The aqueous reduction of divalent mercury by sulfite

Divalent Hg is reduced by sulfite in aqueous solutions. The proposed mechanism involves the formation of an instable intermediate, HgSO₃, which decomposes to produce Hg⁺ which in turn is rapidly reduced to Hg⁰. The overall rate of the reaction is inversely dependent on the concentration of sulfite. This reaction may influence the concentration of Hg in cloud- and rain-water by reducing water soluble Hg²⁺ to volatile Hg⁰. At low concentrations of SO₂(g) (5 μg m⁻³, 25 °C), the rate of the conversion of Hg(SO₃)₂ ²⁻ to Hg⁰ becomes significant (> 1 % h⁻¹) at pH < 5.5. At higher S02 concentrations (500 pg m⁻³), the same rate is expected at pH < 4.5.     

Vineyard soils under organic and conventional management—microbial biomass and activity indices and their relation to soil chemical properties

Eight vineyards in Pfaffenheim (P) and Turckheim (T) close to Colmar, France, forming four pairs of organic and conventional vineyards, were analyzed for microbial biomass and activity indices in relation to important soil chemical properties (carbon, nutrient elements, heavy metals) and also to differences between the bottom and top positions on the vineyard slope. The question was whether the vineyard management affects especially the soil microbiological indices. Three locations were on limestone (P-I, P-II, T-II), one on granite (T-I). The gravel content (>2 mm) ranged from 9 to 47%. The management systems had no significant main effect on the contents of organic C, total N, P, and S. The mean total contents of man-derived heavy metals decreased in the order Cu (164 μg g⁻¹ soil) > Zn (100 μg g⁻¹ soil) > Pb (32 μg g⁻¹ soil). The contents of microbial biomass C varied between 320 and 1,000 μg g⁻¹ soil. The significantly highest content was found at location P-II, the significantly lowest at the moderately acidic location T-I. The contents of microbial biomass N and adenosine triphosphate showed a similar trend. At location T-I, the fungal ergosterol-to-microbial biomass C ratio and the metabolic quotient qCO₂ were significantly highest, whereas the percentage of soil organic C present as microbial biomass C was lowest. Highest percentages of soil organic C present as microbial biomass C and lowest qCO₂ values were found in the organic in comparison with the conventional vineyards. None of the soil microbiological indices was significantly affected by the position on the slope, but all were significantly affected by the management system. This was mainly due to the highest index levels in the organic vineyard location P-II with the longest history in organic management.     

Soil Degradation Due to Vicinal Intensive Hog Farming Operation Located in East Mediterranean

One of the main environmental impacts of concentrated animal feeding operations is the soil degradation in vicinity with the livestock breeding facilities due to substances such as ammonia emitted from the various stages of the process. Owing to the high temperatures of the Mediterranean ecosystems, the evolution of gasses is more extensive and the soil degradation is consequently more severe than those obtained in northern Europe. In this research, the soil degradation effects of a large meat-producing, processing, and packaging unit have been investigated. The investigated intensive hog farming operation (IHFO) is located at a limestone soil coastal area with sea to the north and hills to the south. Soil samples of the upper mineral soil were taken in various distances and directions from the IHFO boundaries. Thirteen experimental cycles were carried out in the duration of 1.5 years starting in March 2009 until October 2010. The soil samples were analyzed on pH and electrical conductivity (EC) values as well as NH₄ ⁺ and NO₃ ⁻ concentrations. Significantly higher concentrations of the two nitrogen forms were observed on samples at increasing proximity downwind from the farm (south). Southern soil average NH₄ ⁺ and NO₃ ⁻ concentrations ranged between 0.4–118 μg NH₄ ⁺-N g⁻¹ soil and 6.1–88.4 μg NO₃ ⁻-N g⁻¹ soil, respectively. The variation of emitted gasses depositions was clearly reflected in the average pH and EC values. Average pH and EC values downwind from IHFO boundaries varied between 7.1–8.2 and 140–268 μS/cm, respectively.     

Critical sulphur concentration and sulphur requirement of microbial biomass in a glucose and cellulose-amended regosol

 The critical S concentration and S requirement of the soil microbial biomass of a granitic regosol was examined. S was applied at the rate of 0, 5, 10, 20, 30 and 50 μg S as MgSO₄·7H₂O, together with either 3000 μg glucose-C or 3333 μg cellulose-C, 400 μg N, and 200 μg P g ^–1 soil and 200 μg K g^–1 soil. Microbial biomass, inorganic SO₄ ^2–-S, and CO₂ emission were monitored over 30 days during incubation at 25  °C. Both glucose and cellulose decomposition rates responded positively to the S made available for microbial cell synthesis. The amounts of microbial biomass C and S increased with the level of applied S up to 10 μg S g^–1 soil and 30 μg S g^–1 soil in the glucose- and cellulose-amended soil, respectively, and then declined. Incorporated S was found to be concentrated within the microbial biomass or partially transformed into soil organic matter. The concentration of S in the microbial biomass was higher in the cellulose- (4.8–14.2 mg g^–1) than in the glucose-amended soil (3.7–10.9 mg g^–1). The microbial biomass C:S ratio was higher in the glucose- (46–142 : 1) than in the cellulose-amended soil (36–115 : 1). The critical S concentration in the microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) was estimated to be 5.1 mg g^–1 in the glucose- and 10.9 mg g^–1 in the cellulose-amended soil. The minimum requirement of SO₄ ^2–-S for microbial biomass formation was estimated to be 11 μg S g^–1 soil and 21 μg S g^–1 soil for glucose- and cellulose-amended soil, respectively. The highest levels of activity of the microbial biomass were observed at the SO₄ ^2–-S concentrations of 14 μg S g^–1 soil and 17 μg S g^–1 soil, for the glucose and cellulose amendments, respectively, and were approximately 31–54% higher during glucose than cellulose decomposition. Received: 20 October 1999     

Comparison of community structures of microbiota at main habitats in rice field ecosystems based on phospholipid fatty acid analysis

The present study compares the community structures of microbiota at different habitats in Japanese rice fields by comparing their phospholipid fatty acid (PLFA) compositions to understand the contribution of different habitats to microbiological diversity. The data were collected from four neighboring rice fields. Comparison was made for the PLFA compositions extracted from the floodwater, percolating water, rice soils under flooded and drained conditions, rice straw (RS) placed in flooded and drained rice soils, RS in the composting process, and RS compost placed in a flooded rice field. Average amounts of PLFAs were 33 μg L⁻¹ in the floodwater, 17.1 μg L⁻¹ in the percolating water from plow layers, 34.6 μg L⁻¹ in the percolating water from subsoil layers, 108 μg g⁻¹ dry weight basis (dw) in flooded rice soils, 382 μg g⁻¹ dw in RS materials, 2,510 μg g⁻¹ dw in RS composts, 2,850 μg g⁻¹ dw in RS composts after application to a flooded rice soil, 222 μg g⁻¹ wet weight basis (ww) in RS in drained rice soils, and 284 μg g⁻¹ ww in RS in flooded rice soils. The total amount of PLFAs to the soil depth of 10 cm was estimated to be about 12 g m⁻². The PLFA compositions were different from each other depending on the habitats. Rice soils were characterized by the predominance of actinomycetes and Gram-positive bacteria in comparison with the other habitats. In contrast, the microbial communities in the floodwater and percolating water were characterized by the predominance of Gram-negative bacteria and eukaryotes (presumably algae), and Gram-negative bacteria, respectively. The microbial community of RS materials was dominated by fungi. Gram-positive bacteria became predominant in RS after application to flooded rice soils, while RS placed in a drained rice field after harvesting rice was characterized by the predominance of Gram-negative bacteria and fungi. The community structures at respective habitats were stable and specific, irrespective of the season of sampling and the duration of decomposition of RS.     

Field-scale variability of topsoil dehydrogenase and cellulase activities as affected by variability of some physico-chemical properties

We have studied spatial field-scale variability of soil dehydrogenase (DH) and cellulase activities (CEL) and their relationship with variability of some physico-chemical properties at the surface horizon of the agricultural field. Soil samples were collected at 50 points from the upper 20 cm of soil. The activity of DH ranged between 0.77 and 1.5 μM TPP·g⁻¹·h⁻¹ while CEL activity ranged from 0.8 to 1.94 μM glucose·g⁻¹·24 h⁻¹. Concentrations of C_ORG and TN varied from 8.5 to 31.7 g·kg⁻¹ and from 0.94 to 3.56 g·kg⁻¹, respectively. The soil data showed that spatial variability and semivariograms describe spherical and linear models with the nugget effect (DH, CEL, C_ORG and TN). Dehydrogenase activity was in the strong variability class, while cellulase activity was situated in the week variability class. Both C_ORG and TN concentrations and pH_KCl values were strongly spatially dependent with the percentage of total variance (sill) presents as nugget variance ranging from 8.9% to 16.1%. Kriged maps displayed the lowest values of CEL activities in the north-east of the area, while the south area showed the highest CEL activity. The DH activity values were irregularly distributed in the surface horizon of the studied soil and this behaviour did not correspond with the spatial distribution of other properties.     

Changes in microbial community structure in soil as a result of different amounts of nitrogen fertilization

The changes in size, activity and structure of soil microbial community caused by N fertilization were studied in a laboratory incubation experiment. The rates of N fertiliser applied (KNO₃) were 0 (control), 100 and 2,000 μg N g⁻¹ soil. Despite no extra C sources added, a high percentage of N was immobilized. Whereas no significant increase of microbial C was revealed during incubation period, microbial growth kinetics as determined by the substrate-induced growth-response method demonstrated a significant decrease in the specific growth rate of microbial community in soil treated with 2,000 μg N g⁻¹ soil. Additionally, a shift in microbial community structure resulting in an increase in fungal biomarkers, mainly in the treatment with 2,000 μg N g⁻¹ soil was visible.     

Mercury speciation in Lake Baikal

Research on mercury (Hg) distribution and speciation was carried out in Lake Baikal, a large, strong-oligotrophic freshwater reservoir in Siberia, Russia, during June 1992 and march 1993. In summer, total Hg in the water column ranged from 0.14 to 0.77 ng Hg/L, with the highest concentrations observed in the central basin of the lake in surface water samples. Labile inorganic Hg was found to be 7 to 20 % of the total Hg content. Highest total Hg concentrations were found in river waters: up to 2 ng Hg/L. Labile methylmercury (MeHg) concentrations ranged from 2 to 38 pg Hg/L in the water column, with the higher concentrations in the central part of the lake, and showing a slight increase in near bottom waters. Labile MeHg makes up 1 to 15 % of the total Hg content in the water column, with larger fractions in deep waters. The slight increase of the MeHg gradient with depth corresponds with the O₂ minimum region. Highest MeHg concentrations were observed in river waters (up to 145 pg Hg/L) and in some bays of the lake (up to 160 pg Hg/L). In these high temperature- and phytoplankton-rich water masses, the MeHg-fraction increased up to 35 % of total Hg. Labile MeHg concentrations in water samples taken in winter in the southern basin (under the ice cover), showed slightly higher concentrations than in summer, possibly due to an early spring bloom. In rainwater, total Hg ranged from 3 to 20 ng Hg/L and MeHg from 0.1 to 0.25 ng Hg/L. In snow, a large fraction of total Hg is bound to particulate matter; concentrations of total Hg ranged from 8 to 60 ng Hg/L and labile MeHg from 0.1 to 0.25 ng Hg/L. Atmospheric Hg was found to be 0.73 to 2.31 ng/m³ as gaseous Hg and 0.005 to 0.02 ng/m³ in its particulate form. Spatial distribution patterns of atmospheric Hg show slightly higher concentrations over the central part of the lake and the Selenga river delta. In winter, atmospheric Hg values (measured in the southern region), ranged from 1.2 to 6.1 ng/m³ as total gaseous Hg and 0.02 to 0.09 ng/m³ as total particulate Hg, and are higher than in summer, probably influenced by coal burning and traffic by the local population. MeHg contents in fish ranged from 20 ng Hg/g dry weight in small Cottocomephorus to 300 ng Hg/g dry weight in pike and trout species, which were caught in organic-rich waters.     

Perchlorate Depositional History as Recorded in North American Ice Cores from the Eclipse Icefield, Canada, and the Upper Fremont Glacier, USA

Temporal depositional rates are important in order to understand the production and occurrence of perchlorate (ClO₄⁻) as limited information exists regarding the impact of anthropogenic production or atmospheric pollution on ClO₄⁻ deposition. Perchlorate concentrations in discrete ice core samples from the Eclipse Icefield (Yukon Territory, Canada) and Upper Fremont Glacier (Wyoming, USA) were analyzed using ion chromatography tandem mass spectrometry to evaluate temporal changes in the deposition of ClO₄ ⁻ in North America. The ice core samples cover a time period from 1726 to 1993 and 1970 to 2002 for the Upper Fremont Glacier (UFG) and Eclipse ice cores, respectively. The average ClO₄ ⁻ concentration in the Eclipse ice core for the time period from 1970 to 1973 was 0.6 ± 0.3 ng L⁻¹, with higher values of 2.3 ± 1.7 and 2.2 ± 2.0 ng L⁻¹ for the periods 1982–1986 and 1999–2002, respectively. All pre-1980 ice core samples from the UFG had ClO₄ ⁻ concentrations <0.2 ng L⁻¹, and the post-1980 samples ranged from <0.2 ng L⁻¹ to a maximum of 2.6 ng L⁻¹ for the year 1992. A significant positive correlation (R = 0.75, N = 15, p < 0.001) of ClO₄⁻ with SO₄²⁻ was found for the annual UFG ice core layers and of ClO₄ ⁻ with SO₄²⁻ and NO₃⁻ in sub-annual Eclipse ice samples (R > 0.3, N = 121, p < 0.002). The estimated yearly ClO₄⁻ depositional flux for the Eclipse ice core ranged from 0.6 (1970) to 4.7 μg m⁻² year⁻¹ (1982) and the UFG from <0.1 (pre-1980) to 1.4 μg m⁻² year⁻¹ (1992). There was no consistent seasonal variation in the ClO₄⁻ depositional flux for the Eclipse ice core, in contrast to a previous study on the Arctic region. The presence of ClO₄⁻ in these ice cores might correspond to an intermittent source such as volcanic eruptions and/or any anthropogenic forcing that may directly or indirectly aid in atmospheric ClO₄⁻ formation.     

Methyl Mercury Production and Distribution in River Water-Sediment Systems Investigated Through Radiochemical Techniques

The toxicological consequences of Hg releases to the environment are largely governed by the conversion ofinorganic Hg to the most toxic methylmercury (MeHg), that is biomagnified through aquatic food chains. To gain further insight on the biological and physico-chemical factors controlling MeHg production and distribution among freshwater sediments and water, we used a sensitive and specific radiochemical procedure, developed at the National Institute for Minamata Disease. Systems containing ²⁰³Hg²⁺-spiked sediment cores (0.7 μg total Hg g^-1 d.w.) and overlying water, both from a pristine mountain stream in Southern Japan, were incubated for 21–38 days in different conditions. Inorganic Hg and MeHg in sediment and water were extracted in dithizone-benzene and measured after separation by thin-layer chromatography. The conversion of added Hg to MeHg was 3.0 to 13.7% in sediments, with a tendency for higher proportions in the top layers. Surprisingly, more MeHg was found in the sediment (11.3%) and water (66.5%) of a system bubbled with air than in one bubbled with nitrogen (4.2 and 44.1%). Artificially increased levels ofbioturbation reduced by half the MeHg concentrationsand % of added total Hg in sediment and water. In allsystems, 55–68% of total Hg and MeHg in water wereassociated to suspended particles >1μm. MeHgbioaccumulation factors (BFs) from water ranged270–8100 and from sediment, 0.2–5.7 (wet weight basis). BFs in relation to water where 3 times higher for MeHg than for total Hg.     

The effects of pH,redox, and salinity on metal release from a contaminated sediment

Levels of total Hg were determined in the muscle tissue of Helicolenus dactylopterus (mean=0.29 ug.g⁻¹ ±0.025 S.E.; range=0.041.10 ug.g⁻¹) and Pontinus kuhlii (mean=0.16 ug.g⁻¹ ±0.092 S.E.; range=0.05-0.50 ug.g⁻¹) caught in the Azores between August 1989 and May 1990. Mercury concentrations were related to sex, length, weight, age, growth and condition of both fish species. There were highly significant positive correlations between Hg levels and these variables. In P. kuhlii a significant sex-related difference in Hg levels was found; the rate of Hg accumulation is significantly faster in females than in males and mean Hg levels were higher in females. Relationships describing Hg vs size and age dependence were determined for both species and sexes and patterns and rates of Hg accumulation were discussed in relation to a number of biological and ecological factors with influence on it. A comparison of Hg content of Azorean and Mediterranean populations of H. dactylopterus was conducted and its suitability as an indicator in pollutant monitoring of benthic compartment of marine ecosystems is discussed.     

Influence of Sediment Preservation on Total Mercury and Methylmercury Analyses

Estuarine and riverine sediments from fourlocations showing different sediment structures wereanalysed as fresh, thawed and lyophilised samples fortotal mercury (TotHg) and methylmercury (MeHg)concentrations, and results were compared to addresseffects of sample preservation on Hg speciation. TotHg was measured by cold vapour atomic absorptionspectrometry (CVAAS). MeHg was isolated bydistillation and ion-exchange and analysed by coldvapour atomic fluorescence spectrometry (CVAFS) afterpreconcentration on a gold trap. No loss of TotHg norMeHg due to lyophilisation was found. Concentrationsof TotHg and MeHg respectively ranged from 92 to 267ng g^-1 dw and 1.1 to 2.9 ng g^-1 dw in freshsamples, from 94 to 215 ng g^-1 dw and 1.1 to 2.8ng g^-1 dw in thawed samples, and from 100 to 256ng g^-1 dw and 1.2 to 3.1 ng g^-1 dw inlyophilised samples. Lyophilised samples showedbetter homogeneity and better MeHg analysisreproducibility compared with wet samples.     

Mercury concentrations in northern pike,Esox lucius L., in small lakes of Evo area,southern Finland

We measured Hg concentrations in northern pike (Esox lucius) from 17 small lakes in Evo forest area, Lammi, southern Finland. The mean Hg concentration in muscle tissue of a 1 kg pike ranged from 0.15 to 1.36 μg g⁻¹ (ww) in the lakes. There was a trend towards higher concentrations in acidic and humic lakes than in circumneutral and clear-water lakes. The Hg content of pike from successive lakes of a lake chain was similar, whereas there were clear differences in the Hg concentrations among seepage lakes and the uppermost lakes of other lake chains. The latter was probably due to special characteristics of the lakes: in one lake pike was the only fish species, two of the lakes were regulated by beaver, and one lake was a groundwater or spring lake. Our observations indicate that Hg concentrations in pike can vary considerably from lake to lake in a small geographical area and that the variation among lakes in the accumulation of Hg in fish largely depends on lake characteristics and on the diet of pike.     

Atmospheric concentrations and deposition of Hg to A deciduous forest atwalker branch watershed,Tennessee, USA

Aerosol and total vapor-phase Hg concentrations in air have been measured at Walker Branch Watershed, Tennessee for ≈ 2 yr. Airborne Hg at this site is dominated by vapor forms which exhibit a strong seasonal cycle, with summer maxima that correspond to elevated air temperature. Concentrations in this forest are near background levels; however, concentrations at a site within 3 km are significantly elevated due to emissions from Hg-contaminated soils. The concentration data have been combined with a recently modified dry deposition model to estimate dry deposition fluxes to the deciduous forest at Walker Branch. Weekly mean modeled V_d values for Hg° ranged from <0.01 (winter) to > 0.1 (summer) cm s¹. Weekly dry deposition fluxes ranged from <0.1 μg m⁻² during winter to > 1.0 μgg m⁻² in the summer. Our dry deposition estimates plus limited measurements of wet deposition in this area indicate that dry deposition may be the dominant input process in this forest, at least during the summer.     

Effects of the addition of forest floor extracts on soil carbon dioxide efflux

Composition and effects of additions of fibric (Oi) and hemic/sapric (Oe + Oa) layer extracts collected from a 20-year-old stand of radiata pine (Pinus radiata) on soil carbon dioxide (CO₂) evolution were investigated in a 94-day aerobic incubation. The ¹³C nuclear magnetic resonance spectroscopy indicated that Oi layer extract contained greater concentrations of alkyl C while Oe + Oa layer extract was rich in carboxyl C. Extracts from Oi and Oe + Oa layers were added to a forest soil at two different polyphenol concentrations (43 and 85 μg g⁻¹ soil) along with tannic acid (TA) and glucose solutions to evaluate effects on soil CO₂ efflux. CO₂ evolution was greater in amended soils than control (deionized water) indicating that water-soluble organic carbon (WSOC) was readily available to microbial degradation. However, addition of WSOC extracted from both Oi and Oe + Oa layers containing 85 μg polyphenols g⁻¹ soil severely inhibited microbial activity. Soils amended with extracts containing lower concentrations of polyphenols (43 μg polyphenols g⁻¹ soil), TA solutions, and glucose solutions released 2 to 22 times more CO₂-C than added WSOC, indicating a strong positive priming effect. The differences in CO₂ evolution rates were attributed to chemical composition of the forest floor extracts.     

The analysis of vegetation for total mercury

Over four hundred vegetation samples were collected for total Hg determination as part of a biogeochemical survey in the Precambrian Shield region near Huntsville, Ontario. An objective of the survey was to obtain accurate data describing the spatial and temporal variation of Hg concentrations in vegetation. Five tree species, clubmosses, mosses, lichen and fungi were collected along three transects each 8 to 10 km long. The samples were digested using a hot H₂SO₄/HNO₃ mixture followed by cold-vapor AAS detection. Very low detection limits (less than 1.0 ng g⁻¹) were achieved by performing the analyses in a clean, Hg-free laboratory. The Hg concentration of coniferous needles did not vary significantly over eight weeks of the summer, but did vary significantly between first and second year growth. In all tree species examined, Hg concentrations in needles/leaves were two to three times as high (by dry weight) as that in twig tissue from the same branch. Differences in Hg content between tissues of different types and ages constituted a major source of within-site variation between plants of the same species.     

Formation and distribution of methylmercury in sediments at a mariculture site: a mesocosm study

Purpose

The aim of the present study was to investigate the differences of methylmercury (MeHg) formation and distribution between mariculture (aquaculture) sediments (MS) and reference sediments (RS) collected from a site in Hong Kong.

Materials and methods

The MS and RS samples were split into four batches, three of which were spiked with HgCl₂ aqueous solution to a concentration of 0.8, ,2 and 8 mg k g^?1 in sediment samples SP1, SP2, and SP3, respectively, while the rest served as a control batch (referred to as C).

Results and discussion

The results showed that the highly Hg-polluted sediment produced greater amounts of MeHg. During the culture period, MeHg concentrations in sediments decreased over time. The decreasing percentage increased in the order of SP3?<?SP2?<?SP1, which might be due to the inhibition of MeHg degradation by high Hg concentrations. The mean value of MeHg concentrations and %MeHg of the total Hg (THg) in MS was significantly lower than those in RS, possibly due to the complexation of Hg with organic ligands, leading to lower Hg bioavailability for methylation bacteria. The distribution coefficient of THg (K_dT) was relatively high in MS compared to RS, indicating that the former had a greater number of binding sites for Hg adsorption.

Conclusions

Methylmercury formation was inhibited in MS, probably due to increased complexation of Hg²⁺ with organic matter and adsorption of Hg to MS. Furthermore, the mean value of K_dT in MS was relatively high when compared to RS, which illustrates that MS sediments have more binding sites than RS for adsorption of Hg.     

Production of Perchlorate by Laboratory Simulated Lightning Process

Perchlorate (ClO₄⁻), a thyroid hormone disruptor, is both naturally occurring and a man-made contaminant increasingly found in a variety of terrestrial environments. The environmental presence of ClO₄⁻ is considered to be the result of atmospheric formation and deposition processes. The ultimate processes, particularly heterogeneous-based reactions, leading to natural ClO₄⁻ formation are not well understood. Oxidation of chlorine species by an energetic source such as lightning is considered to be one of the potential heterogeneous sources of natural ClO₄⁻. Currently, there is very little information available on lightning-induced ClO₄⁻. We designed a laboratory electrical discharge reactor capable of evaluating ClO₄⁻ formation by the oxidation of “dry” sodium chloride (NaCl) aerosols (relative humidity (RH) <70%) in electrical discharge plasma at voltages and energies up to 24 kV and 21 kJ, respectively. Similar to other non-electrochemical ClO₄⁻ production processes, the amount of ClO₄⁻ produced (0.5–4.8 μg) was 3 orders of magnitude lower than the input Cl⁻ (7.1–60.1 mg). The amount of ClO₄⁻ generated increased with peak voltage (V) and theoretical maximum discharge energy with ΔClO₄⁻/ΔV = 0.28 × 10⁻³ μg V⁻¹ (R ² = 0.94) and ΔClO₄⁻/ΔE = 0.44 × 10⁻³ μg J⁻¹ (R ² = 0.83). The total ClO₄⁻ generated decreased with an increase in relative humidity from 2.8 ± 0.1 μg (RH ∼46%) to 0.9 ± 0.1 μg (RH ∼62%) indicating that the presence of moisture inhibits the formation of ClO₄⁻. Additional modifications to the reactor support the hypothesis of ClO₄⁻ formation due to the action of plasma on Cl⁻ aerosols as opposed to direct oxidation on the surface of the electrodes. Finally, the contribution of lightning-induced ClO₄⁻ in North America is calculated to have a wide range from 0.006 × 10⁵ to 5 × 10⁵ kg/year and is within the range of the measured ClO₄⁻ depositional flux in precipitation samples obtained across the USA (0.09 × 10⁵–1.2 × 10⁵ kg/y).     

The Effect of Fire on Mercury Cycling in the Soils of Forested Watersheds: Acadia National Park,Maine, U.S.A.

This study compares mercury (Hg) and methylmercury (MeHg) distribution in the soils of two forested stream watersheds at Acadia National Park, Maine, U.S.A. Cadillac Brook watershed, which burned in 1947, has thin soils and predominantly deciduous vegetation. It was compared to the unburned Hadlock Brook watershed, with thicker soil and predominantly coniferous vegetation. Soils in both watersheds were primarily well drained. The fire had a significant impact on the Cadillac watershed, by raising the soil pH, altering the vegetation, and reducing carbon and Hg pools. Total Hg content was significantly higher (P > 0.05) in Hadlock soils (0.18 kg Hg ha^-1) compared to Cadillac soils (0.13 kg Hg ha^-1). Hadlock O horizon had an average Hg concentration of 134±48 ng Hg g^-1 dry weight, compared to 103±23 ng Hg g^-1 dry weight in Cadillac O horizon. Soil pH was significantly higher in all soil horizons at Cadillac compared to Hadlock soils. This difference was especially significant in the O horizon, where Cadillac soils had an average pH of 3.41±0.22 compared to Hadlock soils with an average pH of 2.99±0.13.To study the mobilization potential of Hg in the O horizons of the two watersheds, batch adsorption experiments were conducted, and the results were modeled using surface complexation modeling. The results of Hg adsorption experiments indicated that the dissolved Hg concentration was controlled by the dissolved organic carbon (DOC) concentration. The adsorption isotherms suggest that Hg is more mobile in the O horizon of the unburned Hadlock watershed because of higher solubility of organic carbon resulting in higher DOC concentrations in that watershed.Methylmercury concentrations, however, were consistently higher in the burned Cadillac O horizon (0.20±0.13 ng Hg g^-1 dry weight) than in the unburned Hadlock O horizon (0.07±0.07 ng Hg g^-1 dry weight). Similarly, Cadillac soils possessed a higher MeHg content (0.30 g MeHg ha^-1) than Hadlock soils (0.16 g MeHg ha^-1). The higher MeHg concentrations in Cadillac soils may reflect generally faster rates of microbial metabolism due to more rapid nutrient cycling and higher soil pH in the deciduous forest. In this research, we have shown that the amount of MeHg is not a function of the total pool of Hg in the watershed. Indeed, MeHg was inversely proportional to total Hg, suggesting that landscape factors such as soil pH, vegetation type, or land use history (e.g., fire) may be the determining factors for susceptibility to high Hg in biota.     

Relation between sulphur concentrations in the Scots pine needles and the air in northernmost Europe

The SO₂ emissions from the Kola Peninsula in Arctic Russia (totalling around 600 Gg(SO₂) yr^–1 at the beginning of the 1990s) produce an atmospheric SO₂ concentration gradient to the northernmost Europe. This gradient covers the range from >50 g m^–3 in the vicinity of the sources to 1 g m^–3 in Finnish Lapland. In the present study, the measured sulphur concentrations in Scots pine needles were compared with the estimated distribution of atmospheric SO₂. The total sulphur concentrations in the needles ranged from 741 to 2017 mg kg^–1. Strongly elevated concentrations (> 1200 mg kg^–1) were found within 40 km from the smelters corresponding to an area where the annual mean atmospheric SO₂ concentration exceeded 10 g m^–3. The foliar sulphur concentrations (total, organic and inorganic) show a high correlation with the estimated mean SO₂ concentration distribution in the air. Consequently the foliar sulphur concentrations reflected the atmospheric sulphur load well. The data presented here show that uptake via stomata is an important deposition pathway also in the arctic conditions with a short growing season.