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Free radicals play a central role in the chemical transformations that characterize both clean and polluted air. Hydroxyl radical (HO) is sunlight's prime agent in maintaining the trace-gas composition of the lower atmosphere. HO is believed to control the atmospheric lifetimes of most trace gases emitted by the biosphere and by human technology. This control is exerted through the chemical reaction of HO with these trace gases, initiating their conversion to a variety of oxidized gases or aerosols subject to removal by precipitation or deposition at the earth's surface. Most water-insoluble gases have atmospheric lifetimes inversely proportional to their HO reactivity. These lifetimes range from hours to decades.
Compounds which react slowly with HO accumulate in the troposphere where they may affect the earth's radiation balance. Such compounds survive long enough to reach and interact with the stratospheric ozone shield. Where anthropogenic emissions are concentrated, the intermediate products of the HO-catalyzed cleansing process are known as urban smog and the final products as acid rain. Much of our current research is directed toward a greater understanding of the processes that control tropospheric HO concentrations and of the reaction mechanisms of HO with other atmospheric trace gases.
In a variation on the technique of laser-excited fluorescence, we have developed with Dr. Thomas Hard a highly sensitive method for measuring the atmospheric concentrations of hydroxyl radical (HO) and hydroperoxyl radical (HO2). In this procedure (FAGE, fluorescence assay with gas expansion), the air sample is expanded to low pressure to suppress the many interfering emissions which complicate the detection of the sub-part-per trillion levels of HO present in sunlit air. Addition of chemical reagents to the expanding air allows chemical modulation of the HO signal or the conversion of HO2 to HO for fluorescent detection. Ox measurements currently employ a copper-vapor pump laser which emits radiation at two wavelengths. One beam is used for HO and HO2 measurements, while the other beam may be employed for direct fluorescence detection of NO2. Since the NO2 fluorescent yield at atmospheric pressure is very low, this excitation can be carried out at much lower pressure than the HO determination. We use a two-chamber fluorescence cell where NO2 excitation occurs in one chamber, and the flowing gas stream moves to a second, detection chamber where its fluorescence is monitored, thereby minimizing extraneous radiation so that ppt levels of NO2
both of which occur in several steps.
A recent field campaign in the Los Angeles Basin utilized FAGE for simultaneous measurements of HO and HO2 concentrations, the first ever such simultaneous measurements made in polluted air. Analysis of these data in light of existing photochemical mechanisms for highly polluted air is almost complete. During some periods, HOx radical concentrations have been found to be in reasonable agreement with polluted air models, while there are significant disagreements during other periods.
Another area of current research involves unraveling the mechanisms whereby atmospheric aromatic hydrocarbons are catalytically oxidized by HO. These compounds, which constitute a
large fraction of gasoline and are common solvents, have an extremely complex sequence of atmospheric reactions. In the past, we have used tandem mass spectrometry to determine the entire range of products formed. More recently, atmospheric pressure ionization (API) mass spectrometry has been used for the direct kinetic study of their oxidation mechanisms. Future work using an ion trap mass spectrometer coupled to an API source is planned. Both chemical kinetic and molecular modeling is used to better understand the neutral and ion-molecule reactions which occur in an API source used for kinetic studies.
Another project uses kinetic modeling and flow-reactor atmospheric simulations to study the very interesting phenomenon of multiple steady-states and chemical hysteresis in atmospheric photochemical systems. In these experiments, we have carried out the first experimental demonstration of hysteresis in a simulated atmospheric reaction system.
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Tropospheric HOx Measurements by FAGE, T.M. Hard, L.A. George, R.J. O'Brien, J. Atm. Sci., 52, 3354, 1995.
Intercomparison of HO Measurements by Radiocarbon and FAGE, M.J. Campbell,
B.D. Hall, J.C. Sheppard, P.L. Utley, R.J. O'Brien, T.M. Hard, L.A. George, J. Atm. Sci., 52, 328, 1995.
R.J. O'Brien, L.A. George (D.D. Dunnette, R.J. O'Brien, Eds.) Chemical Reactivity and its Consequences for Clean
and Polluted Air, The Science of Global Change,
American Chemical Society, 1992.
Tropospheric Hydroxyl Radical, a Challenging Analyte, in Measurement Challenges in Atmospheric Chemistry, R.J. O'Brien, T.M. Hard, Advances in Chemistry Series, 232,
American Chemical Society, 1992.
Tropospheric Hydroxyl Radical: Measurements and Model of Interferences, T.M. Hard, A.A. Mehrabzadeh, C.Y. Chan, R.J. O'Brien, J. Geophys. Res. 97, 9795 1992.
Tropospheric Hydroperoxyl Radical Measurements in Clean and Polluted Air, Hard, T.M., C.Y. Chan, A.A. Mehrabzadeh, R.J. O'Brien, J. Geophys. Res. 97, 9785 1992. |
