Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)←(00001) near-infrared carbon dioxide band

Journal of Quantitative Spectroscopy & Radiative Transfer ] (]]]]) ]]]–]]]

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Contents lists available at ScienceDirect

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Journal of Quantitative Spectroscopy & Radiative Transfer

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journal homepage: www.elsevier.com/locate/jqsrt

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Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band

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D.A. Long a,n, S. Wójtewicz a,c, C.E. Miller b, J.T. Hodges a

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a

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b c

Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MA 20899, USA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

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a r t i c l e i n f o

abstract

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Article history: Received 27 February 2015 Received in revised form 25 March 2015 Accepted 26 March 2015

We present new high accuracy measurements of the (30012)’(00001) CO2 band near 1575 nm recorded with a frequency-agile, rapid scanning cavity ring-down spectrometer. The resulting spectra were fit with the partially correlated, quadratic-speed-dependent Nelkin-Ghatak profile with line mixing. Significant differences were observed between the fitted line shape parameters and those found in existing databases, which are based upon more simplistic line profiles. Absolute transition frequencies, which were referenced to an optical frequency comb, are given, as well as the other line shape parameters needed to model this line profile. These high accuracy measurements should allow for improved atmospheric retrievals of greenhouse gas concentrations by current and future remote sensing missions. & 2015 Published by Elsevier Ltd.

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Keywords: Carbon dioxide Cavity ring-down spectroscopy Line shape effects Line mixing

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1. Introduction

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With the recent launch of the Orbiting Carbon Observatory (OCO-2) [1] and subsequent greenhouse gas remote sensing missions [e.g., NASA's Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) [2] and OCO-3] there is a strong need for high accuracy spectroscopic parameters for near-infrared CO2 spectroscopic bands [3]. Due to the high target accuracy (o1 mmol/mol uncertainty of CO2) of these missions, spectroscopic parameters are needed with relative uncertainties below 0.3% [3]. In the light of these measurement needs, we utilized a recently developed spectroscopic technique, frequencyagile, rapid scanning cavity ring-down spectroscopy (FARSCRDS) [4,5], to produce high accuracy line shape parameters for the (30012)’(00001) CO2 band near 1575 nm. These

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n

Corresponding author. Tel.: þ1 301 975 3298; fax: þ1 301 869 4020. E-mail address: [email protected] (D.A. Long).

room temperature measurements in concert with future temperature-dependent measurements should allow for significantly reduced uncertainties in atmospheric retrievals of CO2 dry air mixing ratios. In order to reach these accuracy targets it is necessary to employ advanced spectroscopic line profiles that include effects such as speed dependence, collisional narrowing, and line mixing. As a result, here we use the partially correlated, quadratic-speed-dependent Nelkin-Ghatak line profile (pCqSDNGP) [6] with line mixing. This profile is equivalent to the Hartmann-Tran profile which has been recently recommended by the IUPAC Task Group [7] and which will be soon incorporated into the HITRAN database [8]. 2. Spectrum analysis The pCqSDNGP, which is a generalization of the wellknown Nelkin-Ghatak hard-collision profile (NGP), accounts for three mechanisms not included in the widely used Voigt

http://dx.doi.org/10.1016/j.jqsrt.2015.03.031 0022-4073/& 2015 Published by Elsevier Ltd.

61 Please cite this article as: Long DA, et al. Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band. J Quant Spectrosc Radiat Transfer (2015), http: //dx.doi.org/10.1016/j.jqsrt.2015.03.031i

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profile, including: line narrowing caused by velocitychanging collisions, speed dependence of line broadening and shifting, and partial correlation between velocity- and phase-changing collisions. For a detailed discussion on the mathematical formalism of the pCqSDNGP see Ref. [9]. In the first-order approximation to line mixing, the local absorption coefficient, α, at optical frequency ν can be calculated as nn o nn o αðνÞ ¼ nSðRe eI pCqSDNGP ðν  ν0 Þ þ Y Im eI pCqSDNGP ðν ν0 Þ Þ;

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The cavity's comb of transmission modes was locked to an I2-stabilized HeNe laser having a long-term stability of 10 kHz [13]. A total of 15 spectra were recorded which spanned the wave number range 6297–6368 cm  1. Each spectrum was recorded in approximately 1 h. A series of example spectra

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ð1Þ n

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where eI pCqSDNGP is the complex representation of the pCqSDNGP profile, n ¼ xa p=ðkB TÞ is the absorber number density, S is the line intensity of the nearest transition at frequency ν0, Y is the dimensionless line mixing term, xa is the absorber molar fraction, p is the total pressure, T is the temperature, and kB is the Boltzmann constant [10]. For the analysis of isolated transitions at low pressure, we set Y¼ 0. In addition to the center frequency, area and Doppler width of the absorption transition, other parameters which can be determined by fits to the measured spectra include the collisional relaxation rate, Γ0, the pressure shift, Δ0, the complex optical frequency of velocity changing collisions, νopt, the broadening speed dependence parameter, aw, the shifting speed dependence parameter, as, and the empirical correlation parameter, ηΓ. The pressure broadening and shifting coefficients are then given by γ ¼ Γ 0 =p and δ ¼ Δ0 =p, and the pressure-normalized line mixing coefficient is given by y¼Y/p.

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3. Experimental approach

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The spectra presented herein were recorded with a frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) spectrometer located at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. This instrument has been described previously [5] and therefore will be only briefly discussed. FARS-CRDS is a variation of traditional cw-CRDS in which an electro-optic modulator (EOM) is used to rapidly step the laser frequency between successive modes of an optical cavity [4]. This is done by offsetting the carrier frequency from the cavity modes such that only a single, selected EOM-shifted laser sideband is resonant with the cavity. Spectra can then be recorded simply by stepping the EOM drive frequency in increments of the cavity's free spectral range. We have demonstrated scanning rates which are limited only by the cavity's response time and a noise-equivalent absorption coefficient (NEA) as low as 6  10  14 cm  1 Hz  1/2 [5,11]. The present instrument used a variation of Pound– Drever–Hall locking [12] in which the laser was continuously locked to the optical cavity via a dual-polarization approach [5]. This approach yielded a relative linewidth between the laser and optical cavity as low as 130 Hz [5]. A fiber-coupled external-cavity diode laser was used with a tuning range of 1570–1630 nm and an output power of 25 mW. The optical cavity had a free-spectral range of nominally 200 MHz and a finesse of about 20,000.

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75 77 79 81 83 85 87 Fig. 1. Example spectra from the FARS-CRDS instrument of the (30012)’(00001) CO2 band. The frequency of each spectral point is directly linked to an octave-spanning, self-referenced optical frequency comb (which is itself linked to a cesium atomic clock).

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99 101 103 105 107 109 111 113 115 Fig. 2. Example air-broadened spectrum for the (30012)’(00001) R8e CO2 transition at a pressure of 129.3 kPa. Also shown are the corresponding residuals when using the pCqSDNGP line profile both with and without line mixing. We note that the high frequency structure in these residual plots is due to under-sampled etalons which only slightly affect the resulting spectroscopic parameters (i.e. o 0.1% for γ). The true technical noise level is far lower. For clarity smoothed residuals are shown by the black curves.

Please cite this article as: Long DA, et al. Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band. J Quant Spectrosc Radiat Transfer (2015), http: //dx.doi.org/10.1016/j.jqsrt.2015.03.031i

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425.4(11) mmol/mol CO2, 1.0150(84) %Ar, and 20.6611(74) % O2 with the remainder being N2. The set of 12 spectra had nominal pressures (kPa) of 7.5, 11.1, 13.4, 14.5, 20.3, 24.7, 31.6, 35.8, 88.2, 108.4, 128.8, and 129.3. The pressure measurements were made with a NIST-calibrated capacitance diaphragm gauge with a full-scale response of 133.3 kPa and a relative standard uncertainty less than 0.1%. Temperature was recorded with a NIST-calibrated 2.4 kΩ thermistor. Furthermore, the room temperature was actively stabilized such that it remained within the range 296.3–296.5 K during the course of these measurements. These air-broadened spectra were fit with the pCqSDNGP [15,16] (see Fig. 2 for an example spectrum and corresponding residuals). The Doppler width was constrained to its

can be found in Fig. 1. The frequency axes of these spectra were directly linked to an octave-spanning, self-referenced optical frequency comb (which was itself referenced to a cesium atomic clock) leading to an absolute frequency uncertainty at each spectral point of only 14 kHz [14]. Three of these spectra were recorded with a pure CO2 sample at low pressure (6.6 Pa) to measure transition frequencies. Note that this sample was isotopically enriched in 12C (99.99% 12C) in order to reduce the effects of isotopic interferences. These spectra were fit using the Voigt profile, as the various narrowing effects are negligible at these extremely low pressures. An additional 12 spectra were recorded using a CO2-inair mixture at several pressures. This sample contained

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Table 1 Line shape parameters determined by multispectrum fitting of the frequency-stabilized cavity ring-down spectra reported in Ref. [17] using the partially correlated, quadratic-speed-dependent, Nelkin-Ghatak line profile for the (30012)’(00001) R16e 12C16O2 transition. The shown standard uncertainties are based upon the reported fit uncertainty as well as the uncertainty in measured sample density. The correlation parameter, ηΓ, was calculated as (νdiff/p–Re {νopt}/p)/γ. We note that νdiff was taken to be 0.00550 MHz Pa  1 [18]. Note that 1 cm  1 atm  1 ¼0.295872152 MHz Pa  1. Line

γ (MHz Pa  1)

Re{νopt}/p (MHz Pa  1)

aw

as

ηΓ

R16e

0.02204(2)

0.000917(9)

0.0884(1)

0.055(1)

0.208(2)

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Table 2 Measured line shape parameters and their corresponding standard fit uncertainties for the (30012)’(00001) 12C16O2 band. The shown pressure broadening parameters (γ) were temperature-corrected from approximately 296.4 K to 296 K using the temperature dependencies found in HITRAN 2012 [8]. Note that 1 cm  1 atm  1 ¼ 0.295872152 MHz Pa  1 and 1 atm  1 ¼ 9.86923 MPa  1.

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1

Line

m

ν0 (cm

P42e P40e P38e P36e P34e P32e P30e P28e P26e P24e P22e P20e P18e P16e P14e P12e P10e P8e P6e P4e P2e R0e R2e R4e R6e R8e R10e R12e R14e R16e R18e R20e R22e R24e R26e R28e

 42  40  38  36  34  32  30  28  26  24  22  20  18  16  14  12  10 8 6 4 2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

6308.737532 6310.883379 6313.002410 6315.094329 6317.158847 6319.195653 6321.204490 6323.185124 6325.137322 6327.060860 6328.955550 6330.821204 6332.657652 6334.464734 6336.242387 6337.990334 6339.708575 6341.396996 6343.055484 6344.684016 6346.282483 6348.623768 6350.147030 6351.640126 6353.103108 6354.535972 6355.938775 6357.311574 6358.654325 6359.967248 6361.250356 6362.503760 6363.727575 6364.921944 6366.087013 6367.222912

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67 69 71 73 75

79 81 83 85 87

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77

15 17

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)

u(ν0) (10 27 19 8 4 7 6 3 3 4 4 3 2 2 2 2 1 2 2 3 3 6 10 9 2 2 1 3 2 3 1 2 2 2 3 2 8

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cm

1

)

γ (MHz Pa 0.02027 0.02033 0.02033 0.02047 0.02053 0.02047 0.02065 0.02089 0.02092 0.02113 0.02130 0.02163 0.02198 0.02243 0.02275 0.02334 0.02376 0.02420 0.02500 0.02547 0.02539 0.02453 0.02595 0.02533 0.02423 0.02382 0.02326 0.02275 0.02231 0.02192 0.02172 0.02142 0.02115 0.02080 0.02077 0.02053

1

)

1

ur(γ) (%)

δ (MHz Pa

2.5 2.5 2.2 0.6 0.7 0.4 0.5 0.5 0.5 0.3 0.4 0.2 0.4 0.2 0.5 0.4 0.2 0.3 0.6 1.8 4.4 5.9 1.5 0.4 0.8 0.4 0.2 0.2 0.3 0.3 0.5 0.2 0.5 0.6 0.3 0.8

 0.00290  0.00189  0.00183  0.00195  0.00207  0.00195  0.00201  0.00192  0.00186  0.00189  0.00210  0.00195  0.00186  0.00183  0.00180  0.00178  0.00172  0.00160  0.00145  0.00142  0.00092  0.00050  0.00151  0.00121  0.00133  0.00151  0.00139  0.00151  0.00148  0.00160  0.00175  0.00175  0.00180  0.00183  0.00183  0.00183

)

1

ur(δ) (%)

y (MPa

9 18 32 7 9 7 2 6 6 4 5 4 1 1 2 3 5 4 3 8 14 68 21 3 2 7 3 5 5 1 3 4 1 2 5 3

– – – – – 0.064 0.060 0.051 0.043 0.029 0.024 0.036 0.018 0.010  0.002 0.000  0.022  0.053  0.064  0.116  0.183 0.264 0.173 0.133 0.117 0.091 0.069 0.042 0.025 0.000  0.010  0.019  0.028  0.073  0.078 –

)

89 91

ur(y) (%)

93 – – – – – 14 14 9 12 7 8 9 7 5 13 9 12 17 19 24 30 31 25 21 21 18 15 11 5 36 8 12 16 19 20 –

Please cite this article as: Long DA, et al. Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band. J Quant Spectrosc Radiat Transfer (2015), http: //dx.doi.org/10.1016/j.jqsrt.2015.03.031i

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expected value, which was calculated based upon the measured temperature, molecular mass and transition frequency. For all transitions, the collisional narrowing parameter and the two speed-dependent parameters were assumed to be independent of rotational quantum number and were constrained to the values determined by fits of the pCqSDNGP to earlier FS-CRDS measurements of the (30012)’(00001) R16e 12 16 C O2 line (see Table 1) [17]. These previous results indicated that collisional narrowing and speed dependent effects must be considered to capture the pressure dependence of the isolated line shape at the signal-to-noise ratios considered in the present study. Further, they revealed that Re{νopt} was nearly five times smaller than the effective frequency of velocitychanging collisions (which was calculated based upon the known mass diffusion parameter [18]), thus indicating the occurrence of correlation between velocity- and phasechanging collisions. These measurements were unable to quantify Im{νopt}/p and hence this term was set to zero in both the earlier and present studies. The lowest eight pressure spectra were fit in a multispectrum analysis (for each transition) and were utilized to determine γ and δ in the absence of line mixing. Subsequently, we fit the first-order line mixing model (Eq. (1)) to the highest four pressure spectra in order to extract the line mixing coefficients. We chose this approach because of the high computational cost of determining the line mixing parameters when simultaneously fitting spectra comprising multiple transitions.

63 65 67 69 71 73 75 77 Fig. 4. Comparison of our measured pressure broadening parameters for the (30012)’(00001) 12C16O2 band to HITRAN 2012 [8], Predoi-Cross et al. [27], and Devi et al. [26]. The shown standard uncertainties are based upon the reported fit uncertainty. We attribute the large, systematic difference between the data sets to the absence of collisional (Dicke) narrowing in the spectral fitting used to produce these parameters. The three highly uncertainty transitions at band center (where the fitting was impaired by interfering transitions) are not shown.

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4. Results and discussion

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The line shape parameters which resulted from the FARS-CRDS measurements can be found in Table 2. Figs. 3– 5 provide comparisons of these parameters to those found in the HITRAN 2012 database [8].

97 99 101 Fig. 5. Comparison of our measured pressure shifting parameters for the (30012)’(00001) 12C16O2 band to HITRAN 2012 [8], Predoi-Cross et al. [27], and Devi et al. [26]. The shown standard uncertainties are based upon the reported fit uncertainty. We attribute the large, systematic difference between the data sets to the absence of collisional (Dicke) narrowing in the spectral fitting used to produce these parameters. The three highly uncertainty transitions at band center (where the fitting was impaired by interfering transitions) are not shown.

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103 105 107 109

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59

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85

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Fig. 3. Comparison of our measured transition frequencies for the (30012)’(00001) 12C16O2 band to HITRAN 2012 [8] (Present – HITRAN 2012). The shown standard uncertainties are based upon the reported fit uncertainty.

As can be seen in Fig. 3, we observed significant differences between our absolute transition frequencies and those found in HITRAN [8]. The HITRAN values are based upon the Fourier-transform infrared spectroscopy (FT-IR) measurements of Miller and Brown [19] and were referenced to earlier measurements of C2H2 and CO transitions in the nearinfrared region [20–23]. We note that our present measurements are directly linked to an absolute frequency reference (a cesium clock) via an optical frequency comb. An offset of about 2 MHz was observed between the two measurements at low-J likely due to the frequency calibration of the FT-IR measurements. This offset is in rough agreement with the quoted uncertainty of 1.5 MHz for the FT-IR measurements.

Please cite this article as: Long DA, et al. Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band. J Quant Spectrosc Radiat Transfer (2015), http: //dx.doi.org/10.1016/j.jqsrt.2015.03.031i

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Table 3 Fitted 12C16O2 spectroscopic constants (in MHz) with the corresponding standard uncertainties.

3 5

5

gs 30012

Gν

Bν

Dν (10  3)

Hν (10  9)

– 190,303,782.258(59)

11,698.469997536 11,585.62864(39)

 3.998627717  2.94262(69)

0.411169933 15.81(32)

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The deviation between the two measurements becomes significantly larger at high-J, reaching as high as approximately 7 MHz at m¼  42. These deviations are in agreement with those we observed earlier for the (30013)’(00001) CO2 band in comparison to these FT-IR measurements [24]. The present transition frequency measurements have been added to an existing global fit that included a large number of CO2 transitions spanning a wide range of energy levels [24]. Essentially, the present transition frequencies were added to this existing global fit to produce the new set of upper state molecular parameters given in Table 3. This global fit exhibited a root-mean-square deviation of only 250 kHz, which is commensurate with our fit uncertainties calculated during the spectral fitting. As can be seen in Fig. 3, the four transitions with J□ ¼13 or 15 exhibit evidence of a slight perturbation likely due to a Fermi resonance [25]. As this perturbation is too weak to enable incorporation into the global fit, these four transitions were excluded from the fit. As a result, for these transitions the frequencies found in Table 2 should be used rather than calculated from the molecular parameters found in Table 3. Significant differences (several percent) were also observed for the pressure broadening parameters in comparison to earlier measurements [26,27] (see Fig. 4). We attribute these differences to the use of the speeddependent Voigt profile (SDVP) in the multispectrum fitting which led to these values. Because the SDVP does not include collisional (Dicke) narrowing, use of this profile can lead to inaccurate, large values for aw. This excess speed dependent narrowing in turn can lead to systematically overestimated pressure broadening parameters. We note that the values for aw reported by Devi et al. [26] were substantially larger (at low-J) than we observed. As a further test of this hypothesis, we repeated our fits using the aw values reported by Devi et al. In this analysis, we retrieved pressure broadening parameters which were larger by several percent than those obtained from fitting the pCqSDNGP to our spectra, consistent with the values reported by Devi et al. For the pressure shifting and line mixing parameters we observed good agreement with the values found in the HITRAN database [8] and the studies by Predoi-Cross et al. [27] and Devi et al. [26] (see Figs. 5 and 6). As a result we see no evidence that these values should not be utilized in future retrievals. We note that high accuracy line intensity measurements were performed with a separate spectrometer and will be presented in a forthcoming publication.

57 5. Conclusions 59 61

67 69

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We have presented high resolution spectra of the (30012)’(00001) CO2 band in the near-infrared region.

71 73 75 77 79 81 83 85 87 Fig. 6. Comparison of our measured air-broadened line mixing parameters to those of Predoi-Cross et al. [27] and Devi et al. [26]. The shown standard uncertainties are based upon the reported fit uncertainty.

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Because these measurements are directly linked to an optical frequency comb, they provide accurate line positions and shifting coefficients. These high fidelity spectra also enabled the investigation of subtle line shape effects which can bias fitted laboratory measurements and remote sensing retrievals. Our analysis combined the partially correlated, quadratic-speed-dependent NelkinGhatak line profile and a first-order line mixing model. We find that both collisional narrowing and speed dependent broadening contribute to the isolated line shapes, while at pressures up to 130 kPa, the addition of line mixing provides an accurate description of the composite absorption spectrum. In comparison to earlier studies which utilized more simplistic line shape models, we observe significant differences in the measured broadening parameters which we assign to the failure to account for collisional narrowing effects.

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Acknowledgments 113 Support was provided by the NIST Greenhouse Gas Q4 115 Measurements and Climate Research Program. S. W. was partially supported by the Foundation for Polish Science 117 TEAM Project co-financed by the EU European Regional Development Fund. Additional support was provided by 119 the Orbiting Carbon Observatory (OCO-2) mission. The research performed at the Jet Propulsion Laboratory 121 (JPL), California Institute of Technology was conducted under contract from the National Aeronautics and Space 123 Administration (NASA).

Please cite this article as: Long DA, et al. Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band. J Quant Spectrosc Radiat Transfer (2015), http: //dx.doi.org/10.1016/j.jqsrt.2015.03.031i

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Please cite this article as: Long DA, et al. Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)’(00001) near-infrared carbon dioxide band. J Quant Spectrosc Radiat Transfer (2015), http: //dx.doi.org/10.1016/j.jqsrt.2015.03.031i