High Altitude OH Experiment (HO_x)

Measurement Description: OH is detected by direct laser-induced fluorescence in the (0 1) band of the 2 2 electronic transition. A pulsed dye-laser system produces frequency tunable laser light at 282 nm. An on-board frequency reference cell is used by a computer to lock the laser to the appropriate wavelength. Measurement of the signal is then made by tuning the laser on and off resonance with the OH transition. Stratospheric air is channeled into the instrument using a double-ducted system that both maintains laminar flow through the detection region and slows the flow from (free stream velocity) 200 m/s to 40 m/s. The laser light is split and directed to two detection axes, where it passes through the stratospheric air in multipass White cells. Fluorescence from OH (centered at 309 nm) is detected orthogonally to both the flow and the laser propagation using a filtered PMT assembly. Optical stability is checked periodically by exchanging the 309 nm interference filter with a filter centered at 302 nm, where Raman scattering of ambient N₂ is observed. HO₂ is measured as OH after chemical titration with nitric oxide: HO₂ + NO -> OH + NO₂. Variation of added NO density and flow velocity, as well as the use of two detection axes, aid in diagnosis of the kinetics of this titration.