Sensitive detection of atmospheric N2O isotopomers using a quantum cascade laser based spectrometer

Accepted Manuscript

Sensitive detection of atmospheric N2 O isotopomers using a quantum cascade laser based spectrometer Ningwu Liu , Linguang Xu , Sheng Zhou , Tianbo He , Lei Zhang , Dianming Wu , Jingsong Li PII: DOI: Article Number: Reference:

S0022-4073(19)30428-5 https://doi.org/10.1016/j.jqsrt.2019.106587 106587 JQSRT 106587

To appear in:

Journal of Quantitative Spectroscopy & Radiative Transfer

Received date: Revised date: Accepted date:

20 June 2019 24 July 2019 24 July 2019

Please cite this article as: Ningwu Liu , Linguang Xu , Sheng Zhou , Tianbo He , Lei Zhang , Dianming Wu , Jingsong Li , Sensitive detection of atmospheric N2 O isotopomers using a quantum cascade laser based spectrometer, Journal of Quantitative Spectroscopy & Radiative Transfer (2019), doi: https://doi.org/10.1016/j.jqsrt.2019.106587

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Highlights 

QCLs are promising spectroscopic sources for developing analytical instrumentation.



Direct absorption spectroscopy with multi-pass cell is a sensitive and calibration-free technique for gas sensing.



A versatile QCL based spectrometer combining with a liquid nitrogen-free preconcentration unit was developed for high sensitivity detection of atmospheric N2O isotopomers.

CR IP T

Measurement precision of 0.59‰ for sit-selective 15N/14N were achieved at the optimal averaging

CE

PT

ED

M

AN US

time of 258s.

AC



1

ACCEPTED MANUSCRIPT

Sensitive detection of atmospheric N2O isotopomers using a quantum cascade laser based spectrometer

CR IP T

Ningwu Liu1, Linguang Xu1, Sheng Zhou1, Tianbo He1, Lei Zhang1, Dianming Wu2, Jingsong Li1,*

1 Laser Spectroscopy and Sensing Laboratory, Anhui University, 230601 Hefei, China

2 Key Laboratory of Geographic Information Sciences, Ministry of Education, School of Geographic Sciences, East China Normal University, 200241 Shanghai, China

Corresponding author: [email protected]

AN US

Tel./Fax: +86 (0) 551-63861490

Abstract: Stable isotope analytical techniques have a crucial role to play in the study of atmospheric pollutant and trace gas sources, sinks and transport processes. Quantum cascade laser absorption spectroscopy (QCLAS) allows the site-selective and high-precision analysis of many isotopic species at trace levels, yet not sensitive enough for atmospheric N 2O mixing ratios. In this study, a quantum cascade laser based spectrometer combining with a liquid nitrogen-free preconcentration unit

M

was developed to demonstrate the applicability of high sensitivity and high precision measurement of atmospheric N 2O isotopomers. Primary laboratory tests have been performed for simultaneous determination of the mixing ratios of the most abundant nitrous oxide isotopic species: 14N15N16O, 15N14N16O and 14N216O. The experiment results showed that site-selective N/14N can be measured with the precision of 0.59‰ with an optimal averaging time of 258 s at 100 ppm. Details of spectral

ED

15

absorption line pairs selection and N2O adsorption/desorption processes are also discussed.

CE

1. Introduction

PT

Keywords: Laser spectroscopy, Quantum cascade laser, Istope analysis, N2O

Nitrous oxide (N2O) is an important trace gas, both a major greenhouse gas and main contributor to stratospheric ozone loss [1]. It plays a major role in the destruction of ozone in the stratosphere and influences the troposphere heat budget from

AC

an atmospheric chemistry point of view. Although the emission of N2O formed by nitrification and denitrification from soil and aquatic environment is well quantified, the strength of N2O sources still remains largely uncertain, due to the complexity of pathways involved [2]. Generally, important information about the biogeochemical cycle of N2O can be obtained by measuring the intramolecular distribution of 15N in atmospheric N2O. N2O is a linear, non-symmetric molecule (N-N-O) with one nitrogen atom at the center (α site) and one at the end (β side). The most abundant N2O isotopic species in the atmosphere are

14

N14N16O(446), 14N15N16O (456, referred to as

15

Nα) and

15

N14N16O (546,referred to as

dance ratios of approximate 0.9903, 0.0037 and 0.0037 [3], respectively.

2

Nβ) with natural abun-

15

ACCEPTED MANUSCRIPT Stable isotopic ratios are commonly expressed as ‘delta values’ in parts per thousand (or per mil, ‰) relative to the reference standard. For nitrogen isotope ratio in N2O, the δ-value is defined as follows [4]:

 Rsampl e  Rr ef   1000 Rr ef  

 

where Rsample and Rref refers to the ratio of 15N/14N in the N2O gas sample to be analyzed and reference standard gas, respectively. In absorption spectroscopy, the isotope ratio can be determined with the ratio of the integrated area A and the absorption intensity S of the major and minor isotopic components:

A 15 n  14S 14 A 14 n  15S 15

CR IP T

Rsample 

where n is the isotope abundance, n and S are available from the high-resolution transmission molecular absorption database (HITRAN), i.e. 3.64093‰ for 15

15

N and 0.990333 for

14

N. Therefore, δ15N denotes the relative difference in per mil (‰) of the

N/14N ratio of the sample versus the reference material, δ15Nα and δ15Nβ denote the site specific ratio of 14N15N16O vs. 14N14N16O

and 15N14N16O vs. 14N14N16O, respectively.

Traditional analytical technique for N2O isotopic measurements at ambient mixing ratios is laboratory-based isotope-ratio

AN US

mass-spectrometry (IRMS) with flask sampling and cryogenic preconcentration [5]. Although IRMS is a well-known method with excellent precision and accuracy, its inherent disadvantages such as costly and time-consuming, bulky equipment, hinder field deployment for in situ and real-time measurements. In contrast, laser absorption spectroscopy has the key advantages of being calibration free, and providing significant advantage of site selectivity combined with high sensitivity and time resolution [6-9]. Uehara et al. demonstrated a high-precision isotope ratio measurements of N2O for 15

N15N16O/15N14N16O/14N14N16O by using three wavelength-modulated 2μm diode lasers and a multi-pass cell (100 m optical

M

path length) with a precision of 0.2-0.6‰ at 5 Torr of pure natural abundance N2O gas [10]. Nakayama et al. measured the absorption spectra of the 3ν3 band of nitrous oxide isotopologues,

14

N15N16O and 15N14N16O, using diode laser cavity ring-

ED

down spectroscopy near 1.5 μm with a precision of 5.1‰ at 32 Torr of cylinder nitrous oxide gas without buffer gas [11]. Unfortunately, the published near infrared (NIR) spectroscopic measurements of N2O isotope ratios were mostly performed in pure N2O sample due to limited sensitivity and precision. Mid-infrared (MIR) laser spectroscopy is particularly effective

PT

for molecules that exhibit strong fundamental vibrations, with line intensities about two orders of magnitude higher than those in the NIR. For example, Wä chter and Sigrist used a difference-frequency system and wavelength modulation spectroscopy combined with balanced path length detection scheme to measure the isotope ratio of 14N15N16O and 15N14N16O and

CE

achieved a precision of 3‰ for a N2O concentration of 825 ppm [12]. Recent advances in laser technology provided a new class of MIR semiconductor laser sources, namely quantum cascade lasers (QCLs), which offers high spectral purity, rela-

AC

tively high power output, and room temperature operation. Recently, Wächter et al. demonstrated a precision of 0.5‰ for δ15N at N2O mixing ratios of 90 ppm with an averaging time of 300 s using a pulsed QCL emitting at 4.6 µm [13]. Li et al. reported site-selective nitrogen isotopic ratio measurement of nitrous oxide using a continuous-wave (CW) QCL with a similar emitting wavelength, and a precision of 3‰ with 90 s averaging time was obtained under a natural-abundance N2O sample of 12.7 ppm [14]. However, it is still not accessible for direct isotopic analysis of atmospheric N2O, since the N2O mole fraction in ambient air is generally in the order of 330 ppb. For many years, sample preconcentration using an adsorbent material has been shown to be a viable technique in mass-spectrometry (MS) and gas-chromatography (GC) for measuring atmospheric trace gas in ambient level, a small trap containing molecular sieve was used for selective adsorption of the analyte of interest with a concentration enhancement factor of several hundreds. Most recently, laser spectroscopy techniques combining with preconcentration technique have been successfully demonstrated for direct isotope ratio measurement of different

3

ACCEPTED MANUSCRIPT atmospheric trace species, such as CH4, C2H2, and C2H4, VOC [15-17]. For atmospheric N2O isotopomers, a pulsed QCL based spectrometer combined with a liquid nitrogen-free preconcentration unit was demonstrated with a temporal resolution of approximate 30 min by researchers at EMPA (Switzerland) [18-20]. However, pulsed QCL operation suffers from some drawbacks, such as high line widths due to thermal chirping, pulse-to-pulse intensity fluctuations, and the generation of nanosecond current pulses requires high-speed driving electronics and detectors as well as fast data acquisition. In this paper, a CW distributed feedback (DFB) QCL based spectrometer was developed and coupled to a home-made preconcentration unit for measuring atmospheric N2O isotopomers in ambient air. Using adsorbents under material-specific optimal conditions, in terms of cooling and heating temperature, gas flow rate, adsorption capacity, trap volume and adsorbent

CR IP T

materials content and spectral absorption line selection, were investigated in details. Finally, nitrous oxide isotopic detection in ambient air was performed and compared with commercial analytical instrument. 2. Experimental setup

The presented system as shown schematically in Fig.1. mainly consists of the following four parts: a QCL spectrometer for

Detector

Pressure controller

Flow counter

Valve 2

Valve 1

M

Standard gas Air Teflon Desiccant filter

Valve 5

Valve 3

Valve 4

Valve 6

Vacuum pump

AN US

concentration measurement, a preconcentration unit, a gas handling system, and a system control and data acquisition unit.

Data acquisition board

Parablic Mirror

Multi pass cell

ED

Preconcentrati on unit High-low temperature chamber

Notebook computer

QCL

T sensor

QCLAS System System Control and Data Acquisition Unit

Gas Handling System

CE

2.1 QCL spectrometer

PT

Fig.1. Diagram of the experimental setup for N2O isotopomer measurements.

A detailed description of this apparatus has been given elsewhere [21]. Here, we focus on recent advances and improvements that make the instrument suitable for N2O isotopomers detection. For high precision isotopic ratio determination with

AC

laser spectroscopy method, the absorption line pairs must be appropriately selected to meet some criteria. For example, the spectral regions were chosen to offer maximum sensitivity for the less abundant

14

N15N16O and

15

N14N16O isotopologues,

comparable line-strength for 14N14N16O to avoid saturation and are relatively free from spectral interference by other molecular species and itself. After extensively theoretical simulation by according to HITRAN database [22], a room temperature CW QCL (Alpes Lasers, Switzerland) with a central radiating wavelength at around 4566 nm was specially selected for simultaneous multiple species gas detection. The QCL wavelength can be tuned between 2185-2202 cm-1 over a temperature range of 243-303 K with a maximal power of 20 mW. This wavelength range permits access to several transitions of both major and minor isotopologues of nitrous oxide. Finally, the best line pairs for 15

14

[email protected] cm-1,

N14N16O@218 8.75601 cm-1 and 14N14N16O @2188.93846 cm-1 were selected, and compared with that reported by previous

4

ACCEPTED MANUSCRIPT work at EMPA (Switzerland) [18], as shown in Fig. 2. As can be seen, the selected major isotopologue of nitrous oxide (@2188.04484 cm-1) will be influenced by the interference of adjacent line

14

N14N16O at 2188.15597 cm-1, and this effect

will be more significant with increasing sample concentration and pressure. Although the susceptibility to spectral overlap could be reduced by decreasing the sample pressure in the absorption cell, this is done at the cost of the magnitude of molecule absorption signal, and hence lowering the sensitivity. 1.00

1.00

0.99

0.99 0.98

14N15N16O

14N14N16O

Trans.

Trans.

15N14N16O 0.97 0.96

0.97

0.95

0.94

0.94

0.93

0.93

2187.9

2188.0

2188.1

2188.2

0.92 2188.6

2188.7

-1

2188.9

2189.0

2189.1

Wavenumber (cm )

1.0 EMPA

This work

0.8

AN US

0.7

Trans.

2188.8

-1

Wavenumber (cm )

0.9

14N14N16O

0.96

0.95

0.92 2187.8

14N15N16O 15N14N16O

CR IP T

0.98

0.6 0.5 0.4

Simulated spectrum

0.3 0.2

Conditions: T=296K; L=6250cm; P=100mbar; C=10ppm

0.1 2187.8

2188.0

2188.2

2188.4

2188.6

2188.8

2189.0

2189.2

2189.4

-1

Wavenumber (cm )

M

Fig. 2. HITRAN simulation of nitrous oxide absorption spectra. Moreover, a compact multiple-pass Herriott cell (AMAC-76, Aerodyne Research Inc.) with a maximal path length of 76 m

ED

was employed for enhancing sensitivity. The newly developed QCL spectrometer can be operated in both detection schemes: calibration-free direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) with second-

2.2 Preconcentration unit

PT

harmonic detection. Here, DAS detection mode was used for isotopic ratio analysis.

The main analytical challenge in the present work is the quantification of 14N15N16O and 15N14N16O isotopologues consid-

CE

ering their very low natural abundance. The amount of adsorbent N2O had to be increased by at least 10-fold than its ambient mixing ratios. Therefore, empirical investigations with various trap models adsorbing atmospheric N2O were made under different conditions. We found that the N2O adsorption capacity shows significantly dependence on tube length and adsorp-

AC

tion content. In addition, a compromise with gas flow rate and time consume has to be considered. Finally, a preconcentration trap with a 20 cm long stainless steel tube and a 4 mm inner diameter was selected here, which is filled with adsorbent material (HayeSep D 100-120mesh, Hayes Separations Inc., USA) and sealed in a temperature-controlled chamber (ESSSDJ701, Ltd CHN) with a temperature tuning range between -70℃and 130℃. Non-silanised glass wool and wire mesh screen was used as a secure stopper at both tube ends. 2.3 Gas handling system The gas handling system comprises a compact oil-free diaphragm pump, two filters, several two-way and three-way valves, a mass flow controller and a pressure controller (MFC), etc. Sampled air passes firstly through a Teflon filter that protects the preconcentration unit and the Herriott cell from dust. The second strainer with drying agent is used to filter wa-

5

ACCEPTED MANUSCRIPT ter vapor after the first Teflon filter, and then air passes through the preconcentration unit and thereafter the gas flow is transferred into our QCL spectrometer for concentration measurement. The air flow rate is controlled by a flow mass flow controller (MCR-2000 slpm ALICAT), and sample pressure in the Herriott cell is maintained by a pressure controller (PC3Series ALICAT) at a constant pressure of around 100 mbar. All the experimental parameters are precisely controlled and monitored by a driving software. Details of parameter setting are discussed in next section. 2.4 Data acquisition unit A data acquisition I/O card (National Instrument, NI USB-6361) connected to a laptop via a USB link was used for

driving the QCL laser and data acquisition. Typically, a 100-Hz saw tooth tuning waveform was generated to sweep the

CR IP T

laser wavelength over absorption features of N2O isotopes. Signal processing was performed by a custom-written LabView-based multi-peak-fitting program for real-time fitting and quantification of the absorption spectra using molecular spectroscopy parameters taken from HITRAN database. Each spectrum was signal-averaged from a 100 individual scans (corresponding to a measurement time of ~1 s.) in order to increase the signal to noise ratio (SNR), and saved for further analysis. 3 Results and discussion

AN US

3.1 Performance of QCL Spectrometer

The precision and stability of the QCL spectrometer’s performance was firstly evaluated using gas cylinders with known N2O concentrations. In Fig. 3 as an example, we present the observed and simulated absorption profiles of N2O lines between 2188.6 and 2189.1 cm-1 at different concentrations, as well as the corresponding residuals (observed minus calculated spectrum). As can be seen, the experimental results are in quite good agreement with the simulation based on HITRAN database. In addition, Allan variance technique was used to analysis the time series data [23], as shown in Fig. 4. It indicates that a

M

short term precision of 0.86% (with 1 Hz sampling rate) for the 14N15N16O/14N14N16O ratio was obtained at a concentration of 100 ppm N2O standard gas, while the Allan variance reached its minimum at an integration time of ~258 s, corresponding to 15

N14N16O/14N14N16O ratio (data not shown). The isotopomer

ED

a precision of 0.59‰. Similar results were obtained for the

concentrations reported here are weighted by the isotopic abundance as given in the HITRAN database [22]. The accuracy and precision of the nitrous oxide isotopic ratio measurement by absorption spectroscopy is mainly limited by the uncertainty

PT

in the line intensity given in the common database, the SNR of the recorded absorption spectrum and difference of ground state energies of the used transition lines. Although the precision with a concentration of 10.8 ppm was 5.7‰ approximately

CE

10 times lower than that of 100 ppm. However, the precision can be further improved by combing digital filter algorithm, for example, the results after the application of adaptive Savitzky-Golay (S-G) filter algorithm [24] are also presented in Fig. 4, a precision enhancement by a factor of 3-5 times was obtained. Therefore, the amount of N2O output mixing ratio near 10 ppm

AC

level was desired for practical application, in order to decrease sample adsorption time, which in turn improve the duty cycle.

6

ACCEPTED MANUSCRIPT

Measure conditions: T=286K L=6250cm P=100mbar C=100ppm

Measure conditions: T=287K L=6250cm P=100mbar C=10.8ppm

1.0

Trans.

Trans.

1.00

0.99

0.98

Residual

0.001 0.000 -0.001 -0.002 2188.7

2188.8

2188.9 -1

2189.0

0.006 0.003 0.000 -0.003 -0.006

2189.1

2188.7

CR IP T

0.8

0.002

Residual

0.9

2188.8

2188.9 -1

2189.0

2189.1

Wavenumber (cm )

Wavenumber (cm )

Fig. 3. Experimentally measured absorption spectra under different concentrations and the Voigt fitted results.

Collected data time(s) 200 Exp.

14N15N/14N

1.2

400

600

S-G Filter

1.0 0.8 0.98 0.96 0.94

0.01

1000

1200

N2O = 10.8ppm (178s) = 5.7‰

M

1E-3

1E-4

N2O = 100ppm (258s) = 0.59‰

ED

2

Allan Variance (  )

800

AN US

0

1E-5

PT

1E-6

1

10

Integration time(s)

100

AC

CE

Fig. 4. Allan variance of the normalized isotope ratio 15Nα/14N at 10.8 ppm and 100 ppm of N2O samples.

7

ACCEPTED MANUSCRIPT

12

Exp. data Linear fit

8

6 Equation

y = Intercept + B1*x^1

No Weightin Weight Residual 0.03532 Sum of Squares 0.99872 Adj. R-Squar

4

CR IP T

Adsorbent N2O (ppm)

10

Value

2 200

400

600

800

Standard Err

B

Intercept

1.3373

B

B1

0.0074 1.54808E-4

1000

Mass of HayeSep D (mg)

1200

0.1321

1400

3.2 Optimization of adsorption and desorption process

AN US

Fig. 5. The relationship between adsorbent materials mass and adsorbent N2O at a cooling temperature of -70.4℃.

In order to achieve a tradeoff between direct isotopic analysis of atmospheric N2O with high precision and high duty cycle, the operation sequence and parameter settings (e.g. gas flow rates, trap temperatures and timing) of the liquid nitrogen-free preconcentration unit were optimized to obtain a threshold value of N2O concentration (i.e. > 10 ppm) during desorption. As mentioned above, the amount of adsorbent N2O shows significantly dependence on the mass of HayeSep D, as demonstrated in Fig. 5. From this figure, a good linear dependence of adsorbent N2O on adsorbent materials mass was found in the range

M

from 200 mg to 1400 mg. With increasing adsorbent materials mass, the gas flow rate decreases significantly. Therefore, the maxima of 1400 mg is selected to ensure a flow rate of 150 sccm (standard cubic centimetre per minute), which enhances

ED

N2O mole fractions by a factor of approximate 40 above it ambient level. In Fig. 6 as an example, we present a typical N2O adsorption and desorption process. First, the preconcentration unit is cooled down to a temperature of approximate -70.℃, then, lab air is continuously sampled through the preconcentration trap

PT

with a flow rate of 150 sccm, after the adsorption process is saturated (Phase I), the preconcentration unit is gradually heated up to room temperature of 20℃. After setting the trap temperature to 20℃, an immediate release of N2O can be observed by our QCL spectrometer (as can be seen in Fig. 6 (b)). The desorption process (phase II) is initialized by setting the sample gas

CE

flow to 15 sccm. During the whole process, N2O concentration was measured by the QCL spectrometer, the gas flow rate and pressure in the multi-pass cell was also monitored synchronously. Measurement procedure was repeated at 10.℃ intervals

AC

within the cooling temperature range between -30.℃ and -70.℃.

8

ACCEPTED MANUSCRIPT

100

80 40

N2O (ppm)

0

b

9 6

Flow rate (sccm)

120

95 12

0.5

b

0.4 0.3 0.2

0 20

0.0 600

900

2100 2400 2700

c

0

Phase I

-20

Phase II

Phase I

Phase II

-40 -60 -80 0

2000

CR IP T

0.1

3

Phase I

Phase II

AN US

Trap temperature (℃)

160

a

N2O (ppm)

Pressure (mbar)

105

4000

6000

8000

10000

12000

14000

Time (s)

Fig. 6. Typical N2O adsorption and desorption process with preconcentration trap and multi-pass cell conditions: (a) sample gas flow and gas pressure; (b) output N2O mixing ratio; (c) trap temperature

M

Table. 1. Summary of experimental conditions and the maximal N2O output mixing ratio. Pressure

Flow rate

Saturation

Maximum

(±0.4℃)

(±0.3mbar)

(±0.4sccm)

Time (min)

N2O (ppm)

-31.6 -40.8

20.1

Adsorption

Desorption

100.0

150.0

15.0

6.0

1.31

20.3

100.0

150.0

15.0

7.4

1.81

19.8

100.0

150.0

15.0

12.9

3.79

CE

-51.5

Desorption

PT

Adsorption

ED

Temperature

20.0

100.0

150.0

15.0

19.6

5.78

-71.4

19.9

100.0

150.0

15.0

45.3

11.75

AC

-61.2

For clarity, the experimental conditions, the maximal N2O output mixing ratio and the time needed to reach saturation adsorption are summarized in Table. 1. To extend our comparison, Fig. 7. are plotted the saturation adsorbent time against preconcentration trap temperature. As shown in the inset of Fig. 6(b), here the saturation adsorption time is defined as the time requirement for N2O output mixing ratio recovers to its input value (for ambient air ~350 ppb). Unfortunately, N2O in ambient air can not be completely adsorbed by the preconcentration trap, mainly due to the limitation of trap temperature. Experimentally, we found that the adsorbent capacity for N2O shows significantly dependence on the trap temperature. The lower the trap temperature, the higher the adsorbent capacity.

9

50

Equation

y = A1*exp(-x/t1) + y0

Adj. R-Square

40

0.99234 Value

Standard Error

B

y0

5.66149

1.41186

B

A1

0.04207

0.03826

B

t1

10.22028

1.32826

30

Exp.data Exponential fitting

20 10 0 12

Equation

y = A1*exp(-x/t1) + y0

Adj. R-Square

0.99063 Value

9

Standard Error

C

y0

0.71854

0.59189

C

A1

0.07265

0.05997

C

t1

13.95118

2.19574

6

Exp.data Exponential fitting

3

0 -30

-40

-50

CR IP T

N2O Concentration(ppm)

Saturation adsorbent time (min)

ACCEPTED MANUSCRIPT

-60

-70

AN US

Preconcentration trap temperature (℃)

Fig. 7. The relationship between saturation adsorbent time (a) and N2O concentration (b) with preconcentration trap temperature.

3.3 Ambient N2O isotopomers detection

Before field deployment, the developed QCL spectrometer was compared with a commercial N2O analytical instrument

M

based-upon off-axis integrated cavity output spectroscopy (LGR, model 914-U027) for measuring a gas cylinder with a certified N2O concentration (9.9 ppm in N2). The averaging results determined by our QCL spectrometer and LGR analyzer are

ED

10.83 ppm and 11.2 ppm, respectively. The slight difference of 3.4% is completely acceptable, considering the influence of spectroscopic parameters for the selected transition lines and potential error arising from calibration standards. Finally, the QCL spectrometer integrated with the preconcentration unit was deployed for ambient N2O isotopomers measurement at

PT

Anhui University campus (Hefei city, China).

Fig. 8 illustrates the measured N2O isotopomers absorption spectra at a sample pressure of 100 mbar and a flow rate of 15 sccm. The data analysis was performed by using a nonlinear least-squares fitting procedure, which integrates the Levenberg–

CE

Marquardt algorithm and the Voigt profile model. The integrated area, in conjunction with the measured gas temperature, pressure, and optical path length, provides a direct measure of the N2O mixing ratio of 12.7 ppm. As can be seen, the fitted residual (observed minus calculated) is less than ±0.003. Finally, the calculated values of 14N15N16O/14N14N16O (R15Nα/14N)

AC

and 15N14N16O/14N14N16O (R15Nβ/14N) ratios are 4.0582‰ and 3.7096‰, respectively.

10

ACCEPTED MANUSCRIPT 0.04

R15N /14N = 4.05818‰ 

14

Absorbance

0.03

15

15

16

N N O

14

R15N /14N = 3.70963‰

16

N N O



0.02

14

14

16

N N O

0.01

0.00

2188.7

2188.6

2188.7

2188.8

2188.9

2188.8

2188.9

Residual

0.000 -0.003 -1

Wavenumber (cm )

2189.0

2189.1

2189.0

2189.1

CR IP T

2188.6 0.003

AN US

Fig. 8. Experimentally recorded ambient N2O isotopomers absorption spectrum and the best-fitted curve with the Voigt profile model.

The long-term reliability was finally evaluated with automated operation for tens of hours, the results are demonstrated in Fig. 9. Based on these measurements, averaged values of (4.06247±0.0441)‰ for (3.72369±0.0375)‰ for

15

14

N15N16O/14N14N16O and

N14N16O/14N14N16O are obtained. Similarly to the use of atmospheric N2 for IRMS, isotopomer

specific ratios here are reported relative to Air-N (i.e. (3676.5±8.1)×10-6) which is used as reference to calculate the δ-value

M

from the isotope ratios [25]. The corresponding δ-values have been calculated to be +115.79‰ for δ15Nα and +22.73‰ for

PT

4.1

ED

δ15Nβ, respectively.

RAir

AC

15

 14

N/ N

3.9

CE

R(15N/14N, ‰)

4.0

RAir

15

 14

N/ N

3.8

3.7

3.6 0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19

Number of period

Fig. 9. Measured ambient air 15Nα/14N and 15Nβ/14N isotope ratios. 4. Conclusion

11

ACCEPTED MANUSCRIPT In this paper, the possibility of using a CW-RT-QCL absorption spectrometer combing with preconcentration technique for high precision determination of the site specific isotope abundance ratio of N2O isotopomers was demonstrated. The best line pairs for 14N15N16O@218 8.68757 cm-1, [email protected] cm-1 and14N14N16O@ 2188.93846 cm-1 was selected for absorption spectroscopy measurements. A liquid nitrogen-free preconcentration unit was developed for high volume preconcentration of N2O from ambient air, and experimentally optimized in terms of cooling temperature, gas flow rate, adsorption capacity, trap volume and adsorbent materials content in details. For the selected adsorbent material of HayeSep D, N2O mole fraction enhancement by a factor of approximate 40 above its ambient level was achieved at desorption temperature of ~ 203 K.

CR IP T

The reliability of our QCL spectrometer was confirmed by comparing with the commercial analytical instrument for nitrous oxide measurements. For 15Nα/14N and 15Nβ/14N ratio detection, the QCL spectrometer can achieve a precision of 0.59‰ with 258 s averaging time for 100 ppm. Finally, long-term reliability was evaluated for measuring the natural abundance of N2O isotopomers in ambient air. The average value of the 14N15N16O/14N14N16O and 15N14N16O/14N14N16O ratios are found to be 4.06247‰ and 3.72369‰, respectively. The corresponding δ-value have been calculated to be +115.79‰ for δ15Nα and +22.73‰ for δ15Nβ, respectively, by referring to Air-N standard.

AN US

Future work will focus on improving the design of preconcentration trap to reach a lower adsorption temperature or update the QCL spectrometer with longer optical path length. Another solution could be to substitute the HayeSep D adsorbent material by a candidate exhibiting a superior selectivity for N2O, i.e. having a larger capacity for N2O, so that the adsorption temperature can be increased. Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China

M

(41875158, 61675005, 61705002), the National Program on Key Research and Development Project (2016YFC0302202) and Anhui Province Natural Science Foundation of China (1808085QF198). Special thanks go to Dr. Lukas Emmenegger

ED

and Dr. Joachim Mohn (EMPA, Laboratory for Air Pollution & Environmental Technology, Switzerland) for their helpful discussion of building the preconcentration unit.

PT

Reference

[1] Ravishankara, A.R.; Daniel, J.S.; Poetmann, R.W.; "Nitrous Oxide (N2O): The Doninant Ozone-Depleting Substance

CE

Emitted in The 21 st Century", Science 326 (2009) 123-125. [2] Stein, L.Y.; Yung, Y.L.; "Production, isotopic composition, and atmospheric fate of biologically produced nitrous oxide", Annu. Rev. Earth. PL. SC. 31 (2003) 329-356.

AC

[3] Werner, R.A.; Brand, W.A.; Coplen, T.B.; "Referencing strategies and techniques in stable isotope ratio analysis", Rapid Commu. Mass SP. 412(2001) 501-519. [4] Weidmann, D.; Wysocki, G.; Oppenheimer, C.; Tittel, F.K.; "Development of a compact quantum cascade laser spectrometer for field measurements of CO2 isotopes", Appl. Phys. B 80(2005) 255-260. [5] Brenninkmeijer, C.A.M.; Röckmann, T.; "Mass spectrometry of the intramolecular nitrogen isotope distribution of environmental nitrous oxide using fragmention analysis", Rapid Commun. Mass SP. 13 (1999) 2028-2033. [6] Bergamaschi, P.; Schupp, M.; Harris, G.W.; "High-precision direct measurements of 13CH4/12CH4 and 12CH3D/12CH4 ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer", Appl. Opt. 33 (1994) 7704–7716.

12

ACCEPTED MANUSCRIPT [7] Uehara, K.; Yamamoto, K.; Kikugawa, T.; Yoshida, N.; "Isotope analysis of environmental substances by a new laserspectroscopic method utilizing different pathlengths", Sensor. Actuat. B 74 (2001) 173-178. [8] Gagliardi, G.; Castrillo, A.; Iannone, R.Q.; Kerste, E.R.T.; Gianfrani, L.; "High-precision determination of the 13

CO2/12CO2 isotope ratio using a portable 2.008- μm diode-laser spectrometer", Appl. Phys. B 77(2003) 119-124

[9] Tsuji, K.; Fujikawa, S.; Yamada, K.; Yoshida, N.; Yamamoto, K.; Kikugawa, T.; "Precise measurement of the 13CH4/12CH4 ratio of diluted methane using a near infrared laser absorption spectrometer", Sensor. Actuat. B 114 (2006) 326-333. [10] Uehara, K.; Yamamoto, K.; Kikugawa, T.; Yoshida, N.; "Site-selective nitrogen isotopic ratio measurement of nitrous oxide using 2μm diode lasers", Spectrochim. Acta A 59 (2003) 957-962.

CR IP T

[11] Nakayama, T.; Fukuda, H.; Kamikawa, T.; Sugita, A.; Kawasaki, M.; Morino, I.; Inoue, G.; "Measurement of the 3v3 band of 14N15N16O and 15N14N16O using continuous-wave cavity ring-down spectroscopy", Appl. Phys. B 88 (2007) 137-140. [12] Wächter, H.; Sigrist, M.W.; "Mid-infrared laser spectroscopic determination of isotope ratios of N2O at trace levels using wavelength modulation and balanced path length detection", Appl. Phys. B 87 (2007) 539-546.

[13] Wächter, H.; Mohn, J.; Tuzson, B.; Emmenegger, L.; Sigrist, M.W.; "Determination of N2O isotopomers with quantum cascade laser based absorption spectroscopy", Opt. Express 16(2008) 9239–9244.

AN US

[14] Li, J.S.; Zhang, L.Z.; Yu, B.L.; "Site-seletive nitrogen isotopic ratio measurement of nitrous oxide using a TE-cooled CW-RT-QCL based spectrometer", SpectrochimicaActa. A Mol. Biomol. Spectrosc. 133(2014) 489-494. [15] Eyer, S.; Stadie, N.P.; Borgschulte, A.; Emmenegger, L.; Mohn, J.; "Methane preconcentration by adsorption: a methodology for materials and condition selection", Emerg. Microbes &Infec.20(2014) 657-666. [16] Pradhan, M.; Lindley, R.E.; Grilli, R.; White, I.R.; Martin, D.; Orr-Ewing, A.J.; "Trace detection of C2H2 in ambient air using continuous wave cavity ring-down spectroscopy combined with sample preconcentration", Appl. Phys. B 90(2008) 1-9.

M

[17] Aziz, M.S.I.; Orr-Ewing, A.J.; "Development and applica-tion of an optical sensor for ethene in ambient air using near infra-red cavity ring down spectroscopy and sample preconcentration", J. Environ. Monitor.14(2012) 3094-3100.

ED

[18] Mohn, J.; Guggenheim, C.; Tuzson, B.; Vollmer, M.K.; Toyoda, S.; Yoshida, N.; Emmenegger, L.; "A liquid nitrogenfree preconcentration unit foe measurement of ambient N2O isotopomers by QCLAS", Atmos. Meas. Tech. 3 (2010) 609– 618.

PT

[19] Mohn, J.; Tuzson, B.; Manninen, A.; Yoshida, N.; Toyoda, S.; Brand, W.A.; Emmenegger, L.; "Site selective real-time measurements of atmospheric N2O isotopomers by laser spectroscopy", Atmos. Meas. Tech. 5 (2012) 1601–1609. [20] Wolf, B.; Merbold, L.; Decock, C.; Harris, B.E.; Six, J.; Emmenegger, L.; Mohn, J,; "First on-line isotopic characteriza-

CE

tion of N2O above intensively managed grassland", Biogeosciences 12(2015) 2517-2531. [21] Li, J.S.; Deng, H.; Sun, J.; Yu, B.L.; Fischer, H. "Simultaneous atmospheric CO, N2O and H2O detection using a single

AC

quantum cascade laser sensor based on dual-spectroscopy techniques", Sensor. Actuat. B 231(2016) 723-732. [22] Rothman, L.S.; Gordon, I.E.; Barbe, A.; Benner, D.C.; Bernath, P.F.; Birk, M.; Bizzocchi, L.; Boudon, V.; Brown, L.R.; Campargue, A.; Chance, K.; Coudert, L.H.; Devi, V.M.; Drouin, B.J.; Fayt, A.; Flaud, J.-M.; Gamache, R.R.; Harrison, J.; Hartmann, J.-M.; Hill, C.; Hodges, J.T.; Jacquemart, D.; Jolly, A.; Lamouroux, J.; LeRoy, R.J., Li, G.; Long, D.; Mackie, C.J.; Massie, S.T.; Mikhailenko, S.; Müller, H.S.P.; Naumenko, O.V.; Nikitin, A.V.; Orphal, J.; Perevalov, V.I.; Perrin, A.; Polovtseva, E.R.; Richard, C.; Smith, M.A.H.; Starikova, E.; Sung, K.; Tashkun, S.A.; Tennyson, J.; Toon, G.C.; Tyuterev, V.G.; Wagner, G.; "The HITRAN2012 molecular spectroscopic database", J. Quant. Spectrosc. Radiat. Transfer 130 (2013) 4–50. [23] Werle, P.; Mucke, R.; and Slemr, F.; "The limits of signal averaging in atmospheric trace-gas monitoring by tunable diode-laser absorption spectroscopy (TDLAS)", Appl. Phys. B 57(1993) 131-139.

13

ACCEPTED MANUSCRIPT [24] Li, J.S.; Deng, H.; Li, P.F.; Yu, B.L.; "Real-time infrared gas detection based on a adaptive Savitzky-Golay algorithm", Appl. Phys.B 120(2015) 207-216. [25] Toyoda, S.; and Yoshida, N.; "Determination of Nitrogen Isotopomers of Nitrous Oxide on a Modified Isotope Ratio

AC

CE

PT

ED

M

AN US

CR IP T

Mass Spectrometer", Anal. Chem. 71(1999) 4711-4718.

14