Impacts of anthropogenic and natural NO(x) 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.