Halogen release from Plinian eruptions and depletion of stratospheric ozone - Volcanism and Global Environmental Change - page 252

depletion, thus testifying to the limited halogen scavenging (Millard et al ., 2006 ;
Rose et al ., 2006 ). Due to the high ascent rates of large Plinian eruption columns,
scavenging due to humid conditions in tropical regions can be partly compensated
by the formation of halogen-bearing ice crystals and even solid salt particles (NaCl
containing Br), preserving the halogens for stratospheric release (Woods et al .,
1985 ; Textor et al ., 2003b ), which indeed has been observed for the tropical
El Chichon 1982 eruption. We therefore assume that between 1% and 25% of
the emitted halogens may have reached the stratosphere ( Table 16.1 ), for which
we consider the range between 1% and 10% as a conservative low estimate and
the range above 25% as an upper limit in the light of recent observations
and modelling results (Rose et al ., 2006 ; Textor et al ., 2003b ). Applying the
10% estimate, the resulting stratospheric injections per CAVA eruption vary

between 0.2 8070 kt Cl. In contrast to rapid scavenging from

the wet troposphere, these inorganic halogen compounds can remain in the dry
stratosphere for as long as the stratospheric turnover time of 3

- 110 kt Br and 1 -

6 years (Waugh and
Hall, 2002 ). For the total effect of halogens on stratospheric ozone the equivalent
effective stratospheric chlorine (EESC) quantity can be used, which is estimated
from tropospheric emissions of chloro

-

uorocarbons and other halogens, assump-
tions on transport timescales into the upper atmosphere and by weighting the
ozone-destroying potential of Br relative to Cl (EESC ¼ Cl þ (60 Br); Danilin

et al ., 1996 ; Montzka et al ., 2011 ).
Relating the hypothetical volcanic halogen burden from an average CAVA
eruption to the observed pre-ozone hole background levels in 1980 for mid-
latitudes, Figure 16.2 illustrates the effect of a single eruption in the past and
future stratosphere. Pre-ozone hole 1980 EESC values are projected to occur in
2046 (Daniel et al ., 2011 ). This implies that an average CAVA eruption in the
future may lead to a significant lofting of the EESC amounts probably for up to 3 to
6 years for all three scenarios. Such an effect becomes even more pronounced
towards the end of the twenty-

rst century due to the amendment of anthropogenic

chloro uorocarbons and halons. Inspecting the total Cl and Br loadings it becomes

obvious that the future EESC evolution is mainly controlled by the total Cl decline
(1980 recovery in 2040), whereas total Br stays above pre-1980 values during
the whole century. Figure 16.3a presents the percentage increase of each halogen
component and the EESC compared to 1980 values, applying the 10% scenario
from Figure 16.2 . If an average CAVA volcano were to erupt, it would lead to
a 50% increase of total Br in the present day (2015), which increases to almost
100% towards 2100 (this does not necessarily mean a correlated increase in ozone
depletion). The relative Cl impact is even higher, given the projected more rapid
decline of total Cl in the twenty- rst century. Here the relative impact more

than doubles between 2015 and 2100 changing from 370% to 770%, whereas