Can atoms break down naturally

Natural ozone killers from the ground

Ozone protects life on earth from high-energy radiation from space. In 1985, scientists first published their discovery that an "ozone hole" had formed over the southern polar atmosphere.

This discovery alarmed scientists and the public alike: the earth's stratospheric ozone layer seemed to be more threatened than had been predicted on the basis of atmospheric models - an indication that the knowledge was simply insufficient and that environmental factors were coming together in an unexpected way. One of these factors, the occurrence of very low temperatures in the winter and spring months, is actually due to a natural process. This discovery illustrates the enormous importance of recording environmental changes - caused by natural and anthropogenic processes - and the need to extend them to the entire world.

Scientific studies have now cleared up the secret of the rapid loss of ozone in the upper part of the atmosphere. Without a doubt, the decrease is due to photochemical, ozone-depleting reactions triggered by reactive chlorine compounds; these are formed in the stratosphere by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and other gaseous halogen compounds.

As their name suggests, chlorofluorocarbons are hydrocarbons in which the hydrogen has been partially or wholly replaced by chlorine and fluorine. Since their introduction around 65 years ago, these gases have been used in some countries as coolants for refrigerators and air conditioning systems, as propellants for aerosol sprays and for foaming plastics. These compounds were originally considered ideal industrial chemicals because they are very stable, chemically inert and therefore non-toxic. But it is precisely this inertia that makes CFCs a possible danger, as they pass through the lower atmosphere unhindered and are only photochemically degraded in the stratosphere. In the process, highly reactive chlorine atoms are formed, which cause rapid ozone depletion.

The ozone (O3) the stratosphere protects humans, the flora and fauna from the high-energy radiation from space. Ozone is formed when high-energy radiation hits an oxygen molecule (O2), releasing two highly reactive oxygen atoms (O), which then combine with nearby oxygen molecules. The ozone formed in this way is repeatedly broken up by photons of ultraviolet and visible light, is formed again immediately and can absorb light again.

Under constant conditions, a dynamic equilibrium is established in that the same amount of ozone is formed per unit of time as is destroyed. Chlorine shifts this equilibrium and reduces the concentration of ozone in the stratosphere as it accelerates the conversion of the ozone into oxygen molecules. More importantly, chlorine acts as a catalyst, meaning it remains unchanged in this process. Therefore, each chlorine atom can destroy up to 100,000 ozone molecules before it is itself deactivated or eventually returns to the lower atmosphere (troposphere), where it is removed from the atmosphere by precipitation or other processes.

Bromine can also destroy ozone extremely effectively: like chlorine, it triggers a chain of reactions and is even 50 times more effective at breaking down ozone. So small amounts of bromine entering the stratosphere are another threat to the ozone layer.

The Mexican physical chemist Mario Molina and his British colleague Sherwood Rowland suspected as early as the 1970s that fluorochlorinated hydrocarbons contribute to the depletion of ozone in the stratosphere. Together with the Dutch meteorologist Peter Crutzen (right) they received the Nobel Prize in Chemistry in 1995.

When the later Nobel Prize winners Mario Molina and Sherwood Rowland put forward their CFC hypothesis on the depletion of ozone in the stratosphere in 1974, the starting shot was given for intensive research into ozone-depleting compounds in our environment. It is now known that the CFCs, which humans put into circulation in large quantities, contribute significantly to the formation of the ozone hole. In the course of these investigations, however, it also became clear that nature had found many ways of practicing "natural chlorine chemistry", so to speak, and had long been emitting large amounts of gaseous halogen compounds into the atmosphere. Above all, simple, volatile organic molecules, such as methanes, which contain a halogen atom (chlorine, bromine, iodine), are produced in considerable quantities.

In terms of quantity, the number 1 naturally formed "organochlorine" is chloromethane. The amount emitted into the atmosphere annually is estimated to be at least four million tons. This is around 100 times more than what is industrially produced. Naturally formed chloromethane is now responsible for 20 percent of the stratospheric chlorine content and is therefore also involved in ozone destruction. In 1950 it was 85 percent and it was not until 1970 that the industrially produced compounds exceeded the stratospheric chlorine content caused by chloromethane.

The 200,000 tons of bromomethane produced every year by nature are also of great importance for the atmosphere, since bromine can destroy ozone even more effectively than chlorine. Both natural compounds, like the synthetic CFCs, are not destroyed in the lower part of the atmosphere (troposphere) and reach the stratosphere unhindered. According to the latest estimates, naturally produced chlorine and bromomethanes are responsible for around 30 percent of ozone depletion.

For a long time it was believed that nature does not produce chlorine chemicals. In 1934 the structure of the lichen constituent diploicin was clarified. This compound is the first natural substance containing chlorine and was dismissed as a unique freak of nature for a long time afterwards.

Over 3,500 natural halogen-organic compounds are now known, which are either produced by bacteria, fungi, algae, insects and other organisms or are caused by abiotic processes in volcanic eruptions, forest fires or the decomposition of dead plant material. Almost every week a new chlorine compound from nature is described.

Vinyl chloride can be cited as a current example. Up to now, vinyl chloride has been regarded as a prime example of dangerous products from the repertoire of industrial chlorine chemistry. It is the most frequently produced organochlorine compound and the starting material for the polymerisation of the plastic PVC. Vinyl chloride is a toxic substance that has caused skin and liver cancer in chemical workers.

Our working group at the Institute for Environmental Geochemistry was able to prove that vinyl chloride is naturally formed in the soil. Why this connection is produced by nature and what effects it has on our environment can only be speculated about at this point in time. Most chlorinated natural substances are certainly not created by chance, but rather fulfill important tasks. It should not be left unmentioned, however, that nature has neither triggered catastrophes like in Seveso or Bhopal nor has it crept into the environment with persistent chemicals such as the insecticide DDT or the polychlorinated biphenyls (PCBs) used as flame retardants. The role of man-made organochlorines should continue to be viewed critically because of their persistence and toxicity.

While the emission levels for synthetic halogen compounds are often known, there are still big question marks in nature. Particularly in the case of the ozone-destroying compounds chloro- and bromomethane, there are no sources for the concentrations measured in the atmosphere. At least four million tons of chloromethane are produced in nature every year. Only half of this can be explained by known sources (forest fires, algae and mushroom production). In the end, two million tons are missing.

The same dilemma is looming with methyl bromide, and only recently the problem of missing sources was headlined in the journal Nature with the headline "The mystery of the missing gases". We have been grappling with this problem in recent years and possibly found the still missing source in the terrestrial environment.

The sources of around two million tonnes of the ozone-depleting compound chloromethane in the atmosphere (graphic above) have so far been a mystery. This still missing source is possibly the soil.
The picture below shows the entry of ozone-depleting compounds into the stratosphere. There the ultraviolet radiation is strong enough to break down the molecules and release chlorine and bromine atoms, which attack ozone.

The soil-forming processes such as rock weathering, mineral formation and new formation, decomposition of organic litter, humus formation and material relocation create the prerequisites for natural chlorine chemistry. When dead plant material is broken down and converted, halogenated hydrocarbons are released. All that is needed is sufficient amounts of humus, salt, water and trivalent iron. The salt formers (chloride, bromide, iodide) are decisive for the type of halomethane: Depending on the concentration in which a halide is available, more or less chlorine, iodine and bromomethane is formed.

Iron plays a decisive role: It is often found in the soil in a trivalent mineral form as iron hydroxide (= ferrihydrite) and can easily react with the humus. So-called redox reactions (oxidation and reduction) between iron minerals and the dead organic substance take place continuously in the soil and create the attack surface for the halogens.

Scientists at the Institute for Environmental Geochemistry have developed a model (picture above) for the natural formation of halogenated gases in the soil: When dead plant material reacts with iron and salts, carbon dioxide and halogenated methanes are formed. These gases can quickly escape into the atmosphere. The proportion of industrially produced CFCs and chlorinated foods in ozone depletion is around 70 percent; natural sources have a 30 percent share.

First, we examined the natural production of halomethanes using water and soil samples from the "Rotwasser" nature reserve in the Hessian Odenwald. According to our assumptions, this process had to take place in the ground all over the world. To find out, additional soil samples were examined in Patagonia in southern Chile and in Hawaii, as these regions are almost completely free from environmental pollution. We wanted to rule out that the detected halogen methanes could possibly be of anthropogenic origin. These investigations were able to impressively prove the natural formation of the substances sought.

Another special feature is that neither microorganisms nor other living beings are necessary for these processes. We were able to find out by sterilizing the soil samples and then examining them again in the medium of water. In fact, after a short time we were able to observe a renewed production of halogen compounds. In addition to the production of ozone-depleting substances in the soil, many other chlorine chemicals can also be formed in the highly complex natural body of soil, which will represent an interesting field of research in the future.

Although soils store an enormous amount of organic matter and also contain all other "ingredients" that are necessary to produce chlorine chemicals, they have not been considered with regard to the formation of halogenated methanes. The enormous potential of producing halogenated methanes and releasing them into the atmosphere is illustrated by the example of global carbon distribution: around 3,000 billion tons of humic substances are stored globally in the soils; about two to three percent of this is broken down by oxidation every year. Most of this is converted into carbon dioxide (CO2) and released into the atmosphere. If only a tiny part, around 0.0001 percent, is volatilized in the form of halogenated methanes, then one could close the gap of the missing sources. In future, it will therefore be necessary to clarify how much chlorine and bromomethane actually comes from the soil.

The natural formation of gaseous chloromethane and bromomethane suggests that there was a natural background to ozone depletion even before humans emitted CFCs. If one goes back a few steps into the past, there could already be a significant formation of organic chlorine about 570 million years ago (Cambrian) when marine organisms conquered the oceans or in the Silurian (about 430 million years ago) when land plants colonized the continents - and bromine compounds in the environment. It is possible that these naturally formed "ozone killers" were in a dynamic equilibrium with the ozone in the stratosphere. Only the CFCs emitted by humans have shifted this balance strongly in the direction of ozone depletion and lead to the formation of the ozone hole.

In September 1987, 23 nations signed a treaty to reduce CFC consumption in Montreal. As industrial emissions continue to be curtailed, CFCs will be removed from the atmosphere over a period of 30 to 50 years and natural sources will once again become more important. The sources of "natural ozone killers" must be examined in detail as the basic building block in the overall puzzle of the chemical processes in the atmosphere.

The formation of an ozone hole in the stratosphere over Antarctica has convinced the international community of the need to work together against a global environmental hazard, and it spurs researchers to study the chemistry and dynamics of the atmosphere. These efforts have already significantly advanced our knowledge of the interactions between ozone and other gases - of natural and anthropogenic origin.

Authors:
Dr. Frank Keppler and Prof. Dr. Heinz Friedrich Schöler,
Institute for Environmental Geochemistry, Im Neuenheimer Feld 236,
69120 Heidelberg, phone (06221) 546003, fax (06221) 545228,
e-mail: [email protected], [email protected]