Estimation of Hg0 exchange between ecosystems and the atmosphere using 222Rn and Hg0 concentration changes in the stable nocturnal boundary layer

The goal of this study was to test a 222Rn/Hg0 method to measure exchange fluxes of Hg0 between ecosystems and the atmosphere complementing gradient-based micrometeorological methods which are difficult to apply when exchange fluxes are very low, during calm nights, and over heterogeneous surfaces. The method is based on absolute concentration changes of Hg0 and of the trace gas 222Rn over several hours in the stable nocturnal boundary layer (NBL) when absolute gas concentrations change according to the source or sink strength of the underlying landscape. 222Rn accumulations were observed in 28 of 66 measured nights in an urban area and in 14 of the 40 nights at a subalpine grassland. Concurrent and significant increases in atmospheric Hg0 concentrations were observed 22 times in the urban area and calculated Hg0 emissions—for the first time measured over a city area—averaged 6.4±0.9 ng m−2 h−1. Concurrent changes in Hg0 in the stable NBLs at the subalpine site were very small and significant only nine times with calculated Hg0 fluxes averaging −0.2±0.3 ng m−2 h−1, indicating that the flux of Hg0 in the grassland was a very small net deposition of atmospheric Hg0 to the ecosystem. At the subalpine grassland the 222Rn/Hg0 method compared reasonably well to a modified Bowen ratio (MBR) method applied during turbulent conditions (−1.9±0.2 ng m−2 h−1 [or −1.7±0.4 ng m−2 h−1 during nights only]). The MBR, however, was not applicable in the urban area due to high surface roughness. We conclude that the 222Rn/Hg0 method—although limited to nighttime periods in our study—can complement gradient-based methods during stable NBL periods and can be used over heterogeneous surfaces when conventional micrometeorological approaches are not applicable.