Line strength and collisional broadening coefficients of H₂O at 2.7 μm for natural gas quality assurance applications

Abstract

We employed tunable diode laser absorption spectroscopy to measure the line strength, the methane (CH₄), ethane (C₂H₆) and the propane (C₃H₈) broadening coefficients for the 523–422 H₂O transition at 3619.61 cm⁻¹. Water amount fractions generated by a stable and accurate humidity transfer standard, traceable to the SI units via the German national humidity standard, were used to calibrate the spectroscopic line strength measurements. We focus on the traceability of the measured line data to the SI and on uncertainty assessments following the guidelines of the Guide to the Expression of Uncertainty in Measurement. We determined the line strength to be (8.42 ± 0.07)×10⁻²⁰ cm⁻¹/(cm⁻² molecule) corresponding to a relative uncertainty of ±0.8%. To the best of our knowledge, we report the first methane, ethane and propane broadening coefficients of (8.037 ± 0.056)×10⁻⁵ cm⁻¹/hPa, (9.077 ± 0.064)×10⁻⁵ cm⁻¹/hPa and (10.469 ± 0.073)×10⁻⁵ cm⁻¹/hPa for the 523–422 H₂O transition at 3619.61 cm⁻¹, respectively. The relative combined uncertainties of the stated CH₄, C₂H₆ and C₃H₈ broadening coefficients are in the ±0.7% range.

Keywords:

• spectroscopy
• water
• line data
• metrology
• uncertainties

Acknowledgements

The authors acknowledge financial support and collaboration in the European Metrology Research Programme (EMRP) projects on ‘Characterization of Energy Gases’ (ENG01-GAS), on ‘Metrology for Chemical Pollutants in Air’ (ENV01-MACPoll) and on ‘Spectral reference data for atmospheric monitoring’ (ENV06-EUMETRISPEC). The authors acknowledge the support of Norbert Böse and Sonja Platzler (all PTB), Steven Wagner (TU Darmstadt) and 3.22 group members.

Additional information

Funding

The work was supported by EMRP which is jointly funded by the EMRP participating countries within EURAMET and the European Union.