Identifying and evaluating urban mercury emission sources through passive sampler-based mapping of atmospheric concentration

Beyond emissions from coal-fired power generation, urban sources of mercury (Hg) to the atmosphere, especially minor fugitive sources, are relatively poorly characterized. To identify urban sources of fugitive Hg emissions, passive air samplers (PASs) were deployed for periods of 4-6 weeks in the summer of 2016 at 145 sites across the Greater Toronto Area (GTA). PASs were also deployed along transects of increasing distance from five sites listed as Hg sources in the National Pollution Release Inventory (NPRI) within or near the GTA. Mean gaseous Hg concentrations in downtown Toronto (1.77 +/- 0.28 ng m(-3)) are slightly, but significantly elevated relative to other parts of the GTA (1.42 +/- 0.20ngm(-3)). Similarly, concentrations at sites close to waste/recycling (1.61 +/- 0.22 ng m(-3)) and hospitals/dental facilities (1.63 +/- 0.21 ng in(-3)) are significantly higher than at sites presumably distant from potential sources (1.37 +/- 0.20 ng m(-3)). Gaseous Hg concentrations are elevated near four of five NPRI source sites, but not near a wastewater treatment plant. Measured or predicted concentrations (using extrapolated transect relationships) close to known Hg sources do not correlate with reported NPRI emissions. For example, the Hg disposal company Aevitas Inc. has the lowest reported NPRI emissions (0.11 kg yr(-1)) among the five sources, but measured (12.3 ng m(-3)) and predicted (60.0 ng m(-3)) concentrations outside the facility are the highest. The PAS's ability to precisely and accurately discriminate small differences in gaseous Hg concentration (<0.2 ng m(-3)) at and near global background concentrations enables the mapping of the spatial concentration variability and the identification of fugitive Hg emission sources.