Nitrogen oxides in the troposphere: Global and regional budgets

The cycle of nitrogen oxides in the troposphere is discussed from both global and regional perspectives. Global sources for NO x are estimated to be of magnitude 50(±25)×10 12 gm N yr −1 . Nitrogen oxides are derived from combustion of fossil fuels (∼40%) and biomass burning (∼25%) with the balance from lightning and microbial activity in soils. Estimates for the rate of removal of NO x based on recent atmospheric and precipitation chemistry data are consistent with global source strengths derived here. Industrial and agricultural activities provide approximately two thirds of the global source for NO x . In North America, sources from combustion of fossil fuels exceed natural sources by a factor of 3–13. Wet deposition removes about one third of the combustion source of NO x over North America, while dry deposition removes a similar amount. The balance is exported from the continent. Deposition of nitrate in precipitation over eastern Canada and the western Atlantic is clearly influenced by sources of NO x in the eastern United States. Major uncertainties in the cycle of NO x are attributed to lack of field measurements and inadequate understanding of heterogeneous processes involving NO x .