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±7μg 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.