Z. Xiao

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.

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.

Influence of Humic Substances on Photolysis of Divalent Mercury in Aqueous Solution

Mercury (II) solutions were irradiated by a simulated sunlight in the presence of humic acid (HA) or fulvic acid (FA). Results show that, under the experimental conditions and the FA and HA chosen, less than 20% of the Hg in solution was photolysed with a rate of (1.63±0.29)×10−2 s−1 (n=23) and the rest of (2.38±0.40)×10−4 s−1 (n=23) depending on the substitutes of humic substances to which Hg were bond. The sunlight photolysis lifetimes were estimated to be 4 and 250 sunlight hours respectively under summer conditions at Stockholm latitude.

Factors affecting the measurement of mercury emissions from soils with flux chambers

Air-surface exchange of mercury (Hg) above an arid geothermal area was measured with three parallel flux chamber experiments. The different experimental designs were intercompared with each other, with regard to the magnitude of the measured Hg fluxes and their response to environmental changes. Qualitatively, the measured Hg fluxes agreed well throughout the diurnal cycle, and in their response to environmental events and experimental manipulations, but quantitatively, there were significant discrepancies between the individual flux results. On average, the three designs yielded Hg fluxes agreeing within a factor of 2, but even more pronounced differences were observed during midday high emission periods and during apparent nighttime deposition events. The chamber flushing rate appears to have a very significant impact on the measured fluxes and on the response behavior to environmental change. This study demonstrates that both experimental differences and small-scale regional variability introduce large uncertainty in the estimation of natural Hg air-surface exchange by different flux chamber techniques. Also, the impact of environmental parameters on Hg air-surface exchange was studied. Rain events led to a strong increase in the Hg emissions, even when the covered soil remained dry, suggesting that the apparent chamber footprint is larger than the actually covered area. Exclusion of sunlight led to decreases in Hg emissions. Statistical analysis revealed the strongest correlations between the measured Hg fluxes and radiation and wind speed. Weaker correlations were observed with air and soil temperature and wind direction (probably due to local Hg sources). Fluxes were also inversely correlated with relative humidity.

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.

Atmospheric mercury deposition in Guizhou, China

The atmospheric deposition of mercury including total gas phase mercury (TGM), gas phase divalent mercury (Hg2+), and gas phase monomethyl mercury (MMHg) was investigated. Samples were collected from central cities and nature reserve areas across the province. Techniques and equipment including moss bags, KCl-coated denuder and GARDIS-1A portable mercury vapor analyzer were employed for sample collection and analysis. The reliability of techniques and analytical methods used for the project was evaluated. The deposition of atmospheric mercury was reported in species in this important mercury polluted province for the first time. The spatial and temporal variability of mercury emissions were investigated and reported. Contribution of coal mercury emissions to the atmosphere mercury deposition of the province was also investigated.

A new sensitive and portable mercury vapor analyzer Gardis-1A

A possibility to build a mercury (Hg) analyzer based on the cold vapor atomic absorption spectrometry (CVAAS) with a sub-picogram detection limit was investigated. Construction of the gold traps and the optical cell, carrier gas flow rate and gold traps heating power were chosen in order to effectively concentrate sampled Hg vapor and to obtain highest possible response in the optical and electronic system. The newly created analyzer is able to detect 0.5 pg of elemental Hg vapor. Optimization of construction resulted in stable performance and good selectivity for gaseous Hg of the analyzer. The instrument is commercially available.

Sampling and determination of gaseous and particulate mercury in the atmosphere using gold-coated denuders

Gold-coated denuders have been used to separate gas phase and particulate phase Hg in air samples. The denuders were 65 cm long with a 0.4 cm i.d. and were capable of removing >99.9% of the vapor phase elemental Hg in an air stream at flow rates of about 1 L min−1. Data were obtained at different sampling locations around the city of Göteborg. The concentration of particulate Hg was found to be within the range of 0.11 to 0.57 ng m−3, which corresponds to 2.8 to 16.9% of the total airborne Hg. Positive correlations between the concentration of particulate Hg and that of soot and suspended particles as well as a negative correlation between the concentration of particulate Hg and relative humidity were observed.

Atmospheric mercury deposition to grass in southern Sweden

This paper summarises the results of a 5-year investigation (1990–1994) using artificially planted rye grass in open ground as a biological monitor for air-borne mercury (Hg) in the south of Sweden. The total Hg deposition was estimated to be 14 ng m−2 h−1, of which about 70% was estimated from dry deposition on the grass surface, during the growing seasons from May to October. Hg content in grass leaves decreased from 0.076 (1990) to 0.041 μg g−1 (1994) with a rate of about 10% per year, which reflects the reduced trend of Hg deposition over this area.

Atmospheric mercury deposition on Fanjing Mountain Nature Reserve, Guizhou, China

Fanjing Mountain Nature Reserve (FMNR) is surrounded with several Hg emission sources within distances of 100-200 km. At the two sites studied, Tongren and Danzai, Hg emission and deposition fluxes, Hg concentration in the air, soil and other samples are all several hundred times higher than at other relatively clean areas. Hg accumulation in soil and moss at FMNR varies with the sampling heights. Total Hg deposition to this area has been estimated to be 115 micrograms m-2 y-1 using moss bag technique. Dry deposition was determined to be about 5.2 micrograms m-2 month-1 during March to June, corresponding to more than 50% of the total deposition.