At all locations the TGM measurements were made using automatic Tekran ® 2537 mercury vapour analyzers (described in detail in Poissant, 1997). The air is typically sampled at flow rates between 1.0 and 1.5 L/min (depending on location) and is passed through a Teflon filter (47 mm diameter; 0.45 µm) at the sample line inlet to remove particulate matter. Inside the analyzer, the mercury in the sample air is pre-concentrated before analysis by amalgamation on gold cartridges (5-30 minute concentration time). Mercury is removed from the gold cartridges by thermal desorption and is detected using Cold Vapour Atomic Fluorescence Spectrophotometry (CVAFS). The analyzer has two gold cartridges which allow alternating accumulation and desorption to occur simultaneously resulting in the continuous measurement of mercury in the air stream at 5-30 minute intervals. The instruments are calibrated daily using an internal mercury source and verified during routine site audits by manual injections of mercury from an external source. The data are quality controlled using the Environment Canada RDMQ (Research Data Management and Quality Control) system.