A real time reference system for wavelength locking in laser induced fluorescence with gas expansion (LIF-FAGE) measurement of atmospheric hydroxyl (OH) radicals

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 thermolysis 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 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 h continuous measurement demonstrated high system stability in OH generation and detection, the observed fluorescence intensity showed a low drift rate of 0.2 % h−1 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.