The development of FAGE to measure partially-speciated peroxy radicals

The Leeds ground-based and airborne FAGE instruments have been widely deployed to measure OH, HO₂ and, more recently, IO radicals. Work has commenced to further develop the FAGE technique to provide partially-speciated measurements of tropospheric RO₂ radicals.

The Leeds FAGE instruments employ laser-induced excitation and fluorescence collection at 308 nm. A sample of ambient air is drawn through a nozzle, forming a supersonic molecular beam expansion, into a region of low pressure. Following laser-induced excitation of OH, the fluorescence passes through a series of optics and is collected using a photomultiplier tube (PMT) before amplification and photon counting.

HO₂ radical measurements are performed in the Leeds ground-based and aircraft FAGE instruments via the addition of NO to the detection cell, resulting in the conversion of HO₂ to OH. A Teflon-lined conversion cell held at ~ 50 Torr and into which reagent gases can be added will be fitted to the detection cell. The addition of NO to the conversion cell, in the presence of O₂, will allow the conversion of RO₂ to HO₂. The HO₂ radicals produced in this manner will be converted to OH in the detection cell, via the addition of NO, and excited and detected using LIF at 308 nm.

Using the proposed the methodology it will be possible to provide sequential measurements of HO₂ and RO₂, where R ≠ H, or, when deployed in conjunction with the ground-based or aircraft instruments, simultaneous measurements of OH, HO₂ and RO₂, where R ≠ H.

The instrument development has been complemented by chemical modelling employing the Master Chemical Mechanism (MCM version 3.1) to predict the optimum conditions for the conversion chemistry.