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Analytical Techniques for Measuring Nitrous Oxide ⇑ Trevor D

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Analytical Techniques for Measuring Nitrous Oxide ⇑ Trevor D Trends in Analytical Chemistry 54 (2014) 65–74 Contents lists available at ScienceDirect Trends in Analytical Chemistry journal homepage: www.elsevier.com/locate/trac Review Analytical techniques for measuring nitrous oxide ⇑ Trevor D. Rapson , Helen Dacres CSIRO Ecosystem Sciences and Food Futures Flagship, GPO Box 1700, 2601 ACT, Australia article info abstract Keywords: Nitrous oxide (N2O) is estimated to contribute about 6% of the global warming effect due to greenhouse Amperometric sensor gases. N2O is also predicted to be the single most important ozone-depleting emission in the twenty-first Emissions measurement century. Great progress has been made in N2O measurement, but there is a critical need for sensors that Fourier-transform infrared can be used to map the spatial variation of N O emissions over a wide area. In this article, we outline FTIR 2 where N O measurement is required, describe advances that have been made in developing sensitive Gas chromatography 2 Laser-absorption spectroscopy analytical techniques and review some promising new technologies. Our aim is to assist both those Nitrous oxide new to N2O measurement, enabling them to select the most appropriate of the available technology, Sensor and to inform those developing new analytical techniques. Spatial variation Ó 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY license. Trace-gas analysis Contents 1. Introduction . ....................................................................................................... 66 2. Methods for measuring nitrous oxide. .................................................................................... 67 2.1. Sampling methods. .......................................................................................... 67 2.1.1. Chamber methods . ............................................................................... 67 2.1.2. Micrometeorological methods ............................................................................... 67 3. Analytical techniques currently used . .................................................................................... 68 3.1. Chromatographic techniques . .......................................................................... 68 3.2. Optical techniques. .......................................................................................... 69 3.2.1. Fourier-transform infrared spectroscopy . ............................................................ 69 3.2.2. Infrared laser-absorption spectroscopy . ............................................................ 69 3.2.3. Multipass cells . ............................................................................... 70 3.2.4. High-finesse optical cavities . ............................................................................... 70 3.2.5. Photoacoustic (PA) trace-gas detection . ............................................................ 70 3.3. Amperometric methods . .......................................................................................... 71 4. New methods under development . .................................................................................... 71 4.1. Amperometric biosensors .......................................................................................... 71 4.2. Optical fibers . .......................................................................................... 71 View metadata, citation and similar papers at core.ac.uk brought to you by CORE 4.3. Modified-SnO2 surfaces. .......................................................................................... 71 5. Conclusions. .......................................................................................................provided by Elsevier - Publisher Connector 71 Acknowledgements . .................................................................................... 71 References . ....................................................................................................... 71 Abbreviations: ECD, Electron-capture detector; QCL, Quantum-cascade laser; CRDS, Cavity ring-down spectroscopy; OA-ICOS, Off-axis integrated cavity-output spectroscopy; IRMS, Isotope-ratio mass spectrometry; PAS, Photoacoustic spectroscopy. ⇑ Corresponding author. Tel.: +61 2 62464104; Fax: +61 2 62464000 E-mail address: [email protected] (T.D. Rapson). 0165-9936 Ó 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY license. http://dx.doi.org/10.1016/j.trac.2013.11.004 66 T.D. Rapson, H. Dacres / Trends in Analytical Chemistry 54 (2014) 65–74 1. Introduction processes are affected by a wide range of factors, such as tem- perature, soil pH, moisture and soil organic carbon. While cap- Nitrous oxide (N2O) is a powerful greenhouse gas. The global turing flux events is challenging, it is critical to allow for warming potential (GWP) of N2O is 298 times greater than CO2 accurate quantification of ecosystem-wide emissions [13,14]. over a 100-year time horizon, and N2O is estimated to contribute about 6% of the global warming effect due to greenhouse gases A large amount of research is being carried out on N2O fluxes [1,2] In the stratosphere, N2O is oxidized to form NO and NO2. from soils, wastewater and aquatic systems. This research aims These nitrogen oxides catalyze the destruction of ozone [3], mak- not only to improve greenhouse-gas accounting, but also to under- ing N2O the single most important ozone-depleting emission in stand the complex processes contributing to N2O fluxes, which the twenty-first century [4]. could be used to inform mitigation strategies. N2O is an intermediate in both bacterial nitrification and deni- Not only has the quantity of N2O emissions been measured, but trification pathways, so N2O emissions are mainly due to bacterial isotopic studies have also been performed, and have provided transformations of nitrogen in soils and oceans (see Figs. 1 and 2). important information about the biogeochemical cycle of N2O 14 14 16 Since the industrial revolution (1750 AD), N2O levels have in- [15–19]. Although N N O is the most abundant species (99%), creased by almost 20%, from 270 ppb to 320 ppb. This increase is both the ratio of 15N/14N and the position of 15N (either attributed to human activities, in particular, the use of synthetic 15N14N16Oor14N15N16O) provide useful information [20–23]. Pho- and organic fertilizers. Other important sources of N2O emissions tolytic decomposition of N2–O in the stratosphere, the major sink are human sewage and the burning of biomass and biofuels [5–7]. of N2O, leads to enrichment of the heavier isotopes of N2O (both 15 18 Under the Kyoto Protocol, signatory countries are required to N and O) [24,25]. Microbial sources generally emit N2O that complete an annual national greenhouse-gas inventory, which, is depleted in 15N and 18O, while site preference (15N14N16Oor 14 15 16 for completeness, should include N2O. Calculating N2O emissions N N O) can indicated whether N2O is produced by nitrifying is a complicated task. In addition to N2O emissions from agricul- or denitrifying bacteria (see Fig. 2) [20,22,26]. ture, there are indirect emissions, such as those caused by leaching To assist these studies, there is a strong need for better analyt- of nitrogen from agricultural fields to aquatic systems, which in- ical methods and cost-effective ways of measuring N2O over wide crease N2O emissions from rivers and estuaries [8,9]. Furthermore, areas and long time-frames. The World Meteorological Organiza- natural systems can act as important sources and sinks of N2O [10]. tion Global Atmosphere Watch (WMO-GAW) guidelines for the The Intergovernmental Panel on Climate Change (IPCC) has set out measurement of N2O recommend accuracy and precision of less guidelines for developing national greenhouse-gas inventories [11], whereby N2O emissions are determined from models that link emissions to activities, such as fertilizer use. Underlying the mod- els are emission factors, which are determined from experimental data. The accuracy of the models depends on the quality of the experimental data on which they are based [10,12]. Measuring N2O emissions experimentally is not a trivial matter. Two main challenges are encountered when trying to measure N2O emissions: first, the low atmospheric concentrations of N2O (320 ppb), roughly a thousand times lower than CO2, and these low con- centrations are outside the detection range of many analytical techniques; and, second, N2O fluxes are episodic and demonstrate very large Fig. 2. Microbial pathways of N2O production. (a) N2O production through temporal and spatial variations, due to the multiple processes nitrification via hydroxylamine. (b) N2O production through nitrifier denitrification. that both produce and consume N2O. The rates of these Figure modified from [19]. Fig. 1. Global sources of N2O for the 1990s. Data for the graph was obtained from [5]. T.D. Rapson, H. Dacres / Trends in Analytical Chemistry 54 (2014) 65–74 67 than 0.1 ppb [27]. This strict requirement has driven the develop- gas sensors are placed on towers that measure wind, temperature ment of new methods for measuring N2O. In addition to these and gas concentrations at one or more points above the soil or veg- accuracy requirements, the ideal N2O sensor would be cheap and etation surface [28,38]. easily deployable to allow for a large number of measurements to be taken over extensive areas [13,28]. 2.1.2.1. Eddy covariance. Eddy covariance is the most direct method Besides the interest in N2O as a greenhouse gas,
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