Using Passive Air Samplers to Quantify Vertical Gaseous Elemental Mercury Concentration Gradients Within a Forest and Above Soil

Gaseous elemental mercury (GEM), the primary form of mercury (Hg) in the atmosphere, can undergo bidirectional exchange with the Earth's surface, with uptake in growing vegetation constituting the dominant atmospheric deposition process. Whereas direction and magnitude of this exchange can be determined using micrometeorological and chamber techniques, these approaches are complex, costly and labor-intensive, and often only yield fluxes over relatively short time-scales at few sites. We demonstrate that an inexpensive and easy-to-use passive air sampler can identify and quantify vertical GEM concentration gradients on scales ranging from centimeters to tens of meters over long time periods (up to 1.5 years) and with coarse temporal resolution (monthly to seasonally). Samplers were deployed above clean and contaminated soil and through a deciduous forest canopy at two sites in Southern Ontario. Significant and seasonally variable gradients of GEM concentrations, both increasing and decreasing with height above ground, were observed and can be explained through the influence of factors such as soil contamination, canopy growth, temperature, solar irradiance, and snow cover. At a minimum, the sampler can identify the GEM flux direction, but, when combined with the Modified Bowen Ratio method, can also be used to estimate the evaporative GEM flux from mercury-contaminated soil. We further demonstrate the application of the approach to contaminated site assessment, using a field in Brescia, Italy, known to contain elevated levels of Hg in soil. By allowing for cost-effective measurements at multiple sites and over extended time periods, passive sampling complements existing techniques for studying atmosphere-surface exchange of GEM.