LINE SHAPE PARAMETERS OF WATER VAPOR TRANSITIONS IN THE 3645−3975 cm−1 REGION V. MALATHY DEVI and D. CHRIS BENNER The College of William and Mary, Williamsburg, VA, U.S.A. ROBERT R. GAMACHE, BASTIEN VISPOEL and CANDICE L. RENAUD University of Massachusetts Lowell, Lowell, MA, U.S.A. MARY ANN H. SMITH NASA Langley Research Center, Hampton, VA, U.S.A. ROBERT L. SAMS and THOMAS A. BLAKE Pacific Northwest National Laboratory, Richland, WA, U.S.A. MJ05 – 72nd International Symposium on Molecular Spectroscopy, June 19-23, 2017

Air-broadened H2O line parameters and their temperature dependences are needed for quantitative analysis of spectroscopic observations of Earth’s atmosphere, both to retrieve atmospheric water vapor abundances and to model the H2O absorption overlapping features of other species. Spectroscopic databases such as HITRAN and HITEMP contain line positions, intensities, and assignments for hundreds of thousands of transitions of H2O and its isotopologues. However, measured line shape parameters (widths, pressure-induced shifts, temperature dependences, etc.) are available for only a small fraction of the transitions listed in the database. Calculations of H2O line shape parameters (e.g., Gamache and Laraia, J. Mol. Spectrosc. 257 (2009) 116-127), necessary for completing the databases, require laboratory measurements for validation. The majority of published measurements of air-, N2-, or O2-broadened line shape parameters for H2O have been for the ν2 band system near 1600 cm-1. Present Work: Analyze air- and self-broadened H2O spectra recorded at 268-353 K to determine temperature dependences of air-broadened line shape parameters for ν1 and ν3 transitions in the 3645 − 3975 cm-1 spectral region.

Experimental Conditions: Bruker IFS 120 HR (Pacific Northwest National Laboratory) Source: Tungsten Beam splitter: CaF2 Detector: InSb Spectral coverage: 2950–4750 cm-1 Maximum path difference(2L): ~125 cm Unapodized resolution (FWHM): 0.008 cm-1 (Two pure H2O spectra at 0.006 cm-1) FTS input aperture size: < 2 mm Number of co-added scans: 66−512 Signal-to-RMS noise: >1500 Cell path lengths: 9.906 and 19.95 cm Broadening gases: H2O, Air Pressures: 0.02 to 613 Torr Temperatures: 268 to 353 K

Summary of Lab Spectra Analyzed Serial # Cell Length (cm) Total gas pressure (Torr) Sample gas temperature (K) Volume mixing ratio of H2O Only H2O 1 19.95 0.130 296.20 1.0 2 0.101 3 0.0235 4 0.0690 5 0.0415 296.21 6 9.906 2.032 298.00 7 4.16 H2O + air 8 150.1 0.0051 9 305.5 0.0072 10 450.4 0.00564 11 100.5 268.20 0.00606 12 252.7 268.19 0.00438 13 400.6 0.00466 14 600.0 0.00376 15 100.8 353.23 0.00775 16 250.3 0.00682 17 0.01026 18 612.9 353.24 0.0200 Notes: 1 atm =101.3kPa = 760 Torr. Spectral resolution was 0.006 cm-1 for #6 and #7, and 0.008 cm-1 for all others.

Examples of recorded spectra All spectra shown were recorded with the 19.95 cm sample cell.

Analysis Nonlinear least squares multispectrum fitting [D. Chris Benner et al., JQSRT 53 (1995) 705-721] is used to retrieve spectroscopic parameters consistent with the entire set of laboratory spectra. All spectra are calibrated to the same wavenumber scale with reference to CO 2-0 line positions. Initial line list taken from HITRAN2012 [L. S. Rothman et al., JQSRT 130 (2013) 4-50]. Voigt line shape is initially assumed, and quadratic speed dependence is included if residuals indicate it is necessary; line mixing is allowed for pairs of lines expected to mix. Room-temperature self-broadened spectra are fit first; then the air- broadened spectra at room, higher, and lower temperatures are added sequentially.

Example Fit of Air- and Self-Broadened H2O Spectra A total of 18 spectra were fit. 170 transitions (tick marks in top panel) in this 17 cm-1 interval Voigt line shape profile with quadratic speed dependence

Example Fit of Air- and Self-Broadened H2O Spectra A total of 18 spectra were fit. Expanded view of a small region of the 20 cm-1 interval fit (3847-3867 cm-1). Positions of H2O transitions indicated by tick marks in top panel. Voigt line shape profile with quadratic speed dependence. Line mixing occurs between transitions marked 1&4, 3&5, 3&4 and 4&5 (line 2 does not mix with others).

Measured Air-Broadened Line Widths and Shifts and their Temperature Dependences Temperature dependence of widths: b(T) = b0 × (T0/T)n, where b0 is the width at T0 = 296 K. Temperature dependence of shifts: δ(T) = δ0 + δ' × (T − T0), where δ0 is the shift at T0 = 296 K.

Measured Self-Broadened Line Widths and Shifts Temperature dependences were not measured because self-broadened spectra were recorded only at room temperature (296 − 298 K).

Comparison with Other Measurements and HITRAN Positions and Intensities, ν3 band only Self-broadened Widths and Shifts, ν1 and ν3 bands References: L. S. Rothman et al., JQSRT 130 (2013) 4-50; Q. Zou and P. Varanasi, JQSRT 82 (2003) 45-98; I. V. Ptashnik et al., JQSRT 177 (2016) 92-107; J. Loos et al., JQSRT (2017) in press [two articles]. Self-shifts are not reported in HITRAN 2012 or in Ptashnik et al.

Comparison with Other Measurements and HITRAN Air-broadened Widths and T-dependences, ν1 and ν3 bands Air-broadened Shifts and T-dependences, ν3 band only References: L. S. Rothman et al., JQSRT 130 (2013) 4-50; Q. Zou and P. Varanasi, JQSRT 82 (2003) 45-98; I. V. Ptashnik et al., JQSRT 177 (2016) 92-107; J. Loos et al., JQSRT (2017) in press [two articles]. Air-broadened parameters are not reported in Ptashnik et al., and air-shift T-dependences are only reported in the present study and in Loos et al.

Measured ν3 Parameters with Line Mixing # Quantum Assignment n (cm-1) S (cm/molecule at 296K)  H2O-air Widtha nb Shifta δ̍ (H2O-air)b Wij H2O-aira H2O-H2Oa 1 3 3 0  4 3 1 3 2 1  4 2 2 3646.463642 3647.138304 1.306e-20 3.296e-20 0.08159(7) 0.08963(4) 0.692(5) 0.731(3) -0.00309(6) -0.00286(3) +0.35(4)e-05 +0.35(4)e-05c 0.0044(1) 0.0(F) 2 3650.636109 3651.365171 4.647e-20 4.852e-20 0.08425(5) 0.09201(5) 0.669(2) 0.762(2) -0.00827(3) -0.00365(3) +2.59(5)e-05 +0.82(6)e-05 0.0029(1) 3 2 2 0  3 2 1 2 1 1  3 1 2 3674.957865 3676.019555 8.725e-20 1.695e-19 0.09319(4) 0.09712(3) 0.726(2) 0.745(2) -0.00396(2) -0.00172(2) +0.78(48)e-06 +0.27(4)e-05 0.0078(1) 0.031(5) 4 5 5 1  5 5 0 5 5 0  5 5 1 3726.617089 3726.625468 3.072e-20 8.140e-21 0.05136(4) 0.05136(4)c 0.636(3) 0.636(3)c -0.00642(2) -0.00642(2)c +2.00(4)e-05 +2.00(4)e-05c 0.026(4) 5 5 4 2  5 4 1 5 4 1  5 4 2 3734.930719 3735.444698 3.534e-20 1.174e-20 0.06713(4) 0.06713(4)c 0.615(2) 0.615(2)c -0.00594(2) -0.00594(2)c +1.79(4)e-05 +1.79(4)e-05c 0.0013(1) 0.121(7) 6 3 2 2  3 2 1 3 3 1  3 3 0 3744.509479 3744.651240 1.031e-19 1.647e-19 0.09475(6) 0.07779(4) 0.735(2) 0.695(2) -0.00557(3) -0.00512(3) +0.54(5)e-05 +0.123(4)e-04 0.0049(0) 0.040(1) 7 3 0 3  2 0 2 3 2 2  2 2 1 3820.738533 3821.764198 8.125e-20 9.662e-20 0.09906(4) 0.09244(3) 0.821(2) 0.777(2) +0.00066(2) -0.00482(2) −0.119(4)e-04 +0.178(4)e-04 0.0025(1) 8 5 0 5  4 0 4 4 1 3  3 1 2 3854.090481 3854.438182 6.179e-20 1.770e-19 0.08615(4) 0.09371(3) 0.688(3) 0.760(2) -0.00128(4) -0.00620(2) −0.21(6)e-05 +1.80(3)e-05 0.0035(1) 0.048(3) 9 4 2 2  3 2 1 3853.966209 1.180e-19 0.09060(5) -0.00483(3) +1.13(4)e-05 0.033(1) 10 0.0094(1) 11 5 1 5  4 1 4 3852.057460 1.800e-19 0.08285(2) 0.664(1) -0.00294(1) +0.48(3)e-05 0.0118(1) 12 7 3 4  6 3 3 7 2 5  6 2 4 3925.134434 3925.175901 6.795e-21 1.094e-20 0.09275(41) 0.09405(30) 0.818(11) 0.875(7) -0.01874(30) -0.00786(27) +0.67(3)e-04 +0.30(2)e-04 0.0170(3) 0.071(2) aWidths, Shifts, and Off-diagonal relaxation matrix element coefficients, Wij , all have the same units (cm-1 atm-1 at 296 K). bT-dependence of the width, n, is unitless, and the T-dependence of the shift, δ̍, has units (cm-1 atm-1 K-1). cValues are constrained in the fits to those listed in the line just above it. F indicates a value fixed in the fits. Values in gray-shaded cells are those retrieved from the fit shown on Slide 8.

Summary We have measured line positions, intensities, air-broadened half-width and pressure shift coefficients and their temperature dependences for strong transitions in the ν3 and ν1 bands of H216O, by multispectrum fitting FTS spectra recorded from 268 to 353 K, using the speed-dependent Voigt line shape. Air-width and air-shift coefficients were measured for over 200 transitions, but their temperature dependences were obtained for only about 100 transitions. Room-temperature self-width and self-shift coefficients were determined for nearly all of the same 200 transitions. Line mixing parameters were experimentally determined for 12 pairs of ν3 transitions. Results are in good agreement with other recent measurements of these parameters, but our results extend to higher J values. Future Work Compare with results of Modified Complex Robert-Bonamy (MCRB) calculations. Provide preliminary results for use in atmospheric retrievals. Final results will be contributed to future spectroscopic database updates (e.g., HITRAN).

Acknowledgments Cooperative agreements and contracts with National Aeronautics and Space Administration have supported the research performed at the College of William and Mary and at NASA Langley Research Center through the Upper Atmosphere Research Program and the AQUA Validation Program. Research performed at the University of Massachusetts Lowell is supported by the National Science Foundation through Grant No. AGS-1156862 and AGS- 1622676. The Department of Energy’s Office of Biological and Environmental Research located at the Pacific Northwest National Laboratory (PNNL) supported part of this research. PNNL is operated for the United States Department of Energy by Battelle under contact No. DE-AC06-76RLO 1830.

Backup Slide: Other experimental details Both specially built absorption cells were made of stainless steel, but electroplated and gold-coated to minimize decomposition and sticking of the gas samples and to minimize reactivity when using corrosive gases such as HCN. Both cells have an inner diameter of 5 cm. The 19.95 cm cell had wedged KCl windows sealed with Viton O-rings, and the 9.906 cm cell had wedged BaF2 windows also sealed to the ends of the cell with Viton O-rings. Cell lengths are known with an uncertainty of 0.01%. In each experimental set up, the absorption cell was mounted inside the sample compartment of the spectrometer to avoid errors associated with beam steering due to cell misalignment. A temperature regulated outer jacket surrounds the cell’s inner tube and is plumbed through the spectrometer’s vacuum bulkhead to a circulating bath (Julabo F25-MD). The cell is actively temperature stabilized by either heating or cooling the circulating fluid (a weak solution of propylene glycol in water). Gas sample pressures are measured with uncertainties of ± 0.05% of full-scale readings of the appropriate MKS Baratron pressure gauges (1, 10 or 1000 Torr range, as appropriate). Cell (gas sample) temperatures are measured with uncertainties of ±0.02 K (RTD sensor). Research grade synthetic air sample was used for the air-broadened H2O spectra. To minimize residual atmospheric H2O signal, the entire spectrometer bench and the sample compartment were kept under vacuum (below 0.03 Torr) using a mechanical vacuum pump. A ½” diameter copper tube, positioned horizontally, ran the length of the moving mirror compartment, and both ends of the tube came out of the spectrometer through vacuum feedthrough fittings. The ends of the tube were bent so they were vertical. This tube was kept filled with liquid nitrogen for the duration of each measurement. Two molecular sieve sorption traps were connected to the sample and detector vacuum chambers, and were cooled to liquid nitrogen temperature during the duration of each measurement.