Seasonal and diurnal patterns of speciated atmospheric mercury at a coastal-rural and a coastal-urban site

Gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) were monitored at a coastal-rural and a coastal-urban site in Nova Scotia, Canada from 2010 to 2011. The three-hour average urban concentrations were 1.67 ± 1.01 ng m−3, 2.1 ± 3.4 pg m−3 and 2.3 ± 3.1 pg m−3 for GEM, GOM and PBM, respectively. The rural site concentrations were 1.38 ± 0.20 ng m−3, 0.4 ± 1.0 pg m−3 and 3.5 ± 4.5 pg m−3, respectively. GEM and GOM concentrations were higher at the urban site in all seasons, while PBM was higher at the rural site in winter. Both sites observed higher GEM and PBM during colder seasons and higher GOM in spring. Seasonal diurnal patterns showed daytime increases and nighttime decreases in GEM, GOM, and PBM. Rural–urban site differences may be attributed to the urban heat island effect resulting in warmer nighttime temperatures and higher GEM concentrations, urban GEM emissions, and enhanced deposition in forested areas leading to faster rural GEM decreases. Wind speeds ≥4 and ≥8 km h−1 were associated with higher rural GEM and lower PBM at night, which could be due to downward mixing from the residual boundary layer and marine airflows from the open ocean as modeled by back trajectories. The back trajectory analysis also found higher rural GEM and lower PBM overnight at both sites for open ocean airflows than all airflow conditions, suggesting the ocean is a source of GEM but not PBM.