Ultra-sensitive measurement of peroxy radicals by chemical amplification broadband cavity-enhanced spectroscopy

Abstract

The PERCA (PEroxy Radical Chemical Amplification) technique, which is based on the catalytic conversion of ambient peroxy radicals (HO₂ and RO₂, where R stands for any organic chain) to a larger amount of nitrogen dioxide (NO₂) amplified by chain reactions by adding high concentrations of NO and CO in the flow reactor, has been widely used for total peroxy radical RO₂* (RO₂* = HO₂ + ΣRO₂) measurements. High-sensitivity and accurate measurement of the NO₂ concentration plays a key role in accurate measurement of the RO₂* concentration. In this paper, we report on the development of a dual-channel chemical amplification instrument, which combined the PERCA method with the incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS), for peroxy radical measurements. The IBBCEAS method is capable of simultaneously measuring multiple species with high spectral identification, and can directly measure NO₂ concentrations with high sensitivity and high accuracy and without interference from other absorbers. The detection sensitivity of the developed PERCA–IBBCEAS instrument for HO₂ radicals was estimated to be about 0.9 pptv (1σ, 60 s) at a relative humidity (RH) of 10%. Considering the error sources of NO₂ detection, CL determination, and the radical partitioning in the air sample, the total uncertainty of RO₂* measurements was about 16–20%.