William H. Schroeder

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.

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.

Transformation processes involving mercury species in the atmosphere — results from a literature survey

Mercury is released into the atmosphere from natural and anthropogenic sources. Once in the air, Hg species will be dispersed, chemically and physically transformed, and transported over varying distances (regional, continental and global scales) while concurrently being subjected to removal processes. Modeling of the aerial transport and deposition of Hg to receptor sites offers an attractive approach for integrating existing experimental data and represents an important tool for advancing our understanding of environmental Hg pollution. Comprehensive modeling efforts of the atmospheric pathways and fate of this heavy metal require knowledge of its atmospheric chemistry. To update and consolidate available information relevant to the atmospheric chemistry of Hg, a computer-assisted search was undertaken of the primary scientific literature published during the last ten years. Selected results from this literature survey are presented in this paper.

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.

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.

Volatilization of mercury from lake surfaces

Abstract Field studies were performed in Canada (at Eagle Lake in north-western Ontario) during July 1986 and subsequently at four oligotrophic forest lakes in south-western Sweden during 1988 and 1989 to determine the extent of mercury volatilization. The Canadian investigations involved simultaneous measurements of total vapour-phase mercury concentrations in air sampled immediately above the water surface and over land nearby. A diurnal cycle was observed for mercury emissions from Eagle Lake, with day-time volatilization rates significantly larger than night-time rates. The Swedish field measurements employed a flux chamber. This device was used to determine the rates at which volatile mercury species were emitted from the lakes. Volatilization occurred from each of the Swedish forest lakes during the warmer seasons of the year (with near-surface water temperatures in the 13–23°C range). For day-time measurements, volatilization rates were generally 3–10 ng Hg m −2 h −1 . Daytime fluxes were, on average, about 2.5-times larger than night-time fluxes. Experiments conducted during the winter season (with water temperatures just above the freezing point) indicated very little, if any, emission of volatile mercury species from the lake surface.

Transport patterns and potential sources of total gaseous mercury measured in Canadian high Arctic in 1995

Abstract Trajectory cluster analysis and the potential source contribution function (PSCF) model have been used to investigate the source–receptor relationship for the total gaseous mercury (TGM) measured in the Canadian High Arctic (Alert, 82.5°N, 62.3°W) during 1995. Cluster analysis of 10-day back-trajectories in 1995 shows that the synoptic flows arriving at Alert are dominated by the air masses from the north. Long-range transport only occurs in the cold seasons while summertime flows tend to circulate in the Arctic Ocean. The potential source regions identified by the PSCF modeling include Eurasia and populated areas in the North America and Europe. Based on the modeling results, it is suggested that the elevated TGM concentrations found in the Arctic summer should be of geological origins, mainly from the evasion of volatile Hg 0 from earths surfaces. In the autumn and winter, mercury is transported to the receptor site from remote anthropogenic sources. The preferred sources of TGM in the spring cannot be clearly determined due to the Arctic springtime mercury depletion, which significantly reduces the number of trajectories contributing to PSCF values. Using TGM data of higher temporal resolution improves the sensitivity of the PSCF modeling results.

Modelling of atmospheric transport and deposition of toxaphene into the Great Lakes ecosystem

Abstract Toxaphene, not extensively used in the Great Lakes basin, has been found in fish, lake water, ambient air and precipitation in this region. It has been suggested that the atmosphere constitutes a primary transport route of toxaphene to the Great Lakes from the major source regions in the southern U.S. Environmental measurements are too few to estimate the input of toxaphene to the Great Lakes basins. The ASTRAP model, used in acid rain research, was modified for simulation of the atmospheric pathway of toxaphene. Based on emission inventories, derived from use patterns in North America for 1976 and 1980, air concentration and deposition of toxaphene to the Great Lakes were estimated. The results confirm that the atmosphere is a major transport route of toxaphene to the Great Lakes region. They also show that toxaphene can be transported to the North Atlantic. Total deposition to the Lakes in 1980 was 3–10 t and annual average air concentrations about 0.5ngm−3. Although the information on physical/chemical properties and emissions is incomplete and air quality and precipitation chemistry measurements of toxaphene are few and uncertain, model predictions show good agreement with the measurements.

Sampling and Determination of Particulate Mercury in Ambient Air: A Review

Total particulate mercury (TPM) may constitute, in general, a small percentage by mass of total atmospheric mercury, but under certain conditions it can be a very significant form of atmospheric Hg, and play an important role in the deposition of mercury to terrestrial and aquatic cosystems. To understand the cycling and distribution of mercury in the environment, accurate determination of the total concentration and speciation of TPM are necessary. This article provides a review of scientific and technical literature published in the past 5 yr pertaining to sampling and analytical methodologies for the determination and chemical speciation of TPM in ambient air. Sampling methods discussed include the conventional filtration method and the diffusion denuder-based technique. Analytical techniques, such as gold-amalgamation/cold-vapor spectrometry, and nuclear methods, are summarized.