Mercury is neurotoxic to wildlife and humans and is distributed globally in its gaseous form via the atmosphere after emission from various industrial processes and combustion sources. In this project, we provide measurement-based evidence that forests constitute major atmospheric mercury sinks via uptake of gaseous mercury. Using micrometeorological instruments deployed on large towers above forests (Figure 1), we present a total of three years of atmospheric mercury deposition measurements over both a coniferous forest in Maine and a deciduous forest in Massachusetts, USA. Measurements show annual gaseous mercury deposition from the atmosphere to forests of 13.4 ± 0.80 μg m⁻² (coniferous forest) and 25.1 ± 2.4 μg m⁻² (deciduous forest), respectively (Figure 2). Gaseous mercury deposition accounts for 62% and 76% of total mercury deposition observed in the two forests (with the remainder originating from deposition via rain and snow and other forms of mercury). Atmospheric gaseous mercury deposition is strongest during daytime and active growing periods, and behaves similarly to atmospheric carbon dioxide uptake (Figure 2 and 3). This provides evidence that vegetation is taking up atmospheric mercury via their leaves similar to carbon assimilation during photosynthesis. We also observe periods of nighttime deposition and variable patterns of exchanges over forest floors/soils (i.e., both deposition and emissions; Figure 3). The patterns observed over forest floors/soils suggest that variable deposition pathways exist across different forest types and/or regions.

The onset of atmospheric gaseous deposition to forests starting in spring led to corresponding atmospheric mercury concentration declines. We propose that in the absence of complex and expensive deployment of tower-based measurement systems as used in this study, simple ambient air mercury monitoring can provide proxy measurements to determine ecosystem mercury deposition. We also tested the performance of passive samplers recently developed for atmospheric monitoring. We found that while passive samplers performed well in tracking seasonal gaseous atmospheric mercury concentrations, they have inherent limitations to quantify ecosystem deposition processes. Finally, estimation of mercury deposition to vegetation using global datasets of vegetation samples analyzed for mercury suggest that plants globally may take up 2,422 ± 483 to 2,705 ± 504 metric tons of atmospheric gaseous mercury each year. This massive amount of mercury deposition would turn over the entire global atmospheric mercury pool of 4,800 Mg about every two years.

This study produced and contributed to 9 peer-reviewed scientific publications which so far generated 214 citations in other published articles. In addition, we published two book chapters, 2 further manuscripts are currently in review and preparation, and we presented over 30 presentations about this study at national and international conferences and seminars. This study provided novel scientific information about the processes and pathways of atmospheric deposition of this global pollutant, and we provided extensive outreach activities to the scientific community and regulatory agencies. The findings of this study are being integrated to constrain models of global and regional mercury cycling and are expected to help in the assessment of the Minamata Convention, an international agreement intended to curb anthropogenic mercury emissions and reduce mercury risks to humans and the environment.

Last Modified: 09/10/2023
Modified by: Roisin Commane