Estimates Of Atmospheric Deposition Research Articles

Estimation of atmospheric deposition utilizing lichen Hypogymnia physodes, moss Hypnum cupressiforme and soil in Bosnia and Herzegovina

Estimation of atmospheric deposition utilizing lichen Hypogymnia physodes, moss Hypnum cupressiforme and soil in Bosnia and Herzegovina

Improving Estimates of Sulfur, Nitrogen, and Ozone Total Deposition through Multi-Model and Measurement-Model Fusion Approaches.

Earth system and environmental impact studies need high quality and up-to-date estimates of atmospheric deposition. This study demonstrates the methodological benefits of multimodel ensemble and measurement-model fusion mapping approaches for atmospheric deposition focusing on 2010, a year for which several studies were conducted. Global model-only deposition assessment can be further improved by integrating new model-measurement techniques, including expanded capabilities of satellite observations of atmospheric composition. We identify research and implementation priorities for timely estimates of deposition globally as implemented by the World Meteorological Organization.

Comparison of northeastern and southeastern U.S. watershed response to the declines in atmospheric sulfur deposition

Since the implementation of the 1970 Clean Air Act and its 1990 amendments, sulfur dioxide (SO2) emissions and subsequent sulfur (S) deposition have been declining across the eastern U.S. Although temporal patterns of deposition have been similar in the northeast (NE) and southeast (SE) regions of the U.S., stream and watershed responses have differed. This is primarily due to the spatial extent of the last glaciation, which terminated in northern Pennsylvania, rendering the soils in the SE more clay-rich and deeper than soils in the NE. Through the use of improved estimates of atmospheric deposition and more highly constrained estimates of stream export, we calculated S mass balances from New Hampshire to Georgia for 8 glaciated NE watersheds and 8 unglaciated SE watersheds that have high frequency (i.e. weekly) chemistry data available. Overall, total atmospheric S deposition declined by 70–90% from the early 1990s to the late 2010s across all sites. The resulting S budgets show that the timing of the conversion of the watersheds from net S retention to net S release differ between regions. The SE watersheds converted recently (late 2000s–2010s), while the NE watersheds have been net exporters of S since the 1990s to early 2000s. This delayed response in the SE is a consequence of the much higher sulfate (SO42−) adsorption capacity of the soils in this region. The flow normalized concentrations of SO42−, which eliminates variability due to discharge, shows that the majority of NE sites converged to similar concentrations by the mid to late 2010s, whereas the SE sites exhibit much greater spatial variability in concentrations. An evaluation of the storage and release of S within watersheds, using changes in adsorbed S and dissolved S (i.e. flow normalized concentration), suggests irreversibility of the S adsorption processes in NE and SE watersheds. The conversion of watersheds to a net release of S indicates improvements in watershed recovery due to the declines in acid deposition across the Eastern U.S.

Atmospheric deposition fluxes over the Atlantic Ocean: a GEOTRACES case study

Abstract. Atmospheric deposition is an important source of micronutrients to the ocean, but atmospheric deposition fluxes remain poorly constrained in most ocean regions due to the limited number of field observations of wet and dry atmospheric inputs. Here we present the distribution of dissolved aluminium (dAl), as a tracer of atmospheric inputs, in surface waters of the Atlantic Ocean along GEOTRACES sections GA01, GA06, GA08, and GA10. We used the surface mixed-layer concentrations of dAl to calculate atmospheric deposition fluxes using a simple steady state model. We have optimized the Al fractional aerosol solubility, the dAl residence time within the surface mixed layer and the depth of the surface mixed layer for each separate cruise to calculate the atmospheric deposition fluxes. We calculated the lowest deposition fluxes of 0.15±0.1 and 0.27±0.13 g m−2 yr−1 for the South and North Atlantic Ocean (>40∘ S and >40∘ N) respectively, and the highest fluxes of 1.8 and 3.09 g m−2 yr−1 for the south-east Atlantic and tropical Atlantic Ocean, respectively. Overall, our estimations are comparable to atmospheric dust deposition model estimates and reported field-based atmospheric deposition estimates. We note that our estimates diverge from atmospheric dust deposition model flux estimates in regions influenced by riverine Al inputs and in upwelling regions. As dAl is a key trace element in the GEOTRACES programme, the approach presented in this study allows calculations of atmospheric deposition fluxes at high spatial resolution for remote ocean regions.

Coastal ocean and shelf-sea biogeochemical cycling of trace elements and isotopes: lessons learned from GEOTRACES.

Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Critical Loads of Acid Deposition for Wilderness Lakes in the Sierra Nevada (California) Estimated by the Steady-State Water Chemistry Model

Major ion chemistry (2000–2009) from 208 lakes (342 sample dates and 600 samples) in class I and II wilderness areas of the Sierra Nevada was used in the Steady-State Water Chemistry (SSWC) model to estimate critical loads for acid deposition and investigate the current vulnerability of high elevation lakes to acid deposition. The majority of the lakes were dilute (mean specific conductance = 8.0 μS cm−1) and characterized by low acid neutralizing capacity (ANC; mean = 56.8 μeq L−1). Two variants of the SSWC model were employed: (1) one model used the F-factor and (2) the alternate model used empirical estimates of atmospheric deposition and mineral weathering rates. A comparison between the results from both model variants resulted in a nearly 1:1 slope and an R 2 value of 0.98, suggesting that the deposition and mineral weathering rates used were appropriate. Using an ANClimit of 10 μeq L−1, both models predicted a median critical load value of 149 eq ha−1 year−1 of H+ for granitic catchments. Median exceedances for the empirical approach and F-factor approach were −81 and −77 eq ha−1 year−1, respectively. Based on the F-factor and empirical models, 36 (17 %) and 34 (16 %) lakes exceeded their critical loads for acid deposition. Our analyses suggest that high elevation lakes in the Sierra Nevada have not fully recovered from the effects of acid deposition despite substantial improvement in air quality since the 1970s.

Influence of canopy budget model approaches on atmospheric deposition estimates to forests

Accurate quantification of total nitrogen and acidifying deposition is a major source of uncertainty in determining the exceedance of critical loads in forest ecosystems. Monitoring of atmospheric deposition is frequently based on throughfall measurements in combination with the canopy budget model to calculate ion-exchange fluxes between the forest canopy and incident rainfall water. Various approaches for each step in the canopy budget model have been reported and compared, but combinations of different approaches were not yet assessed. Therefore, the present study quantified the range of estimated dry deposition and total deposition resulting from all possible combinations of canopy budget model approaches for three typical case studies: (i) total nitrogen and potentially acidifying deposition onto a forest canopy, (ii) the ratio of these deposition variables between adjacent coniferous and deciduous stands and (iii) the parameters of a deposition time trend analysis. The time step, type of precipitation data and tracer ion used in the model had a significant effect on the findings in the three case studies. In addition, including or excluding canopy leaching of weak acids and canopy uptake of nitrogen during the leafless season largely affected the results, while including or excluding canopy uptake of nitrate generally showed no effect. In general, the use of wet-only precipitation data can be recommended, along with sodium as a tracer ion and the inclusion of weak acids. We conclude that further research should focus on the assumptions of inertness of the tracer ion and the equal deposition efficiency of base cations and the tracer ion and on the quantification of weak acids in rainfall and throughfall water. Since local or tree-species specific effects might influence the results obtained in this study, a similar analysis is recommended for other tree species and regions when using the canopy budget model.

A probabilistic estimate of global marine N‐fixation and denitrification

We determine the global rates of marine N‐fixation and denitrification and their associated uncertainties by combining marine geochemical and physical data with a new two‐dimensional box model that separates the Atlantic from the IndoPacific basins. The uncertainties are estimated using a probabilistic approach on the basis of a suite of 2500 circulation configurations of this box model. N‐fixation and denitrification are diagnosed in an inverse manner for each of these configurations usingN*, P*, and the stable nitrogen isotope composition of nitrate as data constraints. Our approach yields a median water column denitrification rate of 52 TgN yr−1 (39 to 66 TgN yr−1, 5th to 95th percentile) and a median benthic denitrification rate of 93 TgN yr−1 (68 to 122 TgN yr−1). The resulting benthic‐to‐water column denitrification ratio of 1.8 confirms that the isotopic signature of water column denitrification has a limited influence on the global mean stable isotopic value of nitrate due to the dilution of the waters with a denitrification signal with the remainder of the ocean's nitrate pool. On the basis of two different approaches, we diagnose a global N‐fixation rate of between 94 TgN yr−1 and 175 TgN yr−1, with a best estimate of 131 TgN yr−1 and 134 TgN yr−1, respectively. Most of the N‐fixation occurs in the IndoPacific suggesting a relative close spatial coupling between sources and sinks in the ocean. Our N‐fixation and denitrification estimates plus updated estimates of atmospheric deposition and riverine input yield a pre‐industrial marine N cycle that is balanced to within 3 TgN yr−1 (−38 to 40 TgN yr−1). Our budget implies a median residence time for fixed N of 4,200 yr (3,500 to 5,000 yr).

Is it possible to estimate atmospheric deposition of heavy metals by analysis of terrestrial mosses?

Here we present a critical review of diverse research studies involving estimation of atmospheric deposition of heavy metals from the concentrations of the contaminants in terrestrial moss. The findings can be summarized as follows: i) significant correlations between the concentrations of contaminants in moss and bulk deposition were observed in only 40.1% of the cases in which the relationship was studied and in only 14.1% of the cases, the coefficient of correlation was > 0.7; ii) some method-related problems were identified (i.e. small sample sizes, elimination of some data from the regression analyses, large distances between the moss sampling sites and the bulk precipitation collectors, differences in times of exposure of the moss samples and collection times for the bulk precipitation), so that the results of the studies may not be completely valid, and iii) evidence was found in the relevant literature that moss does not actually integrate the atmospheric deposition received. We also discuss the reason why, in accordance with the published data, bulk deposition cannot be correctly estimated by determination of the final concentrations of contaminants in the organism, such as the existence of different sources of contamination, the physicochemical characteristics of the sources of deposition, physicochemical processes to which the organism is subjected and the biological processes that take place in the moss. Taking into account the above findings, it was concluded that, except for certain elements and specific cases (i.e. Pb and Cd), atmospheric deposition of elements cannot be accurately estimated from the concentrations of metals and metalloids in moss tissues. However, the analysis of moss does provide information about the presence of contaminants in the atmosphere, their spatial and temporal patterns of distribution and how they are taken up by live organisms. Use of mosses is therefore recommended as a complementary (rather than an alternative) technique in the conventional analysis of bulk deposition of contaminants.

Protocol to Reconstruct Historical Contaminant Loading to Large Lakes: The Lake Michigan Sediment Record of Mercury

Samples of opportunity from Pb-210 dated sediment cores collected from Lake Michigan between 1994 and 1996 were analyzed for mercury. The storage of anthropogenic and total (post-1850) mercury in the lake is calculated to be 186 and 228 t, respectively. By setting the sum of mercury stored in a representative core equal to the mercury storage within the entire lake, the time variation of annual mercury loading to the lake is calculated. The modern (1980-2002) mercury flux to the lake represented by the surface of the core at the time of collection in 1994 was 21.4 microg/m(2)/y. The preindustrial flux (< or =1850) was 3.09 microg/m(2)/y, and the peak flux in 1946 was 53.3 microg/m(2)/y. These yield modern and peak enrichment factors of 6.92 and 17.2, respectively. Modern fluxes exceed published atmospheric deposition estimates and, therefore, include terrestrial point sources, atmospheric deposition to watersheds, and atmospheric deposition to the lake. The modern net mercury load to the lake's sediments was 1157 kg/y in 1994. The atmosphere is estimated to contribute 91% of this load directly to the lake.

Atmospheric Deposition of PBDEs to the Great Lakes Featuring a Monte Carlo Analysis of Errors

The first estimates of atmospheric deposition of polychlorinated diphenyl ethers (PBDEs) into the Great Lakes are presented. Precipitation samples were collected monthly from 2005 to 2006 at five sites located in the Great Lakes region. Volume weighted mean concentrations of total PBDEs in precipitation ranged from 94 +/- 19 ng L(-1) at the urban site of Chicago to 0.65 +/- 0.14 ng L(-1) for the rural site of Sturgeon Point Using gas and particle phase concentrations previously obtained in our laboratory and concentrations in precipitation measured in this study, the total annual net mass transfer rates to each lake and their relative errors were calculated using Monte Carlo analysis. The highest net mass transfer rates for BDE-47, and total PBDEs were measured for Lake Michigan (150 +/- 40 and 310 +/- 79 kg y(-1), respectively) while the highest net mass transfer rate for BDE-209 was measured for Lake Erie (79 +/- 56 kg y(-1)). We found good agreement between atmospheric and sediment net mass transfer rates for Lake Superior, but we found a significant imbalance of BDE-209 for the other two lakes and of BDE-47 for Lake Michigan. These findings suggest that Chicago might be a preferential source of BDE-47 to Lake Michigan and that Cleveland might be a preferential source of BDE-209 to Lake Erie.

Origins of Sediment-Associated Contaminants to the Marais Vernier, the Seine Estuary, France

The Marais Vernier is the largest freshwater wetland in the Seine Estuary in northern France. It is in a heavily urbanized and industrialized region and could be affected by atmospheric deposition and by fluvial input of contaminants in water diverted from the Seine River. To evaluate contaminant histories in the wetland and the region, sediment cores were collected from two open-water ponds in the Marais Vernier: the Grand-Mare, which was connected to the Seine by a canal from 1950 to 1996, and the Petite Mare, which has a small rural watershed. Diversions from the Seine to the Grand-Mare increased sedimentation rates but mostly resulted in low contaminant concentrations and loading rates, indicating that the sediment from the Seine was predominantly brought upstream by tidal currents from the estuary and was not from the watershed. Atmospheric sources of metals dominate inputs to the Petite Mare; however, runoff of metals from vehicle-related sources in the watershed might contribute to the upward trends in concentrations of Cr, Cu, and Zn. Estimates of atmospheric deposition using the Petite Mare core are consistent with measured deposition in the region and are mixed (similar for Hg and Pb; larger for Cd, Cu, and Zn) compared with deposition estimated from sediment cores in the northeastern United States. A local source of PAHs in the watershed of the Petite Mare is indicated by higher concentrations, higher accumulation rates, and a different, more petrogenic, PAH assemblage than in the Grand-Mare. The study illustrates how diverse sources and transport pathways can affect wetlands in industrial regions and can be evaluated using sediment cores from the wetland ponds.

Trends in impact of acidification on groundwater bodies in the Czech Republic; an estimation of atmospheric deposition at the horizon 2015

The acidification of surface and groundwaters is a long standing issue in the Czech Republic which culminated in the second half of the last century by mass extinction of forest cover in many mountainous regions. The total deposition of sulfur at that time was reaching as much as about 108 kg/ha/year. This resulted in a decrease of alkalinity in groundwaters, decrease of pH and on the other hand an increase in concentration of nitrates, sulfates, berylium and aluminium. Desulfurization of power plants and attenuation of heavy industry leads to a decrease in sulfur deposition down to the present mean values around 16.5 kg/ha/year. It is expected that around the year 2010 the sulfur deposition should not have a substantial impact on the environment providing the similar trend would continue. However, a reverse trend can be observed in deposition of nitrogen. The growth of road traffic and more frequent use of earth gas for heating lead to a dramatic rise in nitrogen throughfall of which mean value increased from 15.4 kg/ha/year in 1990 up to 25.7 kg/ha/year in 2001. If such trend continues in future, then the mean values of nitrogen throughfall would attain 37.8 kg/ha/year in 2015, so that the nitrogen would take over the role of sulfur in acidification. Therefore, the problem of acidification in Central Europe is by no means resolved, only the structure of acidification input has been changed.

Pelagic Nitrogen Fixation in Lake Victoria (East Africa)

Between 1994 and 1998 biological nitrogen fixation was measured using the acetylene reduction method at inshore and offshore sites on northern Lake Victoria. Rates of biological N-fixation were high and often exceeded 0.5 μg N/L/h. Average rates of volumetric N-fixation at optimal irradiance were 8 times higher at inshore locations than in the offshore. Rates of annual areal N-fixation, modeled from the N-fixation light-response and light attenuation in the lake were moderate to high (1.8 to 23.1 g N/m 2/y) depending on location within the lake, and were, on average, only twice as high inshore as offshore. Variations in the light extinction coefficient explained a small but significant proportion of the variation in the optimal N-fixation in Lake Victoria. Algal biomass and N-fixation were lower in the more deeply mixing (> 20 m) offshore waters, because of the persistently low mean light intensities (108 ± 41 μE/m 2/s) in the water column most of the year. N-fixation increased with increased light availability, and maximal rates occurred when the lake was shallowly mixing and thermally stratified. At both inshore and offshore stations, minima of algal biomass and N-fixation were consistent with low light availability in July when the lake was most deeply mixing. The ratio of the mean water column irradiance (I 24) to the irradiance at which N-fixation approaches saturation (I k) was often < 1, and provides evidence that N-fixation was light-limited, chronically so in the offshore region. Light limitation of algal growth lessens the algal demand for N, and it constrains algal biomass development and N-fixation more in the offshore compared to the inshore surface waters of Lake Victoria. Biological N fixation is the largest input of fixed N to Lake Victoria, greatly exceeding estimates of atmospheric deposition and river inputs of N.

Comparison of snowpack and winter wet-deposition chemistry in the Rocky Mountains, USA: implications for winter dry deposition

Depth-integrated snowpack chemistry was measured just prior to maximum snowpack depth during the winters of 1992–1999 at 12 sites co-located with National Atmospheric Deposition Program/National Trend Network (NADP/NTN) sites in the central and southern Rocky Mountains, USA. Winter volume-weighted mean wet-deposition concentrations were calculated for the NADP/NTN sites, and the data were compared to snowpack concentrations using the paired t-test and the Wilcoxon signed-rank test. No statistically significant differences were indicated in concentrations of SO 4 2− or NO 3 − ( p>0.1). Small, but statistically significant differences ( p⩽0.03) were indicated for all other solutes analyzed. Differences were largest for Ca 2+ concentrations, which on average were 2.3 μeq l −1 (43%) higher in the snowpack than in winter NADP/NTN samples. Eolian carbonate dust appeared to influence snowpack chemistry through both wet and dry deposition, and the effect increased from north to south. Dry deposition of eolian carbonates was estimated to have neutralized an average of 6.9 μeq l −1 and a maximum of 12 μeq l −1 of snowpack acidity at the southernmost sites. The good agreement between snowpack and winter NADP/NTN SO 4 2− and NO 3 − concentrations indicates that for those solutes the two data sets can be combined to increase data density in high-elevation areas, where few NADP/NTN sites exist. This combination of data sets will allow for better estimates of atmospheric deposition of SO 4 2− and NO 3 − across the Rocky Mountain region.

Element concentrations in the forest moss Hylocomium splendens: variation associated with altitude, net primary production and soil chemistry

Net primary production (NPP) of the forest moss Hylocomium splendens increased significantly along an elevational gradient in the southern Alps of Italy. Extracellularly bound metals (Al, Ca, Co, Cr, Fe, Ni, Mo, Ni, Pb) showed declining concentrations in moss tissue with increasing altitude, presumably because the amount of exchange sites on the cell wall increases less than total biomass. Concentrations of intracellular elements did not vary (Cd, Cu, Mg, Na, Zn), or even increased (K) with altitude. The observed patterns were always independent of precipitation amount and soil concentrations of exchangeable elements. A higher soil nutrient status only enhanced K uptake by the moss. We concluded that variations in moss NPP, associated with elevational gradients, may significantly affect estimates of atmospheric deposition based on moss analysis in mountainous regions.

Identifying potentials for reducing uncertainty in critical load calculations using the PROFILE model

A sensitivity analysis was performed testingweathering rates, critical loads andexceedances for Swedish forest soils using Monte Carlosimulations of the PROFILE model. Different subsetsof input data were investigated with respect to theirpotential to reduce data uncertainty at site level butalso for modified estimates of the 5%-ile critical load andthe 95%-ile exceedance on 150×150 kmresolution. Physical soil properties were of dominantimportance for all sites and yield up to 62%reduction of the output standard deviation in weathering rate.The study showed that the critical ratio ofbase cations to inorganic aluminium (Bc/Al ratio) in the soilsolution was of major importance for reducingdata uncertainty in critical loads and exceedance estimates.The critical Bc/Al ratio was found to beimportant for reducing data uncertainties in modifiedestimates of the 5%-ile critical load and the95%-ile exceedance, in particular in the northern part ofSweden. Atmospheric deposition, uptake andlitterfall were more important for reducing data uncertaintyin the southern part. Physical soil propertiesand especially mineral content were found to be less importantfor reducing data uncertainties in criticalloads and exceedance estimates. The greatest scope forreducing data uncertainties in an applied perspectiveis to improve estimates of atmospheric deposition of anionsand cations as well as uptake and litterfall ofbase cations and nitrogen.

The Effect of Landscape Features on Deposition to Hunter Mountain, Catskill Mountains, New York

Atmospheric deposition to montane ecosystems is higher than to adjacent lowlands. However, because of the heterogeneity of mountainous landscapes, rates of deposition are likely to vary considerably across major landscape features. Estimates of total atmospheric deposition for montane areas in the United States are wide ranging and usually based on models that do not take into account landscape heterogeneity. Thus, little is known about the spatial variability of atmospheric deposition to these high-elevation ecosystems. We identified four landscape features that are likely to control patterns and rates of atmospheric deposition in mountainous terrain, including (1) forest edges or gaps, (2) elevation, (3) aspect, and (4) vegetation type, and we measured patterns of atmospheric deposition across them in the Catskill Mountains of New York State. We measured lead amount in the forest floor as an index of atmospheric deposition, used these data to estimate relative deposition to the Hunter Mountain massif, and with a geographic information system (GIS) database, scaled up atmospheric deposition estimates to the Hunter Mountain landscape. Area-weighted mean deposition to the region above 1000 m (high-elevation) was 13–43% greater than to a nearby, low-elevation site, depending upon how edges were defined. In addition, we identified “hotspots” of deposition, such as high-elevation, coniferous forest edge zones, where atmospheric deposition of pollutants and nutrients was 300% greater (44 kg N·ha−1·yr−1 and 46 kg S·ha−1·yr−1) than to low-elevation forests (11.1 kg N·ha−1·yr−1 and 11.6 kg S·ha−1·yr−1). Our data suggest that estimates of atmospheric deposition to mountainous terrain that do not include landscape heterogeneity may be inadequate. Furthermore, current models of atmospheric deposition cannot predict “hotspot” regions of extremely high deposition. Landscape features such as forest edges, elevation, aspect, and vegetation type should be considered in future modeling efforts.

Sources and Fluxes of Mercury in the St. Lawrence River

A mass balance approach, based essentially on the reconstruction of daily fluxes and circumscribed by strict error calculations, was designed to quantify the main mercury sources for the St. Lawrence and its tributaries, which constitute a large river system. High-frequency samplings were performed over an 18-month period (1955--1996) at the main water inputs and the mouth of the river. Minor tributaries and the Montreal effluent were also sampled. This strategy allowed models to be obtained that relate mercury concentrations in solution and in particles to the hydrological regime. The calculated budget was balanced relative to the calculated errors of the estimates. Gross mercury export from the river was found to be 5.9 kmol yr{sup {minus}1}. Tributaries and internal erosion of the river contributed equally for a total of 75% of this gross load, whereas the Upper St. Lawrence River, which is almost exclusively composed of Lake Ontario waters, accounted for less than 10%, and inventoried anthropogenic point sources accounted for about 5%. Dissolved mercury was mainly from north shore tributaries, and particulate mercury was largely from erosion of the river bed and banks. On the basis of the present results as well as estimates of atmospheric deposition from the literaturemore » it can be inferred that at least 88% of deposited mercury was retained in the watersheds.« less

Atmospheric Inputs of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans to the Great Lakes: Compositional Comparison of PCDD and PCDF in Sediments

Concentrations and accumulations of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were determined in dated sediment cores from three of the Great Lakes and two remotely located “control lakes” to assess how inputs of the compounds vary currently and historically among the lakes. The current PCDD accumulation rates in the Great Lakes ranged from a low of 0.0075 ng/cm 2/yr in Lake Superior to a high of 0.22 ng/cm 2/yr in Lake Ontario. PCDF accumulation rates ranged from a low of 0.00078 ng/cm 2/yr in Lake Superior to a high of 0.23 ng/cm 2/yr in Lake Ontario. The relative importance of atmospheric versus non-atmospheric sources of PCDDs and PCDFs to the lakes was estimated by comparing homolog compositions and accumulation rates among cores and homolog-specific accumulation rates to estimates of atmospheric deposition. The estimates are that the atmosphere currently provides (a) 100% of PCDD and PCDF to Lake Superior, (b) 100% of PCDD to southern Lake Michigan, 33–50% of PCDD to northern Lake Michigan, but only 5–35% of PCDF to Lake Michigan, and (c) only 5–35% of PCDD and < 5% of PCDF to Lake Ontario. By comparing the sediment homolog compositions to those of non-atmospheric sources, likely non-atmospheric sources are suggested.