Direct absorption spectroscopy baseline fitting for blended absorption features

A method for determining the gas concentration from blended absorption features using direct absorption spectroscopy is developed. Blended spectra occur when multiple absorption lines are in close proximity in a laser scan and/or when pressures begin to increase. The blended-feature baseline fitting method estimates the baseline by coupling measured data with simulated fractional transmission at the peaks between absorption features. The method is first validated with synthetically created data. The method is then demonstrated using the absorption lines of CO₂(P41e,P42f,P34e,P43e) and H₂O in the ν₁+ν₃ combination band (3683.5 cm^-1 to 3686.5 cm^-1) in a heated static test cell. Results from the fitting method and wavelength modulation spectroscopy measurements agree within 1%. The method is finally applied to measurements of flue gas emissions from a practical combustion system (wood boiler) and compared to measurements from a hand-held gas analyzer. The mean percent difference of the blended-feature baseline fitting and the wavelength modulation spectroscopy is 2.8% and 5.8% for X_CO2 and X_H2O, respectively, indicating that this method of direct absorption data analysis can be used for spectra with moderate feature blending, which is useful for scan ranges with multiple closely spaced absorption lines and moderate pressures.