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Instrument: ATHOS : Airborne Tropospheric Hydroxides Sensor
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Description
The Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) uses
laser-induced fluorescence (LIF) to measure OH and HO2
simultaneously. OH is both excited and detected with the A2S+(u
'=0) ? X2P +(u "=0) transition near 308 nm. HO2 is first reacted
with reagent NO to form OH and is then detected with LIF.

The ambient air is slowed from the aircraft speed of 240 m/s to
a controlled 8-40 m/s in an aerodynamic nacelle, and is then
pulled by a vacuum pump through a small inlet, up a sampling
tube, and into two low-pressure detection cells. The first cell
is for OH and the second for HO2. Detection occurs in each
detection cell at the intersection of the airflow, the laser
beam multi-passed through White cells, and the detector
field-of-view.

The laser has a 5 kHz pulse repetition frequency, 30 ns long
pulses, and is tuned on and off resonance with the OH transition
to determine OH fluorescence and background signals. The
detector is gated to detect the OH fluorescence after the laser
pulse has cleared the detection cell. A reference cell
containing OH shows when the laser is on and off resonance with
the OH transition.

An in-flight calibration system creates OH and HO2 outside the
detection chamber inlet and is currently used to monitor the
relative sensitivities of the two axes. The absolute
uncertainty, which is determined in the laboratory and
maintained in flight with monitors, is ? 40%. The minimum
detectable mixing ratio (S/N =2, 60 seconds) is 0.015 pptv for
OH and 0.06 pptv for HO2. All data are collected at 5 Hz. OH
and HO2 signals are statistically significant at 5 Hz in plumes,
but they must be integrated for more than 20 seconds in clean
air to get statistical significance. ATHOS can detect OH and HO2
in clear air and light clouds from Earth's surface to the lower
stratosphere.

Additional information available at
http://www-gte.larc.nasa.gov/pem/brune.htm

[Summary provided by NASA]