Concentration-weighted trajectory approach to identifying potential sources of speciated atmospheric mercury at an urban coastal site in Nova Scotia, Canada

Abstract

Regional and local sources contributing to gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) at an urban coastal site in Dartmouth, Nova Scotia, Canada were investigated using the Concentration-Weighted Trajectory model (CWT) and Conditional Probability Function. From 2010–2011, GEM, GOM, and PBM concentrations were 1.67 ± 1.01 ng m⁻³, 2.07 ± 3.35 pg m⁻³, and 2.32 ± 3.09 pg m⁻³, respectively. Seasonal variability was observed, with statistically higher GEM and PBM concentrations in winter and spring and higher GOM in spring. In the CWT, concentrations are the weighting factors for the trajectory residence time in modeled grid cells, which results in the identification of source areas based on the CWT values in the grid cells. Potential source areas were identified in regions with known industrial Hg sources particularly in the fall season, but also in regions without these sources (e.g. Atlantic Ocean, northern Ontario and Quebec). CWTs for GOM and PBM that were associated with ≥ 5 kg industrial Hg emissions from 2010–2011 were statistically larger than those with zero Hg emissions, despite a lack of strong correlations. A large proportion of elevated CWTs (85–97%) was in regions with zero industrial Hg sources indicating the potential role of non-point sources, natural emissions, and residential-scale combustion. Analysis of wind data suggests that a commercial harbor and vehicular traffic were potential local sources. Evaluating modeled source areas against Hg emissions inventories was not an ideal method for assessing the CWT model accuracy because of insufficient data on Hg emissions at more precise locations

Overview of receptor-based source apportionment studies for speciated atmospheric mercury

“…The New York and Milwaukee sites in the Great Lakes region and the Nova Scotia site identified industrial areas in Ohio and the eastern US as potential Hg sources (Han et al, 2007;de Foy et al, 2012;Cheng et al, 2013b). The source areas identified by the RTWC model also revealed that metal industries in Quebec and Ontario, Canada, contributed to TGM in New York (Han et al, 2007).…”

“…CFA, RTWC, and CWT have been used to identify potential sources of TGM and speciated atmospheric Hg contributing to multiple sites in New York (Han et al, 2007), Mexico City (Rutter et al, 2009), Milwaukee, Wisconsin (de Foy et al, 2012; and Dartmouth, Nova Scotia (Cheng et al, 2013b). The New York and Milwaukee sites in the Great Lakes region and the Nova Scotia site identified industrial areas in Ohio and the eastern US as potential Hg sources (Han et al, 2007;de Foy et al, 2012;Cheng et al, 2013b).…”

“…CFA, RTWC, and CWT are also common back trajectory receptor models that have been used to identify potential source areas contributing to speciated atmospheric Hg measurements (Han et al, 2007;Rutter et al, 2009;de Foy et al, 2012;Cheng et al, 2013b). The most apparent difference between CFA/RTWC/CWT and previously described back trajectory receptor models is that the trajectory residence time in the grid cells have been weighted by the observed atmospheric Hg concentrations corresponding to the arrival of each trajectory.…”

“…For CFA or RTWC, logarithmic concentrations are used. τ ij l is the number of trajectory segment endpoints in grid cell (i, j ) for back trajectory l divided by the total number of trajectory segment endpoints for back trajectory l (i.e., residence time of a trajectory in each grid cell); L is the total number of back trajectories over a time period (e.g., entire sampling period or a season) (Cheng et al, 2013b). As the model equation shows, higher atmospheric Hg concentrations would lead to higher source intensity if the trajectory residence time were the same.…”

“…The hourly locations of the particles are counted in all the grid cells that have been overlaid on a map of the study area. The HYS-PLIT back trajectory model using the EDAS 40 km archived meteorological data was used in the CWT studies for speciated atmospheric Hg (Cheng et al, 2013b). Back trajectories of 48 h were generated for each 3 h GEM, GOM, and PBM concentration at a single start height representative of the coastal location.…”