Open Path Spectroscopic Detection of Hydroxyl Radical: A Comparison between Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS) and BBCEAS Coupled with a Fabry–Pérot Interferometer (BBCEAS-FP)

Hydroxyl radical (OH) is the primary atmospheric oxidant, but it is challenging to measure due to its short half-life under atmospheric conditions (approximately 1 s) and low ambient concentrations (approximately 10⁶ molecules/cm³). While several measurement techniques exist, including laser-induced fluorescence with fluorescence assay by gas expansion (LIF-FAGE), differential optical absorption spectroscopy, and chemical ionization mass spectrometry (CIMS), often only LIF-FAGE and CIMS are considered available for field measurements. This study compares the performance of two instruments: a broadband cavity-enhanced absorption spectroscopy (BBCEAS) coupled with a CCD and a BBCEAS coupled with a Fabry-Pérot interferometer (BBCEAS-FP). The semiportable BBCEAS instrument benefits from spectroscopic specificity, avoiding interference from other species, and features inherent calibration, eliminating calibration errors. The effects of aerosols and turbulence on performance were also evaluated. A low-loss optic was used to measure the optical cavity reflectivity in an open-path configuration, and a butane flame served as the OH source during testing. The BBCEAS instrument achieved an extrapolated 1-h detection limit (1σ) of 1.5 × 10⁷ molecules/cm³ for ambient-temperature OH. Applying a Gauss-Hermite filter reduced noise in the extinction spectrum by 2.66 times, lowering the extrapolated detection limit to 4.6 × 10⁶ molecules/cm³. The BBCEAS-FP, a more cost-effective and portable instrument, demonstrated a comparable extrapolated 1-h (1σ) detection limit of 1.5 × 10⁷ molecules/cm³.