Factors affecting the concentration and distribution of gaseous elemental mercury in the urban atmosphere of downtown Toronto

Atmospheric gaseous elemental mercury (GEM) and meteorological parameters were simultaneously monitored at two building rooftops in downtown Toronto, Canada from Oct. 2015 to Oct. 2016. The two buildings are 136 m apart, one is an office building (60 m tall) and the other (30 m tall) mainly contains chemistry and biology laboratories, offices and classrooms. The average mercury concentrations were found to be 1.46 +/- 0.54 ng/m(3) and 1.78 0.89 ng/m(3) for the 60 m and 30 m sites, respectively. Analysis of the data reveals that sporadic events of high mercury concentration are related to local sources; that the higher level of GEM at the 30 m site resulted from a combination of local sources and weaker mixing of air at lower height; regional sources contributed to the loading of GEM in urban atmosphere. Comparing the data set with a data set collected in 2004 from the same location revealed that the average atmospheric GEM concentration in downtown Toronto dropped from 4.5 ng/m(3) to 1.78 ng/m(3) (60% decrease). Decreases in GEM were also observed in the data reported by CAMNet over the same period in rural and remote areas but to a lesser extent (10.5 and 13.9% respectively). The decrease is in line with the key national and provincial policies on mercury use, emission control and management that have been enacted since 2004.