Temperature- and Pressure-Dependent Calibration Transfer Bias of WMS, FWMS, and DAS Methane Retrievals near 1654 nm

Temperature and pressure changes can introduce substantial bias when a tunable diode laser absorption spectroscopy calibration is used away from its reference condition. Here, scanned wavelength modulation spectroscopy (WMS), fixed-wavelength MS (FWMS), and integrated area direct absorption spectroscopy (DAS) are compared for the CH4 R(3) manifold near 1654 nm. To isolate the behavior of the retrieval methods, all three simulations use the same HITRAN2020 line-by-line model, optical depth, laser model, target geometry, and calibration state. The analysis covers temperature variation at constant pressure, pressure variation at constant temperature, and coupled temperature–pressure variation in a closed volume. With temperature held constant, the FWMS bias varies from −5.2% to −0.8% over 0.7–1.1 atm. At constant pressure, it changes from +23.4% to −9.8% between −20 and +40 °C. DAS gives the smallest endpoint bias in the closed-volume case, ranging from +19.1% to −8.0%. Across all 35 operating points, the RMSE is 10.5% for FWMS, 13.4% for WMS, and 14.9% for DAS. The pooled ranking is unchanged for modulation indices between 1.6 and 3.0 and for DAS integration windows between ±10 and ±100 GHz.