This document provides an update of the findings from the 2004 Canada–United States Transboundary Particulate Matter Science Assessment (the 2004 Assessment). Its goal is to present a scientific and technical basis for discussions of adding a particulate matter (PM) annex to the 1991 Canada–United States Air Quality Agreement (the Agreement), to assess the potential impacts of a PM annex and, ultimately, to help determine whether such an annex is currently warranted.
This assessment focuses on the fine particle fraction of PM, or PM2.5, because this size fraction can remain suspended in the air for several days to weeks, can be transported by winds over large distances, and thus is subject to atmospheric transboundary transport in North America.
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