Absolute validation of a diode laser hygrometer via intercomparison with the German national primary water vapor standard

Direct tunable diode laser absorption spectroscopy (dTDLAS) is a powerful diagnostic technique for absolute and accurate gas analysis with highest chemical specificity. Due to its first principles approach, dTDLAS is often claimed to be "calibration-free", but this and the absolute accuracy has not been rigorously validated with respect to a high-accuracy reference. This work describes the first rigorous, side-by-side comparison of a dTDLAS hygrometer-called SEALDH-with a highly accurate, internationally validated, primary reference humidity generator (PHG), which also serves as the German national H2O-standard. This PHG provides a humidified air stream with dew points between -30 A degrees C and +60 A degrees C with an uncertainty of 0.035 K (2 sigma) (equivalent relative H2O mixing ratio uncertainty: 0.4 %). Without any previous calibration, SEALDH was found to accurately reproduce the PHG reference values over the full range from 600 to 20,000 ppmv investigated in the 1-week lab study. Over this range, the SEALDH-PHG relative deviation was in average -1.45 %, the worst case being -2.5 % at 1,000 ppmv, the best -0.2 % at 600 ppmv. As SEALDH's relative uncertainty was metrologically determined to be 4.3 % (k = 2), these deviations are for all concentration steps in full compliance with the PHG reference. Systematic contributions to the relative deviation could be correlated with line shape deviations between the measured line profile and the fitted Voigt line shape. Using this information, SEALDHs absolute accuracy can be improved further to down to an average relative deviation to the PHG of +0.21 %.