CAVIAR water vapour laboratory FTS measurements Dr Robert McPheat CAVIAR Meeting Cosners’ House, 15/12/2009

CAVIAR Measurements As part of the CAVIAR project we are measuring the mid- and near-IR absorption spectrum of water in the MSF. 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.

LPAC & SPAC 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 –0 to 1 bar pressure SPAC –1.2 to 20 metres –Multi-pass reflective optics (White) –Temperature range 300 to 470K –0 to 5 bar pressure

LPAC Measurements Long path-length absorption cell (LPAC) –9m long stainless-steel inner vessel, 8 m mirror separation –Mirrors steerable from outside vacuum vessel for optimising alignment and changing pathlength –Fitted with Vaisala HMP234 humidity sensor at each end –2 fans in centre to ensure even mixing –Temperature measurements by 14 PRTs –3 MKS 690 (1, 10 & 1000 torr) & 625 (1000 torr) Baratrons

LPAC Measurements 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 / CaF 2 beamsplitter / Globar –2000 to cm -1 InSb detector / CaF 2 beamsplitter / Quartz tungsten halogen source

Example LPAC Spectra 17hPa H 2 O + air to 1000hPa

Example LPAC Spectra 17hPa H 2 O + air to 1000hPa

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 cm -1 –Long integration times (~ 12 hours) in mid-IR Up to 13 hPa pure water Room temperature

5 mm Cell 5 mm path length Electropolished stainless steel for low surface adsorption Spherical reservoir volume to increase volume to surface ratio All metal vacuum system Parylene coated KBr windows attached with low vapour pressure epoxy resin 486 measurements covering mid & near IR

5 mm Cell

Mid-IR Spectra Wavenumber /cm

Mid-IR Spectra Wavenumber /cm -1

Near-IR Spectra Wavenumber /cm

Near-IR Spectra Wavenumber /cm -1

SPAC redesign 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 470K –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

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

SPAC upgrade OpticsInnerCooling Jacket

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

Electrical feedthroughs for PRTs moved to bottom of the cell produce a much simplified vacuum / pressure system Electrical feedthroughs

Measurements intended from 300 to 470 K Up to 5000 hPa water vapour (at higher temperatures) Air broadened measurements to 5000 hPa 0.05 to 0.3 cm -1 resolution depending on pressure Argon backgrounds before & after at same pressure as sample for pure vapour spectra. Air background at same total pressure for foreign broadened spectra High temperature SPAC Measurements

Example Spectra at 160 ° C Background 3000 hPa Water vapour 1600Wavenumber / cm

Self Broadened Spectra 3000 hPa Water vapour at 160 °C, ~ 20 m, 0.2 cm Wavenumber / cm % Transmittance

600 mbar water vapour in 5 bar total pressure mixture with air at 160 ° C, ~ 20 m, 0.1 cm -1 resolution Air Broadened Spectra 2500Wavenumber / cm %

Conclusions NIR out of band continuum larger than predicted by CKD Further SPAC spectra required See talk by Igor Ptashnik