Impacts of anthropogenic and natural NOx sources over the U.S. on tropospheric chemistry

We evaluate the impact of anthropogenic and natural NO x sources over the contiguous United States on tropospheric NO x and O 3 levels by using a global 3D chemical transport model. The effects of major U.S. surface NO x emission sources (including anthropogenic, biomass burning, and soil emissions) are compared with that of lightning-produced NO x . Summer lightning is shown to play a dominant role in controlling NO x and O 3 concentrations in the middle and upper troposphere, despite the fact that fossil-fuel burning represents the largest source of NO x over the U.S. Furthermore, the effect of regional U.S. lightning is propagated through large areas of the Northern Hemisphere by atmospheric circulation. The results reveal that a thorough assessment of atmospheric NO x emission sources and their impact is required to devise control strategies for regional and global air pollution.