Diapositive 1

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Methyl Bromide Spectroscopic line parameters
in the 7- and 10-µm region D. Jacquemart1, N.
Lacome1, F. Kwabia-Tchana1, I. Kleiner2 1
Laboratoire de Dynamique, Interactions et
Réactivité Université Pierre et Marie
Curie-Paris6, CNRS, UMR 7075, France 2
Laboratoire Inter-Universitaire des Systèmes
Atmosphériques Universités Paris 12 et Paris 7 ,
CNRS, UMR 7583, FRANCE
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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
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Spectroscopic line parameters in literature
? Previous works concern mainly line positions
analysis (see Graner JMS 198190394-438)
? Two recent works on line positions and
intensities in the 7-µm spectral region
(Kwabia Tchana et al. JMS 2004228441-52
Kwabia Tchana et al. JMS 2006235132-43)
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
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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
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CH3Br in the 10 µm spectra region
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Experimental conditions for spectra recorded
around 10 µm
? Rapid scan interferometer Bruker IFS 120 HR
(LADIR, Paris) (?max 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 ____________________________________
____________________________
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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
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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 20011455-69.) 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
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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 1986119418-25 )
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?R0?2 2.688(6)10?3 Debye2
d62 2. 691(8)10?3 Debye2
d6(2)1.41(4)10-4
AK 5.3(2)10?3
AJ 0
lt(100  (obs  calc) / obs)gt 0.2  3.7
lt(100  (obs  calc) / obs)gt 0.01  3.84 
? weak l-type interactions for the v6 level
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Strong K-dependence
No significant J -dependence
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Ratio of the two calculations for measured
transitions (1200)
RQ(1)
PP(1)
RQ(2)
PQ(2)
PQ(1)
RR(1)
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Ratio of the two calculations for extrapolated
transitions (18000 transitions)
? No line intensity cutoff, but Jmax60 and
Kmax30
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Comparison with measurements
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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
200483243-65.) 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 ?2 band of CH3D.
J Mol Spectrosc 200623535-53 .)
Empirical model has been used to compute measured
self and N2 widths
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Empirical model used to compute measured self and
N2 widths
? For transitions having same value of Jinf
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Empirical model used to compute measured self
widths
? Fit of the two coefficients aJ0 and aJ2
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Empirical model used to compute measured N2
widths
? Fit of the two coefficients aJ0 and aJ2
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Comparison between measured and calculated
self-widths
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Comparison between measured and calculated
N2-widths
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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 0.210-3
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 to 0.8 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
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CH3Br in the 7 µm spectra region
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Experimental conditions for spectra recorded
around 7 µm
? Rapid scan interferometer Bruker IFS 120 HR
(LADIR, Paris) (?max 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 ____________________________
________________
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?2 parallel band
?5 perpendicular band
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Determination of line intensities using a single
spectrum fitting procedure
? Around 320 transitions have been measured
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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 1986119418-25 )
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Dipole Moment Derivatives (in Debye) for the ?2
and ?5 Bands of CH379Br and CH381Br (320 lines
studied)
(a) d calc obs / obs in . (b) Number of
transitions included in the least squares fits.
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Synthetic spectrum
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Synthetic spectrum
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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 110-3 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
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Methyl Bromide will probably be the next new
molecule in HITRAN
The number 40