TEMPERATURE DEPENDENCES OF AIR-BROADENING AND SHIFT PARAMETERS IN THE ν 3 BAND OF OZONE M. A. H. SMITH NASA Langley Research Center, Hampton, VA U.S.A. V. MALATHY DEVI and D. CHRIS BENNER The College of William and Mary, Williamsburg, VA U.S.A. RF08 – 70 th International Symposium on Molecular Spectroscopy, June 22-26, 2015

Why do we need to know the temperature dependences of spectroscopic line parameters? Tropopause temperatures vary with latitude and season, and are typically below 195 K in the tropics. Global Average Temperature Profile Uncertainties in spectroscopic line parameters propagate into retrievals of atmospheric ozone concentrations. Change in retrieved atmospheric O 3 mixing ratio as a function of altitude for a 60% increase in the assumed air-broadened halfwidth [Smith and Gordley, 1983].

3 Present Work: Analyze air-broadened O 3 spectra recorded at K to determine temperature dependences of ν 3 air-broadening and shift parameters needed for atmospheric retrievals. ► T = 297 K, p = 110 torr, VMR = 0.4%, L = 50 cm used for calibration Air-broadened O 3 line parameters and their temperature dependences (175–300 K) in the 9.6 µm region are needed for quantitative analysis of spectroscopic observations of Earth’s atmosphere, both to retrieve ozone abundances and to model the O 3 absorption overlapping features of other species.  Previous lab T-dependence studies have involved mostly B-type bands: Rotational [Larsen et al., 2001], ν 1 [Smith et al., 1997] and ν 2 [Malathy Devi et al., 1997].  However, ν 3 is A-type! One study of N 2 -broadening for 21 ν 3 transitions ( K) [Spencer et al., 1992]. Air-broadened widths and shifts for 9 ν 3 R-branch transitions are included in the [Smith et al., 1997] ν 1 results ( K). [Wagner et al., 2002] reported polynomial representations for ν 3 N 2 - and O 2 - broadened widths and temperature dependences ( K), but no shifts.

4 Lab Spectra Used for Ozone Fits in the ν 3 Band Band pass 1 = cm -1 ; Band pass 2 = cm -1 All spectra were recorded using the McMath-Pierce FTS with KCl beam splitter, 8 mm aperture, and He-cooled As:Si detectors. February 1996 spectra were used in the previous intensity study [Smith et al., 2001]. May 1998 spectra were recorded using a single detector.

5 Analysis Nonlinear least squares multispectrum fitting [Benner et al., 1995] is used to retrieve spectroscopic parameters consistent with the entire set of laboratory spectra.  Voigt line shape is assumed; line mixing is allowed for pairs of lines expected to mix (no mixing observed in intervals fit to date).  All spectra are calibrated to the same wavenumber scale with reference to H 2 O ν 2 line positions.  Room-temperature spectra are fit first; then the lower-temperature spectra are added sequentially down to 213 K.  Noisy spectra from 186 to 160 K are added to the fit last with low weights (poor Signal-to-Noise Ratio due to single detector).

6 Example Fit of Air- and Self-Broadened O 3 Spectra A total of 31 spectra were fit. (Standard Deviation of this fit = 0.592%) These 6 spectra were selected to represent the range of conditions. Noisy spectra (e.g., 857 and 858) were given lower weights in the fits. RunT (Kelvin) P (torr) L (cm) O 3 VMR (%)

7 Measured Line Widths and Shifts and Their Temperature Dependences in A-type O 3 bands P-branch lines, J˝ = 15 – 26, K a ˝ = 0 – 11 (● ν 3 [Present Work]; ■ ν 3 N 2 -broadening [Spencer et al., 1992]; □ 3ν 3 air-broadening [Smith et al., 1994]). R-branch lines, J˝ = 23 – 36, K a ˝ = 2 – 3, ∆K a = 2 (○ ν 3 air-broadening [Smith et al., 1997]).

Two ν 3 transitions with three independent measurements 8 SourceBroadener T range (K)Width Width TdepShiftShift Tdep cm −1 (20,3,18 – 21,3,19) HITRAN08/12air − Spencer et al. (1992)N2N (12)0.68(6) Wagner et al. (2002)air (N 2, O 2 ) Present Workair (2)0.660(10)−0.0017(1)1.8(3)E cm −1 (18,7,12 – 19,7,13) HITRAN08/12air − Spencer et al. (1992)N2N (14)0.71(7) Wagner et al. (2002)air (N 2, O 2 ) Present Workair (2)0.647(16)−0.0018(2)1.8(5)E-05 Note: Units of Width and Shift are cm -1 atm -1 at 296K, units of Shift Tdep are cm -1 atm -1 K -1, and Width Tdep is unitless. Values in parentheses are 1σ statistical errors in units of the last digit quoted.

9 Comparison with Other Fundamentals and Rotational Band Measured values: ν 3 from Present Work, ν 2 from [Malathy Devi et al., 1997], ν 1 from [Smith et al., 1997] and Rotational from [Larsen et al., 2001]. Calculated values are from empirical formulas given by [Wagner et al., 2002] based on their N 2 - and O 2 -broadening measurements at 200 – 298 K. Remember that Rotational, ν 1 and ν 2 are B-type bands, while ν 3 is A-type. Only the lower-state rotational quantum numbers are the same for the transitions compared.

10 Measured ν 3 air-broadening parameters vs. HITRAN HITRAN O 3 air-broadening and shift parameters are the same in the last three editions [Rothman et al., 2005, 2009, 2013]. ◄ (Upper Left) Half widths. ◄ (Lower Left) Temperature dependence exponents of the half widths. ▼ (Below) Pressure-induced line shifts.

11 Summary Present ν 3 results vs. HITRAN Measured half widths are 3-5% larger than HITRAN values, especially for transitions with J ʺ < 18. Measured n values are up to 15-20% smaller than HITRAN values and have a wider range. Measured shift coefficients are mostly larger in magnitude than the constant − cm -1 atm -1 HITRAN default value for ν 3. Shift coefficients with the value are not acceptable (unless measured)! Future Work  Complete the analyses of the laboratory spectra in the entire ν 3 band.  Provide preliminary results for use in atmospheric retrievals and/or comparison to calculated widths and shifts.  Final results will be included in future spectroscopic database updates (e.g., HITRAN).

12 Acknowledgements The research at NASA Langley Research Center, and the College of William and Mary is supported by the Atmospheric Composition Laboratory Research program of the National Aeronautics and Space Administration. We thank Mike Dulick, Detrick Branston, Claude Plymate and the late Jeremy Wagner of the National Solar Observatory for their assistance in recording the spectra at Kitt Peak over many years. NSO is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under contract with the National Science Foundation. We especially thank Charles Solomon and Harry Walthall, now retired from the NASA Langley glass shop, for their assistance in the design, construction and testing of the 50 cm coolable cell and the 9.98 cm cell. We are grateful to the late Dr. Charles Chackerian of NASA Ames Research Center for the loan of the 9.5 cm coolable cell.

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