Water intensities ab initio versus experiment Manfred Birk
Water intensities: ab initio versus experiment Manfred Birk, Georg Wagner Remote Sensing Technology Institute (IMF) Deutsches Zentrum für Luft- und Raumfahrt (DLR) Lorenzo Lodi, Jonathan Tennyson Department of Physics and Astronomy University College London
Introduction Routes to line intensities in spectroscopic databases • Minimal approach: Line intensities are derived from laboratory spectroscopy measurements Disadvantage: not all lines included, precision limited by noise • Effective Hamiltonian approach: Quantum mechanical data reduction of experimental line intensities Only useful when experimental data are reproduced within their precision Advantage: Intensities for lines not in the experiment can be calculated when avoiding large extrapolation towards higher quantum numbers Further advantages: Noise of experimental line intensities is reduced, experimental line intensities are checked for consistency • Pure ab initio approach: Semi-empirical PES, purely ab-initio DMS Advantage: All transitions covered Disadvantage: Experimental validation mandatory
How accurate are ab initio calculations? • Intercomparison between ab initio and experimental line intensities required But: • Experimental line intensities from different labs often differ outside error margins (see below) Thus: • High quality experimental data with known uncertainty needed • Best case: At least two experimental data sets from independent labs agree within error margins
Ab initio calculation of H 2 O line intensities • Lorenzo Lodi, Jonathan Tennyson, and Oleg L. Polyansky, “A global, high accuracy ab initio dipole moment surface for the electronic ground state of the water molecule”, J. Chem. Phys. 135, 034113 (2011). • Quality estimate: 2 different PES and DMS 4 combinations fractional ab initio uncertainty = (largest value/smallest value -1) • The authors state line intensity errors of 1% for most lines validated by average agreement with HITRAN 2008 (ab initio/HITRAN=1. 01± 4. 5%, S>1 e 22, 11% of HITRAN lines with J<13) and agreement of 15 lines measured with CRDS by Lisak and Hodges (NIST) (ab initio/NIST=1. 004± 0. 6%) • But: Systematic line intensity errors in HITRAN 2008 S>1 e-22 are very likely Example: Update 2004 2008 2 line intensities changed up to 6% (see below). Other regions? ? ? • Thus, agreement of 1% between ab initio and HITRAN is not very conclusive This work: Intercomparison of ab initio and high quality experimental water line intensities
DLR measurements – strategy • Goal: Accurate data with defined error margins • Redundancy is important since hidden systematic errors may depend on line width and optical depths. Chi tests and investigation of residuals of redundant data may help to quantify/identify error sources • Line intensities retrieved from many measurements with different optical depths (<4) for redundancy • Combination of pure water and air-broadened measurements used for increasing redundancy (width and optical depth independently selectable) • Influence of instrumental lineshape function is minimized by choosing high resolution • Mostly ambient temperature measurements used
DLR measurements – experimental set-up Water/air mixtures Pure water
DLR measurements – 1 µm region • • Multireflection cell at 85 m High signal-to-noise by narrow band pass (10000 -11000 cm-1) Double-sided interferograms Line intensity analysis included ambient and non-ambient temperature measurements • Line fitting of individual spectra on micro window basis applying speed-dependent Voigt profile yielding line intensities for each measurement and transition
DLR measurements – 1 µm region • Line intensities of up to 11 measurements averaged • Reference: 1 mb pure water measurement at ambient temperature – 5 mb pure water measurement not used as reference due to insufficient spectral resolution • Individual measurements (except reference) scaled for 0% mean measurement omc in averaging • Quality check: Temperature/scaling factor (Sref x scaling factor = Smeas) fit from line intensities of individual measurements using averaged line intensities as reference • Air-broadened measurements: 7 scaling factors less than 1% off 1, 2 scaling factors between 1 and 2%
Linestrength intercomparison in 1 µm region • • NIST: cavity ringdown by Daniel Lisak and Joseph T. Hodges HIT: HITRAN 2008, mainly experimental data by Robert A. Toth Excellent agreement DLR-NIST, mostly <1% HITRAN 2008 shows bias and large scatter
Ab initio vs. experiment 1 µm • Only transitions shown with experimental precision < 1% Vib transition # of lines Mean diff. /% Scatter about mean/ meas Mean( ab initio)/% 121 000 78 +4. 0 1. 9 7. 2 201 000 189 -0. 3 1. 6 2. 1 300 000 76 +4. 3 2. 0 102 000 17 -8. 9 1. 4 7. 1 003 000 12 -0. 7 1. 1 1. 2
Ab initio vs. experiment 1 µm • Bias for entire vibrational bands, cannot be related to experimental error • Mean differences of bands up to 9% • Larger scatter for 3 0 0 0 band • Isolated outliers with up to 30% difference (see below) • Average ab initio uncertainties mostly conservative (exception 3 0 0 0)
Ab initio vs. experiment 1 µm Largest difference for 2 0 1 0 0 0, 12 110, 11 Sab initio - Sexp= -26. 5%, Smean= 0. 6%, =1. 5, Sab initio= 1. 4% PH 2 O/mb Ptot/mb T/K S/% % peak abs (S-Smean)/% (S-Smean)/ S In av. 4. 0 199. 6 296. 0 15. 7 0. 64 -0. 59 -0. 93 1 4. 0 496. 8 296. 5 13. 0 2. 27 -0. 06 -0. 03 1 16. 1 998. 2 315. 9 36. 3 10. 38 2. 11 0. 20 1 4. 0 998. 0 316. 4 10. 7 2. 57 5. 42 2. 00 1 3. 9 997. 7 277. 6 6. 1 2. 84 5. 81 1. 93 1 2. 0 498. 9 317. 3 8. 5 1. 70 -8. 63 -5. 56 0 2. 0 501. 8 277. 6 4. 9 3. 64 10. 47 2. 60 1 0. 8 199. 3 275. 9 2. 6 4. 38 -3. 48 -0. 82 1 1. 0 295. 2 4. 7 2. 46 -2. 23 -0. 93 1
Ab initio vs. experiment 1 µm Subband J’’+10, J’’+1 J’’ 0, J’’ Ab initio: Resonance at J’’=6, experiment: resonance at J’’=11 Apparently, energy level of resonating states not correctly predicted from PES J‘‘ ( Sab initio)/% (Sab initio – Sexp)/% 0 4. 0 0. 0 2 1. 8 0. 3 4 1. 7 0. 1 6 5630. 3 7 8. 7 0. 2 8 1. 6 -0. 6 9 1. 5 -0. 8 11 1. 4 -26. 5
H 216 O linestrength intercomparison in 2 region • HIT 04: HITRAN 2004, mainly experimental data by Robert A. Toth • Lodi: ab initio calculations • DLR: 9 pure water, 16 airbroadened measurements, ambient temperature, Voigt profile analysis • HITRAN 2004 – DLR differences up to 6% for strong and weak lines • Ab initio – DLR agreement < 2% for S > 1 e-23 • 010 000, 020 010 lg(Smin) -24. 5 -24. 0 -23. 5 -23. 0 -22. 5 -22. 0 -21. 5 -21. 0 -20. 5 -20. 0 -19. 5 lg(Smax) -24. 0 -23. 5 -23. 0 -22. 5 -22. 0 -21. 5 -21. 0 -20. 5 -20. 0 -19. 5 -19. 0 Ab initio – DLR % %unc 3. 66 0. 80 4. 23 0. 23 2. 12 0. 10 1. 19 0. 04 0. 76 0. 02 0. 74 0. 02 0. 85 0. 02 1. 13 0. 02 1. 14 0. 02 1. 62 0. 02 1. 74 0. 01 ndata 23 77 94 105 109 72 50 52 51 51 53 HITRAN 2004 - DLR % %unc ndata 5. 90 0. 79 24 5. 46 0. 24 75 3. 56 0. 10 95 1. 87 0. 04 106 0. 91 0. 02 108 0. 27 0. 02 73 -1. 52 0. 01 53 -2. 63 0. 01 50 -2. 73 0. 02 50 -3. 80 0. 01 51 -5. 79 0. 01 55
H 218 O linestrength intercomparison in 2 region % Lodi-DLR • HIT 04: HITRAN 2004, mainly experimental data by Robert A. Toth • Lodi: ab initio calculations lg(Smin) -24. 5 -24. 0 -23. 5 -23. 0 -22. 5 -22. 0 -21. 5 lg(Smax) -24. 0 -23. 5 -23. 0 -22. 5 -22. 0 -21. 5 -21. 0 Ab initio - DLR % %unc 2. 84 1. 09 3. 82 0. 32 1. 00 0. 13 0. 28 0. 06 0. 16 0. 03 0. 27 0. 02 0. 63 0. 03 ndata 12 44 53 50 53 47 17 HITRAN 2004 - DLR % %unc ndata 4. 04 1. 14 11 3. 27 0. 32 45 -1. 20 0. 13 53 -3. 30 0. 06 53 -4. 75 0. 03 55 -5. 11 0. 02 42 -5. 43 0. 03 17
Brand new quick-look 3 results obtained by our Ph. D student Joep Loos • Only single pure water measurement analyzed • Only 21 transitions, line intensity >1 e-19 • Ab initio – experiment: mean -0. 96%, scatter 0. 25% • HITRAN 2008 – experiment: mean -2. 44%, scatter 0. 39%
Conclusion • Good agreement of ab initio and DLR experimental line intensities: • H 216 O, 2, S>1 e-23: <2% • H 218 O, 2, S>1 e-23: <2% • H 216 O, 3, S>1 e-19: <1% (preliminary) • H 216 O, 1 µm region, S>1 e-22: <2% • Agreement of ab initio and DLR data much better than with older HITRAN versions • Good agreement of ab initio and DLR data indicates reliability of both data sources • But there exist overtones and combination bands with biases between ab initio and experiment up to 9%. The origin for the differences can be unambiguously attributed to the ab initio calculation • Furthermore, singular large differences up to 30% (resonances) and vibrational transition specific scatter related to the ab initio calculations exist • Ab initio uncertainties were found to be helpful in assessing data quality. In cases of resonances they may be misleading • Further intercomparison between high quality laboratory measurements and ab initio calculations are required
- Slides: 17