Peroxy radical detection for airborne atmospheric measurements using absorption spectroscopy of NO₂

Abstract. Development of an airborne instrument for the determination of peroxy radicals (PeRCEAS – peroxy radical chemical enhancement and absorption spectroscopy) is reported. Ambient peroxy radicals (HO₂ and RO₂, R being an organic chain) are converted to NO₂ in a reactor using a chain reaction involving NO and CO. Provided that the amplification factor, called effective chain length (eCL), is known, the concentration of NO₂ can be used as a proxy for the peroxy radical concentration in the sampled air. The eCL depends on radical surface losses and must thus be determined experimentally for each individual setup. NO₂ is detected by continuous-wave cavity ring-down spectroscopy (cw-CRDS) using an extended cavity diode laser (ECDL) at 408.9 nm. Optical feedback from a V-shaped resonator maximizes transmission and allows for a simple detector setup. CRDS directly yields absorption coefficients, thus providing NO₂ concentrations without additional calibration. The optimum 1σ detection limit is 0.3 ppbv at an averaging time of 40 s and an inlet pressure of 300 hPa. Effective chain lengths were determined for HO₂ and CH₃O₂ at different inlet pressures. The 1σ detection limit at an inlet pressure of 300 hPa for HO₂ is 3 pptv for an averaging time of 120 s.