Fluorecence Assay by Gas Expansion
We have two instruments with the ability to detect ambient OH, HO2 and IO. The aircraft instrument is primarily run on board the FAAM research aircraft, although has been deployed during a number of ground and ship-based campaigns also. The ground instrument is run from a 20 ft shipping container and has the capability to provide OH reactivity measurements also (Ingham et al., 2009).
OH is measured using the fluorescence assay by gas expansion (FAGE) technique (Whalley et al., 2010). Air is drawn into a cell, held at ~ 1 Torr, via a pinhole nozzle. OH radicals are then electronically excited at ~ 308 nm using a tuneable, pulsed laser. The fluorescence is detected perpendicular to the laser axis using a channel photo-multiplier (CPM) and gated photon counting. HO2 may be detected by titration to OH, via addition of NO, and detected as above.
A similar technique is applied to detect IO (Whalley et al., 2007): IO radicals are electronically excited at ~445 nm followed by off-resonance fluorescence detection at ~520 nm.
Both instruments have an integration time of 1 second.
The limit of detection (LOD) of the ground system is ~1.95x105 molecule cm-3 and ~ 3x105 molecule cm-3 (5 min data collection period) for OH and HO2 respectively, with an associated uncertainty of ~ 15% (1σ). For IO, the LOD is ~ 1x107 molecule cm-3, with an associated uncertainty of 23% (1σ).
The cells are calibrated at regular intervals.
T. Ingham, L.K. Whalley, D. E. Heard et al., A flow-tube based laser-induced fluorescence instrument to measure OH reactivity in the troposphere, Atmos. Meas. Tech., 2, 465-477, 2009
L.K. Whalley, D. E. Heard et al., The chemistry of OH and HO2 radicals in the boundary layer over the tropical Atlantic Ocean, Atmos. Chem. Phys., 10, 1555-1576, 2010
L.K. Whalley, T. Ingham, D.E. Heard et al., Detection of iodine monoxide radicals in the marine boundary layer using laser induced fluorescence spectroscopy, J. Atm. Chem., 58, 19-39, 2007
FAGE group website: http://www.chem.leeds.ac.uk/Atmospheric/Field/fage/fage.html
Aircraft FAGE Contact: Dr Trevor Ingham
Ground FAGE Contact: Dr Lisa Whalley