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