Peroxy radical detection for airborne atmospheric measurements using absorption spectroscopy of NO 2

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 (HO 2 and RO 2 , R being an organic chain) are converted to NO 2 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 NO 2 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. NO 2 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 NO 2 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 HO 2 and CH 3 O 2 at different inlet pressures. The 1σ detection limit at an inlet pressure of 300 hPa for HO 2 is 3 pptv for an averaging time of 120 s.