Unlike most trace species that are emitted only at the surface, NO, sources exist both at the surface and in the free troposphere. Surface sources include both natural and anthropogenic sources; e.g., soil/microbial emission, fossil fuel combustion, and biomass burning. In the free troposphere, NO, sources include lightning, aircraft emissions, and stratosphere-troposphere exchange. Table 1 gives estimated source strengths and uncertainties for each of these sources. While there are still substantial uncertainties in these sources, the global NO, source strength is clearly dominated by surface sources with anthropogenic use of fossil fuels having the greatest contribution. The smaller sources in the free troposphere, however, cannot be trivialized since they are localized in a region of the atmosphere where NO, lifetimes are maximized. Thus, their proportional impact is greater than their absolute source strengths would imply.
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