CAVIAR water vapour laboratory FTS measurements

Dr Robert McPheatCAVIAR 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 / CaF2 beamsplitter / Globar– 2000 to 11000 cm-1

• InSb detector / CaF2 beamsplitter / Quartz tungsten halogen source

Example LPAC Spectra

17hPa H2O + air to 1000hPa

Example LPAC Spectra

17hPa H2O + 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

0.002 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-11250 2000

Mid-IR Spectra

Wavenumber /cm-1

Near-IR Spectra

Wavenumber /cm-11600 6000

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

Optics Inner Cooling 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

Background3000 hPa Water vapour

1600 Wavenumber / cm-1 12000

Self Broadened Spectra

3000 hPa Water vapour at 160 °C, ~ 20 m, 0.2 cm-1

2000 Wavenumber / cm-1 11000

100 %

Tra

nsm

ittan

ce

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

2500 Wavenumber / cm-1 10000

100 %

Conclusions

• NIR out of band continuum larger than predicted by CKD

• Further SPAC spectra required• See talk by Igor Ptashnik