| The natural world, and human technology, emit gases into the atmosphere. These gases include water vapor, carbon dioxide, and a host of trace gases that have direct or indirect effects on climate, agricultural productivity, and human health. Most trace gases are removed from the atmosphere by chemical reaction with hydroxyl radicals, HO (or equivalently, OH).
Atmospheric HO is created by sunlight, acting on ozone and water vapor. HO has a short chemical lifetime, less than 1 second, due to its rapid reactions with other trace gases. Even at noon on a summer day, the net atmospheric HO concentration is only a few parts per quadrillion of air, making its detection a major experimental challenge.
With my colleagues, particularly Bob O'Brien, I've developed a sensitive method for HO detection and quantitative measurement, called FAGE (Fluorescence Assay with Gas Expansion). FAGE uses a tunable ultraviolet pulsed laser, a vacuum pump, photomultipliers, and optical, electronic, and chemical components. Besides HO, FAGE can measure the atmospheric concentrations of other molecules that can be converted to HO by adding a reagent. In that manner, we've used FAGE for HO2 (hydroperoxyl radical) and RO2 (organic peroxy radicals). HO2 and RO2 are products of atmospheric reactions of HO with more abundant trace gases, and are important intermediates in the production of ozone in the lower atmosphere and in urban air.
We've deployed FAGE in atmospheric studies of HO and HO2 at the Oregon Coast, Eastern Washington, and the Los Angeles basin. These studies included measurements of other molecules involved in the production and removal of HO and HO2. These efforts have been supported by EPA, NSF, NASA, and the California Air Resources Board. FAGE has been adopted (and adapted) by several groups of researchers for airborne, shipborne, and land-based atmospheric radical measurements.
The goal of this work is to understand how the atmosphere's normal chemical composition is maintained, and how it responds to pollution.
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| L.A. George, T.M. Hard, and R.J. O'Brien, "Measurements of Free Radicals OH and HO2 in Los Angeles Smog," J. Geophys. Res. 104 11643-11656 (1999).
T.M. Hard, L.A. George, and R.J. O'Brien, "FAGE determination of tropospheric HO and HO2", J. Atmos. Sci. 52 3354-3372 (1995).
T.M.
Hard, A.A. Mehrabzadeh, C.Y. Chan and R.J. O'Brien, "Diurnal HO2 cycles at clean air and urban sites in the troposphere", J. Geophys. Res. 97, 9785-9794 (1992).
T.M. Hard, A.A. Mehrabzadeh, C.Y. Chan and R.J. O'Brien, "FAGE measurements of tropospheric HO with measurements and model of interferences", J. Geophys. Res. 97, 9795-9817 (1992).
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