This article is from the Ozone Depletion:
Stratospheric Chlorine and Bromine FAQ, by Robert Parson
email@example.com with numerous contributions by
23 Volcanoes put more chlorine into the stratosphere than CFC's.
Short Reply: False. Volcanoes account for at most a few percent
of the chlorine in the stratosphere.
Long reply: This is one of the most persistent myths in this
area. As is so often the case, there is a seed of truth at the
root of the myth. Volcanic gases are rich in Hydrogen Chloride, HCl.
As we have discussed, this gas is very soluble in water and is
removed from the troposphere on a time scale of 1-7 days, so we can
dismiss quietly simmering volcanoes as a stratospheric source, just
as we can neglect sea salt and other natural sources of HCl. (In fact
tropospheric HCl from volcanoes is neglible compared to HCl from
sea salt.) However, we cannot use this argument to dismiss MAJOR
volcanic eruptions, which can in principle inject HCl directly into
the middle stratosphere.
What is a "major" eruption? There is a sort of "Richter scale" for
volcanic eruptions, the so-called "Volcanic explosivity index" or
VEI. Like the Richter scale it is logarithmic; an eruption with a
VEI of 5 is ten times "bigger" than one with a VEI of 4. To give a
sense of magnitude, I list below the VEI for some familiar recent
and historic eruptions:
Eruption VEI Stratospheric Aerosol,
Kilauea 0-1 -
Erebus, 1976-84 1-2 -
Augustine, 1976 4 0.6
St Helen's, 1980 5 (barely) 0.55
El Chichon, 1982 5 12
Pinatubo, 1991 5-6 30
Krakatau, 1883 6 50 (estimated)
Tambora, 1815 7 80-200 (estimated)
[Smithsonian] [Symonds et al.] [Sigurdsson] [Pinatubo] [WMO 1988]
[Bluth et al.] [McCormick et al. 1995]
Roughly speaking, an eruption with VEI>3 can penetrate the
stratosphere. An eruption with VEI>5 can send a plume up to 25km, in the
middle of the ozone layer. Such eruptions occur about once a decade.
Since the VEI is not designed specifically to measure a volcano's impact
on the stratosphere, I have also listed the total mass of stratospheric
aerosols (mostly sulfates) produced by the eruption. (Note that St.
Helens produced much less aerosol than El Chichon - St. Helens blew out
sideways, dumping a large ash cloud over eastern Washington, rather than
ejecting its gases into the stratosphere.) Passively degassing volcanoes
such as Kilauea and Erebus are far too weak to penetrate the
stratosphere, but explosive eruptions like El Chichon and Pinatubo need
to be considered in detail.
Before 1982, there were no direct measurements of the amount of HCl
that an explosive eruption put into the stratosphere. There were,
however, estimates of the _total_ chlorine production from an
eruption, based upon such geophysical techniques as analysis of
glass inclusions trapped in volcanic rocks. [Cadle] [Johnston]
[Sigurdsson] [Symonds et al.] There was much debate
about how much of the emitted chlorine reached the stratosphere;
estimates ranged from < 0.03 Mt/year [Cadle] to 0.1-1.0 Mt/year
[Symonds et al.]. During the 1980's emissions of CFC's and related
compounds contributed ~1 Mt of chlorine per year to the
atmosphere. [Prather et al.] This results in an annual flux of >0.3
Mt/yr of chlorine into the stratosphere. The _highest_ estimates
of volcanic emissions - upper limits calculated by assuming that
_all_ of the HCl from a major eruption reached and stayed in the
stratosphere - were thus of the same order of magnitude as human
sources. (There is no support whatsoever for the claim that a
_single_ recent eruption produced ~500 times as much chlorine as a
year's worth of CFC production. This wildly inaccurate number appears
to have originated as an editorial mistake in a scientific encyclopedia.)
It is very difficult to reconcile the higher estimates with the
altitude and time-dependence of stratospheric HCl. The volcanic
contribution to the upper stratosphere should come in sudden bursts
following major eruptions, and it should initially be largest in
the vicinity of the volcanic plume. Since vertical transport in the
stratosphere is slow, one would expect to see the altitude profile
change abruptly after a major eruption, whereas it has maintained
more-or-less the same shape since it was first measured in 1975.
One would also not expect a strong correlation between HCl and
organochlorine compounds if volcanic injection were contributing
~50% of the total HCl. If half of the HCl has an inorganic origin,
where is all that _organic_ stratospheric chlorine going?
The issue has now been largely resolved by _direct_ measurements of the
stratospheric HCl produced by El Chichon, the most important eruption of
the 1980's, and Pinatubo, the largest since 1912. It was found that El
Chichon injected *0.04* Mt of HCl [Mankin and Coffey]. The much bigger
eruption of Pinatubo produced less [Mankin, Coffey and Goldman] [Wallace
and Livingston 1992], - in fact the authors were not sure that they had
measured _any_ significant increase. Analysis of ice cores leads to
similar conclusions for historic eruptions [Delmas]. The ice cores show
significantly enhanced levels of sulfur following major historic
eruptions, but no enhancement in chlorine, showing that the chlorine
produced in the eruption did not survive long enough to be transported
to polar regions. It is clear, then, that even though major eruptions
produce large amounts of chlorine in the form of HCl, most of that HCl
either never enters the stratosphere, or is very rapidly removed from it.
Recent model calculations [Pinto et al.] [Tabazadeh and Turco]
have clarified the physics involved. A volcanic plume contains
approximately 1000 times as much water vapor as HCl. As the plume
rises and cools the water condenses, capturing the HCl as it does
so and returning it to the earth in the extensive rain showers that
typically follow major eruptions. HCl can also be removed if it
is adsorbed on ice or ash particles. Model calculations show that
more than 99% of the HCl is removed by these processes, in good
agreement with observations.
* Older indirect _estimates_ of the contribution of volcanic
eruptions to stratospheric chlorine gave results that ranged
from much less than anthropogenic to somewhat larger than
anthropogenic. It is difficult to reconcile the larger estimates
with the altitude distribution of inorganic chlorine in the
stratosphere, or its steady increase over the past 20 years.
Nevertheless, these estimates raised an important scientific
question that needed to be resolved by _direct_ measurements
in the stratosphere.
* Direct measurements on El Chichon, the largest eruption of
the 1980's, and on Pinatubo, the largest since 1912, show
that the volcanic contribution is small.
* Claims that volcanoes produce more stratospheric chlorine than
human activity arise from the careless use of old scientific
estimates that have since been refuted by observation.
* Claims that a single recent eruption injected ~500 times a year's
CFC production into the stratosphere have no scientific basis
To conclude, we need to say something about Mt. Erebus. In an
article in _21st Century_ (July/August 1989), Rogelio Maduro
claimed that this Antarctic volcano has been erupting constantly
for the last 100 years, emitting more than 1000 tons of chlorine
per day. Mt. Erebus has in fact been simmering quietly for over a
century [ARS] but the estimate of 1000 tons/day of HCl only applied
to an especially active period between 1976 and 1983 [Kyle et al. 1990].
Moreover, that estimate has been since been reduced to 167 tons/day
(0.0609 Mt/year). By late 1984 emissions had dropped by an order of
magnitude, and have remained at low levels since; HCl emissions
_at the crater rim_ were 19 tons/day (0.007 Mt/year) in 1986,
and 36 tons/day (0.013 Mt/year) in 1991. [Zreda-Gostynska et al.]
Since this is a passively degassing volcano (VEI=1-2 in the active
period), very little of this HCl reaches the stratosphere. The
Erebus plume never rises more than 0.5 km above the volcano,
and in fact the gas usually just oozes over the crater rim. Indeed,
one purpose of the measurements of Kyle et al. was to explain high
Cl concentrations in Antarctic snow.