A Relaxed Eddy Accumulation System for Measuring Surface Fluxes of Total Gaseous Mercury

A relaxed eddy accumulation (REA) system was designed to continuously measure total gaseous mercury (TGM) fluxes over a forest canopy. TGM concentration measurements were measured at 5-min intervals with a Tekran model 2537A mercury analyzer located above the forest canopy on a walk-up meteorological tower. Ten-minute averages for upand downdraft mercury concentrations were used to calculate the flux. The multiresolution decomposition technique was used to determine dayand nighttime averaging periods for the turbulent statistics used in the REA technique. This paper documents the REA system for mercury flux measurements and its use over a forest canopy. The REA system response to the averaging times for the turbulent statistics and corrections to upand downdraft concentrations are major considerations when using the technique with the Tekran mercury analyzer over a forest canopy. TGM flux data collected from 18 August to 12 September 2005 are used here to demonstrate the capabilities of the REA system to measure both short(1-h time periods) and long-term flux dynamics. During the demonstration period the TGM median flux was 21.9 32.6 ng m 2 h 1 and the median atmospheric TGM concentrations were 1.34 0.13 ng m 2 h . Maximum short-term TGM evasive fluxes occurred during the daylight hours with minimums during the nighttime. A consistent bimodal emission pattern was observed during the daytime emissions over the canopy. The first peak occurred immediately following the evaporation of the nighttime dew on the canopy and the second peak occurred in the late afternoon.