National Institute of Standards and Technology High precision 2.0 µm Photoacoustic Spectrometer for Determination of the 13CO2/12CO2 Isotope Ratio Z.D. Reed and J.T. Hodges National Institute of Standards and Technology

Photoacoustic Spectroscopy Absorption Excitation Relaxation PL =peak-to-peak laser power (W) Bm = microphone response (V Pa-1 ) n=PA resonator response at frequency f R = efficiency of acoustic relaxation Detection Sound Generation Local Heating

Advantages of Photoacoustic Spectroscopy Optically simple High sensitivity (αmin =1 x 10-10 cm-1 W) Excellent linearity over wide range of concentration (several orders of magnitude) Measures absorption coefficient but not scattering or extinction Sensitivity proportional to laser power Benefits from high powered laser sources

Applications of Photoacoustic Spectroscopy Previously demonstrated for determination of atmospheric CO2 mixing ratio at 1.57 µm [1] Oxygen A-band sensing [2] Widely employed for concentration determination in various species (CO2, CO, HCN, NH3, etc) [3-4] QEPAS (Quartz enhanced Photoacoustic Spectroscopy) previously employed to probe CO2 near 2.0 µm [5] [1] Z. Reed et al App Phys B 117 (2), 2014, 645-657 [2] K.A Gillis, D.K. Havey, J.T. Hodges Rev. Sci. Instruments 81, 2010, 064902 [3] A.A. Kosterev et al App Phys B 100, 2010, 173-180 [4] P. Patimisco, G. Scamarcio, F. Tittel, V. Spagnolo Sensors 14, 2014, 6165 [5] G. Wysoscki, A.A. Kosterev, F.K. Tittel Appl. Phys. B. 85, 301, 2006

13CO2/12CO2 Isotope Ratio Source apportionment (anthropogenic/biogenic sources), biomedical tracing and diagnosis 13CO2 is nominally 1.1237% of 12CO2 concentration Line intensities are ~100x weaker (in same band), not sufficient sensitivity to make measurement near 1.6 µm 2.0 µm CO2 line intensities are ~100x higher than those at 1.6 µm, enabling 13CO2 measurements and reduced averaging time for 12CO2

Stable Isotope Ratio Terminology Carbon Pool δ 13CO2 mole fraction PDB Standard 0‰ 0.011237 Terrestrial Biosphere -26‰ 0.010945 Oceans and Atmosphere -8‰ 0.011142 HITRAN Atmospheric Standard1 -15.8‰ 0.01106 1. P. De Bievre, N.E. Holden, and I.L. Barnes, “Isotopic Abundances and Atomic Weights of the Elements,” J.Phys.Chem.Ref.Data 13, 809-891 (1984).

Dual wavelength PAS approach Probe selected transitions of 12CO2 near 2004nm (4990 cm-1) and 13CO2 near 1998nm (5008 cm-1) Reducing deadtime while tuning, higher duty cycle Decreased averaging time for 12CO2 if using strong lines near 2004nm (vs 1998nm region or 1570nm) Ability to probe 12CO2 and 13CO2 lines with same j, similar lower state energy (temperature dependence)

5x

1σ = 0.13 µV Top pane: Spectrum (symbols) and Galatry profile fit (line) of 384 ppm CO2 in air near 2004nm , measured at 250 torr and 296K Bottom pane: Residuals of fitted spectrum. Note change of scale Top pane: Spectrum (symbols) and GP fit (line) of 384 ppm CO2 in air near 1998nm, measured at 250 torr and 296K Bottom pane: Residuals of fitted spectrum.

Measurement precision of 12CO2 near 2004nm, using a lock-in time constant of 30 ms and 0.1 sec averaging Measurement precision of δ CO2. 12CO2 measured once per 100 seconds for 7.5 seconds, 13CO2 measured using lock-in time constant of 1 second and 7.5 sec averaging

Long Term Averaging Mole fraction of 13CO2 measured with 7.5 s averaging and 1 sec lock-in time constant. Baseline correction (7.5 sec measurement) every 100 seconds. Concentration determination uses HITRAN2012 line intensities. Determination of δ CO2. 12CO2 measured once per 100 seconds for 7.5 seconds, 13CO2 measured using lock-in time constant of 1 second and 7.5 sec averaging

Dual Wavelength Photoacoustic Spectrometer Achieves better than 0.01% precision (10 sec averaging) for 12CO2, 7 ppb limit of detection Achieves better than 0.05‰ precision on δ (13CO2/12CO2) over long term averaging 12CO2 accuracy limited to calibration standards Future work: calibrate δ CO2 against known 13CO2 standards (IR-MS)

Relaxation Dynamics Absorption Excitation Relaxation PL =peak-to-peak laser power (W) Bm = microphone response (V Pa-1 ) n=PA resonator response at frequency f R = efficiency of acoustic relaxation Detection Sound Generation Local Heating

Relaxation Dynamics Relaxation Time Relaxation of excited molecule must be fast compared to acoustic excitation timescale Relaxation rate varies by molecule, transition wavelength, and bath gas Relaxation Time 11 µs atm in dry N2 0.1 µs atm in water G. Wysocki, A.A. Kosterev, F.K. Tittel Appl. Phys. B. 85, 301, 2006

QEPAS Relaxation time (fmod = 32768 Hz) ≪ 30 µs NIST PAS Relaxation time (fmod = 1630 Hz) ≪ 613.5 µs 20 cm Relative relaxation efficiency measured at 2004nm of CO2 in N2 as a function of water concentration Tittle, F. Photonics Spectra, June 2014 G. Wysocki, A.A. Kosterev, F.K. Tittel Appl. Phys. B. 85, 301, 2006

Relaxation Dynamics Normalized Photoacoustic spectrum of 398 ppm CO2 in N2 Simulated spectrum near 2.0 µm of xCO2=400ppm and xwater= 1000ppm, p=250 torr and T=296K

Comparison of PA spectra measured in dry N2 (left) and dry air (right)

Transition Frequency Relative Response (Dry/Wet) R16 4989.967 0.97 R18 4991.26 0.94 R20 4992.513 0.96 R22 4993.74 0.80 R24 4994.94 0.90 R26 4996.107 0.84 Relative Response <1 indicates suppressed signal in dry air, greater relative enhancement due to water Calculated water transitions (right panes) and CO2 transitions (left). Water transitions from MARVEL line list. Tennyson et al, JQSRT 117, 2013, 29-58

Conclusions PAS Spectrometer operating near 2.0 µm Achieves better than 0.01% precision (10 sec averaging) for 12CO2 Achieves better than 0.05‰ precision on δ (13CO2/12CO2) over long term averaging PA system response shows transition specific relaxation Calibration must be transition specific Likely due to overlap with water transitions differentially enhancing lines at low moisture content Currently unknown why air and N2 exhibit different behaviors Funding: NIST Greenhouse Gas Measurements and Climate Research Program