Atmospheric mercury (Hg) inputs to terrestrial ecosystems are considered the major source of mercury pollution in remote ecosystems. A large number of studies have measured depositional fluxes of Hg to terrestrial ecosystems by collecting wet, particulate, litterfall, and throughfall Hg deposition. Fewer studies have attempted to directly measure elemental Hg deposition, the dominant atmospheric Hg species. Here we present methods and results to measure elemental Hg (Hg0) exchange fluxes in terrestrial ecosystems using non-contact micrometeorological methods.

Based on a modified Bowen Ratio method, we report small deposition fluxes of Hg0 in a subalpine grassland in the range of 2 ng m-2 hr-1. Measured Hg0 concentrations with high vertical resolution (five heights) over a second grassland site showed increasing Hg0 concentrations during nights indicative of net deposition of atmospheric Hg0 to the grassland. An almost neutral concentration profile during daytime indicated that the sink strength was less prounounced during daytime periods or that vertical mixing was too high for detection of low daytime fluxes. Measured gradients over uncontaminated systems, however, are extremely small and difficult to detect. A 222Rn/Hg0 accumulation method which tracks absolute changes of Hg0 and of the trace gas 222Rn over serval hours in stable boundary layer conditions supported the notion of small depositional fluxes in grassland ecosystems. Detection of Hg fluxes has proven to be very sensitive using this method but the method was limited to nocturnal boundary layer conditions. Application of the 222Rn/Hg0 accumulation method in an urban area showed emissions of Hg0 averaging 6.5 ng m-2 hr-1 allowing for the first direct measurement of atmospheric Hg fluxes from urban sites. We conclude that micrometeorological methods allow for direct quantification of small Hg0 depositional fluxes, and that Hg0 deposition is a common pathway of atmospheric Hg inputs to vegetated terrestrial ecosystems.