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Methyl Bromide : Spectroscopic line parameters in the 7- and 10- μ m region

Methyl Bromide : Spectroscopic line parameters in the 7- and 10- μ m region D. Jacquemart 1 , N. Lacome 1 , F. Kwabia-Tchana 1 , I. Kleiner 2 1 La boratoire de D ynamique, I nteractions et R éactivité; Université Pierre et Marie Curie-Paris6, CNRS, UMR 7075, France

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Methyl Bromide : Spectroscopic line parameters in the 7- and 10- μ m region

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  1. Methyl Bromide : Spectroscopic line parameters in the 7- and 10-μm region D. Jacquemart1, N. Lacome1, F. Kwabia-Tchana1, I. Kleiner2 1Laboratoire de Dynamique, Interactions et Réactivité; Université Pierre et Marie Curie-Paris6, CNRS, UMR 7075, France 2Laboratoire Inter-Universitaire des Systèmes Atmosphériques; Universités Paris 12 et Paris 7 , CNRS, UMR 7583, FRANCE

  2. Methyl Bromide (CH3Br) → Atmospheric trace gas (≈ 10 pptv) of both natural and anthropogenic origins (oceanic emission, biomass burning, leaded gasoline, agricultural pesticide …) → Major contributor to stratospheric bromine which participates to ozone destruction → Deadly toxic gas for human and animal life when exposed to high concentration

  3. Spectroscopic line parameters in literature → Previous works concern mainly line positions analysis (see Graner JMS 1981;90:394-438) → Two recent works on line positions and intensities in the 7-μm spectral region (Kwabia Tchana et al. JMS 2004;228:441-52 ; Kwabia Tchana et al. JMS 2006;235:132-43) Presented in the second part of this talk → No work on broadening coefficients → No spectroscopic data is available in atmospheric database such as HITRAN or GEISA

  4. CH3Br in our atmosphere → Compare to Cl, Br radicals are 10 time more efficient for the ozone destruction → Compare to CH3Cl, the quantity in our atmosphere is 10 time less → Not yet detected in atmospheric spectra Complete line lists are necessary to detect CH3Br

  5. CH3Br in the 10 μm spectra region

  6. Experimental conditions for spectra recorded around 10 μm → Rapid scan interferometer Bruker IFS 120 HR (LADIR, Paris) (Δmax = 450 cm; FWHM =1.1 103 cm1) Absorbing sample Natural CH3Br 50.54 % of CH379Br 49.46 % of CH381Br Stated purity 99.50 % Experimental conditions S/N ratio 100 __________________________________________________________________ # CH3Br pressure N2 pressure Temperature Absorption path (mbar) (mbar) (K) (cm) __________________________________________________________________ 1 0.4712 0 298.15 415 2 0.8745 0 297.15 415 3 4.738 0 298.15 415 4 7.200 0 298.15 30 5 2.030 25.30 297.55 415 6 3.376 32.90 296.45 415 ________________________________________________________________

  7. scattering (1SD) of 0.04×10–3 cm–1 at 1000 cm–1 < ε > = 1.789(40)×10–6 Preliminary work → Phase correction for each spectrum (Mertz method) → Determination of an average effective iris radius → Wavenumber calibration using NH3 transitions and HITRAN2004 wavenumbers as etalon

  8. Line parameters measurement for transitions having J and K ranging from 0 to 55 and from 0 to 9 → 1200 transitions fitted between 880 and 1050 cm1 of both CH379Br and CH381Br → Use of a multispectrum fitting procedure(Eur Phys J D 2001;14:55-69.) Position, intensity and broadening coefficients of a same line are constrained to be the same during the simultaneous fit of the six spectra. Use of a Voigt profile. For broadening coefficients we assumed that:

  9. → Classical Herman Wallis treatment with |v,ℓ,J,K> as eigenvectors (Watson JKG. Quadratic Herman-Wallis Factors for Symmetric- and Asymmetric- Top Molecules. J Mol Spectrosc 1992;153:211-24.) Two models have been used to analyze measured line intensities → Treatment using the eigenvectors as a linear combination of the zero order basis wavefunction (ℓ-type interactions) (Tarrago G, Delaveau M. Triad vn(A1), vt(E), vt’(E) in C3v Molecules: Energy and Intensity Formulation (Computer Programs). J Mol Spectrosc 1986;119:418-25)

  10. R02 = 2.688(6)103 Debye2 d62 = 2. 691(8)103 Debye2 d6(2)=1.41(4)×10-4 AJ= 0 AK= 5.3(2)103 <(100 × (obs – calc) / obs)> = –0.01 ± 3.84 % ; <(100 × (obs – calc) / obs)> = 0.2 ± 3.7 % → weak ℓ-type interactions for the v6 level

  11. Strong K-dependence No significant J -dependence

  12. Ratio of the two calculations for measured transitions (1200) RQ(1) PP(1) RQ(2) PQ(2) PQ(1) RR(1)

  13. RQ(1) PP(1) RQ(2) PQ(2) PQ(1) RR(1) Ratio of the two calculations for extrapolated transitions (18000 transitions) → No line intensity cutoff, but Jmax=60 and Kmax=30

  14. PQ(1) branch RQ(1) branch Comparison with measurements

  15. Analysis of the measured self and N2 widths → For C3v molecules the J-and K-dependences of the widths have already been observed for: NH3(Nemtchinov V, Sung, K, Varanasi P. Measurements of line intensities and half-widths in the 10-μm bands of 14NH3. JQSRT 2004;83:243-65.) CH3D (Predoi-Cross A, Hambrook K, Brawley-Tremblay S, Bouanich JP, Malathy Devi V, Smith MAH.Measurements and theoretical calculations of N2-broadening and N2-shifting coefficients in the ν2 band of CH3D. J Mol Spectrosc 2006;235;35-53 .) Empirical model has been used to compute measured self and N2 widths

  16. Empirical model used to compute measured self and N2 widths → For transitions having same value of Jinf

  17. Empirical model used to compute measured self widths → Fit of the two coefficients aJ0 and aJ2

  18. Empirical model used to compute measured N2 widths → Fit of the two coefficients aJ0 and aJ2

  19. Comparison between measured and calculated self-widths

  20. Comparison between measured and calculated N2-widths

  21. Conclusion for the 10 μm region based on the fit of 1200 measurements For positions: The average discrepancy obs-calc is equal to (0.001 ± 0.114)×10-3 cm-1, The accuracy is estimated to be better than 0.2×10-3 cm-1. For intensities: The average discrepancy obs-calc is equal to 0.2 ± 3.8 %, The rotational dependence is reproduced with accuracy around 5 %. For widths: The average discrepancy %self is equal to0.8 ± 6.4 %, The average discrepancy % N2 is equal to –0.3 ± 3.3 %. The J and K dependence of the measurement is reproduced with accuracy better than 10 % for the self-broadening coefficients, and around 5 % for the N2-broadening coefficients. → List of these parameters will be proposed to atmospheric databases

  22. CH3Br in the 7 μm spectra region

  23. Experimental conditions for spectra recorded around 7 μm → Rapid scan interferometer Bruker IFS 120 HR (LADIR, Paris) (Δmax = 450 cm; FWHM =1.1 103 cm1) Experimental conditions Absorbing sample Natural CH3Br 50.54 % of CH379Br and 49.46 % of CH381Br Stated purity 99.50 % Experimental conditions (SNR  100) ____________________________________________ # CH3Br pressure Temperature Absorption path (mbar) (K) (cm) ____________________________________________ 1 0.1991 297 415 2 0.2778 296 415 3 0.3415 298 415 4 0.4028 296 415 ____________________________________________

  24. ν2 parallel band ν5 perpendicular band

  25. Determination of line intensities using a single spectrum fitting procedure → Around 320 transitions have been measured

  26. Analysis of line intensities → Treatment using the eigenvectors as a linear combination of the zero order basis wavefunction (ℓ-type interactions) (Tarrago G, Delaveau M. Triad vn(A1), vt(E), vt’(E) in C3v Molecules: Energy and Intensity Formulation (Computer Programs). J Mol Spectrosc 1986;119:418-25)

  27. Dipole Moment Derivatives (in Debye) for the 2 and 5 Bands of CH379Br and CH381Br (320 lines studied) Dipole Moment Derivatives CH379Br CH381Br 0.06705(16) 0.06644(18) 0.037819(72) 0.037500(79) × 103 – 0.1007(92) – 0.111(10) Statistics CH379Br CH381Br 0 ≤ δ(a) < 4% 80 % 72 % of the lines 4% ≤ δ < 7% 16 % 22 % 7% ≤ δ < 10% 2 % 3 % 10% ≤ δ < 17% 2 % 3 % # lines 2(b) 59 58 # lines 5 (b) 93 103 % rms 2 3.0 % 3.3 % % rms 5 2.6 % 3.0 % (a) δ = |calc – obs| / obs in %. (b) Number of transitions included in the least squares fits.

  28. Synthetic spectrum

  29. Synthetic spectrum

  30. Conclusion for the 7 μm region For positions:(based on the fit of 7500 line positions) The average discrepancy obs-calc is equal to (0.002 ± 0.784)×10-3 cm-1, The accuracy is estimated to be better than 1×10-3 cm-1. For intensities:(based on the fit of 300 line intensities) The average discrepancy obs-calc is equal to 0.04 ± 3.9 %, The rotational dependence is reproduced with accuracy around 5 %. For widths: New spectra have been recorded with CH3Br and N2, and will be analyzed to observe or not a vibrational dependence for broadening coefficients. At the present time we suggested that widths obtained in the 10 μm region could be applied to the 7 μm region. → List at 7 μm will be proposed to atmospheric databases

  31. Methyl Bromide will probably be the next new molecule in HITRAN The number 40

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