Jökull - 01.01.2001, Page 70

Þorvaldur Þórðarson and Stephen Self
Figure 4. Simplified vertical section along line A–B on Figure 3 showing the inferred dispersal of the Laki
plumes. Eruption columns produced by explosive activity at the Laki fissures between 8 and 14 June 1783 car-
ried ash, SO  and other volcanic gases to altitudes of 9–12 km. The sulfur–rich volcanic plumes were dispersed
eastward towards Europe by the polar jet stream and at the same time the SO  was gradually converted to sul-
furic aerosols. A large quasi–stationary high–pressure cell (H) was located over Europe between 21 June and 5
July. Due to convergence of airflow at the tropopause level, about 75 wt.% of the Laki aerosol cloud was sucked
into the high–pressure system and re–introduced into the lower atmosphere by the subsiding air masses, spread-
ing in a spiral–like fashion across the continent. – Einfaldað þversnið (A–B á 3. mynd) sem sýnir flutningsferli
og dreifingu brennisteinsmóðunar frá Skaftáreldum. Dagana 8.–14. júní þeytti sprengivirknin brennisteinsoxíði
(SO  ) og öðrum kvikugösum upp í 9–12 km hæð. Gösin dreifðust austur yfir Evrópu með háloftavindum. Við
flutninginn hvarfaðist brennisteinsoxíðið við vatnsgufu í andrúmsloftinu og myndaði agnir af brennisteinssýru
(H  SO ) og þar með sjáanlega móðu. Kyrrstæð lægð lá yfir Evrópu frá 21. júní til 5. júlí þannig að um það bil
75% af móðumassanum sökk til yfirborðs jarðar með niðurstreyminu í hæðarmiðjunni.
then dispersed as sulfur-rich plumes eastward towards
Europe. During transport in the polar jet stream the
SO  was gradually converted to sulfuric aerosols by
reaction with atmospheric moisture to produce 80
megatons of sulfuric aerosols. At these altitudes the
conversion of SO  to H  SO has a time constant of
1–2 weeks (Schoeberl et al., 1993), which most likely
accounts for the 8–10 day time difference between the
loading of the SO  mass and the appearance of visi-
ble haze over Europe. When the haze intensity was
at its peak over Europe between 21 June and 5 July, a
large quasi-stationary high–pressure cell was located
over Europe (Figure 3). Due to convergence of air-
flow at the tropopause level, the Laki aerosol cloud
was sucked into the high–pressure system and re–
introduced into the lower atmosphere by the subsiding
air masses, spreading in a spiral–like fashion across
the continent (Figure 4). As the aerosols were carried
towards the surface they grew in size through con-
tinued reaction with moist air and thereby increased
the opacity (density) of the haze as well as drying
up the atmosphere. This mechanism best explains the
widespread occurrence of the thick haze in Europe be-
tween 21 June and 5 July 1783.
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