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[1] The August 2003 North American electrical blackout provided a unique opportunity to quantify directly the contribution of power plants to regional haze and O3. Airborne observations over central Pennsylvania on August 15, 2003, ∼24 h into the blackout, revealed large reductions in SO2 (>90%), O3 (∼50%), and light scattered by particles (∼70%) relative to measurements outside the blackout region and over the same location when power plants were operating normally. CO and light absorbing particles were unaffected. Low level O3 decreased by ∼38 ppbv and the visual range increased by >40 km. This clean air benefit was realized over much of the eastern U.S. Reported SO2 and NOx emissions from upwind power plants were down to 34 and 20% of normal, respectively. The improvement in air quality provides evidence that transported emissions from power plants hundreds of km upwind play a dominant role in regional haze and O3 production.