Wet deposition of mercury and ambient mercury concentrations at a site in the Lake Champlain basin

The “Great Waters” program, established in the 1990 Clean Air Act Amendments, mandated that atmospheric deposition of hazardous air pollutants to Lake Champlain (including Hg) be assessed. An assessment of the magnitude and seasonal variation of atmospheric Hg deposition in the Lake Champlain basin was initiated in December 1992 with one year of event precipitation collection, as well as collection of vapor and particle phase Hg in ambient air. Samples were collected at the Vermont Monitoring Cooperative air monitoring site at the Proctor Maple Research Center in Underhill Center, VT. The average volume-weighted concentration for Hg in precipitation was 8.3 ng/L for the sampling year and the average amount of Hg deposited with each precipitation event was 0.069 μg/m². The total amount of Hg deposited through precipitation during 1993 was 9.26 μg/m²/yr. A seasonal pattern for Hg in precipitation was evident, with increased concentrations and deposition during spring and summer months. Meteorological analysis indicated the highest levels of Hg in precipitation were associated with regional transport from the south regardless of season, and with transport from the west, southwest and northwest during spring and summer months. Concentrations of ambient vapor phase Hg were typical of rural locations and consistent across seasons. Ambient particulate Hg concentrations averaged 11 pg/m³ with highest concentrations during the winter months.     

An intensive multi-site pilot study investigating atmospheric mercury in Broward County,Florida

An intensive multi-site pilot study of atmospheric Hg was conducted in Broward County, Florida in August and September of 1993. Broward County, which contains the city of Fort Lauderdale, is located in southeastern Florida. The county borders the Florida Everglades on the west and the Atlantic Ocean on the east. A network of four sampling sites was set up for 20 days throughout Broward County to measure Hg in both the vapor phase and the particle phase as well as Hg in precipitation. The mean concentrations of total vapor phase Hg measured at two inland sites were found to be significantly higher (3.3 and 2.8 ng/m³) than that measured at a site located on the Atlantic shore (1.8 ng/m³). The mean concentrations of particle phase Hg collected at the two inland sites (51 and 49 pg/m³) were found to be 50% greater than that measured at the coastal site (34 pg/m³). In addition, event precipitation samples were collected at four sampling sites over the 20 day study period and were analyzed for both reactive and total Hg. The mean concentration of total Hg in the precipitation samples was found to be 44 ng/L, with a range of 14 to 130 ng/L. It was determined that further meteorological analysis and a more complete characterization of the aerosol and precipitation composition are needed to identify the probable source(s) contributing to the increased deposition of Hg.     

Mercury Transport in a High-Elevation Watershed in Rocky Mountain National Park, Colorado

Mercury (Hg) was measured in stream water and precipitation in the Loch Vale watershed in Rocky Mountain National Park, Colorado, during 2001–2002 to investigate processes controlling Hg transport in high-elevation ecosystems. Total Hg concentrations in precipitation ranged from 2.6 to 36.2 ng/L and showed a strong seasonal pattern with concentrations that were 3 to 4 times higher during summer months. Annual bulk deposition of Hg was 8.3 to 12.4 μ g/m² and was similar to deposition rates in the Midwestern and Northeastern U.S. Total Hg concentrations in streams ranged from 0.8 to 13.5 ng/L and were highest in mid-May on the rising limb of the snowmelt hydrograph. Stream-water Hg was positively correlated with dissolved organic carbon suggesting organically complexed Hg was flushed into streams from near-surface soil horizons during the early stages of snowmelt. Methylmercury (MeHg) in stream water peaked at 0.048 ng/L just prior to peak snowmelt but was at or below detection (< 0.040 ng/L) for the remainder of the snowmelt season. Annual export of total Hg in Loch Vale streams ranged from 1.2 to 2.3 μ g/m², which was less than 20% of wet deposition, indicating the terrestrial environment is a net sink of atmospheric Hg. Concentrations of MeHg in stream water and corresponding watershed fluxes were low, indicating low methylation rates or high demethylation rates or both.     

Atmospheric deposition of mercury in Florida: The fams project (1992–1994)

The primary goal of the Florida Atmospheric Mercury Study (FAMS) is to quantify the seasonal and geographical variability in the atmospheric deposition of Hg and other trace elements in central and south Florida. Precipitation, aerosol, and gaseous Hg samples have been collected at seven sites in Florida for periods ranging from 3 to 24 months. The summertime wet season in south Florida accounts for 80 to 90% of the annual rainfall Hg deposition. Depositional rates in south Florida are 30 to 50% higher than those from central Florida. Particle phase measurements range from 2 to 18 pg/m³Hg at all sites. Measurements of monomethylmercury in precipitation range from <0.005 to 0.020 ng/L.     

Particulate mercury in the atmosphere: Its significance,transport, transformation and sources

The importance of participate mercury (Hg(p)) in the transport, chemistry and deposition of this toxic metal has long been underestimated and largely ignored. While it was once believed to constitute a small percentage of total atmospheric mercury, Hg(p) may contribute a significant portion of the deposition of this metal to adjacent natural waters. Recent measurements of Hg(p) in several urban/industrial areas have documented that Hg can be associated with large particles (>2.5 μm) and in concentrations similar to those of the vapor phase Hg (ng/m³). As part of ongoing effort to diagnose the sources, transport and deposition of Hg to the Great Lakes and other Great Waters, the University of Michigan Air Quality Laboratory (UMAQL) has investigated the physical and chemical properties of particulate-phase Hg in both urban and rural locations. It appears that particulate Hg may be the one of the most difficult of the Hg measurements to perform, and perhaps the one of the most important for deposition and source apportionment studies. Particulate Hg concentrations measured in rural areas of the Great Lakes Region and Vermont ranged from 1 to 86 pg/m³ whereas Hg(p) levels in urban/industrialized areas were in the range 15 pg/m³ to 1.2 ng/m³.     

Litterfall Mercury in Two Forested Watersheds at Acadia National Park, Maine, USA

Litterfall can be an important flux of mercury (Hg) to soils in forested landscapes, yet typically the only available data to evaluate Hg deposition is from precipitation Hg monitoring. Litterfall was collected at 39 sampling sites in two small research watersheds, in 2003 and 2004, and analyzed for total Hg. Four vegetation classes were designated in this study as hardwoods, softwoods, mixed and scrub. The mean litter Hg concentration in softwoods (58.8 ± 3.3 ng Hg g^?1 was significantly greater than in mixed (41.7 ± 2.8 ng Hg g^?1 and scrub (40.6 ± 2.7 ng Hg g^?1, and significantly lower than in hardwoods (31.6 ± 2.6 ng Hg g^?1. In contrast, the mean weighted litter Hg flux was not significantly different among vegetation classes. The lack of a significant difference in litter Hg flux between hardwoods and softwoods was attributable to the large autumnal hardwood litter Hg flux being balanced by the higher softwood litter Hg concentrations, along with the higher chronic litterfall flux throughout the winter and spring in softwoods. The estimated annual deposition of Hg via litterfall in Hadlock Brook watershed (10.1 μg m^?2 and Cadillac Brook watershed (10.0 μg m^?2 was greater than precipitation Hg deposition and similar to or greater than the magnitude of Hg deposition via throughfall. These results demonstrate that litterfall Hg flux to forested landscapes can be at least as important as precipitation Hg inputs.     

A Preliminary Assessment of Wet Deposition and Episodic Transport of Total and Methyl Mercury from Low Order Blue Ridge Watersheds,S.E. U.S.A.

Results from a preliminary sampling program designed to investigate total (THg) and methyl Hg (MeHg) deposition, cycling and transport at the Coweeta Hydrologic Laboratory western North Carolina are presented. Wet deposition samples were collected in June and July 1994 and throughfall, seep and streamwaters were intensively collected during and after a rainfall event in June 1994. All water samples were collected using ultra clean trace sampling protocol. Low elevation Watershed 18 streamwater THg concentrations peaked with discharge, increasing 6 fold to 9 ng L^-1. High elevation Watershed 27 which received less than one half the precipitation Watershed 18 received during the event, exhibited THg concentrations only 1.3 times over base flow conditions. Methyl Hg concentrations remained near detection limits (≤ 0.025 ng L^-1) in both streams. Dissolved MeHg concentrations were higher in shallow seep (0.097 ng L^-1), throughfall (0.135 ng L^-1) and precipitation (0.16 – 0.035 ng L^-1) than streamwaters. Initial estimates of annual THg and MeHg deposition and transport indicate >90% retention of Thg and a >80% retention or demethylation of wet deposition MeHg is occurring in these low order watersheds.     

Mercury Transport in a Low-Arctic River in Kobbefjord, West Greenland (64° N)

Mercury (Hg) transport was studied in a river in Kobbefjord, near Nuuk in West Greenland, during the 2009 and 2010 summer periods. The river drains an area of 32?km², and the Kobbefjord area is considered representative to low-Arctic West Greenland. The river water origins from both precipitation and melting of small glaciers and annual water discharges for 2009 and 2010 were estimated to be 29 and 26 million?m³, respectively. Mean Hg concentrations (±SD) were 0.46?±?0.17 and 0.26?±?0.17?ng?L^?1 for 2009 and 2010. The annual Hg transport was estimated to 14 and 6.4?g, corresponding to a transport rate of 0.45 and 0.20?g Hg km^?2?year^?1 from the river basin. The highest Hg concentrations (up to 1.0?ng?L^?1) and discharges were measured in spring 2009 along with melting of extensive amounts of snow deposited during the 2008?C2009 winter period. In contrast, the following 2009?C2010 winter period was relatively dry with less snowfall. This indicates that a major fraction of the Hg in this area is likely to come from Hg deposited along with winter precipitation (as wet deposition) released upon snowmelt. Also, the results show that while Hg concentrations were low in Kobbefjord River compared to other sub-Arctic/Arctic rivers, the annual Hg transport rates from the basin area were within the range reported for other sub-Arctic/Arctic areas.     

Mercury Air/Surface Exchange Kinetics of Background Soils of the Tahquamenon River Watershed in the Michigan Upper Peninsula

Air/surface exchange of mercury was investigated over background soils at five sites in the Tahquamenon River watershed in the Michigan Upper Peninsula in the summer of 1998. Measurements of Hg fluxes were performed during middayperiods using the ORNL Teflon dynamic flux chamber. Mean Hg emission fluxes were 1.4±0.3–2.4±1.0 ng m^-2 hr^-1 for three shaded forest sites and 7.6±1.7 ng m^-2 hr^-1 for an open field site. Hg dry deposition was observed at a heavily shaded forest site overwet soils (mean = –0.3±0.2 ng m^-2 hr^-1). Theoverall mean Hg flux was 1.4±1.4 ng m^-2 hr^-1for the four shaded forest sites. The Hg fluxes observed at these sites are similar to those found at other northern background sites. Significant, rapid response of Hg emission to solar radiation was observed over these background soils. Artificial irrigation over these soils induced immediate andmeasurable increases in Hg emission. Soil temperature was found to be less influential to Hg air/surface exchange over these heavily shaded forest background soils than we have seen elsewhere.     

Historical deposition rates of mercury in scandinavia estimated by dating and measurement of mercury in cores of peat bogs

Historical deposition rates of Hg were determined in 7 ombrotrophic bogs located far from direct sources in Sweden and Norway. The peat bog cores were dated using ²¹⁰Pb. Based on the dating result 10 to 12 slices from each core were analyzed for Hg. In Southwestern Scandinavia (Rörvik) the deposition rate has increased from about 10 to about 30 μg Hg m^?2 yr^?1 since the beginning of this century. In Northwestern Norway (Överbygd) the deposition rate has increased from about 10 to about 35 μg Hg m^?2 yr^?1 since the 1960's. In Southeast Sweden (Aspvreten) the deposition rate decreased since the beginning of this century. There are no significant trends in the historical deposition rates at the two other stations in middle Scandinavia. A comparison between recent Hg deposition rates measured by peat bog analysis, wet precipitation and mosses shows good agreement at all stations except those in Northern Norway were local dry deposition of Hg may be an important source of Hg.     

Resuspension of Trace Metals in Belgrade Under Conditions of Drastically Reduced Emission Levels

Deposition rates of trace metals (Pb, Cu, Zn, Cd) were determined at the Meteorological Station in New Belgrade (H _s =74 m; φ=44°49′N and λ=20°24′E) in the hydrological year 1992/93 (1 November-31 October). The dominant characteristics of the observation period were strong drought and drastically reduced pollutant emissions. Traffic and local sources operated at 0–10% of their capacities. Samples of dry and wet precipitation were taken by an automatic collector at a height of 2 m at weekly intervals. Two petri polycarbonate dishes of 93.5 mm diameter, facing upward, and one, facing downward, were used for dry precipitation sampling. Determination in a ‘clean room’ by differential pulse stripping voltammetry provided the low detection thresholds of 0.1, 1.0, 0.5 and 2.0 ng mL^-1,i.e. the measurement of minimal dry deposition rates of 0.02, 0.2, 0.1 and 0.4 μg m^-2d^-1 for Cd, Pb, Cu and Zn, respectively. The mean annual deposition rates, expressed in μg m^-2d^-1, were found to be as follows: 4.5 (down), 26 (up) and 37 (wet) for Pb, 2.1 (down), 13 (up) and 36 (wet) for Cu, and 3.1 (down), 26 (up) and 95 (wet) for Zn. No values above 0.2 μg m^-2d^-1 were found for the Cd dry deposition rate and no Cd concentration higher than 1 ng mL^-1 for precipitation with an amount >0.5 mm at the weekly interval was detected. Using the XREDS method with a scanning electron microscope, Si, Al, Fe matrix elements and Mg, Ca, S, K, P, Cu, Zn. W were identified as minor constituents of the single spherical particles and of the agglomerates present. Pb could not be categorized as a minor constituent of the coarse particles suspended in the air of New Belgrade on the basis of the previous analysis. Analysis of meteorological data showed road dust transport by outgoing SE-ESE wind from the old city of Belgrade (H _s =100–250 m) as a possible Pb source at the chosen site, which was located in a park. A comparative measurement of Pb deposition on linden leaves was carried out in the vegetation period (1 April–31 October) of 1993 and 1994. The Pb distribution on leaves differed in the two growing seasons, while the sum of Pb deposition rates on the downward and upward oriented petri dishes remained practically equal. The Pb accumulation on linden leaves was higher in 1994, first because of an intensive soil supply by rain water. Besides, ‘yellow’ rain was reported during the night 15/16 April, on 16 and 17 April 1994. A Fe concentration of 2.3 μg mL^-1 and a Pb concentration of 312 ng mL^-1 with a total precipitation of 12.6 mm were found in the weekly wet sample of 13–20 April. Dry deposition rates of 17 and 84 μg m^-2d^-1 for Cd and Pb, respectively, were measured on the upward facing dishes two weeks later (27 April–4 May). The synoptic analysis confirmed the development of the Saharan cyclogenesis and its influence on the Balkan peninsula in the period of 14 to 17 April 1994. In this region the impact of dustfall originating from the Saharan storm was evident either during wet deposition or through resuspension processes.     

Long-term changes in concentration and deposition of atmospheric mercury over Scandinavia

Samples for measurements of total gaseous mercury (Hg) in air have been collected since 1980 in south-western part of Scandinavia. A collection program for precipitation samples used to determine changes in depositional fluxes of total Hg has been in operation since 1987. A comparison of today's total gaseous Hg levels in air and the total Hg concentrations in precipitation with the ones found earlier, shows a clear decrease with time. At the Swedish west-coast, yearly average air concentrations and median levels of 3.3 and 3.1 (1980–1984), 3.2 and 2.8 (1985–1989), and 2.7 and 2.6 ng Hg/m³ (1990–1992), respectively, were found. Increased average and median winter concentrations were always found, with levels at 3.7 and 3.4, 3.7 and 3.3, and 3.0 and 2.7 ng Hg/m³ for the respective time period. Higher winter values were expected due to increased anthropogenic emissions and changes in the mixing height of the atmosphere. The corresponding total wet deposition rates decreased from 27 (1987–1989) to 10 μg Hg/m² yr. (1990–1992). A finding of special interest was the decreased number of episodic events of high total gaseous Hg levels in air, from 1990 and further on. In addition, the frequency distribution of the concentrations of Hg in air seems to be different for these years compared to the other two time periods. A frequency distribution of air concentrations of Hg more resembling a normal distribution was found for the years 1990 to 1992. The decrease of the atmospheric burden of total gaseous Hg and deposition of total Hg are most probably connected to lower emissions in source areas on the European continent. It seems logical to state that the problem of high Hg depositional fluxes to Scandinavia, is best solved by abatement strategies on the regional scale.     

Mechanisms of deposition of methylmercury and mercury to coniferous forests

Deposition of methylmercury (MeHg) and mercury (Hg) to a coniferous forest have been investigated using field measurements. Samples of open field (OF) wet deposition, throughfall (TF) and litterfall (LF) have been collected and analyzed for MeHg and Hg during the period November 1991 to April 1994. Average concentrations in TF were 22.8 and 0.38 ng L^?1, for Hg and MeHg, respectively. Concentrations in OF precipitation were 11.9 and 0.37 ng L^?1, for Hg and MeHg, respectively, during the same period. Considerable differences were found for Hg in TF and OF which was attributed to a dry deposition of Hg. Hg in LF contributes a deposition of equal size as in TF. The relations between OF, TF and total Hg deposition were approximately 1∶1,5∶3. A decrease in OF Hg was found over the three year period studied. MeHg deposition in OF was also found to decrease during the same period whereas the TF MeHg showed a slight increase. Dry deposition of MeHg is also an important process in a coniferous forest although the flux to the forest floor is not via TF but rather as MeHg in LF.     

Mercury in the Swedish environment — Recent research on causes,consequences and corrective methods

During the last decade a new pattern of Hg pollution has been discerned, mostly in Scandinavia and North America. Fish from low productive lakes, even in remote areas, have been found to have a high Hg content. This pollution problem cannot be connected to single Hg discharges but is due to more widespread air pollution and long-range transport of pollutants. A large number of waters are affected and the problem is of a regional character. The national limits for Hg in fish are exceeded in a large number of lakes. In Sweden alone, it has been estimated that the total number of lakes exceeding the blacklisting limit of 1 mg Hg kg-1 in 1-kg pike is about 10 000. The content of Hg in fish has markedly increased in a large part of Sweden, exceeding the estimate background level by about a factor of 2 to 6. Only in the northernmost part of the country is the content in fish close to natural values. There is, however, a large variation of Hg content in fish within the same region, which is basically due to natural conditions such as the geological and hydrological properties of the drainage area. Higher concentrations in fish are mostly found in smaller lakes and in waters with a higher content of humic matter. Since only a small percentage of the total flow of Hg through a lake basin is transferred into the biological system, the bioavailability and the accumulation pattern of Hg in the food web is of importance for the Hg concentrations in top predators like pike. Especially, the transfer of Hg to low trophic levels seems to be a very important factor in determining the concentration in the food web. The fluxes of biomass through the fish community appear to be dominated by fluxes in the pelagic food web. The Hg in the lake water is therefore probably more important as a secondary source of Hg in pike than is the sediment via the benthic food chain. Different remedy actions to reduce Hg in fish have been tested. Improvements have been obtained by measures designed to reduce the transport of Hg to the lakes from the catchment area, eg. wetland liming and drainage area liming, to reduce the Hg flow via the pelagic nutrient chains, eg. intensive fishing, and to reduce the biologically available proportion of the total lake dose of Hg, eg. lake liming with different types of lime and additions of selenium. The length of time necessary before the remedy gives result is a central question, due to the long half-time of Hg in pike. In general it has been possible to reduce the Hg content in perch by 20 to 30% two years after treatments like lake liming, wetland liming, drainage area liming and intensive fishing. Selenium treatment is also effective, but before this method can be recommended, dosing problems and questions concerning the effects of selenium on other species must be evaluated. Regardless how essential these kind of remedial measures may be in a short-term perspective, the only satisfactory long-term alternative is to minimize the Hg contamination in air, soil and water. Internationally, the major sources of Hg emissions to the atmosphere are chlor-alkali factories, waste incineration plants, coal and peat combustion units and metal smelter industries. In the combustion processes without flue gas cleaning systems, probably about 20 to 60% of the Hg is emitted in divalent forms. In Sweden, large amounts of Hg were emitted to the atmosphere during the 50s and 60s, mainly from chlor-alkali plants and from metal production. In those years, the discharges from point sources were about 20 to 30 t yr 1. Since the end of the 60s, the emission of Hg has been reduced dramatically due to better emission control legislation, improved technology, and reduction of polluting industrial production. At present, the annual emissions of Hg to air are about 3.5 t from point sources in Sweden. In air, more than 95% of Hg is present as the elemental Hg form, HgO⁰. The remaining non-elemental (oxidized) form is partly associated to particles with a high wash-out ratio, and therefore more easily deposited to soils and surface waters by precipitation. The total Hg concentration in air is normally in the range 1 to 4 ng m-3. In oceanic regions in the southern hemisphere, the concentration is generally about 1 ng m^?3, while the corresponding figure for the northern hemisphere is about 2 ng m-3. In remote continental regions, the concentrations are mainly about 2 to 4 ng m^?3. In precipitation, Hg concentrations are generally found in the range 1 to 100 ng L^?1. In the Nordic countries, yearly mean values in rural areas are about 20 to 40 ng L^?1 in the southern and central parts, and about 10 ng L^?1 in the northern part. Accordingly, wet deposition is about 20 (10 to 35) g km^?2 yr^?1 in southern Scandinavia and 5 (2 to 7) in the northern part. Calculations of Hg deposition based on forest moss mapping techniques give similar values. The general pattern of atmospheric deposition of Hg with decreasing values from the southwest part of the country towards the north, strongly suggests that the deposition over Sweden is dominated by sources in other European countries. This conclusion is supported by analyses of air parcel back trajectories and findings of significant covariations between Hg and other long range transported pollutants in the precipitation. Apart from the long range transport of anthropogenic Hg, the deposition over Sweden may also be affected by an oxidation of elemental Hg in the atmosphere. Atmospheric Hg deposited on podzolic soils, the most common type of forest soil in Sweden, is effectively bound in the humus-rich upper parts of the forest soil. In the Tiveden area in southern Sweden, about 75 to 80% of the yearly deposition is retained in the humus layer, chemically bound to S or Se atoms in the humic structure. The amount of Hg found in the B horizon of the soils is probably only slightly influenced by anthropogenic emissions. In the deeper layers of the soil, hardly any accumulation of Hg takes place. The dominating horizontal flow in the soils takes place in the uppermost soil layers (0 to 20 cm) during periods of high precipitation and high groun water level in the soils. The yearly transport of Hg within the soils has been calculated to be about 5 to 6 g km^?2. The specific transport of total Hg from the soil system to running waters and lakes in Sweden is about 1 to 6 g km^?2 yr¹. The transport of Hg is closely related to the transport of humic matter in the water. The main factors influencing the Hg content and the transport of Hg in run-off waters from soils are therefore the Hg content in soils, the transport of humic matter from the soils and the humus content of the water. Other factors, for example acidification of soils and waters, are of secondary importance. Large peatlands and major lake basins in the catchment area reduce the out-transport of Hg from such areas. About 25 to 75% of the total load of Hg of lakes in southern and central Sweden originates from run-off from the catchment area. In lakes where the total load is high, the transport from run-off is the dominating pathway. The total Hg concentrations in soil solution are usually in the range 1 to 50, in ground water 0.5 to 15 and in run-off and lake water 2 to 12 ng L^?1, respectively. The variation is largely due to differences in the humus content of the waters. In deep ground water with a low content of humic substances, the Hg concentration is usually below 1 ng L^?1. The present amount and concentrations of Hg in the mor layer of forest soils are affected by the total anthropogenic emissions of Hg to the atmosphere, mainly during this century. Especially in the southern part of Sweden and in the central part along the Bothnian coast, the concentrations in the mor layer are markedly high. In southern areas the anthropogenic part of the total Hg content is about 70 to 90%. Here, the increased content in these soils is mainly caused by long-range transport and emissions from other European countries, while high level areas in the central parts are markedly affected by local historical emissions, mainly from the chlor-alkali industry. When comparing the input/output fluxes to watersheds it is evident that the present atmospheric deposition is much higher than the output via run-off waters, on average about 3 to 10 times higher, with the highest ration in the southern parts of Sweden. Obviously, Hg is accumulating in forest soils in Sweden at the present atmospheric deposition rate and, accordingly, the concentrations in forest soils are still increasing despite the fact that the emissions of Hg have drastically been reduced in Sweden during the last decades. The increased content of Hg in forest soils may have an effect on the organisms and the biological processes in the soils. Hg is by far the most toxic metal to microorganisms. In some regions in Sweden, the content of Hg in soils is already today at a level that has been proposed as a critical concentration. To obtain a general decrease in the Hg content in fish and in forest soils, the atmospheric deposition of Hg has to be reduced. The critical atmospheric load of Hg can be defined as the load where the input to the forest soils is less than the output and, consequently, where the Hg content in the top soil layers and the transport of Hg to the surface waters start to decrease. A reduction by about 80% of the present atmospheric wet deposition has to be obtained to reach the critical load for Scandinavia.     

Polluted precipitation and the geochronology of mercury deposition in lake sediment of northern Minnesota

Elevated Hg levels in game fish from wilderness lakes in northern Minnesota led to the present study of sediment cores from two lakes to ascertain the source and history of Hg deposition. Natural background levels of Hg were found to range from 0.03 to 0.06 μg g^?1, with cultural levels as high as 0.16 μg g^?1. Reconstructed geochronologies reveal a dramatic two-fold increase in Hg flux, from 0.008 to 0.017 μg cm^?2 yr^?1, occuring after the year 1880, suggesting an anthropogenic influence. No industrial or geologic source of Hg is found in the study watersheds. The entire historical increase in Hg flux can be accounted for by atmospheric loading provided that 1/5 of all the Hg presently supplied to the watershed via precipitation is ultimately deposited in lake sediment. Hg levels in fish are not correlated with Hg levels in lake sediment, although there is a link to acid-sensitivity of lake water, amount of acid-neutralizing geologic material exposed in the watershed, and watershed area/lake volume ratio. Thermal stratification of lake water and a complexation-adsorption mechanism are proposed to account for variations in Hg levels observed in sediment collected from different sites.     

Preliminary analysis of atmospheric scavenging processes in the industrial region of Cubatao,Southeastern Brazil

During a German-Brazilian research project on vegetation damage, atmospheric scavenging processes were investigated, considering transport and local emissions. For this goal, radar precipitation data was evaluated together with analyses of fractionated rain samples of one stratiform and one convective rain event. The temporal variation of pollutant concentration in rain water and the meteorological conditions were compared for both rain events at two stations: one at the top of the Serra do Mar mountain ridge ( 900 m) with minor influence of local emissions and one at sea level, close to the high indutrialized area of Cubatao. At the station close to the industrial plants, major influence of the local sources on wet deposition was determined. During the stratiform rain event, an additional influence of transport could be observed. On the top of the Serra do Mar, major influence of the local sources could only be observed during the convective rain event, while transport is mainly responsible for wet deposition during the stratiform event.     

Output of methylmercury from a Catchment in northern Sweden

The runoff output of methylmercury (MeHg) from the 50 ha Svartberget Catchment in northern Sweden was studied during 1993. These outputs are compared to those from the Gårdsjön Catchment in the southwestern part of Sweden. Although the wet deposition of MeHg is several times higher in southern Sweden the output of MeHg per unit area from the two catchments was comparable (0.12 g /km_* ²yr). Furthermore, the concentration of MeHg (0.4 ng/L) in the Svartberget Catchment was more than twice that from the Gårdsjön Catchment. These results suggest that the contemporary wet deposition of MeHg in itself is not a good indicator of runoff concentration or output per unit area. DOC transport and catchment characteristics such as wetland area, or possibly other forms of contemporary atmospheric deposition may all be more important for explaining MeHg output from the terrestrial ecosystem.     

Particle-size distribution of PAH in the air of a remote Norway spruce forest in northern Bavaria

The aim of the present study was to elucidate thedistribution of particulate polycyclic aromatichydrocarbons (PAH) in the air of a remote Norwayspruce (Picea abies (L.) karst.) stand.The study encompassed a total of twenty differentcongeners. Particles in ambient air were alternativelycollected by two different Berner cascade impactors atthe field site in Northern Bavaria over a 14-weeksperiod from the end of April through to the end ofJuly 1994 and subsequently analyzed for their PAHcontent. The concentrations of total suspendedparticulate matter (TSP) ranged from 9–31 μg m^-3,suggesting that the field site is an area with a lowlevel of airborne particles. There was a negativerelationship of total particle concentrations with theamount of canopy precipitation due to particle washout at precipitation events. The distribution of theparticle masses on the different size fractions witha maximum in the 1-μm range is typical for remoteareas and indicates a medium-range transport. Theaerosol-bound PAH load decreased from spring sampleswith 2–4 ng m^-3 to values <1 ng m^-3 in the summersamples. Concurrently, the proportion of low molecularweight congeners in the total PAH load declined.Frequently, the highest PAH concentrations (referringto the air) were found in the 0.1–3 μm sizeseparates (accumulation mode). The results suggestthat apart from PAH input to the soil with litter, dryand wet deposition of aerosol particles is animportant pathway of PAH contamination of acid forestsoils in the Fichtelgebirge mountain range.     

ACID DEPOSITION DURING TWO CONTRASTING FRONTAL RAINFALL EVENTS

Analysis of the chemical composition of rain at high temporal resolution provides additional information on wet deposition processes. High resolution data was obtained using a microprocessor-based acid rain monitor at two sites in SW Scotland and SE England. Meteorological details of the transport and wet deposition processes during two frontal rain events were examined and related to rainfall composition. Rapid depletions of ion concentrations during heavy rainfall in the first event were interpreted using a rainfall scavenging model. The sub-event data for the second event showed the influence of frontal discontinuities. Increasing ionic concentrations during this second event were attributed both to the change in air mass, and to diminished upwind precipitation scavenging.     

Changes in Concentrations of Major Elements and Trace Metals in Northeastern U.S.-Canadian Sub-Alpine Forest Floors

The forest floor at high elevation spruce-fir sites from southern Vermont, U.S. to the Gaspé Peninsula, Québec, Canada was sampled and analyzed in 1979 and re-sampled and analyzed in 1996 to study temporal changes in the impacts of atmospheric pollutants. We determined organic matter mass, pH, and concentrations of Al, Ca, Fe, K, Mg, Na, Cd, Cu, Hg, Pb, and Zn for the litter (L = fresh litter plus Oi horizon) and fermentation plus humic horizons (F+H) (= Oe plus Oa horizons) of the forest floor. There were no trends for Al or Fe concentrations in the 1979 or 1996 L along the transect. Several sites had significantly lower Al and Fe values in 1996 than in 1979, likely indicating less mineral soil in the 1996 samples. The 1996 concentrations of Ca in L increased along the transect from 0.22% dry weight (dw) in Vermont to 0.60% dw in Québec. Concentrations of Mg in L were relatively constant along the transect. Neither Ca nor Mg changed at sites from 1979 to 1996, indicating unchanged base status. Concentrations of Cd did not vary spatially along the transect but decreased at all sites from 1979 to 1996. Cu and Zn did not vary spatially or with time. In 1979, the concentration of Hg in L ranged between 150 and 300 μg kg^?1 dw, with no spatial gradient. By 1996, Hg concentrations were 25 to 50% lower in L, with decreases generally proportional to the concentration in 1979. The concentration of Pb in 1979 L decreased significantly from 200 mg kg^?1 dw in southern Vermont to 60 mg kg^?1 dw in Québec. By 1996, the Pb concentration in L ranged between 32 and 66 mg kg^?1 dw with no spatial trend along the transect. Decreases in Pb concentrations at sites were proportional to the absolute value in 1979. The concentrations of Cd, Hg, and Pb have declined in litter from 1979 to 1996, indicating a decline in atmospheric deposition. Higher Hg and Pb accumulation rates to the southwest are suggested for the past as indicated by (F+H) concentrations and inventories of Hg and Pb. The decline of Pb in L is consistent with the decreased use of leaded gasoline starting in the 1970s; the declines in Cd and Hg probably reflect lower emissions over the same period. Declining concentrations of Cd, Hg, and Pb in L parallel those documented in recent lake and peat sediments in the northeastern United States.