HOx
Instrument: High-altitude OH Experiment (HOx)
Principal Investigator: James G. Anderson
Organization:
Department of Chemistry and
Department of Earth and Planetary Science
Harvard University
40 Oxford St.
Cambridge, MA 02138
Principle of Operation:
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 beam-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 orthogonal 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 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.
Measurements of ozone (by uv absorption) and water vapor (by photofragment
fluorescence) are made as diagnostics of potential photochemical interference
from the mechanism: O3 + hv (282 nm) --> O(1D) + O2, followed by:
O(1D) + H2O --> OH + OH