blank solutions left untreated over the same time course consistently showed no change or slight increases in concentrations, and therefore, the instability must be created by the presence of other species found in the ambient urban air. Until this is resolved, it is recommended that sample solutions be reduced within 7 days from the time of collection.
Figure 4 details the collection of samples with the mobile setup over various field campaigns in urban, near-road, and on-road settings. The NOx concentration range spans three orders of magnitude, and the isotope ratios range from -1 to -13&#8240;. This sampling set includes 51 on-road samples taken over 52 hr, covering over 4,000 km, and in a myriad of driving conditions (e.g., heavy stop-and-go traffic to very light traffic at high speeds on the highway). The sampling took place on roads in and between 6 major cities, including Providence, RI, Philadelphia and Pittsburgh, PA, and Cleveland, Columbus, and Cincinnati, OH. Average vehicle speeds ranged from 12.4 km/hr to 119.7 km/hr. The near-roadside samples (N = 27) were obtained at a monitoring site at I-95 in Providence, RI. The ambient urban air samples (N = 44 samples taken over 117.5 hr) were taken from two rooftop locations in Providence, RI, one near the I-95, I-195 interchange and one 775 meters away from the interchange site. This represents the first steps towards building new capabilities to resolve the ranges of isotopic signatures from NOx sources, in this case, vehicle emissions and ambient urban sources. The variations in daytime on-road and road-side &#948;15N-NOx (Figure 4) were not correlated with variations in driving conditions and occurred across relatively constant vehicle fuel-class traffic counts. A more detailed discussion of the variations of isotopic signatures due to vehicle fuel types is the subject of another manuscript (Miller, D.J., et al. 2016. J. Geophys. Atmos. Submitted).
Finally, Table 1 details field and laboratory collections where two collection systems were deployed at the same time to test reproducibility. The comparisons show excellent agreement for the isotopic data, quantified here as the absolute deviation between the two data points for each collection period. The data are displayed from urban air collections at a rooftop location in Providence, RI; near-roadside collections in Providence, RI; and from collections in a laboratory-based smog chamber at the University of Massachusetts, Amherst.
<img alt="Figure 1" src="/files/ftp_upload/54962/54962fig1.jpg" /> 
Figure 1: Collection Schematic and Image. (A) Diagram of the Automated NOx Collection System. Gray is airflow, blue is water/solution flow, green is electronics connections, yellow is the frit, and purple is the permanganate solution. The syringe pump is used to add and remove rinsing solution (ultrapure water) and to add new solution for the start of sample collection (the syringe pump is a commercially available stepper motor syringe pump with a 50 ml syringe, a 5-port distribution valve, and driver/control boards equipped with an RS-232 serial interface). The sample is removed manually via the black valve at the bottom of the gas washing bottle. (B) Picture of the NOx collection system and NOx box in the mobile laboratory. <a href="http://cloudflare.jove.com/files/ftp_upload/54962/54962fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54962/54962fig2.jpg" /> 
Figure 2: Collection Efficiency of the Automated Collection System. (A) The NOx concentrations calculated from the NO3- concentrations measured in solution and the flow data compared against the median concentration measured by a chemiluminescent NOx concentration analyzer at a rooftop site in Providence, RI. The error bars are the standard deviation (&#177;1&#963;) of the solution-based NOx mixing ratio estimates derived from the propagated errors of the pooled standard deviations of the quality controls (0.4 &#181;M) across colorimetric concentration measurement runs and the flow rate uncertainty (&#177;1%). The NOx analyzer concentration uncertainties are &#177;5%. (B) The time series of NOx concentration distributions during diurnal measurements at a rooftop site in Providence, RI. The boxes represent the 25th, 50th, and 75th percentiles. The whiskers represent the extremes without outliers. <a href="http://cloudflare.jove.com/files/ftp_upload/54962/54962fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54962/54962fig3.jpg" /> 
Figure 3: Comparison of the reduction times for NOx samples collected at Brown University in May and July 2015. (A) The results are recorded as deviations from the first reduction, performed within 1 day of sampling. May samples are displayed as triangles and July samples as circles, with colors denoting different collection periods. Samples in (B) were pre-treated with ammonium chloride prior to air collection to test the interference of NH4+ in solution over time. The dashed lines represent the expected overall precision of the isotopic collection method, expressed as a standard deviation of &#177;1.5%. <a href="http://cloudflare.jove.com/files/ftp_upload/54962/54962fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54962/54962fig4.jpg" /> 
Figure 4: The &#948;15N-NOx (&#8240;) and NOx concentration of samples collected in ambient urban air, on-road, and near-road sites. The types of samples are delineated by different colors, and represent a range of conditions (see the text) and NOx concentrations. <a href="http://cloudflare.jove.com/files/ftp_upload/54962/54962fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
 
&#160;
<table fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Sample Name</td>
			<td>System Number</td>
			<td>Collection Date</td>
			<td>Hours of Collection</td>
			<td>Temperature (&#186;C)</td>
			<td>[NO3-] (&#181;M)</td>
			<td>Blank/total N</td>
			<td>&#948; 15N (&#8240;)</td>
			<td>Deviation&#160;&#948; 15N (%)</td>
		</tr>
		<tr>
			<td rowspan="2">Urban Air PVD 1</td>
			<td>1</td>
			<td rowspan="2">10/8/2013 - 10/9/2013</td>
			<td rowspan="2">6.75</td>
			<td rowspan="2">15.8</td>
			<td>14.43</td>
			<td>0.3</td>
			<td>-0.6</td>
			<td rowspan="2">0.7</td>
		</tr>
		<tr>
			<td>2</td>
			<td>16.78</td>
			<td>0.26</td>
			<td>-1.3</td>
		</tr>
		<tr>
			<td rowspan="2">Urban Air PVD 2</td>
			<td>1</td>
			<td rowspan="2">11/6/2013 - 11/7/2013</td>
			<td>2.5*</td>
			<td rowspan="2">17.1</td>
			<td>30.86</td>
			<td>0.2</td>
			<td>-7.7</td>
			<td rowspan="2">1</td>
		</tr>
		<tr>
			<td>2</td>
			<td>5.25</td>
			<td>37.05</td>
			<td>0.17</td>
			<td>-6.7</td>
		</tr>
		<tr>
			<td rowspan="2">Urban Air PVD 3</td>
			<td>1</td>
			<td rowspan="2">11/20/2013 - 11/21/2013</td>
			<td rowspan="2">8.9</td>
			<td rowspan="2">3.28</td>
			<td>44.29</td>
			<td>0.14</td>
			<td>-7.1</td>
			<td rowspan="2">0.4</td>
		</tr>
		<tr>
			<td>2</td>
			<td>29.66</td>
			<td>0.21</td>
			<td>-6.7</td>
		</tr>
		<tr>
			<td rowspan="2">Near Roadside 1</td>
			<td>1</td>
			<td rowspan="2">8/14/2014 - 8/15/2014</td>
			<td rowspan="2">29</td>
			<td rowspan="2">19.2</td>
			<td>13.3</td>
			<td>0.37</td>
			<td>-9.47</td>
			<td rowspan="2">0.69</td>
		</tr>
		<tr>
			<td>2</td>
			<td>16.4</td>
			<td>0.3</td>
			<td>-10.16</td>
		</tr>
		<tr>
			<td rowspan="2">Near Roadside 2</td>
			<td>1</td>
			<td rowspan="2">8/17/2014 - 8/18/2014</td>
			<td rowspan="2">30</td>
			<td rowspan="2">21.85</td>
			<td>9.4</td>
			<td>0.68</td>
			<td>-8.95</td>
			<td rowspan="2">1.56</td>
		</tr>
		<tr>
			<td>2</td>
			<td>11.6</td>
			<td>0.55</td>
			<td>-7.39</td>
		</tr>
		<tr>
			<td rowspan="2">Near Roadside 3</td>
			<td>1</td>
			<td rowspan="2">5/25/2015</td>
			<td rowspan="2">3.5</td>
			<td rowspan="2">20</td>
			<td>6.86</td>
			<td>0.51</td>
			<td>-7.67</td>
			<td rowspan="2">0.86</td>
		</tr>
		<tr>
			<td>2</td>
			<td>9.49</td>
			<td>0.42</td>
			<td>-8.53</td>
		</tr>
		<tr>
			<td rowspan="2">Near Roadside 4</td>
			<td>1</td>
			<td rowspan="2">5/26/2015</td>
			<td rowspan="2">2.75</td>
			<td rowspan="2">25.56</td>
			<td>6.07</td>
			<td>0.656</td>
			<td>-8.7</td>
			<td rowspan="2">1.57</td>
		</tr>
		<tr>
			<td>2</td>
			<td>6.49</td>
			<td>0.61</td>
			<td>-7.13</td>
		</tr>
		<tr>
			<td rowspan="2">Smog Chamber 1</td>
			<td>1</td>
			<td rowspan="2">8/26/2014 - 8/27/2014</td>
			<td rowspan="2">24.4</td>
			<td rowspan="2">21</td>
			<td>24.392</td>
			<td>0.27</td>
			<td>-12.28</td>
			<td rowspan="2">0.33</td>
		</tr>
		<tr>
			<td>2</td>
			<td>33.2</td>
			<td>0.2</td>
			<td>-12.61</td>
		</tr>
		<tr>
			<td rowspan="2">Smog Chamber 2</td>
			<td>1</td>
			<td rowspan="2">8/27/2014 - 8/28/2014</td>
			<td rowspan="2">19.8</td>
			<td rowspan="2">21</td>
			<td>10.96</td>
			<td>0.54</td>
			<td>-10.22</td>
			<td rowspan="2">1.25</td>
		</tr>
		<tr>
			<td>1</td>
			<td>14.245</td>
			<td>0.41</td>
			<td>-11.47</td>
		</tr>
		<tr>
			<td>Smog Chamber 3</td>
			<td>1</td>
			<td>8/28/2014 - 8/29/2014</td>
			<td>24.2</td>
			<td>21</td>
			<td>7.476</td>
			<td>0.8</td>
			<td>-5.86</td>
			<td>1.27</td>
		</tr>
	</tbody>
</table>
Table 1: Reproducibility of samples collected at the same time using two identical collection systems. *The collection had to be stopped due to a clogged filter. Urban Air PVD (PVD = Providence, RI) 1-3 were previously published4. Near Roadside represents roadside collections in Providence, RI; Smog chamber represents samples collected from air inside a smog chamber at the University of Massachusetts, Amherst4.

Watch this Scientific Journal Video about Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx at JoVE.com

Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx (Video) | JoVE

Previous work suggests that the nitrogen isotopic composition of atmospheric nitrogen oxides might distinguish the influence of different sources in the environment. We report on an automated, mobile, field-based method for the high collection efficiency of atmospheric NOx for N isotopic analysis at an hourly time resolution.

Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NOx

Nitrogen oxides (NOx = NO + NO2) are a family of atmospheric trace gases that have great impact on the environment. NOx concentrations directly influence ...

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JoVE Journal

Environment

The Use of High-resolution Infrared Thermography (HRIT) for the Study of Ice Nucleation and Ice Propagation in Plants

The Use of High-resolution Infrared Thermography HRIT for the Study of Ice Nucleation and Ice Propagation in Plants (Video) | JoVE

Freezing events that occur when plants are actively growing can be a lethal event, particularly if the plant has no freezing tolerance. Such frost events ...

The Use of an Automated System (GreenFeed) to Monitor Enteric Methane and Carbon Dioxide Emissions from Ruminant Animals

The Use of an Automated System GreenFeed to Monitor Enteric Methane and Carbon Dioxide Emissions from Ruminant Animals (Video) | JoVE

Ruminant animals (domesticated or wild) emit methane (CH4) through enteric fermentation in their digestive tract and from decomposition of manure during ...

A Simple Method for Automated Solid Phase Extraction of Water Samples for Immunological Analysis of Small Pollutants

A Simple Method for Automated Solid Phase Extraction of Water Samples for Immunological Analysis of Small Pollutants (Video) | JoVE

A new method for solid phase extraction (SPE) of environmental water samples is proposed. The developed prototype is cost-efficient and user friendly, and ...

Modification and Application of a Leaf Blower-vac for Field Sampling of Arthropods

Modification and Application of a Leaf Blower-vac for Field Sampling of Arthropods (Video) | JoVE

Rice fields host a large diversity of arthropods, but investigating their population dynamics and interactions is challenging. Here we describe the ...

Biofilm Removal Using Carbon Dioxide Aerosols without Nitrogen Purge

Biofilm Removal Using Carbon Dioxide Aerosols without Nitrogen Purge (Video) | JoVE

Biofilms can cause serious concerns in many applications. Not only can they cause economic losses, but they can also present a public health hazard. ...

Protocol for Microplastics Sampling on the Sea Surface and Sample Analysis

Protocol for Microplastics Sampling on the Sea Surface and Sample Analysis (Video) | JoVE

Microplastic pollution in the marine environment is a scientific topic that has received increasing attention over the last decade. The majority of ...

Extraction of Organochlorine Pesticides from Plastic Pellets and Plastic Type Analysis

Extraction of Organochlorine Pesticides from Plastic Pellets and Plastic Type Analysis (Video) | JoVE

Plastic resin pellets, categorized as microplastics (&#8804;5 mm in diameter), are small granules that can be unintentionally released to the environment ...

Techniques for Investigating the Anatomy of the Ant Visual System

Techniques for Investigating the Anatomy of the Ant Visual System (Video) | JoVE

This article outlines a suite of techniques in light microscopy (LM) and electron microscopy (EM) which can be used to study the internal and external eye ...