Progress in water vapour spectroscopy at RAL

1. Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Annual Meeting Imperial College, 16/12/2008

2. Measurements so far in the mid- and near-IR. These measurements are intended to target the weak water vapour continuum. Long path lengths and / or high vapour pressures. Measurements carried out at moderate (0.01cm-1) & high resolution (0.002cm-1) using the Bruker IFS125 HR. Initial measurements revealed a need for better line data too. MSF Measurements

3. Long path lengths and / or high vapour pressures. Use Long pathlength absorption cell (LPAC) / Short pathlength absorption cell (SPAC). LPAC 32 to 640 metres in IR, over 1 km in visible Multi-pass reflective optics (White) Room temperature to 190K 0 to 1 bar pressure SPAC 1.2 to 20 metres Multi-pass reflective optics (White) Existing temperature range 200 to 350K 0 to 5 bar pressure LPAC & SPAC

4. Measurement conditions Room temperature, ~ 290 K Water pressures up to 0.75 SVP (< 20 hPa) Resolution up to 0.002cm-1 but mainly at 0.01cm-1 Water/air mixtures at total pressures of 150, 500 & 1000 hPa Pathlengths of 32, 128 & 512m Made measurements in 3 spectral ranges covering mid/near-IR 650 to 2000 cm-1 MCT detector / KBr beamsplitter / Globar 1800 to 5500 cm-1 InSb detector / CaF2 beamsplitter / Globar 2000 to 11000 cm-1 InSb detector / CaF2 beamsplitter / Quartz tungsten halogen source LPAC Measurements

5. Example LPAC Spectra 17hPa H2O + air to 1000hPa

6. Water vapour partial pressure limited by vapour pressure at room temperature Increase temperature to observe weak lines & continuum Desired upper temperature for water vapour measurements 450K All parts of the cell & filling lines need to be at same temperature to avoid condensation in cold spots. Need 200ºC, 5 bar pressure gauge Other measurements using SPAC require lower temperatures therefore SPAC redesigned for -200 to 200ºC temperature range Other requirements 5 bar maximum pressure Mirrors adjustable from outside Accurate temperature & humidity measurement Corrosion resistant SPAC redesign

7. Insulating Vacuum Helium Electrical heaters Liquid Nitrogen Sample Gas & White Optics

8. SPAC upgrade Optics Inner Cooling Jacket

9. High temperature pressure gauge & filling line Temperature & pressure logging Temperature control Outer vacuum jacket Detector box Transfer optics vacuum tank

10. SPAC upgrade modifications • Additional base heaters added • Problems with thermal expansion • Can be used for hot measurements without cooling jacket but modifications required

11. SPAC upgrade problems • Heating capacity limited by rating of spectrometer room ring main. • Use several extension leads from elsewhere • Original window seals (Indium) melted. • Initially replaced with gold wire which failed on temperature cycling. • PTFE seals have been installed successfully. • Long delays getting a working pressure gauge suitable for 5 bar & 200 °C • Ready to begin measurements beginning of new year

12. Water line parameter measurements • Although CAVIAR is targeted at improving the water vapour continuum, errors in line parameters contribute to the measured continuum • First measurements with upgraded Bruker IFS120 HR • Upgraded electronics / moving mirror to same specification as IFS125 • High resolution measurements 0.002 cm-1 • Long integration times (~ 12 hours) in mid-IR

13. Water line parameter measurements • 5 cm cell to avoid saturation of strong lines • Required sample pressures <0.5 hPa • Absorption of water on cell walls causes difficulty in maintaining stable sample pressures • Large variation of sample pressure with room temperature • Filling line all stainless steel and as short as possible • Measurements completed 1000 to 2000 cm-1, 2000 cm-1 to 9000 cm-1 next.

14. Example Spectra Transmission Wavenumber /cm-1