Volcanoes and the Ozone Layer
Ozone layer depletion and appearance of large ozone holes over Antarctica in the last decades of the 20th century are often attributed only to anthropogenic factors, whereas dynamics of global natural phenomena is not taken into account. Volcano eruptions, for instance, strongly affect processes taking place on the Earth. There are grounds to believe that increased volcanic activity and, as a consequence, long-lasting volcanogenic perturbation of the stratosphere at the end of the past century are the main reasons for the ozone anomalies being observed
“SPECIAL-PURPOSE” LAYEROzone is an allotropic modification of oxygen, О3. Although the amount of ozone in the atmosphere is very small (about a millionth fraction of a per cent), its role in the Earth’s life is great. Ozone absorbs hard ultraviolet radiation of the Sun, which is of great danger for the biosphere. More than 85 % of atmospheric ozone comes to the stratosphere from the tropopause, whose height increases from 8 km over the poles to 18 km over the equator, up to 50 km, forming an ozone layer, the ozonosphere. The height of the ozone layer maximum, depending on latitude, increases from 15 km in the polar zones to 25 km in the tropical belt. The most intensive formation and destruction of ozone is in the tropical stratosphere, where solar radiation is maximal. Ozone lifetime in the tropics is only several hours, where, therefore, the ozone content is minimal. A part of the ozone synthesized in the tropical latitudes is transported via stratospheric circulation to the high latitudes, where its lifetime increases to 100 days, which allows it to accumulate. Therefore, ozone content in the stratosphere of polar and subpolar latitudes is higher than in the tropical belt
In the last quarter of the 20th century, thanks to spacecrafts, observations of the Earth’s ozone layer became global. In the first decades of observations, monitoring revealed a tendency toward extensive depletion of stratospheric ozone. Although the ozone layer destruction observed had no unambiguous scientific explanation, the anthropogenic concept was actively popularized. Ozone-destroying industrial freons were declared to be responsible for the problem revealed. In the last decades of the 20th century, the motto was overall struggle with this “evil.”
Underlying it was the article of English scientists published in 1985 in Nature, reporting a considerable decrease in the daily values of the total ozone content (TOC) over the Antarctic stations Faraday and Halley in October 1984. This value turned out to be half the average values for the previous five years. This fact had a great public response due to the authors’ hypothesis that there was a direct interrelation between life-threatening ozone holes and industrial freons destroying the ozone layer.
PLINIAN AND STROMBOLIAN ERUPTIONSPlinian-type volcano eruptions are characterized by strong explosive ejections with a VEI index of not less than 4 (the Volcanic Explosivity Index determines the volcanic eructation volume according to the eight-point scale). The erupted columns of ashes and gases are capable of going through the tropopause and reaching stratospheric heights, spreading the ejections to long distances. The duration of eruptions varies from several hours to several days. This type of eruptions was named after the ancient Roman author Pliny the Younger, who described in detail the Vesuvius eruption in the year 79 AD.
Strombolian-type volcano eruptions are characterized by rather small pulsating ejections (mostly in the form of volcanic bombs). Intervals between stronger eruptions may be as long as several years. In any case, the height of ejections does not exceed 2 km and the VEI index is not more than 2. This eruption type was named after the active volcano located on Stromboli island off the north coast of Sicily
However, the first observations of ozone holes in Antarctica were made much earlier. In 1957—1959 at the Halley station, a device constructed by G. Dobson detected a considerable decrease in TOC during the Antarctic spring (September, October) and restoration of its initial value only by the beginning of the Antarctic summer (the end of November). While processing ozone measurements at the Dumont d’Urville station, the French researchers P. Rigaud and B. Leroy found that on October 18, 1958 the TOC value was by a factor of 1.5 lower than that registered in 1984.
Moreover, quite recently substances with a structure similar to that of industrial freons and in a concentration comparable to the current one were found by Ukrainian researchers in air bubbles frozen in the Antarctic ice mass. However, their occurrence depth indicates that this event took place several thousand years ago.
The anthropogenic concept of ozone layer destruction does not offer a convincing mechanism for the transport of industrial freons from the northern hemisphere (where most of them are produced and consumed) to the South Pole. In this case, the fact that the amount of natural freons entering the atmosphere as a result of volcanic activity and from the ocean surfaces is by several orders of magnitude higher than that of technogeneous freons is ignored.
When solving a fundamental multi-parametric problem such as ozone layer destruction, one should take into account not only chemical processes occurring in the stratosphere, but also the atmosphere dynamics, relations between the Sun and the Earth, and interaction of the atmosphere and the ocean. It goes without saying that among the global phenomena that should not be overlooked are the Earth’s volcanic eruptions.
Volcanic eruptions and stratospheric ozone
Earth’s volcanic activity features an extremely wide range of eruption energies and eructation volumes. During plinian-type eruptions, a great amount of ash and sulphur dioxide enters the stratosphere. The sulphur dioxide gives rise to a long-lived sulfuric acid aerosol forming in the upper part of the ozone layer. On the volcanogenic aerosol surfaces, the stratospheric ozone is destroyed in heterogeneous reactions, which are most intensive on the surfaces of a fine (“young”) aerosol.
The plinian-type volcanic eruptions result in a global perturbation of the ozonosphere in the tropical latitudinal belt. The sulfuric acid aerosol formed in the stratosphere is scattered by zonal winds across the tropical belt and gradually drawn by the meridional circulation to the polar zones, shrouding the entire planet.
ANTHROPOGENIC CONCEPT OF OZONE LAYER DESTRUCTIONFirst assumptions about an important significant role of chlorine in the destruction of stratospheric ozone were made in 1974 by a few American scientists, who believed that the presence of chlorine in the stratosphere was linked to gas ejections from volcanoes and solid-propellant rockets. In the same year, the American chemists F. Rowland and M. Molina declared that there was a hypothetical danger of ozone layer destruction by industrial freons accumulating in the atmosphere and releasing chlorine during their disintegration in the stratosphere under the action of solar radiation.
Industrial production of chlorofluorocarbons (CFCs), or freons, was started by the chemical corporation Du Pont de Nemours in 1930. For many years, freons were widely used as coolants in coolant units, in spray and foam cans. The USSR was one of world leaders in the production of freons.
In 1985, a hypothesis was put forward that the ozone hole formation over Antarctica resulted from the atmospheric pollution by industrial freons, which initiated a wide campaign for the protection of the Earth’s ozone layer against anthropogenic action; unexpectedly, this campaign was actively supported by Du Pont de Nemours top management. It was in those years that the corporation developed the production of alternative coolants, namely, chlorine-free hydrofluorocarbons (HFCs), and, since the production of freons was prohibited, it became the monopolist in the market of “ozone-saving” HFC coolants production.
In 1985, the Vienna Convention for the Protection of the Ozone Layer was adopted. Two years later, the Montreal Protocol was signed to regulate the use of ozone-destroying substances in industry and in everyday life.
In 1995, F. Rowland and M. Molina, jointly with the Dutch chemist P. Crutzen, received the Nobel Prize “for… work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone”
In the second half of the 1980s, on the initiative of Academician V. E. Zuev, a multichannel lidar system was created providing remote access to real-time information about the distributions of ozone, temperature, and aerosols in the stratosphere. Decades-long observations of the dynamics of these parameters during aerosol perturbations of the stratosphere and in the periods of absence of such perturbations allowed us to analyze in detail the effect of the stratospheric volcanogenic aerosol on the state of the ozone layer and classify the processes causing its depression. Previously, this effect was studied poorly. For instance, the period of volcanogenic depression of the stratospheric ozone was considered to be not longer than several months. According to our investigations, aerosol perturbations of the stratosphere with eruption products getting into it play a prominent role in the ozone layer destruction within 2 or 3 years, especially after the eruption of tropical-belt volcanoes.
The average frequency of the plinian-type eruptions is once in every several tens of years or even once in a hundred of years. However, during the period from 1979 to 1994, more than ten such eruptions occurred in the tropical belt alone (i.e., every 1.5 years on average). This high volcanic activity led to an extremely long period of continuous stratospheric pollution by the volcanogenic aerosol. When, by the end of the century, the stratosphere cleaned itself from this pollutant, the stratospheric ozone content quickly increased to the previous level.
Of all factors affecting the ozonosphere, this dynamic variation in the last quarter of the past century was typical only for the aerosol content of the stratosphere. This means that the main regulator of the ozone layer state during this period was the massive volcanogenic aerosol perturbation of the stratosphere and not the technogenous freons, whose content in the atmosphere (due to their long lifetime) varied insignificantly.
Antarctica’s ozone hole
The activity of volcanic eruptions is also directly related to the Antarctic ozone anomaly. The volcanogenic aerosol plays the role of condensation nuclei in the formation of particles of polar stratospheric clouds (PSCs) under anomalously low temperatures in the stratosphere over Antarctica in the winter-spring polar period. On the solid surfaces of PSC particles, chlorine is reduced from molecules-reservoirs, which initiates the formation of ozone holes. This anomalous regime with temperatures below 195 K forms due to the isolation of the stratospheric air masses inside the circumpolar vortex.
At higher temperatures, PSC particles are not formed, and ozone is not destroyed. This is why in 1986, 1988, and 2002 ozone holes over the Antarctic station Halley were small and short-lived. The situation is the same in the Arctic stratosphere, where anomalously low temperatures, as well as ozone holes, are observed rarely and for a very short time, although the amount of chlorine compounds in the Arctic is not smaller than in Antarctica.
The presence of condensation nuclei is closely related to volcanogenic perturbations of the stratosphere. Aerosol perturbations of the Antarctic stratosphere that took place after a series of volcanic eruptions in the tropical belt early in the 1980s enhanced the stratospheric ozone destruction in this period and resulted in the formation of an ozone hole registered in October 1984 at the Halley station.
However, the strongest aerosol perturbation of the Antarctic stratosphere that caused a long-lasting depression of the ozone layer took place in 1991—1993, after the three exceedingly powerful eruptions of Pinatubo (the Philippines), Cerro-Hudson, and Lascar (Chile) volcanoes with a volcanic explosivity index of about 6, 5, and 4, respectively, which occurred in less than two years. As a consequence, in the polar spring of 1993 a very deep and long-lasting ozone hole was observed practically at all Antarctic stations.
Another supplier of volcanogenic sulphuric acid aerosol to the Antarctic stratosphere is Mount Erebus Volcano (Antarctica, 3,794 m high), which has been active since 1972. Erebus eruptions belong to the Strombolian type. However, according to the description by Harun Taciev (Haroun Tazieff), a well-known volcanist who investigated this volcano in 1974, gas jets consisting mostly of sulfur dioxide, hydrogen chloride, and methane are ejected during its upward eruptions with a speed of over 700 km/hour. At such speeds, the gas jets quickly reach stratospheric heights, where sulfuric acid aerosol, with the condensation nuclei of PSC particles, forms from sulfur dioxide. Hydrogen chloride frozen in these particles provides efficient heterogeneous reactions (described above) on their surfaces. Methane, blocking the chlorine cycle, also contributes to the accumulation of hydrogen chloride in the stratosphere.
HOW OZONE HOLES APPEAR AND VANISHOn the solid surfaces of particles of polar stratospheric clouds, containing frozen hydrogen chloride, HCl, heterogeneous reactions in the gas phase result in chlorine release from molecules-reservoirs of chlorine nitrate, ClONO2, with simultaneous capture in the solid phase of nitrogen dioxide in the form of frozen nitric acid. When the polar winter comes to an end, chlorine molecules dissociate into atoms under the action of solar radiation. The appearance of free chlorine in the spring period catalyzes the reactions of ozone decay in the chlorine cycle. Contributing to the ozone depression is the boundedness of nitrogen dioxide, which blocks the formation of chlorine. Thus, when the Antarctic spring comes, an ozone hole forms at the boundaries between PSC fields, which vanishes with the evaporation of cloud particles when stratosphere temperature increases by the end of the spring period
A considerable activation of Erebus was observed in 1995—2007. Frequent and strong eruptions of the volcano in the second half of the 1990s prevented further increase in the spring TOC values evident after the 1993 minimum. Series of extremely powerful eruptions were observed in 2000 and 2006. It is in these years that an ozone hole of almost 30 million km2, the largest one in the entire observation period, was recorded.
Contributing to the record size of the ozone hole in 2000 were aerosol condensation nuclei from ion clusters that originated in the Antarctic stratosphere after an extremely strong solar flare just in the period of winter-spring PSC formation. Ion clusters have an activated (charged) surface, which intensifies the ozone destruction processes.
Nowadays, there are attempts to use the scenario of combating ozone layer destruction to solve another global problem. The anthropogenic factor is now declared to be responsible for climate warming. Once again, we unreasonably underestimate the role of global natural phenomena, in particular, variations in СО2 exchange between the atmosphere and the ocean in the conditions when its surface temperature is increasing and the activity of volcanoes whose gas ejections contain a lot of carbon dioxide is growing.
Unfortunately, the situation is aggravated by inconsistent current-day scientific data on the supply of volcanogenic carbon dioxide to the atmosphere. Geologists believe that volcanoes eject not less than several billion tons of СО2 annually; the value used by climatologists in their models is by an order of magnitude lower, namely, 0.3 billion tons.
In this situation, we shouldn’t ignore the fact that in the last 200 years (the period of industrial revolution) the annual number of volcanic eruptions taking place on the Earth has increased almost four-fold. A noticeable growth both in the number of powerful volcanic eruptions and in the carbon dioxide content in the atmosphere in the last quarter of the 20th century was synchronous.
VOLCANOES AND THE OZONE LAYER IN RETROSPECTThe bioindication method of reconstructing TOC by the density of annual rings of dark coniferous trees has made it possible to “decipher” the centuries-long behavior of the ozone layer in the sub-Arctic latitudinal belt and reveal the cyclicity of TOC variations with the periods that largely coincide with the periods of solar cycles. It has turned out that variations with the periods of 22 and 66 years manifest themselves most clearly. The first cycle coincides with the well-known Hayle solar cycle. The second one is, most probably, linked to the major cycle of the solar system reflecting the gravitational interaction between its main bodies, namely, the Sun, Jupiter, and Saturn.
Periods of volcanogenic depressions of the ozone layer in the subarctic latitudinal belt are characterized by negative deviations from the main cyclic variations. At the negative phase of cyclic variations, these deviations increase, and at the positive phase, they decrease. This is why the ozone layer depression after a most powerful 1883 eruption of Krakatau volcano revealed itself only slightly. On the contrary, the 1991 Pinatubo volcano eruption, whose strength was much less, coincided with the minimum of cyclic variations of the ozonosphere. Under the conditions of long-lasting continuous volcanogenic depression of the ozone layer, summation of negative TOC variations, owing to the synergetic effect, caused an anomalous TOC decrease in the 1990s
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The article contains volcano photos from the site www.ngdc.noaa.gov/hazard/volcano.shtml of the Department of Commerce, the National Oceanic and Atmospheric Administration, and the National Environmental Satellite Data, and Information Service of the USA