21Keywords 22 ammonia in ambient air, traceability, reference gas standards, optical transfer standard, validation and testing 23 infrastructure 24 Abstract 25The environmental impacts of ammonia (NH 3 ) in ambient air have become more evident in the recent decades, 26 leading to intensifying research in this field. A number of novel analytical techniques and monitoring 27 instruments have been developed, and the quality and availability of reference gas mixtures used for the 28 calibration of measuring instruments has also increased significantly. However, recent inter-comparison 29 measurements show significant discrepancies, indicating that the majority of the newly developed devices and 30 reference materials require further thorough validation. There is a clear need for more intensive metrological 31 research focusing on quality assurance, intercomparability and validations. MetNH3 (Metrology for ammonia in 32 ambient air) is a three-year project within the framework of the European Metrology Research Programme 33 (EMRP), which aims to bring metrological traceability to ambient ammonia measurements in the 0.5 -34 500 nmol/mol amount fraction range. This is addressed by working in three areas: 1) improving accuracy and 35 2 stability of static and dynamic reference gas mixtures, 2) developing an optical transfer standard and 3) 36 establishing the link between high-accuracy metrological standards and field measurements. In this article we 37 describe the concept, aims and first results of the project. 38

Abstract. Nitrous oxide (N 2 O) is an important and strong greenhouse gas in the atmosphere. It is produced by microbes during nitrification and denitrification in terrestrial and aquatic ecosystems. The main sinks for N 2 O are turnover by denitrification and photolysis and photo-oxidation in the stratosphere. In the linear N=N=O molecule 15 N substitution is possible in two distinct positions: central and terminal. The respective molecules, 14 N 15 N 16 O and 15 N 14 N 16 O, are called isotopomers. It has been demonstrated that N 2 O produced by nitrifying or denitrifying microbes exhibits a different relative abundance of the isotopomers. Therefore, measurements of the site preference (difference in the abundance of the two isotopomers) in N 2 O can be used to determine the source of N 2 O, i.e., nitrification or denitrification. Recent instrument development allows for continuous positiondependent δ 15 N measurements at N 2 O concentrations relevant for studies of atmospheric chemistry. We present results from continuous incubation experiments with denitrifying bacteria, Pseudomonas fluorescens (producing and reducing N 2 O) and Pseudomonas chlororaphis (only producing N 2 O). The continuous measurements of N 2 O isotopomers reveals the transient isotope exchange among KNO 3 , N 2 O, and N 2 . We find bulk isotopic fractionation of −5.01 ‰ ± 1.20 for P. chlororaphis, in line with previous results for production from denitrification. For P. fluorescens, the bulk isotopic fractionation during production of N 2 O is −52.21 ‰ ± 9.28 and 8.77 ‰ ± 4.49 during N 2 O reduction.The site preference (SP) isotopic fractionation for P. chlororaphis is −3.42 ‰ ± 1.69. For P. fluorescens, the calculations result in SP isotopic fractionation values of 5.73 ‰ ± 5.26 during production of N 2 O and 2.41 ‰ ± 3.04 during reduction of N 2 O. In summary, we implemented continuous measurements of N 2 O isotopomers during incubation of denitrifying bacteria and believe that similar experiments will lead to a better understanding of denitrifying bacteria and N 2 O turnover in soils and sediments and ultimately hands-on knowledge on the biotic mechanisms behind greenhouse gas exchange of the globe.

Nitrous oxide (N₂O) is an important and strong greenhouse gas in the atmosphere and part of a feed-back loop with climate. N₂O is produced by microbes during nitrification and denitrification in terrestrial and aquatic ecosystems. The main sinks for N₂O are turnover by denitrification and photolysis and photo-oxidation in the stratosphere. The position of the isotope ¹⁵N in the linear N&amp;thinsp;=&amp;thinsp;N&amp;thinsp;=&amp;thinsp;O molecule can be distinguished between the central or terminal position (isotopomers of N₂O). It has been demonstrated that nitrifying and denitrifying microbes have a different relative preference for the terminal and central position. Therefore, measurements of the site preference in N₂O can be used to determine the source of N₂O i.e. nitrification or denitrification. Recent instrument development allows for continuous (on the order of days) position dependent <i>δ</i>¹⁵N measurements at N₂O concentrations relevant for studies of atmospheric chemistry. We present results from continuous incubation experiments with denitrifying bacteria, <i>Pseudomonas fluorescens</i> (producing and reducing N₂O) and <i>P. chlororaphis</i> (only producing N₂O). The continuous position dependent measurements reveal the transient pattern (KNO₃ to N₂O and N₂, respectively), which can be compared to previous reported site preference (SP) values. We find bulk isotope effects of &amp;minus;5.5&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;0.9 for <i>P. chlororaphis</i>. For <i>P. fluorescens</i>, the bulk isotope effect during production of N₂O is &amp;minus;50.4&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;9.3 and 8.5&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;3.7 during N₂O reduction. The values for <i>P. fluorescens</i> are in line with earlier findings, whereas the values for <i>P. chlororaphis</i> are larger than previously published <i>δ</i>¹⁵N_<i>bulk</i> measurements from production. The calculations of the SP isotope effect from the measurements of <i>P. chlororaphis</i> result in values of &amp;minus;6.6&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;1.8. For <i>P. fluorescens</i>, the calculations results in SP values of &amp;minus;5.7&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;5.6 during production of N₂O and 2.3&amp;thinsp;‰&amp;thinsp;±&amp;thinsp;3.2 during reduction of N₂O. In summary, we implemented continuous measurements of N₂O isotopomers during incubation of denitrifying bacteria and believe that similar experiments will lead to a better understanding of denitrifying bacteria and N₂O turnover in soils and sediments and ultimately hands-on knowledge on the biotic mechanisms behind greenhouse gas exchange of the Globe.

Quartz-enhanced photoacoustic sensing is a promising method for low-concentration trace-gas monitoring due to the resonant signal enhancement provided by a high-Q quartz tuning fork. However, quartz-enhanced photoacoustic spectroscopy (QEPAS) is associated with a relatively slow acoustic decay, which results in a reduced spectral resolution and signal-to-noise ratio as the wavelength tuning rate is increased. In this work, we investigate the influence of wavelength scan rate on the spectral resolution and signal-to-noise ratio of QEPAS sensors. We demonstrate the acquisition of photoacoustic spectra from 3.1 μm to 3.6 μm using a tunable mid-infrared optical parametric oscillator. The spectra are attained using wavelength scan rates differing by more than two orders of magnitude (from 0.3 nm s−1 to 96 nm s−1). With this variation in scan rate, the spectral resolution is found to change from 2.5 cm−1 to 9 cm−1. The investigated gas samples are methane (in nitrogen) and a gas mixture consisting of methane, water, and ethanol. For the gas mixture, the reduced spectral resolution at fast scan rates significantly complicates the quantification of constituent gas concentrations.

We demonstrate the usefulness of a nanosecond pulsed single-mode mid-infrared (MIR) optical parametric oscillator (OPO) for Photoacoustic (PA) spectroscopic measurements. The maximum wavelength ranges for the signal and idler are 1.4 µm to 1.7 µm and 2.8 µm to 4.6 µm, respectively, and with a MIR output power of up to 500 mW. Making the OPO useful for different spectroscopic PA trace-gas measurements targeting the major market opportunity of environmental monitoring and breath gas analysis. We perform spectroscopic measurements of methane (CH4) nitrodioxide (NO2) and ammonia (NH3) in the 2.8 µm to 3.7 µm wavelength region. The measurements were conducted with a constant flow rate of 300 ml/min, thus demonstrating the suitability of the gas sensor for real time trace gas measurements. The acquired spectra are compared with data from the Hitran database and good agreement is found. Demonstrating a resolution bandwidth of 1.5 cm 1 . An Allan deviation analysis shows that the detection limit for methane at optimum integration time for the PA sensor is 8 ppbV (nmol/mol) at 105 seconds of integration time, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 2.9×10 −7 W cm −1 Hz −1/2 .

A photoacoustic (PA) sensor based on higher order acoustic modes is demonstrated. The PA sensor is designed to enhance the gas-detection performance and simultaneously suppress ambient noise sources (e.g. flow noise, electrical noise and external acoustic noise). Two microphones are used and positioned such that the PA signals are (π) out of phase. Ambient acoustic noise are approximately in the same phase and will be subtracted and thus improve the SNR. In addition, by placing the gas in-and outlets so that the gas flows through the node of the first higher order membrane mode the coupling of flow noise is approximately 20 dB lower compared with flow through the fundamental mode at 5 L/min. The noise reduction and thus the increase in sensitivity is demonstrated by measuring vibrational lines of methanol and methane using a broadband interband cascade laser emitting radiation at 3.38 µm. A signal-to-noise improvement of 20 (26 dB) using higher order modes are demonstrated compared with the fundamental mode. The maximum normalized noise equivalent absorption coefficient is 1.1 ×10 −6 W cm −1 Hz 1/2 for a flow of 5 L/min. It is anticipated that the noise cancelation strategy may find use in many practical industrial and environmental PA sensors, where ambient noise sources plays a crucial role for the absolute sensitive.

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