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

2 Outline 1.Quick overview of BBCRDS 2.BBCRDS Measurements Measurement rationale near-IR (750 nm) results Orange (612 nm) results

3 Traditional CRDS 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)

4 BB (Broadband) CRDS 30km 60km 90km Laser Pulse

5 Spectral Regions Investigated Orange 612 nm cm -1 Near-IR 750 nm cm -1

6 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.

7 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.

8 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

9 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

10 Investigation of Pfeilsticker (Science 03) Dimer Signal

11 near – IR Theoretical Spectra Position, Shape and Width of dimer feature are uncertain. In the following analysis, we assume a Lorentzian shape

12 Deriving Detection Limits Fit a Lorentzian function (variable centre and width) to the measured residual. Leads to an upper limit for water dimer absorption.

13 Fitted Peak Absorption to Measurement Residual Measured High K eq Low K eq Plot above shown for Lorentzian centred at cm -1 Residual from measurement at 312K [H 2 O] = 1.41x10 18 molecules/cm 3

14 High K eq Low K eq 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)

15 Retrieved Upper Linestrength Curtiss et al (79) K eq Low K eq High K eq Plot shows data for cm -1

16 Retrieved Upper Linestrength (2) Surface calculated using Curtiss et al (79) K eq

17 Effects of Water Monomer Database Plot shows data for cm -1

18 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.

19 Orange Region Theoretical Spectra Spectra cover region in between two water absorption bands – very weak monomer absorption. 610nm = cm -1

20 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. Neither database gives perfect quantitative agreement with hygrometer measurement (6.1x10 18 molecules/cm 3 ) UCL_08 = 6.6x10 18 molecules/cm 3 Hitran 06 = 8.2x10 18 molecules/cm 3

21 Orange Residual Spectra

22 Fitted Peak Absorption to Measurement Residual Plot above shown for Lorentzian centred at cm -1 Residual from measurement at 361K [H 2 O] = 6.1x10 18 molecules/cm 3 High K eq Low K eq Measured

23 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 K eq Low K eq

24 Retrieved Upper Linestrength Curtiss et al (79) K eq Low K eq High K eq Plot shows data for cm -1

25 Retrieved Upper Linestrength (2)

26 Effects of Water Monomer Database Plot shows data for cm -1

27 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. Water monomer databases contain significant discrepancies. Despite monomer structure in residuals, upper limit for water dimer absorption excludes the higher theoretical predictions. Hopefully these measurements (with their assumptions) will provide a useful constraint for further theoretical work. Summary