The Hydroxyl Radical; Sources and Measurement.
Hydroxyl Radical Source.
It has been known for over 30 years that the hydroxyl radical is the main driving force behind the daytime reactions of hydrocarbons in the troposphere. The main source of hydroxyl in both clean and polluted air is the photolysis of ozone. Relatively small concentrations (15-30 ppb) of ozone occur in the remote troposphere and it is these that provide the source. The photolysis of ozone (wavelengths below 310 nm) produce a singlet oxygen atom, which may react with a water molecule to yield hydroxyl radicals;
or is collisionally quenched to a triplet oxygen atom (O(3P)). The reaction between a triplet oxygen atom and an oxygen molecule will regenerate ozone. It is worth noting that in polluted air, photodissociation of nitrous acid is a significant source of hydroxyl radicals.
Measuring Atmospheric Concentrations of the Hydroxyl Radical.
The tropospheric hydroxyl radical concentration is rather low and very variable. By application of monthly mean concentrations values ranging from (0.1 to 25) x 105 radicals cm-3 depending on solar irradiation, latitude, altitude, temperature and concentrations of H2O, CO, O3, CH4 and NOx have been calculated. A global average tropospheric hydroxyl radical concentration of 8.1 x 105 radicals cm-3 has been determined based on the tracer 1,1,1-trichloroethane (CH3CCl3). In-situ measurements of tropospheric hydroxyl radical concentrations at ground level have been carried out using a number of different techniques, e.g. long path absorption spectroscopy (LPAS), ion-assisted analysis (IAA), differential optical absorption spectroscopy (DOAS), and a fluorescence assay with gas expansion (FAGE) based on laser induced fluorescence (LIF) of the hydroxyl radical. The direct techniques, LAPS and DOAS, measure the attenuation of a laser beam (@ 308 nm) in a long horizontal path, ~20 km. DOAS also measures the absorption of a reference beam (@ 307.99 nm) to account for interferences and scattering. The indirect method, IAA, determines hydroxyl radical concentrations by drawing air samples through a flow reactor in which hydroxyl radicals react with 34SO2 to form H34SO3 radicals, subsequently leading to the formation of H234SO3 in the presence of O2 and H2O. H2SO4 is then measured by mass spectrometry employing a chemical ionisation technique. In the FAGE technique, samples of the air to be analysed are introduced into the analytical chamber through a pin hole nozzle, expanding the gas to a pressure of a few torr. After the expansion zone, the sample enters the excitation zone where hydroxyl radicals are excited by a laser, (@ 308 nm), the excited hydroxyl radicals relax to the ground state emitting fluorescent radiation (@ 308 nm). This fluorescence is detected perpendicular to the excitation beam using a photomultiplier.
The range of values observed in these studies is 0.2 - 8 x 106 radicals cm-3 , with mean daytime concentrations of approximately 2 x 106 radicals cm-3 .
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Last modified on Tue Apr 14 15:06:20 1998