OH
and HO2 measurements with the AirborneTropospheric
Hydrogen Oxides Sensor (ATHOS)
William Brune, Monica
Martinez, Hartwig Harder, and Robert Lesher
503 Walker Building;
Department of Meteorology;
Pennsylvania State University;
University Park, PA 16802;
phone: 814-865-3286
fax:814-865-3663
e-mail: brune@essc.psu.edu
Instrument Description
ATHOS uses laser-induced fluorescence (LIF) to measure OH and
HO2 simultaneously. OH is both excited and detected
with the A2S +(v'=0) --> X2P +(v"=0)
transition near 308nm. HO2 is first reacted with reagent
NO to form OH and is thendetected 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 througha 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 occursin each detection
cell at the intersection of the airflow, the laser beammulti-passed
through White cells, and the detector field-of-view.
The laser has a 3 kHz pulse repetition frequency, 30 ns long pulses,
andis tuned on and off resonance with the OH transition to determine
OH fluorescenceand background signals. The detector is gated to
detect the OH fluorescenceafter the laser pulse has cleared the
detection cell. A reference cell containingOH shows when the laser
is on and off resonance with the OH transition.
An in-flight calibration system creates OH and HO2 outside
thedetection chamber inlet and is currently used to monitor the
relative sensitivitiesof the two axes. The absolute
uncertainty, which is determinedin the laboratory and maintained
in flight with monitors, is ± 40%,
with 90% confidence. The minimum detectable mixing ratio(S/N =2,
60 seconds) is <0.005 pptv (or <105 molecules
cm-3)for OH and <0.06 pptv for HO2 above
2 km altitude. All data are collected at 5 Hz. OH andHO2 signals
are statistically significant at 5 Hz in plumes, butthey must be
integrated for more than 20 seconds in clean air to get statisticalsignificance.
ATHOS can detect OH and HO2 in clear air and lightclouds
from Earth's surface to the lower stratosphere.
Reference Publications
Brune, W.H., P.S. Stevens, and J.H. Mather, Measuring OH and HO2 in
the troposphere by laser-induced fluorescence at low pressure, J.
Atmos.Sci., 52, 3328-3336, 1995.
Brune, W.H., I.C. Faloona, D. Tan, A.J. Weinheimer, T. Campos,
B.A. Ridley,S.A. Vay, J.E. Collins, G.W. Sachse, L. Jaegle, and
D. J. Jacob, Airbornein situ OH and HO2 observations
in the cloud-free troposphereand lower stratosphere during SUCCESS, Geophys.
Res. Lett., 25, 1701,1998.
Tan, D., I. Faloona, W. H. Brune, A. Weinheimer, T. Campos, B.
Ridley, S. Vay, J. Collins, and G. Sachse, In situ meaurements
of HOx in aircraft exhaust plumes and contrails during
SUCCESS, Geophys. Res. Lett., 25, 1721, 1998.
|