Atmospheric mercury deposition over the land surfaces and the associated uncertainties in observations and simulations: a critical review

Abstract

Abstract. One of the most important processes in the global mercury (Hg) biogeochemical cycling is the deposition of atmospheric Hg, including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM), to the land surfaces. Results of wet, dry, and forest Hg deposition from global observation networks, individual monitoring studies, and observation-based simulations have been reviewed in this study. Uncertainties in the observation and simulation of global speciated atmospheric Hg deposition to the land surfaces have been systemically estimated based on assessment of commonly used observation methods, campaign results for comparison of different methods, model evaluation with observation data, and sensitivity analysis for model parameterization. The uncertainties of GOM and PBM dry deposition measurements come from the interference of unwanted Hg forms or incomplete capture of targeted Hg forms, while that of GEM dry deposition observation originates from the lack of a standardized experimental system and operating procedure. The large biases in the measurements of GOM and PBM concentrations and the high sensitivities of key parameters in resistance models lead to high uncertainties in GOM and PBM dry deposition simulation. Non-precipitation Hg wet deposition could play a crucial role in alpine and coastal regions, and its high uncertainties in both observation and simulation affect the overall uncertainties of Hg wet deposition. The overall uncertainties in the observation and simulation of the total global Hg deposition were estimated to be ± (25–50) % and ± (45–70) %, respectively, with the largest contributions from dry deposition. According to the results from uncertainty analysis, future research needs were recommended, among which a global Hg dry deposition network, unified methods for GOM and PBM dry deposition measurements, quantitative methods for GOM speciation, campaigns for comprehensive forest Hg behavior, and more efforts in long-term Hg deposition monitoring in Asia are the top priorities.

Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest

“…S6) which averaged 4.1 ± 5.6 pg •m −3 and yields a GOM deposition flux of 1.9 ± 2.6 μg • m −2 • yr −1 if extrapolated to a full year. This estimated GOM deposition at Harvard Forest is smaller than an average GOM deposition of 6.4 μg • m −2 • yr −1 estimated across North America (35), which is reasonable given that the North American average includes urban sites with higher GOM concentrations from anthropogenic emissions while our forest ecosystem is a rural site. We did not measure particulate mercury (PHg) deposition but assume an upper limit of 1.1 μg • m −2 • yr −1 of deposition, which is the average PHg deposition reported across North America (35).…”

“…This estimated GOM deposition at Harvard Forest is smaller than an average GOM deposition of 6.4 μg • m −2 • yr −1 estimated across North America (35), which is reasonable given that the North American average includes urban sites with higher GOM concentrations from anthropogenic emissions while our forest ecosystem is a rural site. We did not measure particulate mercury (PHg) deposition but assume an upper limit of 1.1 μg • m −2 • yr −1 of deposition, which is the average PHg deposition reported across North America (35). These comparisons strongly suggest that GEM dry deposition was by far the dominant deposition pathway in this forest accounting for 76% of a total annual mercury deposition of about 33.1 μg • m −2 • yr −1 (Table 1).…”

Soil–atmosphere exchange flux of total gaseous mercury (TGM) at subtropical and temperate forest catchments

“…4). Additionally, refrozen ice/snow layers are characterized by elevated Hg concentrations, and the deposited Hg from atmosphere could be potentially released to meltwater (Zhang et al, 2012;Perez-Rodriguez et al, 2019), which is consistent with our results that atmospheric Hg deposition could be released to meltwater during snow melt. Our observations through the annual climatic cycle reduce the uncertainty and bias of temporal patterns of soil-air Hg fluxes.…”

BrHgO• + CO: Analogue of OH + CO and Reduction Path for Hg(II) in the Atmosphere

“…Mercury is a neurotoxin which is transported globally by the atmosphere . Mercury deposits from the atmosphere to ecosystems most efficiently when it is in the form of Hg­(II) compounds. , Unfortunately, we are still uncovering major aspects of the redox chemistry of atmospheric mercury. − This complicates efforts to predict the impact of emissions reductions on the spatial distribution of mercury entry into ecosystems. , …”