The objectives of the study were to identify sources of mercury (Hg) measured in downtown Toronto, Canada using Positive Matrix Factorization (PMF), Principal Components Analysis (PCA), ratio analysis, back trajectories, and correlation analyses. Atmospheric Hg species (i.e., GEM, PHg<2.5, RGM) and air pollutants measured from December 2003 to November 2004 were the input variables. Model results suggest industrial sources (chemical production, metal production, sewage treatment), rather that coal combustion, were the major contributors to measured Hg levels. Both the PMF model and PCA identified factors that cannot be characterized using the National Pollutant Release Inventory emissions data. Correlation analyses revealed direct emissions were the sources of RGM in spring, summer and fall, and the occurrence of GEM oxidation by ozone in the summer. Elevated Hg events are attributed to emissions from regional point sources, photochemical processes involving ozone, and potentially urban sources near the sampling site.