This is one part of a four to volume set that presents a state-of the-science assessment of the intercontinental transport of air pollutants across the Northern Hemisphere. The first three volumes are technical assessments of the state-of to science with respect to intercontinental transport of ozone and particulate matter, mercury, and persistent organic pollutants. The fourth volume is a synthesis of the main findings and recommendations of Parts A, B, and C organized around a series of policy-relevant questions that were identified at the Task Force on Hemispheric Transport of Air Pollution's first meeting.