Measurements of total mercury flux over a forest canopy for model development

This dissertation presents the development of a relaxed eddy accumulation (REA) system to quantify the seasonal total gaseous mercury (TGM) flux forest canopy. This study represents the first use of the REA technique to measure TGM fluxes over a forest canopy and the first seasonal dataset of TGM flux data over a forest canopy. TGM flux and concentration measurements were taken on a 40 m tall micrometeorology instrument tower in a red maple (Acer Rubrum) forest on the University of Connecticut research farm in Coventry Connecticut (Lat. 41° 47\u27 30 N, Long. 72° 22\u27 29 W, 162 m in elevation). Site specific parameters such as fetch, turbulent averaging periods, and coordinate rotations were investigated and the use of flux footprint models, the multiresolution decomposition technique, and planar fit rotations are recommended. The REA technique is shown to be a reliable and robust method for TMG mercury flux measurements over a forest canopy. ^ The direction and magnitude of TGM fluxes were seasonally dynamic. During the fall leaf off conditions and the winter the site was a sink for atmospheric mercury, during periods of spring snow melt and surface heating the site was a source of TGM to the atmosphere, and there was little net accumulation or evasion of mercury during the growing season. However, during the growing season there was a significant trend from net deposition after leaf out to net evasion in the late summer to the fall litter fall. Stomatal uptake can account for approximately 80% of the mercury accumulation in the leaves and appears to be responsible for much of the dry deposition to the canopy. Atmosphere-cuticle and atmosphere-stomata compensation points were measured. Anthropogenic emissions from agricultural processes were first documented in this thesis when elevated mercury concentrations were measured during the tilling of an adjacent agricultural field. Analysis of the microclimate of the field and concentrations in the media clearly show that the elevated concentrations were from the disturbance of the soil in the field. The conclusions of the seasonal flux study are presented as testable hypotheses in a two-film atmosphere-canopy resistance model.