OH and HO₂ measurements with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS)

William Brune, David Tan, Ian Faloona, Robert Lesher, Christopher Frame, Thomas Kovacs, and James Simpas

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⁺(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.

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, Airborne in situ OH and HO₂ observations in the cloud-free troposphere and 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.

A schematic of ATHOS.  Top view is looking aft; Bottom view is looking starboard.