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During an airborne study in the Southeast United States, measured mixing ratios of biogenic hydrocarbons were systematically lower in air masses containing enhanced nitrogen oxides from power plants, which we attribute to increased concentrations of hydroxyl (OH) radicals within the power plant plumes. Plume transects at successively further downwind distances provide a decreasing gradient of nitrogen oxides (NO_x) concentrations, which together with the implied loss rates of isoprene, constrains the OH dependence on NO_x. We find that OH concentrations were highest at nitrogen dioxide concentrations near 1–2 ppbv and decreased at higher and at lower concentrations. These findings agree with the dependence of OH on NO_x concentrations expected from known chemical reactions but are not consistent with some studies reporting direct OH measurements higher than expected in regions of the …