Unprecedented decrease in deposition of nitrogen oxides over North America: the relative effects of emission controls and prevailing air-mass trajectories

As one of the main forms of reactive nitrogen delivered by anthropogenic sources, atmospheric emissions of nitrogen oxides and their subsequent deposition has significantly perturbed the natural nitrogen cycle in sensitive receiving ecosystems worldwide. In North America, despite of decades of increasingly stringent regulations of emissions, decreases in the deposition of nitrogen oxides were not observed until the turn of the century. Analysis of available deposition data and trends at various spatial scales revealed that the decrease took place at a continental scale, but is particularly evident in the eastern side of the continent, where there was an unprecedented 50 % decrease in deposition. The magnitude, timing and geographical extension of the observed changes in deposition resulted from a combination of successfully and relatively coordinated application of emission controls in major contributing regions and increasingly lower amounts of nitrogen oxides being transported from source to receptor areas, thereby extending the effects of emission controls over large geographical scales.