Ensemble Assessments of Atmospheric Emissions and Impacts.

The impacts of emissions to the atmosphere are customarily addressed on four different spatial scales: local (up to a few tens of km), regional (up to a few hundred km), continental (up to two or three thousand km), and global. In North America, vigorous efforts to reduce emissions are reflected for some atmospheric constituents in decreases in average ambient concentrations on local and regional scales. The same successes are not occurring on continental and global scales, however. Most species in the atmosphere have many sources, and most sources emit many different species; the range in lifetimes of those species results in impacts on all spatial scales. As a consequence, assessments need to be done in ensemble, for all sources and emittants of significance. In this paper, a methodology for such an assessment is presented and discussed.