Heavy metal deposition in Scandinavia

Maps showing the regional differences in the atmospheric deposition of Pb, Cd, Hg, Cr, Ni, Cu, Zn, and Fe have been prepared for Finland, Norway and Sweden from data obtained by means of moss analysis, a method previously shown to be a sensitive and reliable tool in measuring heavy metal deposition. In all elements the minima were recorded in northern Norway, the maxima in south-central or south-western Sweden and southernmost Norway. The greatest regional differences were measured in Pb and Cd.     

Occurrence and turnover of atmospheric mercury over the nordic countries

A seasonal variation of both particle and gaseous Hg concentrations in the atmosphere is present in south-western Sweden. An average gaseous Hg level of 3.7 ng m^?3 is found in winter, compared to 2.8 ng m^?3 in summer. A weak decreasing south-north gradient for gaseous Hg in air over the Nordic countries is also present, with yearly average values from 3.2 to 2.8 ng m^?3. A gradient for particulate Hg is less clear. An air parcel trajectory sector classification of gaseous Hg levels in air, and to some extent the particulate associated Hg, clearly demonstrates the increased concentrations in the southern sectors, especially in south-western Sweden where the gaseous Hg increase is about I ng m^?3. These observations are consistent with an influence from the European continent. The average concentrations of Hg in precipitation at the various stations show a pronounced decreasing south-north gradient. A major portion of the total Hg present in precipitation is associated with particles. For the southern stations, a strong correlation between Hg and sulfate, or pH, is present suggesting a connection between Hg in precipitation and anthropogenic activities.     

Linkages between atmospheric mercury deposition and the methylmercury content of marine fish

Enhanced Hg deposition to productive marine systems may result in concurrent increases in monomethyl Hg (MMHg) concentrations of marine fish. Consequently, it is important to understand what effects an increasing Hg supply may have on the marine food chain. A simple ocean model is employed to estimate the fraction of total Hg inputs which is required to sustain “average” marine fish MMHg concentrations annually. Calculations show that upwelling zones require 20% of total annual Hg inputs, coastal zones 5%, and open-ocean regions only 0.02%. The value for coastal areas is similar to that calculated for the acidified basin of Little Rock Lake, Wisconsin, a small fresh water seepage lake. These calculations point to Hg source strength and rates of particle scavenging as being key factors in controlling the rate of transport to sites of methylation (and subsequent entry into the marine food chain). If biological variables (scavenging rates, primary productivity) remain constant while anthropogenically-derived Hg deposition increases, it is likely that concentrations in marine biota (including fish) will rise in accord.     

Cycling of methyl mercury and mercury — Responses in the forest roof catchment to three years of decreased atmospheric deposition

Studies of the biogeochemistry of total mercury (Hg) and methyl mercury (MeHg) in the Lake Gårdsjön watershed have shown that the atmosphere is the most important source of Hg and MeHg in the ecosystem. Soils are accumulating most of the deposited Hg and MeHg, but transport of Hg and MeHg from the forested catchments into the lake ecosystems is enough to explain elevated concentrations of MeHg in fish in more than 10 000 Swedish lakes. An experimental roof was constructed to study effects of decreased atmospheric input on an entire forested catchment. The experiment started in April 1991, and decreases in the output of both MeHg and Hg occurred during 1991, 1992 and 1993. Runoff fluxes from the control catchment during the pre-treatment period were related to the experimental catchment using regression analyses. Since April 1991, after three year experiment, predicted compared to measured fluxes showed that Hg output decreased by 32% and MeHg by 28%. The decrease in Hg was most obvious during high water flows in winter/spring while MeHg decreased during all seasons of the year. The decreased input of Hg and MeHg to the Forest Roof Catchment is the most probable explanation to the rapid decrease in output of Hg and MeHg by runoff from the catchment basin.     

Pre-industrial atmospheric deposition of mercury: Uncertain rates from lake sediment and peat cores

Lacustrine sediment cores from depositional areas have frequently been used to estimate pre-industrial rates of atmospheric Hg deposition. However, this approach tends to result in overestimates, partly because of Hg inputs from the catchment, partly because of a horizontal redistribution of sediments within lakes. Peat core studies may suffer from a vertical migration of Hg due to water table fluctuations. A natural Hg deposition rate around 2 μg m^?2 y^?1 is suggested to be more realistic than values of 3 to 12 μg m^?2 y^?1 reported from recent studies. The anthropogenic impact on the present Hg deposition may have been underestimated accordingly.     

Lidar measurements of atmospheric mercury

Mercury is the only atmospheric pollutant that is present in the atmosphere in atomic form. The optical resonance line at 254 nm can be used for absorption measurements using different optical remote sensing techniques. Range-resolved Hg mapping can be performed using the differential absorption lidar (DIAL) technique. We have used the lidar technique both for mapping of industrial plumes and for background concentration measurements. Our studies also include Hg of geophysical origin.     

汞沉淀对黑麦草中汞含量及其分布的影响

A grass(Italian rye grass)experiment with complete desing of three factos(Hg,grass and exposure)and two levels(with and withou the factor tested)was carried out in Goeteborg,Sweden from May to September,1995,toward understanding the effect of mercury deposition on plant Hg content and its distribution.It has been found that besides the two general pathways of Hg absorption,i.e.root uptake and foliar absorption,mercury deposition can aslo signifcantly incease Hg content and total load in grass.Such effects seem to be confined only within the above-ground parts of grass plant.Estimate has been made to make clear of the contributions of different pathways to Hg Content and total Hg load in the upper part of grass.Results showed that the contribution of Hg deposition accounted for 27%-32%,of Hg content in the above-ground parts of grass plant without adding Hg to the soil.decreasing with the increase of soil and/or air Hg concentrations.The increment of Hg load in the upper part of grass plant caused by Hg deposition during an interval of two weeks varied between 0.01-0.07μg pt^-1,contributing to 17%-48% of the total Hg load in grass plant exposed.     

Computing atmospheric transport and deposition of heavy metals over Europe: Country budgets for 1985

The Heavy Metal Eulerian Transport (HMET) model has been used to calculate the exchange of As, Cd, Pb and Zn between European countries in 1985. The model was run separately for each emitter country and the computed deposition field was used to calculate the contribution of the emitter to each receptor country. The results of these computations are presented in the form of a country budget matrix for each metal. Accuracy of such computations is dependant on the size and linearity of the numerical method applied to the transport equation. Exchange of heavy metals due to atmospheric transport over Europe is significant. Approximately 30% to 90% of the heavy metals emitted from each country is deposited in other countries. The remaining mass is deposited in European seas, Atlantic Ocean and transported outside the model domain. The largest part of the emission from each country is deposited in the same country. The next largest fraction is transported to the nearest neighbors. The results indicate also a significant long range ransport of heavy metals to the Soviet Union. This is partly justified by the size and location of this receptor country, as well as, the prevailing meteorological conditions in Europe. However, this large transport to USSR is slightly overestimated due to some artificial properties of the numerical method applied to basic model equations. In addition to the country budget, export versus import and emission versus deposition of metals were analyzed for each country. The largest positive difference between export and import was found for Poland, German Federal Republic and Yugoslavia (As, Cd and Zn), and United Kingdom, Italy and Belgium (Pb). The Soviet Union and Czechoslovakia are the countries where import of all metals is significantly larger than export. When emission versus deposition of heavy metals is analyzed, the Soviet Union has much higher emissions than deposition of all metals compared to other European countries.     

Long-term changes in topsoil nematode populations in grazed pasture under elevated atmospheric carbon dioxide

Increasing atmospheric CO₂ concentrations are expected to affect ecosystems processes, and while a New Zealand study reported a response in soil biological activity after 4 years of CO₂ enrichment, apparently reflecting increased populations of Longidorus elongatus, similar findings have not been reported from other sites. Soil microfauna in 0–10 cm soil under a sheep-grazed pasture on a sand was assessed quarterly in FACE rings that were either at ambient CO₂ or had been exposed to 475 μl l⁻¹ CO₂ for some 9 years. Although the area had been subject to a severe drought and microfaunal populations were lower than previously found, the effects of elevated CO₂ on microfaunal populations were broadly similar to those at 4 years. Average populations of the root-feeding L. elongatus increased from 67,000 to 233,000 m⁻² (3.48×) compared with a 4.26× increase after 4 years; microbial-feeding nematodes increased slightly, while predacious nematodes showed a 2.0× increase. A pot experiment showed an additive effect of elevated CO₂ and L. elongatus abundance in reducing specific root length. That similar effects have been found 4 and 9 years after CO₂ enrichment commenced suggests they are real, and emphasises the difference to other sites around the world where much lower responses to elevated CO₂ have been found. This, in part, reflects the unique combination of soil, plant and soil biological conditions at each site and confirms the strong effect of soil type and vegetation on soil biological processes. Just as the effects of global climate change on a given region are idiosyncratic, so it seems are the effects of elevated CO₂ on soil and ecosystem processes. In part, this reflects our limited understanding of below-ground processes.     

Is total mercury concentration a good predictor of methyl mercury concentration in aquatic systems?

Methyl mercury (MeHg) concentrations were compared to total mercury (THg) concentrations in a variety of types of aqueous samples collected at the Experimental Lakes Area during 1991 through 1993. In several streams, an experimentally flooded wetland, and peat pore water, there was no relationship between MeHg and THg concentrations. %MeHg (compared to THg) ranged from < 1% to over 90%. In three ELA lakes, as in groups of lakes from other regions, a linear relationship between MeHg and THg concentration was found. However, these relationships differed by a factor of three from one region to another. This study shows that THg inputs and/or concentrations are not very useful in predicting MeHg concentrations, and that factors within ecosystems are very important in controlling MeHg concentrations.     

An update of the mercury inventory and atmospheric mercury fluxes to and from Finland

Finland, the northern-most agricultural/industrial country in the world, has been for some time steadily improving environmental mercury (Hg) research. This paper focuses upon Hg recovery during zinc production, uses of Hg, updating of information on Hg emissions and atmospheric transport of Hg to and from Finland. The recovery of Hg as a by-product of zinc production began in 1970. The highest amount of recovered Hg was noted to be 160 t in 1989. Total uses of Hg in different sectors were about 7.4 t in 1992, which had decreased by 50% since the year 1987. In 1992, the estimated Hg emission to air was 2 t yr^?1, whereas to water and land the emission was about 0.20 and 3.67 t yr^?1, respectively. Natural Hg emission in Finland was estimated to be about 0.4 t yr^?1 (range: 0.3 to 0.5 t yr^?1). In addition, an initial attempt was made to estimate the atmospheric Hg flux to and from Finland; these values were noted to be 2.7 and 2.1 t yr^?1, respectively.     

大气氮素沉降研究进展

人为干扰下的大气氮素沉降已成为全球氮素生物化学循环的一个重要组成部分。作为营养源和酸源, 大气氮沉降数量的急剧增加将严重影响陆地及水生生态系统的生产力和稳定性。本文从大气氮沉降对土壤和水体环境、农业和森林生态系统以及生物多样性等方面综述了近年来国内外大气氮素沉降的研究现状及其对生态系统的影响, 并总结探讨了前人采用的大气沉降氮测定方法, 展望了我国大气氮沉降的研究前景。     

Forest canopy transformation of atmospheric deposition

Solute fluxes to the ground in open plots and under the forest canopy of different species were investigated in a number of long-term ecosystem studies in West Germany. From the canopy flux balance, rates of interception deposition and canopy/deposition interactions were assessed. Chemically, both open precipitation and throughfall are dilute solutions of H₂SO₄ and HNO₃ and their salts. For the sites investigated, mean pH in bulk precipitation ranged from 4.1 to 4.6, and in throughfall from 3.4 to 4.7. The increase in acidity after canopy passage at most sites indicates considerable interception deposition of strong acids to the forest stands, exceeding the rate of H+ buffering in the canopy. Evidence for buffering processes can be directly deduced from the fact that on sites with high soil alkalinity and high foliage base status, throughfall pH is usually higher than precipitation pH. Furthermore, the same idea can be concluded from changes in solution composition after canopy passage: the H⁺/SO _inf4 ^sup2? ratio is decreasing at most sites, while alkali earth cations from exchange processes occur in throughfall (Ca²⁺/SO _inf4 ^sup2? ratio increases). Solution composition and element flux data are presented for each of the sites, and the regional, orographical and site specific (species composition, ecosystem state) differentiations are discussed. A method for the assessment of total deposition and of canopy interactions such as H⁺-buffering and cation leaching is described, and results of calculations are shown. From these calculations it is concluded that forest ecosystems in Germany receive mean H⁺ loads of ca. 1 to 4 keq H⁺ · ha^?1 · a^?1 from atmospheric deposition. Acidity deposition rates seem to be related to a few key factors such as regional characteristics and ecosystem characteristics.     

Input of pesticides by atmospheric deposition

Sampling and analytical methods were developed to examine the input of various pesticides on noncultivated areas of the FAM (Research Network on Agroecosystems) Research Station Scheyern. Off-target drift from pesticide application on nearby cultivated land, as well as input due to long-range atmospheric transport, were measured. The wet deposition was determined by a cooled wet-only sampler. Bulk samplers and specially designed samplers with glass-fiber surface were used for total deposition measurements. Analysis of pesticides was carried out using liquid/liquid or solid-phase extraction and high performance liquid chromatography-UV (HPLC-UV) or gas chromatography-nitrogen/phosphorus-sensitive detector/MS (GC-NPD/MS) detection. Obtained results demonstrated that for several compounds, total deposition, i.e. the sum of wet and dry deposition, was marginally higher than wet deposition alone. In contrast, total deposition data of pesticides having been applied near the sampling site exceeded wet deposition values by orders of magnitude. In addition to direct drift, determined as droplets depositing near pesticide application areas, an indirect drift represented by particle-associated or gaseous transport was observed, both of which contributed considerably to total deposition. Therefore, to determine the input of pesticides to nontarget areas in the close vicinity of pesticide application, direct and indirect drift, and background deposition must be considered.     

The acidifying potential of atmospheric deposition in Canada

It is proposed that the value of ([SO₄ ^??] - [Ca⁺⁺ + Mg⁺⁺]) in precipitation is a suitable way to describe the acidifying potential (AP) of the wet desposition. In eastern North America, the AP of precipitation varies from 20 to 85% of the total sulphate, the remainder of the sulphate being neutralized H₂SO₄, sulphate from salts in dust or from sea salt. The AP ranges from 20 to 80% of the H⁺ in the wet deposition. The rest of the H⁺ is contributed by the net effect of N compounds. Ammonium and nitrate ions from ammonia and NO_Xemissions do not represent a net acidic loading to the terrestrial ecosystem if they are taken up by vegetation. However, when N leaches from watersheds in the form nitrate, it constitutes an acidifying demand on the ecosystem. Therefore, the overall net acidifying potential (NAP) applied to the terrestrial ecosystem is defined by the value of ([SO₄ ^??] - [Ca⁺⁺ + Mg⁺⁺]) in precipitation plus [NO₃ ^??] in runoff from the watershed.     

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.     

International field intercomparison of atmospheric mercury measurement methods

To determine the extent of comparability of sampling and analytical procedures for atmospheric mercury (Hg) being used by different scientific groups around the world and hence the compatibility of measurement results, the Atmospheric Environment Service (AES) co-ordinated a field intercomparison study in Windsor, Ontario, over a period of 5 days- during Sept./Oct.,1993. This study brought together 2 groups (University of Michigan Air Quality Laboratory; Chemistry Institute of GKSS) which performed conventional (manual) sample collection procedures for total gaseous mercury (TGM) and for particulate-phase mercury (PPM), followed by cold-vapor atomic fluorescence spectrophotometric (CVAFS) analysis in the respective laboratories. Two other groups (Ontario Hydro, and the Ontario Ministry of Environment & Energy) each operated a novel mercury vapor analyzer produced by Tekran Inc. of Toronto. As is the case for the manual methods, this analyzer also uses gold amalgamation and CVAFS. During the intercomparison, meteorological parameters (air temperature, barometric pressure, wind speed/direction and relative humidity) were obtained at the study site.     

The precise measurement of concentration gradients of mercury in air over soils: A review of past and recent measurements

Measurements of the atmospheric concentration gradients of mercury (Hg) vapor over soils can be used to determine the direction and magnitude of exchange rates of Hg if certain assumptions are met. However, these gradients are quite small and require highly precise sampling to achieve accurate data. We have developed a sampling and analysis procedure which allows quantification of gradients over background soils. With this procedure we can now measure atmospheric Hg at ambient levels with a precision of ～0.5 to 2% (expressed as relative standard error). This level of precision is well above those published in earlier gradient studies. In our recent studies, gradients measured between 25 and 165 cm above background forest soils at Walker Branch Watershed, Tennessee were quite small, ranging from 0.02 to 0.39 ng/m³ (expressed as concentration differences). These gradients indicated that Hg emission was about 3 times more frequent than dry deposition. Gradients measured over soils at Lake Gårdsjön, Sweden were generally smaller but also indicated bidirectional fluxes. By comparison, gradients above Hg-contaminated soils in Tennessee were far larger as expected, ranging from 0.12 to 5.60 ng/m³. These gradients consistently indicated emission of Hg. A number of tests were performed to validate that these gradients were true indications of Hg exchange rates.     

Atmosperic deposition of mercury in Norway: temporal and spatial trends

Data from a study of Hg in the terrestrial environment in Norway are presented. Analyses of moss samples and comparison with wet deposition data indicate that dry deposition of gaseous Hg is important at northern latitudes. The Hg content of the humus layer of natural soils shows only moderate regional differences, but is to some extent dependent on atmospheric deposition from long-range transport and local point sources. Peat cores from ombrotrophic bogs provide information about recent and pre-industrial levels of Hg deposition, the ratio between levels varying from less than 2 to almost 10 depending on the location. Surprisingly high levels in surface peat in the northern part of the country support the conclusion indicated from moss analysis concerning the significance of dry deposition.