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.