Total gaseous mercury concentration and flux over a coastal saltmarsh vegetation in Connecticut, USA

The objective of this study is to characterize the ambient total gaseous mercury (TGM) concentration (C) at a coastal salt marsh and the TGM flux (F) between the marsh and the atmosphere using data collected on a near-continuous basis from 5 Jun to 8 July, 1997 and from 8 March to 23 July, 1998. In general, C was lower in the afternoon than at night, which is believed to be a result of diurnal variation in the mixing efficiency of the atmospheric boundary layer. The lack of a significant upward trend from March to July 1998 implies that TGM was removed from the boundary layer at a rate equal or greater than the surface emission. Three episodes of low C (0-1 ng m(-3)) occurred in July 1998, each lasting about 3 h. The TGM flux, F, was determined with the micrometeorological aerodynamic method. An episodic event of large positive F (emission) occurred in early spring of 1998 and appears to have been triggered by ice melting. It is proposed that three competing mechanisms - Hg vaporization, oxygen transport via roots, and diffusion into plants via stomata - may have played a role in the Hg exchange over other periods, resulting in bi-directional exchange of Hg with the atmosphere. Extrapolation of the 1998 data to the whole year suggests a weak sink removing 4 +/- 7 mu g Hg m(-2) yr(-1) from the atmosphere, a rate similar in magnitude to wet or particulate deposition to the site but much smaller than the rate of Hg accumulation in the sediment. (C) 2000 Elsevier Science Ltd. All rights reserved.