Impacts of anthropogenic and natural NOx sources over the US on tropospheric chemistry

We evaluate the impact of anthropogenic and natural NOx sources over the contiguous United States on tropospheric NOx and O-3 levels by using a global 3D chemical transport model. The effects of major U.S. surface NOx emission sources (including anthropogenic, biomass burning, and soil emissions) are compared with that of lightning-produced NOx. Summer lightning is shown to play a dominant role in controlling NOx and O-3 concentrations in the middle and upper troposphere, despite the fact that fossil-fuel burning represents the largest source of NOx 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 NOx emission sources and their impact is required to devise control strategies for regional and global air pollution.