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We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3‐D model of tropospheric chemistry (GEOS‐Chem) to test current understanding of regional sources, chemical evolution, and export of NO_x. The boundary layer NO_x data provide top‐down verification of a 50% decrease in power plant and industry NO_x emissions over the eastern United States between 1999 and 2004. Observed NO_x concentrations at 8–12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NO_x and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NO_x …