The behavior of mercury in the atmosphere depends upon its form, or specie.  Elemental mercury (Hg^o) is typically not very reactive with global lifetime of a few months to a year and is thought to be transported significantly in the troposphere.  Reactive gaseous mercury (RGM) species, are not well characterized chemically but are thought to be gaseous Hg(II)-bearing molecules such as HgCl_2(g).  RGM species are notable for being quickly deposited from the atmosphere to the surface and are thought to be readily available for conversion to methylmercury, a highly toxic form of mercury.  Particulate mercury (Hg-P) is also quickly deposited and is often found in high concentrations near combustion sources.  Although much lower in proportion than Hgo, the greater reactivity and deposition rates of RGM and Hg-P make them a larger environment concern.  Chemical reactions that occur in the atmosphere can transform mercury between these various species.