The atmosphere is a major pathway for the transport and deposition of PCDD/Fs. Atmospheric transport is the primary distribution pathway transferring PCDD/Fs via deposition in terrestrial and aquatic ecosystems. Particle size is again a major factor in controlling the residence time and removal mechanisms of PCDD/Fs. PCDD/Fs are highly fat-soluble and poorly soluble in water. Owing to their lipophilicity, they are typically found in the environment bound to PM. Once sorbed onto the particles, PCDD/Fs do not leach or volatilize to a significant degree because in this form they are highly stable. PCDD/Fs are removed from the atmosphere primarily by photodegradation or by dry or wet de position. Photodegradation applies primarily to those chlorinated hydrocarbons not bound on PM. The deposited and accumulated PCDD/Fs on surfaces can revolatilize depending on vapor pressure of the compound, surface characteristics, and surface temperature In this way, the more volatile congeners can undergo numerous cycles of deposition and reemission during atmospheric transport.