| 28 Dec 2025
A real time reference system for wavelength locking in laser induced fluorescence with gas expansion (LIF-FAGE) measurement of atmospheric hydroxyl (OH) radicals
Abstract. The hydroxyl radical (OH) plays a central role in atmospheric chemistry, however, its accurate measurement by laser induced fluorescence with gas expansion (LIF-FAGE) is unavoidably compromised by wavelength drift of the excitation laser. To overcome this limitation, a real time reference system for active wavelength locking has been developed and systematically characterized in this work. Stable, and high concentration OH radicals were generated through thermocatalytic decomposition of ambient air on a heated filament within a low pressure cell. The excitation source was a 308 nm laser produced by frequency doubling the output of a DCM-ethanol dye laser pumped by an Nd:YAG laser. The induced fluorescence was monitored in real time using a non-gated photomultiplier tube (PMT). The wavelength locking program, implemented with a closed loop feedback mechanism, dynamically adjusted the laser wavelength to the optimal OH excitation line. Through comprehensive characterization of the key parameters, including laser power, filament operating conditions (current, voltage), and cell environment (pressure/inlet flow rate, inlet gas relative humidity), an optimal operational window of the reference system has been identified. A 12-hour continuous measurement demonstrated high system stability in OH generation and detection, the observed fluorescence intensity showed a low drift rate of 0.2% per hour during the first nine hours. The good robustness of the reference system, and its integrated wavelength locking program, enable long term and accurate ambient OH radical quantification in LIF-FAGE measurements.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
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The hydroxyl (OH) radical is the pivotal oxidant in the atmosphere, its accurate measurement is essential for understanding atmospheric oxidation capacity, and thus air quality, and climate change. However, the accuracy of OH measurements using the laser-induced fluorescence (LIF) technique is unavoidably compromised by wavelength drift of the excitation laser. Therefore, a reference system capable of monitoring the laser output wavelength and actively locking it to the optimal OH excitation line is crucial for long-term, reliable measurements. In this manuscript, Chen et al. develop a compact real-time reference system for wavelength locking in LIF-FAGE measurements. Through comprehensive characterization of key parameters, an optimal operational window for stable and high-concentration OH generation and detection is identified. The system's high stability is convincingly demonstrated by continuous measurements over 12 hours, with exceptionally low drift (0.2% per hour) during the first 9 hours. The detailed description of this system will be valuable for other researchers aiming to implement wavelength-locking techniques to improve the stability of their measurement systems. Overall, I recommend its publication after considering the following minor comments: