Use of Passive Sampling Methods to Understand Sources of Mercury and Other Pollutants to High Elevation Sites in the Western United States

Abstract:

Passive samplers have been used for atmospheric gaseous oxidized mercury (GOM) concentration and dry deposition measurements over the last decade; however, there are still uncertainties associated with these methods. This study investigated GOM ambient air concentrations (box sampler) and dry deposition (surrogate surface) at 10 sites in California and Nevada. Measured and modeled (multiple-resistance model) GOM dry deposition were compared. Higher GOM concentrations and dry deposition were observed at high elevation sites (>2000 m) relative to those at low elevation sites (<2000 m) (n= 5-27 depending on the site), except for one that was located in a topographic bowl. High dry deposition was associated with high wind speed, turbulence, and input from the free troposphere at high elevation sites. Dry deposition of GOM at two high elevation sites was calculated using the passive sampler data and a multiple-resistance model. A previously developed relationship between the sampling rate of the passive sampler and GOM concentrations was used to estimate dry deposition and a scaling factor of 3 was used to adjust derived concentrations due to the underestimation by KCl-coated denuder measurements. The GOM dry deposition velocity at these two sites was ~2.2 cm/s without canopy resistance; however, the velocities were ~0.4 and 0.90 cm/s as α=β=2 and α=β=10, respectively. The GOM dry deposition fluxes measured by surrogate surface at two sites ranged from 2.3 to 2.9 ng m^-2/ hr^-1. With the scaling factor of 3, modeled deposition was similar to measured; however, dry deposition did not correlate consistently with either model (α=β=2 and α=β=10). This implies that different GOM compounds were present. Based on trajectory (72-h) probability analyses and10-day back-trajectory analyses high GOM dry deposition at these sites were attributed to Asian air mixed with regional/local urban emissions.