High-sensitive cavity ring-down spectroscopy for hydroperoxyl radical measurement based on tunable narrow-linewidth laser using linear cavity optical feedback

The hydroperoxyl radical (HO2) is an important oxidant, playing a crucial role in atmospheric chemistry processes such as free radical cycling and ozone formation. However, the direct measurements of HO2 using laser absorption techniques present significant challenges due to its low atmospheric concentration, high reactivity, and short lifetime. In this paper, a high-sensitive narrow linewidth cavity ring-down spectroscopy (CRDS) system for HO2 detection was developed. By employing dual locking of feedback phase and laser current to the resonant cavity, the probe laser linewidth was successfully narrowed from 2.3 MHz to 12.3 kHz and the frequency noise was reduced by 4 to 5 orders of magnitude. Through cavity length adjustments, a continuous laser frequency tuning range of 4.5 GHz was achieved. This narrow linewidth laser significantly heightened trigger efficiency of the CRDS system. The Allan deviation analysis at a fixed laser frequency demonstrated that an improved sensitivity of 3.24×10-11 cm-1 could be obtained with an averaging time of 0.48 s, which corresponded to an HO2 detection limit of 1.29×108 molecule/cm3. The developed system featured a simple design and provided a valuable technical reference for future research on direct HO2 measurements.