1. BBCRDS Measurements of Water Vapour: Inferred Upper Limits for Water Dimer Absorption in the 610 and 750 nm regions 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.Introduction Why BBCRDS is suited to this application Why we are interested in water dimers 2.BBCRDS Measurements Attempt to reproduce measurements of Pfeilsticker et al. Constraints on water dimer absorption in 750 nm region. Constraints on water dimer absorption in 610 nm region.

3. BBCRDS – In Situ DOAS BBCRDS combines the benefits of CRDS with multi species ability of DOAS (Differential Optical Absorption Spectroscopy). Can achieve very long effective path lengths in laboratory (many 10s km) under well defined conditions (T, p, [H 2 O]). Can make extremely sensitive absorption measurements of gaseous water vapour over a wide range of conditions and hence look for signs of water dimer absorption at (near) visible wavelengths. Why are we interested in Water Dimers? It has been suggested that water dimers could be responsible for (at least part of) the water vapour continuum absorption (and hence anomalous absorption problem). Relative effect of water dimers could be particularly large in the overtone spectral region as the weaker lines are less likely to be saturated – unlike in the fundamental regions.

4. Water Dimer Abundance Dimer formation is an equilibrium process: H 2 O + H 2 O (H 2 O) 2 [Dimer] = K eq (T) [monomer] 2 Experimental Theoretical Assuming RH of 85%

5. Pfeilsticker Absorption Feature K.Pfeilsticker, A.Lotter, C.Peters and H.Bösch, Science (2003), 300, 2078. ×2.8 H 2 O Pfeilsticker et al. absorption feature not present in BBCRDS laboratory spectra. ×2.8 2 (H 2 O) 2

6. Transitions used to (attempt to) detect water dimers Hydrogen bond in water dimer red-shifts the OH b stretch. OH b transitions predicted to occur away from strongest monomer absorption. – towards the edges of ro-vibrational bands. Useful for detection. Dimer cross-sections plotted assuming a Lorentzian profile with a HWHM of 25 cm -1. L&K 99 S&K 03 S et al. 07

7. Treatment of water monomer absorption UCL database contains many water monomer transitions that are absent from HITRAN 08.

8. 750 nm region monomer database differences Essential to use an accurate representation of water monomer absorption (see later). UCL_08 appears to be superior to (all versions of) HITRAN in each of the spectral regions investigated.

9. Inferring Upper Limit for Dimer Absorption HWHM = 2 cm -1 HWHM = 20 cm -1 HWHM = 100 cm -1 Residual from measurement at 312 K [H 2 O] = 1.41×10 18 molecules/cm 3 using UCL_08 monomer data. Sequentially fit Lorentzians centred at 13494 cm -1 (741.1 nm) (L&K 99) with HWHM 1,2,5,10…100 cm -1 to the measured residual.

10. Inferring Upper Limit for Dimer Absorption Largest possible dimer signal present in measured spectra only consistent with theoretical predictions (we assume that the calculated linestrength is correct) at large HWHMs and then only for the lowest K eq estimate. Inferred Maximum Dimer Signal (from residual)

11. Inferring Upper Limit for Dimer Absorption 13494 cm -1 (741.1 nm) L&K 99 13404 cm -1 (746 nm) S&K 03 13231 cm -1 (755.8 nm) S et al. 07 For each of the 3 theoretical predictions of the central position of the OH b dimer transition, these measurements show that the HWHM of the transition must be rather wider than adopted for dimer transitions in other spectral regions.

12. Effect of Spectral Databases (750 nm region) Inferred maximum dimer signal averaged over measurement interval (736 – 760 nm) UCL_08 data Expected peak absorption using linestrength of 2.8×10 -22 cm molecule -1 – the smallest estimate. Inferred maximum dimer signal averaged over measurement interval (736 – 760 nm) HITRAN 04 data Using HITRAN monomer data instead of UCL_08 data results in significantly larger inferred upper limit for water dimer absorption.

13. As in the 750 nm region, HITRAN, contains a poor representation of the water monomer absorption – UCL_08 is much better but discrepancies still remain. 610 nm region monomer database differences

14. Inferring Upper Limit for Dimer Absorption (610 nm) 16312 cm -1 (613 nm) S&K 03 16330 cm -1 (612.3 nm) L&K 99 These measurements place an upper limit for water dimer absorption that is significantly lower than the theoretical predictions. Residuals from measurement of 6.1 ×10 18 molecules/cm 3 at 361 K.

15. Effect of Spectral Databases (610 nm region) Inferred maximum dimer signal averaged over measurement interval (608 – 622 nm) UCL_08 data Expected peak absorption using linestrength of 4.0×10 -23 cm molecule -1 – the smallest estimate. Inferred maximum dimer signal averaged over measurement interval (608 – 622 nm) HITRAN 04 data Using HITRAN monomer data instead of UCL_08 data results in significantly larger inferred upper limit for water dimer absorption.

16. Summary The CAVIAR consortium for funding. It is essential to accurately treat the water monomer absorption to reveal any water dimer signals that may be present underneath; - HITRAN not good for weak absorption. - UCL_08 (largely from ab initio) generally much better. Using the UCL_08 spectral data….. In 750 nm region, the maximum dimer absorption inferred from BBCRDS measurements is only consistent with the smallest estimate of K eq and then only for transitions that have large HWHM. In 610 nm region, the maximum dimer absorption inferred from BBCRDS measurements is significantly lower than the theoretical estimates – Implying that the HWHM of any dimer transitions in this spectral region must be significantly larger than adopted for other transition in other spectral regions. Acknowledgements

17. Inferring Upper Limit for Dimer Absorption Range of theoretical linestrengths for this spectral region Average Inferred linestrength Smallest K eq Largest K eq