We use the Regional chEmical trAnsport Model to analyze the contributions of lightning and anthropogenic NOx on ozone concentrations over the United States and the western North Atlantic Ocean from June to August 2005. Tropospheric NO2 columns from OMI, tropospheric O3 columns derived from OMI and MLS measurements, and vertical O3 profiles from TES over the region are used in the analysis. With a 50% reduction in the industrial and electrical power generation NOx emissions from 1999 EPA NEI and a parameterization of lightning-produced NOx based on concurrent NLDN, CAPE, and cloud mass flux data, REAM generally captures the spatial distribution of lightning flash rates, and OMI NO2 and OMI-MLS O3 column enhancements with high correlation coefficients (0.6-0.9). The model results show that over the United States, the contribution of anthropogenic NOx emissions to summertime tropospheric O3 declines significantly due to the 50% reduced emissions. This results in the dramatic reduction in the relative impact of fossil-fuel NOx emissions over continental outflow regions. In the convective outflow regions over North America and the western North Atlantic Ocean, the contribution of lightning NOx production on tropospheric O3 in the summer is comparable to that of anthropogenic NOx emissions. The impact of NOx produced by lightning is becoming larger as fossil-fuel combustion NOx emissions decrease. Upper tropospheric O3 enhancements due to lightning NOx production are indirectly shown in the vertical profiles of the TES O3 measurements with the help of REAM simulations. This result suggests that TES measurements could be used to constrain lightning-derived tropospheric O3 enhancements, which play a critical role in climate change.