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 HO_x in aircraft exhaust plumes and contrails during SUCCESS, Geophys. Res. Lett., 25, 1721, 1998.