11 Collisional effects on spectral shapes and remote sensing H. Tran LISA, CNRS UMR 7583, Université Paris-Est Créteil and Université Paris Diderot

22 Outline Introduction: Atmospheric observation and spectral shape Isolated line : - The Doppler, Lorentzian and Voigt profiles - Limits of the Voigt profile - Laboratory studies - Influence on atmospheric retrieval Line-mixing for closely spaced lines - Line-mixing effects - Laboratory studies - Influence on spectroscopic data extraction - Influence on atmospheric retrieval Conclusions

33 Measured spectra (satellite, balloon, ground,…) - P, T profiles - Molecule mole fraction profiles - Cloud altitude, aerosol - etc. … Input spectroscopic data (Position, Intensity, spectral shape,…) Spectral shapes  Collisional (pressure) effects on the spectral shape Radiative transfer Forward model Introduction: Atmospheric retrievals

44 Pressure DopplerDoppler+CollisionCollisions GaussienVoigtLorentzian Comparaison between the Gaussian, the Lorentz and the Voigt profiles. Introduction: The Voigt profile

55 Bernath et al., GRL, 32, L15S01 (2005) ACE spectra at different tangent altitudes (1-0) R(0) 3 R(9) Introduction: isolated lines and closely spaced lines

66 Isolated lines

77 Measured and adjusted spectra using the Voigt profile. H 2 O/N 2, 3 band, 11 1,11  10 1, ,11  10 0,10 Limits of the Voigt profile

88 Limits of the Voigt profile It neglects: 1.The velocity changes induced by collisions. The detailed balance:  change from v to v’ v.  reduction of the Doppler broadening  Dicke narrowing effect 2.The speed-dependences of the collisional width  (v) and shift  (v) of the line. This also (in general) leads to a narrowed line

99 - For the velocity changes effect: The Galatry (soft collisions) and Rautian (hard collisions) line-shapes. Introducing a parameter VC (or  ) - the velocity changing collisions frequency. - For the speed dependences of the line-width and shift: The speed-dependent Voigt profile, assuming a simple quadratic dependence on the absolute speed or polynomial dependence on the relative speed Simple and widely used non-Voigt approaches

10 In situ absorption spectrum of tropospheric H 2 O recorded by balloonborne diode laser and its fits using the Voit profile and the Hard Collision profile Durry et al, JQSRT 94, 387,2005 Influence of non-Voigt effects on atmospheric retrievals

11 Barret et al, JQSRT 95, 499, 2005 HF profile retrieved from ground-based absorption FTIR measurements (Jungfraujoch station) in the (1-0) R(1) micro-window by using the Voigt profile and the Soft Collision model, compared with the HALOE profiles smoothed. Influence of non-Voigt effects on atmospheric retrievals

12 (Diode laser) Measured absorption of pure H 2 O (left) and H 2 O in air (right) and their differences with those adjusted using the VP, HC, and SDVP HC Influence of non-Voigt effects on line parameters determination

13 Ratios of the self-broadening coefficients and intensity obtained by the HC (o) and SDVP (  ) to those obtained by the VP, for 13 H 2 O lines in the near-infrared. Influence of non-Voigt effect on line parameters

14 VC rate in red (SC) Rohart et al, J Mol Spectrosc 251, 282, 2008 Dependence of VC on pressure Relaxation of the GHz line of O 3 in collision with O 2 at 240 K. Remaining problems with these non-Voigt approaches  Need to take into account both Dicke and speed dependence effects

15 2 limits Hard collision model Soft collision model : memory parameter  KS more realistic than these two extreme cases and more suitable for intermediate cases Velocity changing by the Keilson and Storer model

16 With the model we compute the line shape. Then we fit the calculated spectra (as the measured ones) by Voigt profiles and look at the residuals. Pure H 2 O, 296K, band:  measured,  calculated 6 06   4 14 Model-experiment comparison : pure H 2 O

17 H 2 O/N 2, 296K, 3,  ,  doublet H 2 O/air, 296K, , 5 15  4 14 line  measured,  calculated Model-experiment comparison: H 2 O/air

18 Model-experiment comparison: H 2 /N 2 Collisional width 296 K, Q(1)  m = 0.92,  0 = 0.41 Bonamy et al., Eur. Phys. J. D, K, Q(1)  m = 0.95,  0 = 0.50

19 Non-isolated transitions: Line-mixing effects

20 In some cases, for closely spaced lines, the Voigt profile fails when P increases. It predicts shapes that are too broad. Collisions induce transfers of populations between the levels of the two lines that lead to transfers of intensity between the lines. Line-mixing effects

21 Line-mixing effects: Absorption coefficient  k populations d k matrix element of radiation-matter coupling tensor  L 0 matrix of positions W relaxation operator. All effects of collisions. Independent of  within the impact approximation (not too far in the wings) W lk  0  Line coupling between |k> and |l> W lk =0  No line coupling (Lorentz )

22 For moderate line overlapping, a first order perturbation approach is possible. Then we only need to know one coupling parameter (Y, related to the W matrix elements) per line Line-mixing effects: Relaxation matrix Relations for W

23 Nadir looking instruments onboard satellites Greenhouse gases Observation SATellite (GOSAT, JAXA-NIES, in orbit) Orbiting Carbon Observatory (OCO 2, NASA) MicroCarb (CNES, under study), CarbonSat (ESA, under study) Spectral regions and aims - CO 2 from 1.6  m (weak) and 2.1  m (strong) bands - Air mass from O 2 A band (0.76  m) - CH 4 from 2 3 band (near 1.7  m) - aerosols from CO 2 and O 2 bands Detection/quantifying sinks and sources (1 ppm for x CO2, 0.3 %) → Extreme accuracy of spectra modelling. Huge constraints on the spectroscopic data and the prediction of pressure effects (collisions and spectral-shape) Line-mixing and remote sensing: Monitoring GreenHouse Gases from space

24 Sza 79.9°, Park Falls, region 2.1  m Wrong air-mass (and time) dependences CO 2 retrieved Sza 79.9°, Park Falls, region 1.6  m CO 2 : ground-based measurements

25 Sza 79.9°, Park Falls, 1.27  m band Sza 79.9°, Park Falls, A band Wrong time (and air-mass) dependences O 2 : Ground-based measurements

26 Spectra: air mass 5.7, Park Falls Wrong time and airmass dependences → Largely erroneous conclusions on sinks and sources CH 4 : Ground-based atmospheric solar absorption

27 No use CO 2 bands: Scaling model, self consistent model for all bands. Adjustment of model in 720 cm -1 Q branch. No use of present NIR bands CH 4 : Semi-classical model. Self consistent model for 3, 4 and 2 3 bands. Adjustment of model in 3 band at high pressure. No use of present 2 3 band No use O 2 A-band: Scaling model. Model developed from O 2 A band at elevated pressure. No use of low pressure spectra Collision induced absorption: From analysis of O 2 A band at elevated pressures Modeling of line-mixing effect

28 O 2 /N 2 A band 76.0 atm 47.6 atm 28.1 atm ___ measurement, ___ Line-mixing ___ Lorentz CO 2 /N 2 Line-mixing effects: Laboratory studies

29 Line-mixing effects: Influence on spectroscopic parameters retrieval CH 4 /N 2, 2 3, R(6), 296 K

observed-(LM+CIA) Residual wavenumber (cm ) observed-Voigt Transmission Influence on retrievals: O 2 ground-based measurements Spectra: Sza 79.9°, Park Falls O 2 A band region O 2 A band: Relative errors on surface pressures retrieved from atmospheric spectra

31  Fits of a ground based transmission spectra in the region of the band of CO 2 Significant errors (10%) on the CO 2 atmospheric amount. Sinks and sources !!!  Influence on retrievals: CO 2 ground-based measurements

32 Influence on retrievals: CH 4 ground-based measurements Methane amounts retrieved from atmospheric spectra

33 Jupiter ISO/SWS spectrum in the 4 band region of CH 4 Influence on planetary spectra

34 Summary Spectral shape has become a key issue for high precision soundings (OCO, ACE, …) When line is isolated: - The Voigt profile fails to model isolated line-shape -> need to take into account Dicke narrowing and speed dependence effects (velocity effects) - Influence of velocity effects on atmospheric retrieval is quite small (but few studies available) When lines are closely spaced (especially at high pressure): - line-mixing can be observed - Increasing evidences of influence of line-mixing for remote sensing - Need to take into account both line-mixing and velocity effects

35 -General Equations -Isolated Lines -Line-mixing -Far wings -Collision Induced processes -Consequences for applications -Remaining problems