Ambient concentrations of speciated mercury (Hg) have been measured at many locations across Canada. Mercury in the atmosphere is measured in three operationally-defined forms - gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and particulate-bound mercury (PBM). Under most conditions, GEM (or Hg0) is the predominant species in the air (~95-99%), while RGM and PBM concentrations are typically two orders of magnitude lower, i.e., lesser than 5% of the total atmospheric mercury concentration (Schroeder and Munthe, 1998). Reactive gaseous mercury is thought to consist of compounds such as HgCl2, HgBr2, Hg(OH)2 (Lin and Pehkonen, 1999), although the exact composition is unknown.