Subject: volcanic chlorine and the ozone layer
My mother who is an environmental activist, recently read that the chlorine emissions from volcanoes is more of an hazard to the ozone layer than man made chlorine. She would like to know if this is the case and where she could get more information to substantiate or contradict these allegations.
The question you asked about volcanically-derived chlorine and the
ozone layer is a good one, since this is an area of active debate
(as are most issues related to the ozone layer).
First, as you may be aware, it has been hypothesized that chlorine-
and fluorine-bearing compounds (particularly chlorofluorocarbons)
break-down due to the action of ultra-violet light in the upper
atmosphere to form radicals, a highly reactive non-charged form of
certain chemical elements and molecules. These quickly react with
ozone, another electronically peculiar molecule, causing it to
decompose. Ozone is of course important to life on earth as it
effectively shields the planet's surface from harmful uv waves.
It is also possible for other chlorine-bearing molecules to
decompose in a similar manner in the upper atmosphere, although the
relative rates of decomposition depend on many variables.
Volcanic eruptions account for a large flux of chlorine from land to
the atmosphere on a yearly basis. This is in addition to chlorine
that enters the atmosphere from sea spray, industrial processes and
biological gases. All of these inputs occur near or at the base of
the atmosphere (the planet's surface). Very little of the material
emited from volcanoes makes it up into the higher reaches of our
atmosphere (the stratosphere) where it could affect the ozone layer,
however. Most of it is believed to be depositied lower down (in
the troposphere), where it then rained out back to the surface of
the earth. Only during fairly rare, large, explosive eruptions,
such as occured a few years back at Mt. Pinatubo, do large amounts
of volcanic gases reach the stratosphere.
So why do chlorofluorocarbons reach the upper atmosphere when they
too are only input at the base of the atmosphere? Because the
latter are much more stable in the lower atmosphere, so they become
well distributed and make their way to the stratosphere via
atmospheric circulation. On the other hand, chlorine from volcanoes
is usually emitted as hydrochloric acid (HCl), chlorine gas (Cl2)
or volatile compounds such as lead chloride (PbCl2). Each of these
is far more water soluble and/or reactive in the lower atmosphere
(as compared to chlorofluorocarbons) so these volcanic gases tend
not to be as uniformly distributed in the atmosphere following
injection by a volcano.
So, does volcanic chlorine affect the ozone layer or not? Well,
possibly it does, since even if only a small amount of the total
chlorine input to the atmosphere from this source makes it to the
stratosphere, it could still be a significant portion of the total
amount of reactive chlorine-bearing compounds there. However, the
rate of volcanic activity over a 100 to 1000 year interval has not,
as far as we can tell, changed drastically for much of the past
million years; the level of ozone in the upper atmosphere has also
not changed drastically over this period (until recently). Thus, it
may be true that chlorine from volcanoes helps keep the production
of ozone from oxygen gas in check by limiting it's build up, but it
is not likely that our present ozone woes are due to this source of
chlorine.
One final point that is probably worth noting is that it is important
to remember that our monitoring of the exact nature of the ozone layer
has only begun recently, and we have a very poor handle on what, if
any, natural causes there may be for its density to change over short
time intervals. Thus, although the case for the role of
chlorofluorocarbons is convincing, it is difficult to say if even it is
the culprit (or at least if it did or didn't have help from other
gases). My own opinion is that a responsible society should
always act to curtail the addition of substances deemed potentially
hazardous even if there is compelling but not necessarily absolute
evidence (which is the case for chlorofluorocarbons), since the balance
of nature is far too complex a thing to risk toying with.
Dr. Ken Rubin, Asistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822
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