Quantum cascade laser based absorption spectroscopy for direct monitoring of atmospheric N2O isotopes

Accepted Manuscript Quantum cascade laser based absorption spectroscopy for direct monitoring of atmospheric N2O isotopes

Sheng Zhou, Ningwu Liu, Lei Zhang, Tianbo He, Jingsong Li PII: DOI: Reference:

S1386-1425(18)30681-4 doi:10.1016/j.saa.2018.07.028 SAA 16297

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

30 March 2018 17 June 2018 9 July 2018

Please cite this article as: Sheng Zhou, Ningwu Liu, Lei Zhang, Tianbo He, Jingsong Li , Quantum cascade laser based absorption spectroscopy for direct monitoring of atmospheric N2O isotopes. Saa (2018), doi:10.1016/j.saa.2018.07.028

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Quantum cascade laser based absorption spectroscopy for direct monitoring of atmospheric N2O isotopes Sheng Zhoua,b,*, Ningwu Liu a,b, Lei Zhang a,b, Tianbo He a,b, Jingsong Lia,b,* Laser Spectroscopy and Sensing Laboratory, Anhui University, 230601 Hefei, China

b

Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of

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a

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Education, Anhui University, 230601 Hefei, China Corresponding authors:

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Sheng Zhou, E-Mail: [email protected]

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Jingsong Li, E-Mail: [email protected]

Abstract

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A compact high-resolution spectroscopic sensor using a thermoelectrically (TE) cooled continuous-wave (CW) room temperature (RT) quantum cascade laser (QCL) operating at 4.6 μm, is 15

N14N16O and

N14N16O). To enable a high-precision analysis of N2O isotopic species at ambient mixing ratios, a

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employed for simultaneous detection of three main isotopic species (14N15N16O,

liquid nitrogen-free preconcentration unit is built to trap and load atmospheric N2O. The absorption 14

N15N16O, 15N14N16O, and

14

N14N16O between 2188.6 cm-1 and 2189 cm-1 are measured,

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spectra of

and the respective ratios of the rare to the abundant isotopologues abundances are demonstrated.

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Moreover, spectroscopic parameters of pressure-broadening coefficient for selected absorption lines have been determined, and a good agreement is obtained by comparing with HITRAN database.

Key words: Laser spectroscopy, QCL, Isotope analysis, Nitrous oxide

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Graphical Abstract

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A liquid nitrogen-free preconcentration unit is built to trap and load atmospheric N2O.

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CW-QCLs are promising spectroscopic sources for developing analytical instrumentation based

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

Three main isotopic species (14N15N16O, 15N14N16O and

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N14N16O) and the respective ratios of

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the rare to the abundant isotopologues abundances in the atmosphere are measured

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simultaneously.

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

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on high-resolution laser absorption spectroscopy.

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1 Introduction Nitrous oxide, commonly known as laughing gas, has significant medical uses in surgery and dentistry for its anaesthetic and pain reducing effects [1]. However, it is found to be a major scavenger of stratospheric ozone with an impact comparable to that of Chlorofluorocarbons (CFCs), which influences the troposphere heat budget [2]. Although the predominant sources of N2O on

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nitrification and denitrification from soil and aquatic environments are well quantified, the strengths of different sources and sinks of N2O for a global budget still remain largely uncertain, due to the

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complexity of pathways involved [3-5]. Important information about the biogeoche ical cycle of N2O

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can be obtained by measuring the intramolecular distribution of N2O isotopes ratio in atmospheric. Knowledge of the specific isotopic signatures of N2O associated with atmospheric sources, sinks, and

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other influences can provides valuable additional information, leading to a reduction in N2O budget uncertainty [6-8]. The 14N15N16O (456) and 15N14N16O (546) with one nitrogen atom at the center (α

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site) and one at the end (β site) are two most abundant N2O isotopic species in the atmosphere containing one heavy isotope of nitrogen, usually referred to as 15Nα and 15Nβ, respectively [9]. The

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corresponding isotope ratios are usually reported in the δ-notation, where δ15Nα denotes the relative

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difference in per mil (‰) of the 15N/14N ratio of the sample vs. the reference standard, thus δ15Nαand δ15N β denote the site specific ratio of

14

N15N16O vs.

14

N14N16O and

15

N14N16O vs.

14

N14N16O,

respectively. Important information about the biogeochemical cycle of nitrous oxide (N2O) can be

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obtained by measuring its three main isotopic species (14N15N16O, 15N14N16O and 14N14N16O) and the respective site-specific relative isotope ratio differences δ15Nα and δ15Nβ in the atmosphere [10].

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Currently, the only standard analytical technique for N2O isotope measurement at ambient concentration is the laboratory-based isotope-ratio mass-spectrometry (IRMS) [11-14]. However, the 14

N15N16O and

15

N14N16O are unaccessible to be directly discriminated, since only isotopologues

with different mass numbers can be distinguished. Recently, an off-line pre-concentration system for the measurement of atmospheric N2O samples has been developed by standard dual inlet isotope ratio mass spectrometry (IRMS) [15]. However, hundreds of liters atmospheric air samples at standard temperature and pressure (STP) is required with difficult sampling techniques to obtain the needed μmol quantities of N2O. Similarly, optical Fourier transform infrared (FTIR) based N2O

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isotopic measurements also require μmol quantities of N2O, which limits the high-frequency detection [16]. Besides, the mass spectrometric analysis involves the complex operation, which makes it incapable of field deployment for in situ and real-time measurements. Recently, the high-resolution laser absorption spectroscopy has developed into an alternative analytical technique for determining the stable isotope abundance ratios with inherent advantages of

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high sensitivity, high time resolution and excellent precision [17]. The different isotopic molecular species can be distinguished definitely without need of destructive and laborious in sample

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preparation. Mid-infrared (mid-IR) molecular fingerprint region are suitable for trace gas analysis

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since most of the atmospheric species have their strong fundamental vibrational transitions in this spectral region [18-20]. The advances in semiconductor laser technology, particularly the quantum

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cascade lasers (QCLs), provided a more effective way for highly sensitive and selective detection of trace gases [21-23]. The major advantage of QCLs is their ability to operate over a wide spectral range,

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providing an opportunity for simultaneous detection of mixed gases [24]. The spectroscopic detection technique has been widely used in N2O isotope ratios detection, most

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of these reported applications are done in pure N2O or synthetic air, since it is a high challenge for

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practical atmospheric study [25-27]. Although the monitoring of the N2O isotopic species and site-specific relative isotope ratio differences has been reported, the pulsed QCL is used as an optical source, which requires a wide frequency bandwidth infrared detector and a high-speed driving

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electronics as well as a fast data acquisition [28]. The wide line-width of pulsed QCL shows a high pulse-pulse intensity fluctuation. In this paper, we demonstrate the precise and simultaneous

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determination of three main isotopic species (14N15N16O,

15

N14N16O and

14

N14N16O) in the

atmosphere at concentration as low as several ppm using a TE-cooled, CW-RT distributed feedback (DFB) QCL based spectrometer. To enable a high precision analysis of N2O isotopic species at ambient mixing ratios, we built and optimized a liquid nitrogen-free preconcentration unit to trap and load atmospheric N2O. The results indicate that the laser spectroscopy combined with a preconcentration unit is feasible for the measurement of isotopic composition of trace gases in atmosphere. 2 Experimental Details

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2.1 Spectral Range Selection For high precision determination of isotopic ratios using laser absorption spectroscopy, the absorption lines near 2188.7 cm-1 have been selected based on the following considerations: (i) both 14

N15N16O and

15

N14N16O have absorption features with distinctive structures and absorption lines

should be located within a single current scan of the laser. Thus, the potential nonlinear effect of laser

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power within the tuning of the laser source can be avoided (ii) there should be no interference from other transitions of the same or other atmospheric species, especially for water vapor. The spectral

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overview of 15N-isotopes of N2O in the spectral region covering scanning range taken from HITRAN

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database [28] is shown in Fig. 1(a).

Fig. 1 (a) The absorption lines from HITRAN database for 15N-isotopes of N2O, respectively in the spectral region

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covering 2185.2-2191.7 cm-1. The dashed region represents the selected absorption lines of N2O isotopes. (b) The simulated absorption cross-section of N2O isotopes in the selected region at a 100 mbar pressure and room temperature.

The dashed region represents the selected absorption lines for N2O isotope-ratio detection. Indeed, N-isotopes of N2O from the profile of

14

N14N16O

absorption due to strong overlapping. To resolve the absorption spectra of 14N15N16O and

15

N14N16O

it is difficult to identify the absorption peaks of

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under a strong interference from adjacent lines of 14N14N16O, the pressure in the sample cell has to be reduced. Figure 1(b) gives the simulated absorption cross-section of N2O isotopes in the selected

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region at a 100 mbar pressure and room temperature. 2.2 Spectrometer Desgin The CW-QCL based spectroscopic setup is schematically shown in Fig. 2. It consists of a thermoelectrically (TE) cooled CW room temperature (RT) QCL operating in the wavelength region around 4.6 μm, a multi-pass absorption cell, a TE-cooled room temperature detector and a beam spliter,

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as well as a series of flat mirrors and off-axis parabolic mirrors for beam transfer and collection. The laser wavelength is controlled by changing the device temperature with a peltier driver and the

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maximum peak output power is 30 mW. The laser frequency is scanned across absorption lines using a

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triangular wave at a typical frequency of 100 Hz. The linewidth of laser is estimated to be less than 0.001 cm-1, and then the laser line profile induced broadening can be neglected. The laser beam is

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firstly collimated and sent through a compact multiple-pass Herriott cell with a maximal optical path length of 62.5 m. A ZnSe beam splitter is used to co-align a visible red light (632.8 nm) with the QCL

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beam to facilitate the beam alignment. The light that transmitted through multi-pass cell is focused by a parabolic mirror into a TE-cooled, high-speed IR photovoltaic detector (PVI-4TE-5, Vigo, Poland)

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working at room-temperature, thus eliminating the need for liquid nitrogen cooling, simplifying daily

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use of the system and allowing long-term automated operation. The processed data were subsequently acquired using a DAQ card (National Instruments, USB-6259) and displayed by a Labview (National Instruments) interface in a laptop.

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2.3 Preconcentration unit

A preconcentration system is designed and optimized for the preconcentration of N2O isotopic

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species in atmosphere and their subsequent quantification by laser spectroscopy. The gas flow is significantly regulated by mass flow controller and a relatively mild adsorbent trap is placed in a low-temperature refrigeration unit reaching <-75oC. The process chart during trap loading with N2O in atmosphere is also illustrated in Fig. 2. The preconcentration trap consists of a stainless steel tubing (ID 4mm, OD 6 mm) filled with HayeSep D (100-20 mesh, Hayes Separations Inc., USA). The trap is attached to a refrigeration unit located in a chamber and the temperature can down to <-75oC after cooling the temperature.

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Fig. 2. Schematic of a CW DFB-QCL based sensor system for isotopic composition detection in atmosphere.

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3 Results and discussions 3.1 Analysis of the 15N isotopic of N2O

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In order to evaluate the performance of the CW-QCL based spectroscopic for sensitive, accurate quantitative measurements of 15N isotopes, the interference-free molecular transitions of 15N14N16O, 14

N15N16O isotopes located at ~2188.69 cm-1 and ~2188.76 cm-1, as shown in the HITRAN simulated

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spectra (Fig. 1), are probed. Figure 3 shows an example of the high-resolution spectrum of N2O

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isotopes when certified calibration mixture gas in N2 is injected into the multi-pass cell with a pressure of 100 mbar. The spectral lines were fitted with the Voigt line-shape functions and the For baseline substraction, a high-order

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residual (1σ, or one time of standard deviation) is 7x10-4.

polynomial function is fitted to the areas of the recorded spectrum without atmospheric absorption 14

N15N16O and

15

N14N16O isotopes with different concentrations are injected into the

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features. The

multi-pass cell. Then, the acquired the absorption (-ln(I/I0)) across the selected molecular transitions are fitted. The integrated area under each absorption curve (cm-1) is plotted (Fig. 4) against the different concentrations. According to the linear regression plot, the line-integrated absorption intensity are determined to be 3.2x10-21 cm/molecule, 2.91x10-21 cm/molecule and 2.5x10-21 cm/molecule, respectively, which are in excellent agreement with the values taken from the HITRAN database.

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Fig. 3. Absorption of 14N and 15N-isotopes of N2O centered at ~2188.688 cm-1, ~2188.756 cm-1 and ~2188.938 cm-1

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and the spectra are fitted with Voigt function under 100mbar pressure and room temperature, respectively. The

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measured data and fitted curves are shown in the upper trace and the residuals in the lower trace.

Fig. 4. The linear regression plot between the area of absorption and molecular concentration of 15

14

N and

N-isotopes of N2O, respectively

The pressure broadening effect on the absorption spectra of

14

N and 15N-isotopes of N2O at the

designated wavenumbers is analyzed, because it has a direct effect on the sensitivity and selectivity of the spectroscopy sensor. The absorption spectra for the isotopes are then measured with the experimental setup at different pressures and part of the results are shown in Fig. 5. A significant reduction of the peak height of isotopic N2O spectra is observed with the decreasing pressure in the

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cell. The spectra of such absorption lines are fitted with Voigt lineshape function. Figure 6 shows the plots of the half-width half-maxima (HWHM in cm-1) of the absorption lines of 14N and 15N-isotopes of N2O against the different pressures (in mbar). The gradient of the linear regression of the plot provided the pressure-broadening coefficient of 0.08 cm-1atm-1, 0.079 cm-1atm-1 and 0.082 cm-1atm-1, which are in good agreement with the HITRAN values of 0.079 cm-1atm-1, 0.078 cm-1atm-1, 0.085

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cm-1atm-1.

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Fig. 5. Variation of absorption spectra of 14N and 15N-isotopes of N2O, respectively at different cavity pressures.

Fig. 6. The linear regression plot between HWHM of absorption spectra and sample pressures for 15

14

N and

N-isotopes of N2O, respectively

3.2 Monitoring 14N and 15N-isotopes of N2O in atmosphere Finally, the potential of the CW-QCL based spectroscopic sensor for real-time monitoring of 14N and

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N-isotopes of N2O concentrations in ambient air is evaluated. A vacuum pump is used to control and

maintain the pressure of the system. Before the experiment, the multi-pass cell and the inlet line as well as the preconcentration trap are purged by back-flushing with 200 sccm (standard cubic centimetre per minute) of high purity nitrogen in the room temperature to assure the absence of residual trace gas. Air sampling analysis begins with air introduced into the temperature-stabilized all stainless steel system

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controlled by the integrating mass flow controller (MFC) (Alicat, USA). The sample size and flow rate are optimized as a balance between trapping capacity, lessening time required for sample extraction,

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and optimizing the amount of pre-concentrated N2O to achieve the maximize measurement precision.

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A diaphragm pump behind the multi-pass cell assists in maintaining a pressure differential across the pressure controller, regardless of any potential upstream pressure transients. After cooling the trap

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down to -70 oC, the atmosphere air is pumped through the absorbent trap at a flow rate of 150 sccm and kept for around 20 min, and then the temperature rises to the room temperature. The atmosphere air at

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a flow rate of 15 sccm is used to initiate desorption and measured by the built spectroscopic sensor. Figure 7 depicts the absorption spectra of isotopic N2O in ambient air. These raw absorption

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scans (green stars) are recorded with 100 Hz scan repetition rate and pre-averaged by co-adding 20

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subsequent spectral scans which yielded an effective spectrometer repetition rate of five measurements per second (5 Hz). The isotope ratio can thus be determined from the ratio of the integrated areas A and the absorption line intensities S(T) of the major and minor isotopic components:

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Ax S a / n a R  [C ] / [C ]  a  x x A S /n x

a

(1)

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where R represents the ratio of the rare to the abundant isotopologues abundances, C is the number density of absorbing species (in mol/cm3), x refers to the rare isotopic species (14N15N16O and 15

N14N16O),

a

represents

the

abundant

isotopic

component

(14N14N16O),

S(T)

is

the

temperature-dependent molecular line absorption intensity in cm-1/(mol·cm-2) described in parts 3.1 and n is the coefficient of temperature dependence of air-broadened halfwidth which is assumed to be transition dependent. The isotope abundance ratios of 14N15N16O vs. 14

14

N14N16O and

15

N14N16O vs.

N14N16O, are estimated to be 3.9‰ and 3.78‰, respectively. The concentration of N2O is increased

to 10.83 ppm validated by the preconcentration unit, and the signal to noise ratio (SNR) is estimated to

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be 31.5. The temperature shift may introduce a systematic deviation in the isotope ratio measurements. The error of the derived δ-values proportional to the relative error of the temperature ΔT/T with a relationship:  

E '' T KT T

(2)

where k is the Boltzmann constant, ΔE" is the lower state energy difference between the two

15

Nα and

15

Nβ. The dependence of the selected line strengths on

temperature is -3.3‰/K, -3.1‰/K and +15‰/K for

14

N15N16O at 2188.68757 cm-1,

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shift would be 18‰ for both

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ro-vibrational lines and T is the absolute temperature. In our case, for a difference of 1 K, the apparent

15

N14N16O at

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2188.75601 cm-1 and 14N14N16O at 2188.93846 cm-1, respectively. During these measurements, a slight drift in temperature (ΔT = ±0.3 K) was observed. The

15

Nα and

15

Nβ ratio measurements had a

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comparable precision (1.14‰), which indicates that temperature variation has effect on the instrumental precision.

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This system has been used to provide an assessment of N2O isotopic composition in the atmosphere and interpret the utility and feasibility of tropospheric N2O isotopic observations without a

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great deal of chemical preparation. The results indicates that the present mid-IR CW-QCL based

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spectroscopic sensor allows for the high precision detection of subtle changes in isotopic composition on short or long-term scales and is available for the measurement of site-specific

15

N composition

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which is not straightforward in IRMS systems. This isotopic measurement capability shows great potential in situ observations with precision and frequency for the detection and interpretation of

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variable isotopic signals. Besides, it has enormous potential to be applied in both environmental sensing and the non-invasive breath analysis for medical diagnostic purposes.

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15

N and

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Fig. 7. Absorption of

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N-isotopes of N2O in air under 100mbar pressure. The green stars are the

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measurement data, the red line is the multi-line Voigt fit, and the blue line is the associated fit residual.

Conclusion

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We have developed and tested a high-resolution spectroscopic sensor coupled with a widely-tunable CW-QCL for molecular spectroscopy. We built, optimized and validated a liquid nitrogen-free preconcentration unit to increase N2O mixing ratios by a factor of 34 from ambient levels

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to about 10.83 ppm. The key element is the combination of a relatively mild adsorbent with a low

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temperature refrigeration unit, achieving trapping temperature of approximately -75oC. We have validated the system for direct quantitative estimation of 14N and 15N-isotopes of N2O concentrations

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in ambient air. Finally, the spectroscopic sensor can easily be adapted for monitoring several other isotopic species with mid-IR fundamental absorption bands such as CO, CO2, H2O, C2H2 and H2O2.

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Acknowledgements

The authors gratefully acknowledge the financial support from the National Program on Key Research and Development Project (2016YFC0302202), the National Natural Science Foundation of China (61440010, 61675005, 61705002), the Natural Science Foundation of Anhui Province (1508085MF118), the Key Science, Technology Development Program of Anhui Province (1501041136) and the Doctoral Start-up Foundation of Anhui University (J01003264).

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[29]L.S. Rothman, I.E.Gordon, Y.Babikov, A.Barbe, D.Chris Benner, P.F.Bernath, M.BirkfL.Bizzocchi,

PT E

V.Boudon, L.R.Brown, A.Campargue, K.Chance, E.A.Cohen, L.H.Coudert, V.M.Devi, B.J.Drouin, A.Fayt, J.-M.Flaud, R.R.Gamache, J.J.Harrison, J.-M.Hartmann, C.Hill, J.T.Hodges, D.Jacquemart, A.Jolly, J.Lamouroux, R.J.Le Roy, G.Li, D.A.Long, O.M.Lyulin, C.J.Mackie, S.T.Massie, S.Mikhailenko,

CE

H.S.P.Müller, O.V.Naumenko, A.V.Nikitin, J.Orphal, V.Perevalov, A.Perrin, E.R.Polovtseva, C.Richard, M.A.H.Smith, E.Starikova, K.Sung, S.Tashkun, J.Tennyson, G.C.Toon, Vl.G.Tyuterev, G.Wagner, The

AC

HITRAN 2012 Molecular Spectroscopic Database, J. Quant. Spectrosc. Ra. 130, (2013), 4-50.