20
R. Ebinghaus et al.
Studies in Siberia reveal that the average content of Hg in ores varies from 0.061.2% (Obolensky 1996), while the mercury content in minerals of those ore
deposits ranges from 0.01 to 2.700 ppm (Obolensky 1996). From these
concentration data, it has been estimated that natural emission due to surface
degassing of mercury in Siberia alone (approximate surface area 10 7 km
2 )
amounts to about 40 t year-I (Obolensky 1996). If volcanoes and forest fires are
included, the estimate rises to 1700 t year-I (Nriagu 1989), while 1290 t year-I
were calculated as the sum of soil vapor flux, volcanic and geothermal activities
(Varekamp and Buseck 1986). To put these numbers into context, the mercury
pool in a I-m-deep layer of the continents (total area I.S x 10 8 km" assumed
density 2 g cm- 3 , background Hg concentration so ng g-I) is I.S X 10 7 tons.
These estimates demonstrate that degasing from mineralisation zones (ore
bodies, geothermal areas, primary and secondary geochemical haloes) is a major
contribution to the total continental Hg emissions, and seems to be comparable in
magnitude to emissions from volcanic and geothermal activity. Other sources like
forest fires or vapor emissions from background soils also contribute significantly
to natural continental mercury emissions, but it seems that particulate export in
the form of dust is not one of them. The reason for this seems to be that soil
particles are fairly coarse (10 pm) and although they might be resuspended by
wind, they will settle quickly and generally not be transported very far in the
atmosphere. Typical concentrations of particles of this size in background
atmosphere are 25 ~lg m- 3 (Finlayson-Pitts and Pitts 1986) and assuming they
have an Hg content similar to background soils (so ng g-I), this kind of
particulate Hg would have a global average concentration of 2.S pg m -3 in
ambient air, which is 3 orders of magnitude lower than total gaseous Hg in
background air (Ebinghaus et al. 1995). This also indicates that particulate
emission ofHg from mineralized soils to the atmosphere is not likely to be a major
source of atmospheric Hg and is much less important than gaseous emission.
4.2
Volcanic Eruptions and Other Geothermal Activities
Though volcanic eruptions may contribute significantly to the natural emission
of mercury, there are only a few studies to report the contribution from this
source. Global estimates for Hg release from volcanoes total 830 t year -I, with the
vast majority being contributed by active, erupting volcanoes (800 t year-I) and
only small amounts (30 t year-I) coming from passive degassing of volcanoes
(Varekamp and Buseck 1986). The fluxes of Hg in the gases from eruptive plumes
at Mt. Etna, Italy, and Kuala, Hawaii, and in fumarolic gases at Kilauea, Hawaii,
and at White Island, New Zealand, have been estimated by measuring the Hg/S
ratios and then correlating those to the sulfur flux from these sources (Fitzgerald
1996). These results were scaled up globally and it was concluded that the annual
mercury flux from volcanic activity is between 20 and 90 t year-I and represents
only a small fraction «3%) of the annual mercury emissions from anthropogenic
sources (Fitzgerald 1996). This result agrees well with the above-mentioned
emission from passive degassing volcanoes, but active volcanic eruptions appear
R. Ebinghaus et al.
Studies in Siberia reveal that the average content of Hg in ores varies from 0.061.2% (Obolensky 1996), while the mercury content in minerals of those ore
deposits ranges from 0.01 to 2.700 ppm (Obolensky 1996). From these
concentration data, it has been estimated that natural emission due to surface
degassing of mercury in Siberia alone (approximate surface area 10 7 km
2 )
amounts to about 40 t year-I (Obolensky 1996). If volcanoes and forest fires are
included, the estimate rises to 1700 t year-I (Nriagu 1989), while 1290 t year-I
were calculated as the sum of soil vapor flux, volcanic and geothermal activities
(Varekamp and Buseck 1986). To put these numbers into context, the mercury
pool in a I-m-deep layer of the continents (total area I.S x 10 8 km" assumed
density 2 g cm- 3 , background Hg concentration so ng g-I) is I.S X 10 7 tons.
These estimates demonstrate that degasing from mineralisation zones (ore
bodies, geothermal areas, primary and secondary geochemical haloes) is a major
contribution to the total continental Hg emissions, and seems to be comparable in
magnitude to emissions from volcanic and geothermal activity. Other sources like
forest fires or vapor emissions from background soils also contribute significantly
to natural continental mercury emissions, but it seems that particulate export in
the form of dust is not one of them. The reason for this seems to be that soil
particles are fairly coarse (10 pm) and although they might be resuspended by
wind, they will settle quickly and generally not be transported very far in the
atmosphere. Typical concentrations of particles of this size in background
atmosphere are 25 ~lg m- 3 (Finlayson-Pitts and Pitts 1986) and assuming they
have an Hg content similar to background soils (so ng g-I), this kind of
particulate Hg would have a global average concentration of 2.S pg m -3 in
ambient air, which is 3 orders of magnitude lower than total gaseous Hg in
background air (Ebinghaus et al. 1995). This also indicates that particulate
emission ofHg from mineralized soils to the atmosphere is not likely to be a major
source of atmospheric Hg and is much less important than gaseous emission.
4.2
Volcanic Eruptions and Other Geothermal Activities
Though volcanic eruptions may contribute significantly to the natural emission
of mercury, there are only a few studies to report the contribution from this
source. Global estimates for Hg release from volcanoes total 830 t year -I, with the
vast majority being contributed by active, erupting volcanoes (800 t year-I) and
only small amounts (30 t year-I) coming from passive degassing of volcanoes
(Varekamp and Buseck 1986). The fluxes of Hg in the gases from eruptive plumes
at Mt. Etna, Italy, and Kuala, Hawaii, and in fumarolic gases at Kilauea, Hawaii,
and at White Island, New Zealand, have been estimated by measuring the Hg/S
ratios and then correlating those to the sulfur flux from these sources (Fitzgerald
1996). These results were scaled up globally and it was concluded that the annual
mercury flux from volcanic activity is between 20 and 90 t year-I and represents
only a small fraction «3%) of the annual mercury emissions from anthropogenic
sources (Fitzgerald 1996). This result agrees well with the above-mentioned
emission from passive degassing volcanoes, but active volcanic eruptions appear