Ashu Dastoor - Academia.edu (original) (raw)

Papers by Ashu Dastoor

Research paper thumbnail of Chapter 2 Where Does Mercury in the Arctic Environment Come From, and How Does it Get There?

Very little of the Hg present in the Arctic is derived from pollution sources within this region;... more Very little of the Hg present in the Arctic is derived from pollution sources within this region; most is transported in from anthropogenic and natural sources outside the Arctic (AMAP, 2005). Previous AMAP assessments (AMAP, 1998,

Research paper thumbnail of Aurams Runs During the Pacific2001 Time Period — a Model/Measurement Comparison

Air Pollution Modeling and Its Application XVI, 2004

Research paper thumbnail of A Factor of 3 to 8 Provides a Reasonable Rationale for the Discrepancy of a Factor of 2 – 10 Determined

Zhang et al. (2012a), in a recent report, compared model estimates and new observations of oxidis... more Zhang et al. (2012a), in a recent report, compared model estimates and new observations of oxidised and particulate mercury species (Hg 2+ and Hg p) in the Great Lakes region and found that the sum of Hg 2+ and Hg p varied between a factor of 2 to 10 between measurements and model. They suggested too high emission inputs and too fast oxidative conversion of Hg 0 to Hg 2+ and Hg p , as possible causes. This study quantitatively explores in detail the uncertainties in measurements, in addition to the above concerns and speciation of mercury near emission sources in the model to better understand these discrepancies in the context of oxidized mercury, i.e. gaseous (Hg 2+) and particulate (Hg p) mercury. These include sampling efficiency, composition of sample, interfering species and calibration errors for measurements and in-plume reduction processes. Sensitivity simulations using Global/Regional Atmospheric Heavy Metals Model (GRAHM) were performed to analyze the role of in-plume reduction on ambient concentrations and deposition of mercury in North America. The discrepancy between simulated and observed concentrations of Hg 2+ and Hg p was found to be reduced when a ratio for Hg 0 :Hg 2+ :Hg p in the emissions was changed from 50:40:10 (as specified in the original inventories) to 90:8:2 to account for in-plume reduction of Hg 0 processes. A significant reduction of the root mean square error (e.g., 19.22 to 11.3 pg m −3 for New Jersey site NJ54) and bias (67.8 to 19.3 pg m −3 for NJ54) for sampling sites in the Eastern United States and Canada, especially for sites near emission sources was found. Significant improvements in the spatial distribution of wet deposition of mercury in North America was noticed. Particularly, over-prediction of wet deposition near anthropogenic sources of mercury was reduced by 43 %. On a regional scale, estimated wet deposition improved by a factor of 2 for areas with more than 12 µg m −2 yearly average wet deposition. Model sensitivity simulations show that the measured concentration of oxidized mercury is too low to be consistent with measured wet deposition fluxes in North America.

Research paper thumbnail of Vegetation uptake of mercury and impacts on global cycling

Nature Reviews Earth & Environment, 2021

In this review, we synthesize the current knowledge on mercury (Hg) content and sources in foliag... more In this review, we synthesize the current knowledge on mercury (Hg) content and sources in foliage and vegetated ecosystems and the importance of vegetation to global Hg cycling. By means of a global database of over 35,000 samples across 416 sites, we discuss global Hg concentrations in all major tissues, and mechanisms of vegetation Hg uptake. Hg in aboveground vegetation largely originates from uptake of atmospheric gaseous elemental Hg (Hg(0)), whereas Hg in roots originates from a combination of uptake from soil and foliage-to-root transport. Vegetation Hg uptake from the atmosphere and transfer to soils is the major Hg source in all biomes. Using model sensitivity analyses with and without global vegetation present, we show that vegetation Hg uptake modulates atmospheric Hg(0) seasonality in the northern hemisphere and interhemispheric gradient. We estimate that vegetation uptake the global Hg pool in the atmosphere by approximately 660 Mg and reduces the Hg deposition to global oceans, which in the absence of vegetation might receive an additional 960 Mg yr-1. We discuss future research needs to better constrain vegetation uptake mechanisms and their controlling physiological and environmental variables, improve model processes and address effects of climate and land use changes.

Research paper thumbnail of Aerosols in an urban cold climate: Physical and chemical characteristics of nanoparticles

Urban Climate, 2020

Atmospheric aerosols are of significant importance in climate change and health research. This st... more Atmospheric aerosols are of significant importance in climate change and health research. This study provides a 2-years meta dataset of real-time aerosol number and size distribution measurements in a model cold-climate city of Montreal (Canada). We provide selected information on the chemical composition and morphology of aerosols (~10 nm to 10 μm). We deployed a suite of complementary techniques such as aerosol particle sizers, MOUDI, S/TEM imaging, EDS, QQQ-ICP-MS/MS, IC, and TOC analyzers. The highest geomean nanoparticles (< 100 nm) number density (9260 ± 1.6 number/cm 3) was observed in the winter. A broad range of metals, including emerging contaminants, was quantified in all aerosol sizes include Pb, Cd, Ni, Zn, As, Al, Mn, Co, and Se, with concentrations ranging from 0.01 to 67.64 ng/m 3. Nanosize particles included Ni, Cd, Pb, Zn. Iron exhibited the highest concentrations consistently in two modes (< 180 nm and > 1 μm). S/TEM-EDS confirmed the abundance of nanoparticles, single and clusters, with multifaceted morphologies and compositions. We identified (a) emerging nanoparticles, (b) secondary organic aerosols, (c) bioaerosols, and (d) combustion particles. Diurnal, weekly, monthly, seasonal, and yearly variations of aerosols are provided. We herein discuss the implications of these results on air quality and climate modelling.

Research paper thumbnail of Total Mercury and Methylmercury in Lake Water of Canada’s Oil Sands Region

Environmental Science & Technology, 2018

Increased delivery of mercury to ecosystems is a common consequence of industrialization, includi... more Increased delivery of mercury to ecosystems is a common consequence of industrialization, including in the Athabasca Oil Sands Region (AOSR) of Canada. Atmospheric mercury deposition has been studied previously in the AOSR; however, less is known about the impact of regional industry on toxic methylmercury (MeHg) concentrations in lake ecosystems. We measured total mercury (THg) and MeHg concentrations for five years from 50 lakes throughout the AOSR. Mean lake water concentrations of THg (0.4−5.3 ng L −1) and MeHg (0.01−0.34 ng L −1) were similar to those of other boreal lakes and <5% of all samples exceeded Provincial water quality guidelines. Lakes with the highest THg concentrations were found >100 km northwest of oil sands mines and received runoff from geological formations high in metals concentrations. MeHg concentrations were highest in those lakes, and in smaller productive lakes closer to oil sands mines. Simulated annual average direct deposition of THg to sampled lakes using an atmospheric chemical transport model showed <2% of all mercury deposited to sampled lakes was emitted from oil sands activities. Consequently, spatial patterns of mercury in AOSR lakes were likely most influenced by watershed and lake conditions, though mercury concentrations in these lakes may be perturbed with future development and climatic change.

Research paper thumbnail of How important is biomass burning in Canada to mercury contamination?

Atmospheric Chemistry and Physics Discussions, 2017

Wildfire frequency has increased in past four decades in Canada, and is expected to increase in f... more Wildfire frequency has increased in past four decades in Canada, and is expected to increase in future as a result of climate change (Wotton et al., 2010). Mercury (Hg) emissions from biomass burning are known to be significant; however, the impact of biomass burning on air concentration and deposition fluxes in Canada has not been previously quantified. We use estimates of burned biomass from FINN (Fire Inventory from NCAR) and vegetation-specific Emission Factors (EFs) of mercury to investigate the spatiotemporal variability of Hg emissions in Canada. We use Environment and Climate Change Canada's GEM-MACH-Hg (Global Environmental Multi-scale, Modelling Air quality and Chemistry model, mercury version) to quantify the impact of biomass burning in Canada on spatiotemporal variability of air concentrations and deposition fluxes of mercury in Canada. We use North American gaseous elemental mercury (GEM) observations (2010–2015), GEM-MACH-Hg, and an inversion technique to op...

Research paper thumbnail of Multi-model study of mercury dispersion in the atmosphere: Atmospheric processes and model evaluation

Atmospheric Chemistry and Physics Discussions, 2016

Current understanding of mercury (Hg) behaviour in the atmosphere contains significant gaps. Some... more Current understanding of mercury (Hg) behaviour in the atmosphere contains significant gaps. Some key characteristics of Hg processes including anthropogenic and geogenic emissions, atmospheric chemistry, and air-surface exchange are still poorly known. This study provides a complex analysis of processes governing Hg fate in the atmosphere involving both measurement data from ground-based sites and simulation results of chemical transport models. A variety of long-term measurements of gaseous elemental Hg (GEM) and reactive Hg (RM) concentration as well as Hg wet deposition flux has been compiled from different global and regional monitoring networks. Four contemporary global-scale transport models for Hg were applied both in their state-of-the-art configurations and for a number of numerical experiments aimed at evaluation of particular processes. Results of the model simulation were evaluated against measurements. As it follows from the analysis the inter-hemispheric gradient of G...

Research paper thumbnail of Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

Chemical Reviews, 2015

Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver c... more Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver colored d-block heavy metal exhibits noble-gas-like characteristics and solidifies at a temperature of −38.83 °C.(1) Elemental mercury is the only metal that is liquid at room temperature and has an "ideal" high-energy surface. The average surface tension value of mercury based on different experimental measurements is 466 ± 33 mN m-1 .(2) Due to the versatility of mercury, it is used as a catalyst, as an electrode material in electrochemistry, in optical spectroscopy, as reflective liquid in liquid mirror telescopes, and in medicine, e.g., as amalgam in dental fillings. Yet, due to the varying, but significant, acute and chronic toxicity of mercury compounds, its usage has been of recent concern. Among single events with a large number of human casualties were the Minamata incident due to the release of highly toxic MeHg in the industrial wastewater from the 1930s to the 1960s in Minamata, Japan, and, in 1971 the Iraqi disaster, where organo-mercury fungicide treated seed grains were consumed by humans. Medical symptoms of mercury poisoning vary from parathesia to ataxia, loss of vision, insanity, paralysis coma, congenital deformities affecting fetuses, and death.(3) Because of the urgency to limit mercury related disasters, in 2013 an international treaty was signed by 128 countries, to control and "where feasible" reduce atmospheric emissions from point source categories (e.g., coal fired power plants and smelters). The treaty also called for additional mercury research.(4) The atmosphere is the fastest moving fluid in the Earth's environment. Many mercury species have appreciable vapor pressures at Earth surface temperatures, allowing them to enter the atmosphere and, depending on the mercury species involved, undergo long-and short-range transport, atmospheric physical and chemical transformation, as well as feedback with Earth's surface (see Figure 1), which is the main subject of this review paper.

Research paper thumbnail of Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

Chemical Reviews, 2015

Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver c... more Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver colored d-block heavy metal exhibits noble-gas-like characteristics and solidifies at a temperature of −38.83 °C.(1) Elemental mercury is the only metal that is liquid at room temperature and has an "ideal" high-energy surface. The average surface tension value of mercury based on different experimental measurements is 466 ± 33 mN m-1 .(2) Due to the versatility of mercury, it is used as a catalyst, as an electrode material in electrochemistry, in optical spectroscopy, as reflective liquid in liquid mirror telescopes, and in medicine, e.g., as amalgam in dental fillings. Yet, due to the varying, but significant, acute and chronic toxicity of mercury compounds, its usage has been of recent concern. Among single events with a large number of human casualties were the Minamata incident due to the release of highly toxic MeHg in the industrial wastewater from the 1930s to the 1960s in Minamata, Japan, and, in 1971 the Iraqi disaster, where organo-mercury fungicide treated seed grains were consumed by humans. Medical symptoms of mercury poisoning vary from parathesia to ataxia, loss of vision, insanity, paralysis coma, congenital deformities affecting fetuses, and death.(3) Because of the urgency to limit mercury related disasters, in 2013 an international treaty was signed by 128 countries, to control and "where feasible" reduce atmospheric emissions from point source categories (e.g., coal fired power plants and smelters). The treaty also called for additional mercury research.(4) The atmosphere is the fastest moving fluid in the Earth's environment. Many mercury species have appreciable vapor pressures at Earth surface temperatures, allowing them to enter the atmosphere and, depending on the mercury species involved, undergo long-and short-range transport, atmospheric physical and chemical transformation, as well as feedback with Earth's surface (see Figure 1), which is the main subject of this review paper.

Research paper thumbnail of Mercury simulations within GMOS: Analysis of short-term observational episodes

E3S Web of Conferences, 2013

A number of contemporary chemical transport models for mercury are applied within the framework o... more A number of contemporary chemical transport models for mercury are applied within the framework of the EU GMOS project to study principal processes of mercury transport and transformations in the atmosphere. Each model is involved in simulation of short-term episodes corresponding to particular Hg measurement campaigns in Europe and other regions. In order to evaluate different physical and chemical mechanisms the models perform sensitivity runs with various parameterizations and/or combinations of considered processes. The modeling results are compared to detailed measurements of Hg species (Hg 0 /TGM, RGM, HgP) with high temporal resolution (hours) aiming at reproduction of short-term temporal variability of Hg air concentration.

Research paper thumbnail of Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone

Atmos. Chem. Phys., 2014

To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs... more To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. This paper constitutes Part 1 of the study, describing a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br 2 , is found to be a dominant process of reactive bromine formation in the Published by Copernicus Publications on behalf of the European Geosciences Union. 4102 K. Toyota et al.: Air-snowpack exchange of bromine and ozone calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low in the course of such events to be detected unambiguously by satellite nadir measurements. www.atmos-chem-phys.net/14/4101/2014/ Atmos. Chem. Phys., 14, 4101-4133, 2014

Research paper thumbnail of Chemical cycling and deposition of atmospheric mercury: Global constraints from observations

Journal of Geophysical Research, 2007

We use a global 3-D model of atmospheric mercury (GEOS-Chem) to interpret worldwide observations ... more We use a global 3-D model of atmospheric mercury (GEOS-Chem) to interpret worldwide observations of total gaseous mercury (TGM) and reactive gaseous mercury (RGM) in terms of the constraints they provide on the chemical cycling and deposition of mercury. Our simulation including a global mercury source of 7000 Mg yr À1 and a TGM lifetime of 0.8 years reproduces the magnitude and large-scale variability of TGM observations at land sites. However, it cannot capture observations of high TGM from ship cruises, implying a problem either in the measurements or in our fundamental understanding of mercury sources. Observed TGM seasonal variation at northern midlatitudes is consistent with a photochemical oxidation for Hg(0) partly balanced by photochemical reduction of Hg(II). Observations of increasing RGM with altitude imply a long lifetime of Hg(II) in the free troposphere. We find in the model that Hg(II) dominates over Hg(0) in the upper troposphere and stratosphere and that subsidence is the principal source of Hg(II) at remote surface sites. RGM observations at Okinawa Island (Japan) show large diurnal variability implying fast deposition, which we propose is due to RGM uptake by sea-salt aerosols. Observed mercury wet deposition fluxes in the United States show a maximum in the southeast, which we attribute to photochemical oxidation of the global Hg(0) pool. They also show a secondary maximum in the industrial Midwest due to regional emissions that is underestimated in the model, possibly because of excessive dry deposition relative to wet (dry deposition accounts for 68% of total mercury deposition in the United States in the model, but this is sensitive to the assumed phase of Hg(II)). We estimate that North American anthropogenic emissions contribute on average 20% to U.S. mercury deposition.

Research paper thumbnail of Parameterization of gaseous dry deposition in atmospheric chemistry models: Sensitivity to aerodynamic resistance formulations under statically stable conditions

Atmospheric Environment, 2016

Stability-correction algorithms for r a show notable discrepancies at Ri B > 0.2. Some of the atm... more Stability-correction algorithms for r a show notable discrepancies at Ri B > 0.2. Some of the atmospheric chemistry models compute L, u * and r a inconsistently. Choice of r a formulation impacts the simulated surface chemical exchange rates.

Research paper thumbnail of Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone

Atmos. Chem. Phys., 2014

To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs... more To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. This paper constitutes Part 1 of the study, describing a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br 2 , is found to be a dominant process of reactive bromine formation in the Published by Copernicus Publications on behalf of the European Geosciences Union. 4102 K. Toyota et al.: Air-snowpack exchange of bromine and ozone calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low in the course of such events to be detected unambiguously by satellite nadir measurements. www.atmos-chem-phys.net/14/4101/2014/ Atmos. Chem. Phys., 14, 4101-4133, 2014

Research paper thumbnail of Behavior of mercury in snow from different latitudes

Journal de Physique IV (Proceedings), 2003

Research paper thumbnail of Spatial Trends and Historical Deposition of Mercury in Eastern and Northern Canada Inferred from Lake Sediment Cores

Environmental Science & Technology, 2009

Recent and historical deposition of mercury (Hg) was examined over a broad geographic area from s... more Recent and historical deposition of mercury (Hg) was examined over a broad geographic area from southwestern Northwest Territories to Labrador and from the U.S. Northeast to northern Ellesmere Island using dated sediment cores from 50 lakes (18 in midlatitudes (41-50°N), 14 subarctic (51-64°N) and 18 in the Arctic (65-83°N)). Distinct increases of Hg over time were observed in 76% of Arctic, 86% of subarctic and 100% of midlatitude cores. Subsurface maxima in Hg depositional fluxes (µg m-2 y-1) were observed in only 28% of midlatitude lakes and 18% of arctic lakes, indicating little recent reduction of inputs. Anthropogenic Hg fluxes adjusted for sediment focusing and changes in sedimentation rates (∆F adj,F) ranged from-22.9 to 61 µg m-2 y-1 and were negatively correlated (r)-0.57, P < 0.001) with latitude. Hg flux ratios (FRs; post-1990)/pre-1850) ranged from 0.5 to 7.7. The latitudinal trend for Hg ∆F adj,F values showed excellent agreement with predictions of the global mercury model, GRAHM for the geographic location of each lake (r) 0.933, P < 0.001). The results are consistent with a scenario of slow atmospheric oxidation of mercury, and slow deposition of reactive mercury emissions, declining with increasing latitude away from emission sources in the midlatitudes, and support the view that there are significant anthropogenic Hg inputs in the Arctic.

Research paper thumbnail of Mercury in Arctic marine ecosystems: Sources, pathways and exposure

Environmental Research, 2012

Mercury in the Arctic is an important environmental and human health issue. The reliance of North... more Mercury in the Arctic is an important environmental and human health issue. The reliance of Northern Peoples on traditional foods, such as marine mammals, for subsistence means that they are particularly at risk from mercury exposure. The cycling of mercury in Arctic marine systems is reviewed here, with emphasis placed on the key sources, pathways and processes which regulate mercury levels in marine food webs and ultimately the exposure of human populations to this contaminant. While many knowledge gaps exist limiting our ability to make strong conclusions, it appears that the long-range transport of mercury from Asian emissions is an important source of atmospheric Hg to the Arctic and that mercury methylation resulting in monomethylmercury production (an organic form of mercury which is both toxic and bioaccumulated) in Arctic marine waters is the principal source of mercury incorporated into food webs. Mercury concentrations in biological organisms have increased since the onset of the industrial age and are controlled by a combination of abiotic factors (e.g., monomethylmercury supply), food web dynamics and structure, and animal behavior (e.g., habitat selection and feeding behavior). Finally, although some Northern Peoples have high mercury concentrations of mercury in their blood and hair, harvesting and consuming traditional foods have many nutritional, social, cultural and physical health benefits which must be considered in risk management and communication.

Research paper thumbnail of Assessment of modeled mercury dry deposition over the Great Lakes region

Environmental Pollution, 2012

Three sets of model predicted values for speciated mercury concentrations and dry deposition flux... more Three sets of model predicted values for speciated mercury concentrations and dry deposition fluxes over the Great Lakes region were assessed using field measurements and model intercomparisons. The model predicted values were produced by the Community Multiscale Air Quality Modeling System for the year 2002 (CMAQ2002) and for the year 2005 (CMAQ2005) and by the Global/Regional Atmospheric Heavy Metals Model for the year 2005 (GRAHM2005). Median values of the surface layer ambient concentration of gaseous elemental mercury (GEM) from all three models were generally within 30% of measurements. However, all three models overpredicted surface-layer concentrations of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) by a factor of 2e10 at the majority of the 15 monitoring locations. For dry deposition of GOM plus PBM, CMAQ2005 showed a clear gradient with the highest deposition in Pennsylvania and its surrounding areas while GRAHM2005 showed no such gradient in this region; however, GRAHM2005 had more hot spots than those of CMAQ2005. Predicted dry deposition of GOM plus PBM from these models should be treated as upper-end estimates over some land surfaces in this region based on the tendencies of all the models to overpredict GOM and PBM concentrations when compared to field measurements. Model predicted GEM dry deposition was found to be as important as GOM plus PBM dry deposition as a contributor to total dry deposition. Predicted total annual mercury dry deposition were mostly lower than 5 mg m À2 to the surface of the Great lakes, between 5 and 15 mg m À2 to the land surface north of the US/Canada border, and between 5 and 40 mg m À2 to the land surface south of the US/Canada border. Predicted dry deposition from different models differed from each other by as much as a factor of 2 at regional scales and by a greater extent at local scales.

Research paper thumbnail of Long range transport of mercury to the Arctic and across Canada

Atmospheric Chemistry and Physics, 2010

This study is the most extensive study to date on the transport of mercury to the Arctic. Moreove... more This study is the most extensive study to date on the transport of mercury to the Arctic. Moreover, it is the first such study to use a fully-coupled, online chemical transport model, Environment Canada's Global/Regional Atmospheric Heavy Metals model (GRAHM), where the meteorology and mercury processes are fully integrated. It is also the only study to date on the transport of mercury across Canada. We estimated source attribution from Asia, North America, Russia and Europe at six arctic verification stations, as well as three subarctic and eight midlatitude Canadian stations. We have found that Asia, despite having transport efficiencies that were almost always lower than those of North America and often lower than those of Russia, was the dominant source of gaseous atmospheric mercury at all verification stations: it contributed the most mercury (29-37% at all stations, seasons and levels considered), its concentrations frequently explained nearly 100% of the variability in the concentrations produced by the simulation performed with full global emissions, particularly in the absence of local sources, and it generated the most long range transport (LRT) events, causing 43%, 67% and 75% of the events at the arctic, subarctic and midlatitude stations, respectively. For the Arctic, Russian transport efficiencies tended to be the strongest, as expected, while European and Asian efficiencies were lower and higher, respectively, than those found in the literature. This disagreement is likely produced by mercury's long lifetime relative to that of other pollutants. The accepted springtime preference for the trans-Pacific transport of Asian pollution was evident only in the midlatitude group of stations, being masked in the arctic and subarctic groups by the occurrence of atmospheric mercury depletion events. Some

Research paper thumbnail of Chapter 2 Where Does Mercury in the Arctic Environment Come From, and How Does it Get There?

Very little of the Hg present in the Arctic is derived from pollution sources within this region;... more Very little of the Hg present in the Arctic is derived from pollution sources within this region; most is transported in from anthropogenic and natural sources outside the Arctic (AMAP, 2005). Previous AMAP assessments (AMAP, 1998,

Research paper thumbnail of Aurams Runs During the Pacific2001 Time Period — a Model/Measurement Comparison

Air Pollution Modeling and Its Application XVI, 2004

Research paper thumbnail of A Factor of 3 to 8 Provides a Reasonable Rationale for the Discrepancy of a Factor of 2 – 10 Determined

Zhang et al. (2012a), in a recent report, compared model estimates and new observations of oxidis... more Zhang et al. (2012a), in a recent report, compared model estimates and new observations of oxidised and particulate mercury species (Hg 2+ and Hg p) in the Great Lakes region and found that the sum of Hg 2+ and Hg p varied between a factor of 2 to 10 between measurements and model. They suggested too high emission inputs and too fast oxidative conversion of Hg 0 to Hg 2+ and Hg p , as possible causes. This study quantitatively explores in detail the uncertainties in measurements, in addition to the above concerns and speciation of mercury near emission sources in the model to better understand these discrepancies in the context of oxidized mercury, i.e. gaseous (Hg 2+) and particulate (Hg p) mercury. These include sampling efficiency, composition of sample, interfering species and calibration errors for measurements and in-plume reduction processes. Sensitivity simulations using Global/Regional Atmospheric Heavy Metals Model (GRAHM) were performed to analyze the role of in-plume reduction on ambient concentrations and deposition of mercury in North America. The discrepancy between simulated and observed concentrations of Hg 2+ and Hg p was found to be reduced when a ratio for Hg 0 :Hg 2+ :Hg p in the emissions was changed from 50:40:10 (as specified in the original inventories) to 90:8:2 to account for in-plume reduction of Hg 0 processes. A significant reduction of the root mean square error (e.g., 19.22 to 11.3 pg m −3 for New Jersey site NJ54) and bias (67.8 to 19.3 pg m −3 for NJ54) for sampling sites in the Eastern United States and Canada, especially for sites near emission sources was found. Significant improvements in the spatial distribution of wet deposition of mercury in North America was noticed. Particularly, over-prediction of wet deposition near anthropogenic sources of mercury was reduced by 43 %. On a regional scale, estimated wet deposition improved by a factor of 2 for areas with more than 12 µg m −2 yearly average wet deposition. Model sensitivity simulations show that the measured concentration of oxidized mercury is too low to be consistent with measured wet deposition fluxes in North America.

Research paper thumbnail of Vegetation uptake of mercury and impacts on global cycling

Nature Reviews Earth & Environment, 2021

In this review, we synthesize the current knowledge on mercury (Hg) content and sources in foliag... more In this review, we synthesize the current knowledge on mercury (Hg) content and sources in foliage and vegetated ecosystems and the importance of vegetation to global Hg cycling. By means of a global database of over 35,000 samples across 416 sites, we discuss global Hg concentrations in all major tissues, and mechanisms of vegetation Hg uptake. Hg in aboveground vegetation largely originates from uptake of atmospheric gaseous elemental Hg (Hg(0)), whereas Hg in roots originates from a combination of uptake from soil and foliage-to-root transport. Vegetation Hg uptake from the atmosphere and transfer to soils is the major Hg source in all biomes. Using model sensitivity analyses with and without global vegetation present, we show that vegetation Hg uptake modulates atmospheric Hg(0) seasonality in the northern hemisphere and interhemispheric gradient. We estimate that vegetation uptake the global Hg pool in the atmosphere by approximately 660 Mg and reduces the Hg deposition to global oceans, which in the absence of vegetation might receive an additional 960 Mg yr-1. We discuss future research needs to better constrain vegetation uptake mechanisms and their controlling physiological and environmental variables, improve model processes and address effects of climate and land use changes.

Research paper thumbnail of Aerosols in an urban cold climate: Physical and chemical characteristics of nanoparticles

Urban Climate, 2020

Atmospheric aerosols are of significant importance in climate change and health research. This st... more Atmospheric aerosols are of significant importance in climate change and health research. This study provides a 2-years meta dataset of real-time aerosol number and size distribution measurements in a model cold-climate city of Montreal (Canada). We provide selected information on the chemical composition and morphology of aerosols (~10 nm to 10 μm). We deployed a suite of complementary techniques such as aerosol particle sizers, MOUDI, S/TEM imaging, EDS, QQQ-ICP-MS/MS, IC, and TOC analyzers. The highest geomean nanoparticles (< 100 nm) number density (9260 ± 1.6 number/cm 3) was observed in the winter. A broad range of metals, including emerging contaminants, was quantified in all aerosol sizes include Pb, Cd, Ni, Zn, As, Al, Mn, Co, and Se, with concentrations ranging from 0.01 to 67.64 ng/m 3. Nanosize particles included Ni, Cd, Pb, Zn. Iron exhibited the highest concentrations consistently in two modes (< 180 nm and > 1 μm). S/TEM-EDS confirmed the abundance of nanoparticles, single and clusters, with multifaceted morphologies and compositions. We identified (a) emerging nanoparticles, (b) secondary organic aerosols, (c) bioaerosols, and (d) combustion particles. Diurnal, weekly, monthly, seasonal, and yearly variations of aerosols are provided. We herein discuss the implications of these results on air quality and climate modelling.

Research paper thumbnail of Total Mercury and Methylmercury in Lake Water of Canada’s Oil Sands Region

Environmental Science & Technology, 2018

Increased delivery of mercury to ecosystems is a common consequence of industrialization, includi... more Increased delivery of mercury to ecosystems is a common consequence of industrialization, including in the Athabasca Oil Sands Region (AOSR) of Canada. Atmospheric mercury deposition has been studied previously in the AOSR; however, less is known about the impact of regional industry on toxic methylmercury (MeHg) concentrations in lake ecosystems. We measured total mercury (THg) and MeHg concentrations for five years from 50 lakes throughout the AOSR. Mean lake water concentrations of THg (0.4−5.3 ng L −1) and MeHg (0.01−0.34 ng L −1) were similar to those of other boreal lakes and <5% of all samples exceeded Provincial water quality guidelines. Lakes with the highest THg concentrations were found >100 km northwest of oil sands mines and received runoff from geological formations high in metals concentrations. MeHg concentrations were highest in those lakes, and in smaller productive lakes closer to oil sands mines. Simulated annual average direct deposition of THg to sampled lakes using an atmospheric chemical transport model showed <2% of all mercury deposited to sampled lakes was emitted from oil sands activities. Consequently, spatial patterns of mercury in AOSR lakes were likely most influenced by watershed and lake conditions, though mercury concentrations in these lakes may be perturbed with future development and climatic change.

Research paper thumbnail of How important is biomass burning in Canada to mercury contamination?

Atmospheric Chemistry and Physics Discussions, 2017

Wildfire frequency has increased in past four decades in Canada, and is expected to increase in f... more Wildfire frequency has increased in past four decades in Canada, and is expected to increase in future as a result of climate change (Wotton et al., 2010). Mercury (Hg) emissions from biomass burning are known to be significant; however, the impact of biomass burning on air concentration and deposition fluxes in Canada has not been previously quantified. We use estimates of burned biomass from FINN (Fire Inventory from NCAR) and vegetation-specific Emission Factors (EFs) of mercury to investigate the spatiotemporal variability of Hg emissions in Canada. We use Environment and Climate Change Canada's GEM-MACH-Hg (Global Environmental Multi-scale, Modelling Air quality and Chemistry model, mercury version) to quantify the impact of biomass burning in Canada on spatiotemporal variability of air concentrations and deposition fluxes of mercury in Canada. We use North American gaseous elemental mercury (GEM) observations (2010–2015), GEM-MACH-Hg, and an inversion technique to op...

Research paper thumbnail of Multi-model study of mercury dispersion in the atmosphere: Atmospheric processes and model evaluation

Atmospheric Chemistry and Physics Discussions, 2016

Current understanding of mercury (Hg) behaviour in the atmosphere contains significant gaps. Some... more Current understanding of mercury (Hg) behaviour in the atmosphere contains significant gaps. Some key characteristics of Hg processes including anthropogenic and geogenic emissions, atmospheric chemistry, and air-surface exchange are still poorly known. This study provides a complex analysis of processes governing Hg fate in the atmosphere involving both measurement data from ground-based sites and simulation results of chemical transport models. A variety of long-term measurements of gaseous elemental Hg (GEM) and reactive Hg (RM) concentration as well as Hg wet deposition flux has been compiled from different global and regional monitoring networks. Four contemporary global-scale transport models for Hg were applied both in their state-of-the-art configurations and for a number of numerical experiments aimed at evaluation of particular processes. Results of the model simulation were evaluated against measurements. As it follows from the analysis the inter-hemispheric gradient of G...

Research paper thumbnail of Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

Chemical Reviews, 2015

Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver c... more Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver colored d-block heavy metal exhibits noble-gas-like characteristics and solidifies at a temperature of −38.83 °C.(1) Elemental mercury is the only metal that is liquid at room temperature and has an "ideal" high-energy surface. The average surface tension value of mercury based on different experimental measurements is 466 ± 33 mN m-1 .(2) Due to the versatility of mercury, it is used as a catalyst, as an electrode material in electrochemistry, in optical spectroscopy, as reflective liquid in liquid mirror telescopes, and in medicine, e.g., as amalgam in dental fillings. Yet, due to the varying, but significant, acute and chronic toxicity of mercury compounds, its usage has been of recent concern. Among single events with a large number of human casualties were the Minamata incident due to the release of highly toxic MeHg in the industrial wastewater from the 1930s to the 1960s in Minamata, Japan, and, in 1971 the Iraqi disaster, where organo-mercury fungicide treated seed grains were consumed by humans. Medical symptoms of mercury poisoning vary from parathesia to ataxia, loss of vision, insanity, paralysis coma, congenital deformities affecting fetuses, and death.(3) Because of the urgency to limit mercury related disasters, in 2013 an international treaty was signed by 128 countries, to control and "where feasible" reduce atmospheric emissions from point source categories (e.g., coal fired power plants and smelters). The treaty also called for additional mercury research.(4) The atmosphere is the fastest moving fluid in the Earth's environment. Many mercury species have appreciable vapor pressures at Earth surface temperatures, allowing them to enter the atmosphere and, depending on the mercury species involved, undergo long-and short-range transport, atmospheric physical and chemical transformation, as well as feedback with Earth's surface (see Figure 1), which is the main subject of this review paper.

Research paper thumbnail of Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

Chemical Reviews, 2015

Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver c... more Mercury is a fascinating fluid metal that exists as liquid at 25 °C (0 °C = 273 K). This silver colored d-block heavy metal exhibits noble-gas-like characteristics and solidifies at a temperature of −38.83 °C.(1) Elemental mercury is the only metal that is liquid at room temperature and has an "ideal" high-energy surface. The average surface tension value of mercury based on different experimental measurements is 466 ± 33 mN m-1 .(2) Due to the versatility of mercury, it is used as a catalyst, as an electrode material in electrochemistry, in optical spectroscopy, as reflective liquid in liquid mirror telescopes, and in medicine, e.g., as amalgam in dental fillings. Yet, due to the varying, but significant, acute and chronic toxicity of mercury compounds, its usage has been of recent concern. Among single events with a large number of human casualties were the Minamata incident due to the release of highly toxic MeHg in the industrial wastewater from the 1930s to the 1960s in Minamata, Japan, and, in 1971 the Iraqi disaster, where organo-mercury fungicide treated seed grains were consumed by humans. Medical symptoms of mercury poisoning vary from parathesia to ataxia, loss of vision, insanity, paralysis coma, congenital deformities affecting fetuses, and death.(3) Because of the urgency to limit mercury related disasters, in 2013 an international treaty was signed by 128 countries, to control and "where feasible" reduce atmospheric emissions from point source categories (e.g., coal fired power plants and smelters). The treaty also called for additional mercury research.(4) The atmosphere is the fastest moving fluid in the Earth's environment. Many mercury species have appreciable vapor pressures at Earth surface temperatures, allowing them to enter the atmosphere and, depending on the mercury species involved, undergo long-and short-range transport, atmospheric physical and chemical transformation, as well as feedback with Earth's surface (see Figure 1), which is the main subject of this review paper.

Research paper thumbnail of Mercury simulations within GMOS: Analysis of short-term observational episodes

E3S Web of Conferences, 2013

A number of contemporary chemical transport models for mercury are applied within the framework o... more A number of contemporary chemical transport models for mercury are applied within the framework of the EU GMOS project to study principal processes of mercury transport and transformations in the atmosphere. Each model is involved in simulation of short-term episodes corresponding to particular Hg measurement campaigns in Europe and other regions. In order to evaluate different physical and chemical mechanisms the models perform sensitivity runs with various parameterizations and/or combinations of considered processes. The modeling results are compared to detailed measurements of Hg species (Hg 0 /TGM, RGM, HgP) with high temporal resolution (hours) aiming at reproduction of short-term temporal variability of Hg air concentration.

Research paper thumbnail of Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone

Atmos. Chem. Phys., 2014

To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs... more To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. This paper constitutes Part 1 of the study, describing a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br 2 , is found to be a dominant process of reactive bromine formation in the Published by Copernicus Publications on behalf of the European Geosciences Union. 4102 K. Toyota et al.: Air-snowpack exchange of bromine and ozone calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low in the course of such events to be detected unambiguously by satellite nadir measurements. www.atmos-chem-phys.net/14/4101/2014/ Atmos. Chem. Phys., 14, 4101-4133, 2014

Research paper thumbnail of Chemical cycling and deposition of atmospheric mercury: Global constraints from observations

Journal of Geophysical Research, 2007

We use a global 3-D model of atmospheric mercury (GEOS-Chem) to interpret worldwide observations ... more We use a global 3-D model of atmospheric mercury (GEOS-Chem) to interpret worldwide observations of total gaseous mercury (TGM) and reactive gaseous mercury (RGM) in terms of the constraints they provide on the chemical cycling and deposition of mercury. Our simulation including a global mercury source of 7000 Mg yr À1 and a TGM lifetime of 0.8 years reproduces the magnitude and large-scale variability of TGM observations at land sites. However, it cannot capture observations of high TGM from ship cruises, implying a problem either in the measurements or in our fundamental understanding of mercury sources. Observed TGM seasonal variation at northern midlatitudes is consistent with a photochemical oxidation for Hg(0) partly balanced by photochemical reduction of Hg(II). Observations of increasing RGM with altitude imply a long lifetime of Hg(II) in the free troposphere. We find in the model that Hg(II) dominates over Hg(0) in the upper troposphere and stratosphere and that subsidence is the principal source of Hg(II) at remote surface sites. RGM observations at Okinawa Island (Japan) show large diurnal variability implying fast deposition, which we propose is due to RGM uptake by sea-salt aerosols. Observed mercury wet deposition fluxes in the United States show a maximum in the southeast, which we attribute to photochemical oxidation of the global Hg(0) pool. They also show a secondary maximum in the industrial Midwest due to regional emissions that is underestimated in the model, possibly because of excessive dry deposition relative to wet (dry deposition accounts for 68% of total mercury deposition in the United States in the model, but this is sensitive to the assumed phase of Hg(II)). We estimate that North American anthropogenic emissions contribute on average 20% to U.S. mercury deposition.

Research paper thumbnail of Parameterization of gaseous dry deposition in atmospheric chemistry models: Sensitivity to aerodynamic resistance formulations under statically stable conditions

Atmospheric Environment, 2016

Stability-correction algorithms for r a show notable discrepancies at Ri B > 0.2. Some of the atm... more Stability-correction algorithms for r a show notable discrepancies at Ri B > 0.2. Some of the atmospheric chemistry models compute L, u * and r a inconsistently. Choice of r a formulation impacts the simulated surface chemical exchange rates.

Research paper thumbnail of Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone

Atmos. Chem. Phys., 2014

To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs... more To provide a theoretical framework towards a better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. This paper constitutes Part 1 of the study, describing a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br 2 , is found to be a dominant process of reactive bromine formation in the Published by Copernicus Publications on behalf of the European Geosciences Union. 4102 K. Toyota et al.: Air-snowpack exchange of bromine and ozone calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low in the course of such events to be detected unambiguously by satellite nadir measurements. www.atmos-chem-phys.net/14/4101/2014/ Atmos. Chem. Phys., 14, 4101-4133, 2014

Research paper thumbnail of Behavior of mercury in snow from different latitudes

Journal de Physique IV (Proceedings), 2003

Research paper thumbnail of Spatial Trends and Historical Deposition of Mercury in Eastern and Northern Canada Inferred from Lake Sediment Cores

Environmental Science & Technology, 2009

Recent and historical deposition of mercury (Hg) was examined over a broad geographic area from s... more Recent and historical deposition of mercury (Hg) was examined over a broad geographic area from southwestern Northwest Territories to Labrador and from the U.S. Northeast to northern Ellesmere Island using dated sediment cores from 50 lakes (18 in midlatitudes (41-50°N), 14 subarctic (51-64°N) and 18 in the Arctic (65-83°N)). Distinct increases of Hg over time were observed in 76% of Arctic, 86% of subarctic and 100% of midlatitude cores. Subsurface maxima in Hg depositional fluxes (µg m-2 y-1) were observed in only 28% of midlatitude lakes and 18% of arctic lakes, indicating little recent reduction of inputs. Anthropogenic Hg fluxes adjusted for sediment focusing and changes in sedimentation rates (∆F adj,F) ranged from-22.9 to 61 µg m-2 y-1 and were negatively correlated (r)-0.57, P < 0.001) with latitude. Hg flux ratios (FRs; post-1990)/pre-1850) ranged from 0.5 to 7.7. The latitudinal trend for Hg ∆F adj,F values showed excellent agreement with predictions of the global mercury model, GRAHM for the geographic location of each lake (r) 0.933, P < 0.001). The results are consistent with a scenario of slow atmospheric oxidation of mercury, and slow deposition of reactive mercury emissions, declining with increasing latitude away from emission sources in the midlatitudes, and support the view that there are significant anthropogenic Hg inputs in the Arctic.

Research paper thumbnail of Mercury in Arctic marine ecosystems: Sources, pathways and exposure

Environmental Research, 2012

Mercury in the Arctic is an important environmental and human health issue. The reliance of North... more Mercury in the Arctic is an important environmental and human health issue. The reliance of Northern Peoples on traditional foods, such as marine mammals, for subsistence means that they are particularly at risk from mercury exposure. The cycling of mercury in Arctic marine systems is reviewed here, with emphasis placed on the key sources, pathways and processes which regulate mercury levels in marine food webs and ultimately the exposure of human populations to this contaminant. While many knowledge gaps exist limiting our ability to make strong conclusions, it appears that the long-range transport of mercury from Asian emissions is an important source of atmospheric Hg to the Arctic and that mercury methylation resulting in monomethylmercury production (an organic form of mercury which is both toxic and bioaccumulated) in Arctic marine waters is the principal source of mercury incorporated into food webs. Mercury concentrations in biological organisms have increased since the onset of the industrial age and are controlled by a combination of abiotic factors (e.g., monomethylmercury supply), food web dynamics and structure, and animal behavior (e.g., habitat selection and feeding behavior). Finally, although some Northern Peoples have high mercury concentrations of mercury in their blood and hair, harvesting and consuming traditional foods have many nutritional, social, cultural and physical health benefits which must be considered in risk management and communication.

Research paper thumbnail of Assessment of modeled mercury dry deposition over the Great Lakes region

Environmental Pollution, 2012

Three sets of model predicted values for speciated mercury concentrations and dry deposition flux... more Three sets of model predicted values for speciated mercury concentrations and dry deposition fluxes over the Great Lakes region were assessed using field measurements and model intercomparisons. The model predicted values were produced by the Community Multiscale Air Quality Modeling System for the year 2002 (CMAQ2002) and for the year 2005 (CMAQ2005) and by the Global/Regional Atmospheric Heavy Metals Model for the year 2005 (GRAHM2005). Median values of the surface layer ambient concentration of gaseous elemental mercury (GEM) from all three models were generally within 30% of measurements. However, all three models overpredicted surface-layer concentrations of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) by a factor of 2e10 at the majority of the 15 monitoring locations. For dry deposition of GOM plus PBM, CMAQ2005 showed a clear gradient with the highest deposition in Pennsylvania and its surrounding areas while GRAHM2005 showed no such gradient in this region; however, GRAHM2005 had more hot spots than those of CMAQ2005. Predicted dry deposition of GOM plus PBM from these models should be treated as upper-end estimates over some land surfaces in this region based on the tendencies of all the models to overpredict GOM and PBM concentrations when compared to field measurements. Model predicted GEM dry deposition was found to be as important as GOM plus PBM dry deposition as a contributor to total dry deposition. Predicted total annual mercury dry deposition were mostly lower than 5 mg m À2 to the surface of the Great lakes, between 5 and 15 mg m À2 to the land surface north of the US/Canada border, and between 5 and 40 mg m À2 to the land surface south of the US/Canada border. Predicted dry deposition from different models differed from each other by as much as a factor of 2 at regional scales and by a greater extent at local scales.

Research paper thumbnail of Long range transport of mercury to the Arctic and across Canada

Atmospheric Chemistry and Physics, 2010

This study is the most extensive study to date on the transport of mercury to the Arctic. Moreove... more This study is the most extensive study to date on the transport of mercury to the Arctic. Moreover, it is the first such study to use a fully-coupled, online chemical transport model, Environment Canada's Global/Regional Atmospheric Heavy Metals model (GRAHM), where the meteorology and mercury processes are fully integrated. It is also the only study to date on the transport of mercury across Canada. We estimated source attribution from Asia, North America, Russia and Europe at six arctic verification stations, as well as three subarctic and eight midlatitude Canadian stations. We have found that Asia, despite having transport efficiencies that were almost always lower than those of North America and often lower than those of Russia, was the dominant source of gaseous atmospheric mercury at all verification stations: it contributed the most mercury (29-37% at all stations, seasons and levels considered), its concentrations frequently explained nearly 100% of the variability in the concentrations produced by the simulation performed with full global emissions, particularly in the absence of local sources, and it generated the most long range transport (LRT) events, causing 43%, 67% and 75% of the events at the arctic, subarctic and midlatitude stations, respectively. For the Arctic, Russian transport efficiencies tended to be the strongest, as expected, while European and Asian efficiencies were lower and higher, respectively, than those found in the literature. This disagreement is likely produced by mercury's long lifetime relative to that of other pollutants. The accepted springtime preference for the trans-Pacific transport of Asian pollution was evident only in the midlatitude group of stations, being masked in the arctic and subarctic groups by the occurrence of atmospheric mercury depletion events. Some