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The hydroxyl radical (OH) is one of the most important oxidants in theatmosphere, as it is involved in many reactions that affect regional airquality and global climate change. Because of its high reactivity,measurements of OH radical concentrations in the atmosphere are difficult,and often require careful calibrations that rely on the production of aknown concentration of OH at atmospheric pressure. The Indiana University OHinstrument, based on the Fluorescence Assay by Gas Expansion technique(FAGE), has been calibrated in the laboratory using two differentapproaches: the production of OH from the UV-photolysis of water-vapor, andthe steady-state production of OH from the reaction of ozone with alkenes.The former technique relies on two different actinometric methods to measure theproduct of the lamp flux at 184.9 nm and the photolysis time. This quantityderived from N2O actinometry was found to be 1.5 times higher than that derived from O2actinometry. The water photolysis and ozone-alkene techniques are shown to agreewithin their experimental uncertainties (respectively 17% and 44%),although the sensitivities derived from the ozone-alkene technique weresystematically lower by 40% than those derived from the water-vapor UV-photolysis technique using O2 actinometry. The agreement between the twodifferent methods improves the confidence of the water-vapor photolysis methodas an accurate calibration technique for HOx instruments. Becauseseveral aspects of the mechanism of the gas phase ozonolysis of alkenes arestill uncertain, this technique should be used with caution to calibrate OHinstruments

atmospheric chemistry and physics

Technical note: Measuring tropospheric OH and HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; by laser-induced fluorescence at low pressure. A comparison of calibration techniques