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Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO₂, collectively HO_x) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, on which expensive air pollution control strategies are based, must accurately predict these radical concentrations. We have used the fluorescence assay with gas expansion (FAGE) technique to carry out the first simultaneous, in situ measurements of these two radicals in highly polluted air during the Los Angeles Free Radical Experiment. A complete suite of ancillary measurements was also made, including speciated hydrocarbons, carbon monoxide, aldehydes, nitric oxide, nitrogen dioxide, and ozone along with meteorological parameters. Using this suite of measurements, we tested the ability of a lumped chemical mechanism to accurately predict radical concentrations in polluted air. Comparison of …