1 Analysis of BBCRDS Spectra: Inferred Upper Limits for Water Dimer Absorption A.J.L. Shillings 1 , S.M. Ball 2 and R.L. Jones 1 1 University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge, CB1 2EW 2 University of Leicester, Department of Chemistry, Leicester, LE1 7RH
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1 Analysis of BBCRDS Spectra: Inferred Upper Limits for Water Dimer Absorption A.J.L. Shillings 1, S.M. Ball 2 and R.L. Jones 1 1 University of Cambridge,
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Analysis of BBCRDS Spectra: Inferred Upper Limits for Water Dimer
Absorption
A.J.L. Shillings1, S.M. Ball2 and R.L. Jones1
1University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge, CB1 2EW2 University of Leicester, Department of Chemistry, Leicester, LE1 7RH
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Outline
1. Quick overview of BBCRDS
2. BBCRDS Measurements
• Measurement rationale
• near-IR (750 nm) results
• Orange (612 nm) results
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Traditional CRDS
))/exp(( 0 ctttII ot
Xttc o
111
•CRDS very sensitive technique - very long path lengths are achievable
•BUT: Tricky to unambiguously identify a particular species form a single measurement (Other Absorbers?)
•SOLUTION: BBCRDS combined benefits of CRDS with multi species ability of DOAS (Differential Optical Absorption Spectroscopy)
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BB (Broadband) CRDS
30km
60km
90km
Laser Pulse
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Spectral Regions Investigated
Orange
612 nm 16340 cm-1
Near-IR
750 nm 13400 cm-1
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Rationale of Approach
• Record Spectra of high concentration water vapour in air at elevated temperatures.
•Calculate (explicitly treat non Beer-Lambert behaviour) and then subtract water monomer contribution to reveal ‘contiuum’ features below.
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near-IR Database Comparison
•Subtle (but potentially significant) differences between databases
•New linelist from UCL (J.Tennyson et al) seems to do best job but some monomer structure remains.
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Deriving Residuals from Measurements
•Fit includes calculated water monomer spectrum and a high order polynomial.
•Fitted Polynomial then added onto residual from full fit = everything apart from water monomer
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Deriving Residuals from Measurements
•Seems to suggest UCL_08 database includes a component of the ‘continuum’ absorption in this region
•Residual from Hitran_04 broadly consistent with MT_CKD continuum – but also clearly contains errors
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Investigation of Pfeilsticker (Science ‘03) Dimer Signal
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near –IR Theoretical Spectra
•Position, Shape and Width of dimer feature are uncertain.
•In the following analysis, we assume a Lorentzian shape
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Deriving Detection Limits
•Fit a Lorentzian function (variable centre and width) to the measured residual.
•Leads to an upper limit for water dimer absorption.
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Fitted Peak Absorption to Measurement Residual
Measured
High Keq
Low Keq
eqKOHthlinestrengTheory
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Plot above shown for Lorentzian centred at 13404 cm-1
Residual from measurement at 312K [H2O] = 1.41x1018 molecules/cm3
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High Keq
Low Keq
•Structure results from water monomer mis-fitting.
•Yellow (S&K 03), Red (L&K 99) and Green (S&K 07) dots show lower and upper predicted peak dimer absorbance from theory.
Peak absorbance fitted to
measurement residual
Fitted Peak Absorption to Measurement Residual (2)
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Retrieved Upper Linestrength
eqKOH
absorptionpeakthlinestreng
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Curtiss et al (79) Keq
Low Keq
High Keq
Plot shows data for
13404 cm-1
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Retrieved Upper Linestrength (2)
Surface calculated using Curtiss et al (79) Keq
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Effects of Water Monomer Database
Plot shows data for
13404 cm-1
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near-IR Conclusions
• UCL_08 database appears to give the best quality spectral fits in this spectral region.
• However, some structure due to poorly characterised water monomer absorption remains.
• Measurements suggest UCL_08 database can account for a (significant) proportion of the MT_CKD continuum in this region.
• Using this database, BBCRDS measurements give an upper limit for water dimer absorption very much at the lower end of the theoretical predictions.
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Orange Region Theoretical Spectra
Spectra cover region in between two water absorption bands – very weak monomer absorption.
610nm = 16393 cm-1
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Orange Database Comparison
•UCL_08 data again seems to be doing a better job, but discrepancies remain.
•Widely used Hitran (06) data appears to be very poor in this weak region between the main water bands.
Plot above shown for Lorentzian centred at 16236 cm-1
Residual from measurement at 361K [H2O] = 6.1x1018 molecules/cm3
High Keq
Low Keq
eqKOHthlinestrengTheory
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Measured
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Fitted Peak Absorption to Measurement Residual (2)
•Structure results from water monomer mis-fitting.
•Yellow (S&K 03), Red (L&K 99) dots show lower and upper predicted peak dimer absorbance from theory.
Peak absorbance fitted to
measurement residual
High Keq
Low Keq
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Retrieved Upper Linestrength
Curtiss et al (79) Keq
Low Keq
High Keq
eqKOH
absorptionpeakthlinestreng
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Plot shows data for
16236 cm-1
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Retrieved Upper Linestrength (2)
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Effects of Water Monomer Database
Plot shows data for
16236 cm-1
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Orange Conclusions
•UCL_08 database seems to do a better job than Hitran but discrepancies still remain.
•Despite structure introduced by water monomer database errors, BBCRDS measurements exclude the upper theoretical predictions for water dimer absorption.