John Munthe

Atmospheric mercury—An overview

Abstract This paper presents a broad overview and synthesis of current knowledge and understanding pertaining to all major aspects of mercury in the atmosphere. The significant physical, chemical, and toxicological properties of this element and its environmentally relebant species encountered in the atmosphere are examined. Atmospheric pathways and processes considered herein include anthropogenic as well as natural sources of Hg emissions to the atmosphere, aerial transport and dispersion (including spatial and temporal variability), atmospheric transformations (both physical and chemical types), wet and dry removal/deposition processes to Earths surface. In addition, inter-compartmental (air-water/soil/vegetation) transfer and biogeochemical cycling of mercury are considered and discussed. The section on numerical modelling deals with atmospheric transport models as well as process-oriented models. Important gaps in our current knowledge of mercury in the atmospheric environment are identified, and suggestions for future areas of research are offered.

Recovery of Mercury-Contaminated Fisheries

Abstract In this paper, we synthesize available information on the links between changes in ecosystem loading of inorganic mercury (Hg) and levels of methylmercury (MeHg) in fish. Although it is widely hypothesized that increased Hg load to aquatic ecosystems leads to increases in MeHg in fish, there is limited quantitative data to test this hypothesis. Here we examine the available evidence from a range of sources: studies of ecosystems contaminated by industrial discharges, observations of fish MeHg responses to changes in atmospheric load, studies over space and environmental gradients, and experimental manipulations. A summary of the current understanding of the main processes involved in the transport and transformation from Hg load to MeHg in fish is provided. The role of Hg loading is discussed in context with other factors affecting Hg cycling and bioaccumulation in relation to timing and magnitude of response in fish MeHg. The main conclusion drawn is that changes in Hg loading (increase or decrease) will yield a response in fish MeHg but that the timing and magnitude of the response will vary depending of ecosystem-specific variables and the form of the Hg loaded.

International field intercomparison measurements of atmospheric mercury species at Mace Head, Ireland

Eleven laboratories from North America and Europe met at Mace Head, Ireland for the period 11–15 September 1995 for the first international field intercomparison of measurement techniques for atmospheric mercury species in ambient air and precipitation at a marine background location. Different manual methods for the sampling and analysis of total gaseous mercury (TGM) on gold and silver traps were compared with each other and with new automated analyzers. Additionally, particulate-phase mercury (Hgpart) in ambient air, total mercury, reactive mercury and methylmercury in precipitation were analyzed by some of the participating laboratories. Whereas measured concentrations of TGM and of total mercury in precipitation show good agreement between the participating laboratories, results for airborne particulate-phase mercury show much higher differences. Two laboratories measured inorganic oxidized gaseous mercury species (IOGM), and obtained levels in the low picogram m-3 range.

Atmospheric mercury species over central and Northern Europe. Model calculations and nordic air and precipitation network for 1987 and 1988

A chemical scheme based upon current knowledge of physicochemical forms and transformation reactions of atmospheric mercury has been implemented into a regional pollutant dispersion model for Europe. Existing databases for anthropogenic mercury emissions in Europe have been updated for 1987 and 1988 using new information on source data from eastern European countries including the former German Democratic Republic. Concentrations of total gaseous and particle associated mercury in air and mercury in precipitation calculated by the model are compared with observed values at Roervik in southwestern Sweden, Aspvreten, south of Stockholm and other locations of the Nordic network, on a daily basis. The results show that the model is capable of simulating long-range transport of mercury from Central Europe to Scandinavia including discrete events with peak concentrations in air and precipitation in the range of 10 ng m−3 and 100 ng l−1, respectively. Coinciding observed and calculated peak concentrations indicate that exceptionally high mercury emissions, most probably from chlor-alkali industry and lignite coal combustion in East Germany and Czechoslovakia, must have occurred in 1987 and 1988.

The aqueous oxidation of elemental mercury by ozone

Relative consumptions of aqueous Hg0 and S(IV) due to reactions with O3 have been used to estimate the rate constant for the reaction of Hg0 and O3. Ratios In([Hg0]0/[Hg0t])In([S(IV)]0/[S(IV)t]) =k(Hg0+O3)k(S(IV)+O3) ranging from 10 were measured in the pH range 4.5–9.5. The results were interpreted in terms of a second-order reaction of Hg0 and O3 with a rate constant k = (4.7±2.2) × 107 M−1s−1 which is independent of pH and temperature. Steady-state concentrations of dissolved inorganic mercury in atmospheric waters (rain and clouds) have been estimated using the rate constant determined in this work and previously reported rate constants for the reduction of Hg2+ by S(IV). At gas-phase concentrations of SO2>0.5 ppb, concentrations of dissolved inorganic mercury in the range 1–25 pM (0.2–5 ngl−1) are predicted at different pH and gas-phase concentrations of O3, which agrees reasonably well with measured concentrations. At lower SO2 concentration, the calculated values are unreasonably high, which indicates that other reducing processes may be of importance under these conditions.

Cycling of mercury between water, air, and soil compartments of the environment

The propensity to resist chemical, photo-chemical or biological degradation processes, coupled with a high degree of mobility — due to favorable physical-chemical properties such as relatively high vapor pressure and low solubility in water - bestows upon some chemical substances the necessary pre-requisites for extensive cycling among environmental compartments. Certain inorganic and organometallic species of Hg exhibit the aforementioned characteristics. This paper presents field measurement data, collected in Canada and Sweden, that provide information concerning the direction and magnitude of the material fluxes associated with transfer processes of Hg in the natural environment. The results of the field measurements reported here point to the importance of the natural phenomena of emission or re-emission (i.e., volatilization processes) involving the release of volatile Hg species from lake and soil surfaces into the overlying air, at least under warm weather conditions. Consequently, volatilization fluxes of Hg from natural surfaces should be taken into account when constructing biogeochemical cycles for this element and when calculating mass balances or budgets for Hg on local, regional and global scales.

Intercomparison of methods for sampling and analysis of atmospheric mercury species

An intercomparison for sampling and analysis of atmospheric mercury species was held in Tuscany, June 1998. Methods for sampling and analysis of total gaseous mercury (TGM), reactive gaseous mercury (RGM) and total particulate mercury (TPM) were used in parallel sampling over a period of 4 days. The results show that the different methods employed for TGM compared well whereas RGM and TPM showed a somewhat higher variability. Measurement results of RGM and TPM improved over the time period indicating that activities at the sampling site during set-up and initial sampling affected the results. Especially the TPM measurement results were affected. Additional parallel sampling was performed for two of the TPM methods under more controlled conditions which yielded more comparable results.

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.

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)22− 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.

Xylem sap as a pathway for total mercury and methylmercury transport from soils to tree canopy in the boreal forest

Conifer needles are an important link in the cycling of Total Mercury (THg) and Methylmercury (MeHg) in the boreal ecosystem due to the high THg and MeHg concentrations in litterfall. Translocation within the tree of Hg from soils to the crown canopy has been assumed to be a minor source of the Hg in litterfall. This paper, however, is the first to present direct observations of THg/MeHg transport from the soil via xylem sap. Xylem sap concentrations of THg and MeHg were measured in sap drained from different levels along the boles of freshly cut 100 year old Norway spruce (Picea abies) and Scots pine (Pinus sylvestris). The trees came from a mixed stand growing on podzolized till soils at the Svartberget Forest Research Station in N. Sweden. Soil solution concentrations of THg and MeHg at different levels in the soil profile were measured for comparison.Concentrations of THg in xylem sap ranged from 10–15 ng L-1 in both the Scots pine and Norway spruce. Concentrations of MeHg varied from 0.03 ng L-1to 0.16 ng L-1, with higher values in Scots pine than Norway spruce. If these concentrations are representative of the transport from soils to needles in xylem sap at this site, then only 3% of the MeHg in litterfall (0.12 mg ha-1 yr-1) and 11% of the THg (26 mg ha-1 yr-1) can originate via this pathway. The upward transport via xylem sap is larger relative to the open field inputs (84% of THg and 17% of MeHg). Comparison of soil solution and xylem sap THg/MeHg suggested some degree of THg exclusion during water uptake in Scots pine and Norway spruce, but MeHg exclusion only in Norway spruce.