Development of a cavity ring-down spectrometer toward multi-species composition

This work presents the development of a cavity ring-down spectrometer (CRDS) designed for the detection of several molecules relevant for air pollution, including the second overtone of ro-vibration transitions from CO at 1.58 µm and NO at 1.79 µm. A unique feature of this CRDS is the use of custom mirrors with a reflectivity of about 99.99% from 1.52 to 1.80 µm, enabling efficient laser coupling into the cavity while ensuring a minimum detectable absorbance of 1.1 × 10-10 cm-1 within an integration time of about 1.2 s. In this work, the successful implementation of the current CRDS is demonstrated in two different wavelength regions. At 1.79 µm, the transitions R17.5 and R4.5 of the second overtone of NO are detected. At 1.58 µm, carbon dioxide and water vapor from untreated ambient air are measured, serving as an example to investigate the suitability of a post-processing procedure for the determination of the molar fraction in a multi-species composition. This post-processing procedure has the benefit of being calibration-free and SI-traceable. Additionally, CRDS measurements of gas mixtures containing CO and CO2 are also shown. In the future, the advantages of the developed cavity ring-down spectrometer will be exploited in order to perform fundamental studies on the transport processes of heterogeneous catalysis by locally resolving the gas phase near a working catalytic surface. The possibility to cover a broad wavelength region with this CRDS opens up the opportunity to investigate different catalytic reactions, including CO oxidation and NO reduction.