Magnitude and pathway of gaseous atmospheric mercury deposition in forests, 2019 - 2021
Project Team
PI: Daniel Obrist, UMass Lowell. co-PI: Roisin Commane, Columbia University
Postdoctoral Researcher: Jamie Harrison
Columbia Students: Charlotte Kwong
Publications
D. Obrist, E. Roy, J. Budney, J. Harrison, C. Kwong, J. W. Munger, H. Moosmüller, C. Romero, S. Sun, J. Zhou, R. Commane, Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest, Proceedings of the National Academy of Sciences, 118 (29) e2105477118; DOI: 10.1073/pnas.2105477118, 2021 Link
Project Summary
Overview
The goal of this project is to quantify net atmosphere-surface exchange of gaseous elemental mercury (Hg[0]) in two remote forests for one full year each, providing the first such records in non-polluted forests and leading to transformative progress in constraining Hg(0) sinks (atmospheric dry deposition) and sources (e.g., re-emissions) across landscapes. Dry Hg(0) deposition is now considered the dominant Hg source in vegetated ecosystems, accounting for 54-94% of total Hg. In the absence of direct measurements, Hg(0) deposition is inferred from litterfall and passive membranes, yet these are not ideal proxies for net Hg(0) deposition as they don’t account for re-emission. Stable Hg isotopes provide powerful tools to source apportion Hg in ecosystems, including Hg(0), but can’t provide temporal resolution of sinks and sources nor controlling variables (weather, climate, biological variables). The first objective is to quantify the magnitude and temporal dynamics of net gaseous dry Hg(0) deposition (sum of gross deposition minus emission) in two forests with different seasonalities, a deciduous temperate forest and an evergreen subtropical rain forest. Net Hg(0) deposition will be measured using micrometeorological measurements on large towers, the only available method for direct, non-intrusive and time-extended measurements of net Hg(0) exchange at the ecosystem level encompassing all underlying sinks and sources. The second objective is to partition Hg(0) fluxes into canopy and soil contributions via deployment of two corresponding flux systems. One system will be deployed above the forest canopy to measure ecosystem-level Hg(0) exchange; a second system will be deployed below the canopy to quantify soil contributions. Canopy Hg(0) fluxes will be calculated by difference. Flux partitioning will provide annual, seasonal and diurnal Hg(0) sink (e.g., to canopies) and source strengths (e.g., from soils) needed to constraint Hg(0) deposition in global and regional chemical transport models. The third goal is to elucidate pathways of deposition by comparing Hg(0) fluxes to those of carbon dioxide, ozone, water vapor, and carbonyl sulfide. All these trace gases have different sinks and sources in ecosystems and vary in their degree of canopy, stomatal, mesophyll and soil contributions to fluxes. Comparison among fluxes, including seasonality, diurnality and component fluxes will allow to quantify the degree to which Hg(0) exchange is coupled to photosynthetic activity, stomatal conductance, enzymatic activity within leaves, external cuticular uptake and soil exchange.
Intellectual merit
The finding that atmospheric Hg(0) is the dominant Hg source in ecosystems, largely driven by plant uptake and transfer to soils, calls for a need to focus monitoring strategies to include atmospheric Hg(0) deposition. Terrestrial runoff serves as an important Hg source to rivers, lakes and oceans, even dominating such as in the Arctic Ocean, and constraining pathways and magnitudes of atmospheric Hg(0) deposition will constrain biological and human Hg exposures. As the entire global atmospheric Hg pool cycles through vegetation every 4 to 5 years, the large terrestrial Hg(0) sink serves as critical feedback control on boundary layer and tropospheric Hg(0) dynamics. Climate and land use change is expected to modify terrestrial Hg(0) sinks and the study will yield data needed for projecting future Hg burdens. In summary, the study will be a milestone to improve the current understanding and future predictions of coupling between ecosystem and atmospheric Hg dynamics and biological impacts.
Project News
QCLS Analyzer deployed at Howland Forest to measure CO2 and OCS gradients above and below the canopy. We also added a soil chamber to our measurements at this site to examine soil fluxes. This work made possible in collaboration with Mary Whelan at Rutgers University. - May 2021
COVID-19 related shutdowns limited our activities in 2020 until late in the season but Jamie made it into the field thanks to a collaboration with Mary Whelan, Rutgers University. - October 2020
Charlotte presents her summer work - August 2019
Group visit to Harvard Forest - June 2019
Our Projects have been supported by funding from the National Science Foundation (NSF) Atmospheric Chemistry Program