Atmospheric monitoring

The so-called ozone layer is located between 10 and 50 km above the Earth's surface and contains approximately 90 per cent of all atmospheric ozone. Under undisturbed conditions stratospheric ozone is formed as the result of a photochemical equilibrium involving oxygen molecules, oxygen atoms and solar radiation. The ozone layer protects life on the Earth's surface since ozone is the only efficient absorbent of the ultraviolet-B radiation (wavelengths 280 to 310 nm) from the sun. UV-B radiation is harmful to organisms in many ways (see Chapter 16).

The abundance of ozone is often measured through its total amount in an atmospheric column going from the ground to the top of the atmosphere. Ground-based and satellite observations have shown a decrease of total column ozone in winter in the northern hemisphere (an average depletion of the global ozone layer of 3 per cent in the period 1979­91, but with large latitudinal and seasonal differences). At mid-latitudes over Europe, the decline of the ozone column is about 6 to 7 per cent during the last decade. The decline is strongest in winter and early spring, when there is relatively little UV-B (Table 28.1). The 1991 Scientific Assessment (WMO, 1991) showed, for the first time, evidence of significant decreases in spring and summer in both the northern and southern hemispheres at middle and high latitudes, as well as in the southern winter. These downward trends were larger during the 1980s than the 1970s (see Figure 28.1). Recent evaluations show that seasonal averages of total ozone over Europe were 10 per cent lower than long-term averages in winter 1991/92, and 13 per cent lower in winter 1992/93 (Bojkov et al, 1993).

The 'ozone hole' observed above Antarctica since the end of the 1970s is an extreme case of ozone layer depletion, with column reduction of about 55 per cent in October (ie, southern hemisphere spring) 1987 and 1989­93. At the 13 to 21 km level (lower stratosphere), the depletion was nearly complete, with loss of 95 per cent. There is great concern that substantial ozone decline could be produced in the northern high latitude regions, where the densely populated regions of Eurasia and North America could be under direct risk.

Continuous stratospheric ozone depletion trends, and the possible occurrence of an Arctic ozone hole, should be of great concern for Europe, with potential effects on human health, plants, animals and the food supply. Europe's responsibility is emphasised by the fact that Europe contributes approximately one third of the global annual emissions of ozone-depleting substances (see Chapters 4 and 14). The issue of stratospheric ozone depletion has stimulated interest and dialogue among the scientific community, policy makers and the public. Scientific concern was already expressed in 1974, but effects stronger than predicted were observed in 1985, presenting the public with the alarming image of a hole in the sky. The first concrete international measures aimed at limiting ozone depletion were agreed in 1985 and 1987. Two important questions now are whether the agreed measures are sufficient for the atmosphere to reach its 'pre-hole' conditions, and what is the extent of any irreversible damage that has occurred to humans and ecosystems on Earth.

The increase of levels of chlorine (Cl) (Figure 28.2)and bromine (Br) has been shown to be the cause of the accelerating depletion of stratospheric polar ozone. The concentration of chlorine and bromine in the atmosphere was 1.5 to 2 parts per billion by volume (ppbv) chlorine equivalent in 1975, a few years before the 'ozone hole' started to appear (WMO/UNEP, 1989), while the present concentration is 3.9 ppbv. The artificial sources of chlorine and bromine are chlorofluorocarbons (CFCs or freons) and bromofluorocarbons (halons). These are almost exclusively artificial industrial products, first manufactured in the 1930s. CFCs are used as propellants in aerosols, coolants in refrigerators and air-conditioning units, foaming agents in the production of insulating and packaging materials, and cleaning agents. Halons are used particularly in fire extinguishers.

Direct emissions into the stratosphere of nitrogen oxides (NO_x) from aircraft also contribute to ozone layer depletion through NO_x-catalysed ozone removal and possible additional enhanced formation of polar stratospheric clouds that activate chlorine-catalysed destruction of ozone. Besides NO_x, engine emissions from aircraft include water vapour, unburned hydrocarbons, carbon monoxide, carbon dioxide and sulphur dioxide.

It has been established that important reactions involved in the destruction of stratospheric ozone are enhanced by the presence of solid particles such as ice crystals in polar stratospheric clouds, or small liquid droplets such as the sulphate aerosol particles observed in the lower stratosphere. As the results of heterogeneous reactions occurring on the surface of these particles, the unreactive and more abundant forms of chlorine molecules are partly converted into reactive species. The extreme ozone loss rates observed in the Antarctic lower stratosphere are due to suitable external conditions yielding high levels of active chlorine molecules during a sufficiently long time at the required low temperature for catalytic destruction of stratospheric ozone.

Extensive observation campaigns in the northern hemisphere during winter 1991­92 (the EC-sponsored European Arctic Stratospheric Ozone Experiment ­ EASOE) revealed low levels of NO_x (this helps to maintain high levels of active chlorine for ozone destruction), and enhanced levels of artificial chlorine compounds in the stratosphere in forms capable of destroying ozone. Many monitoring stations reported their lowest ever mean values of column ozone for winter months, this being explained partly by the meteorological conditions. However, no feature was shown in the northern hemisphere which could appropriately be called an ozone hole. Calculated rates of ozone loss were at times of the order of 0.5 per cent per day (considered large), but the subsequent temperature rise, which led to a decrease in the levels of reactive chlorine compounds, precluded a major ozone loss. Nevertheless, measurements during the winter 1991­92 EASOE campaign indicated the potential of the chlorine already in the stratosphere to cause significant ozone loss.

The phenomenon of stratospheric ozone depletion by CFCs can be exacerbated by random increases in the presence of particles in the stratosphere following major volcanic eruptions such as those of El Chichon (Mexico, April 1982), Mount Pinatubo (Philippines, June 1991) and Mount Hudson (Chile, August 1991). In effect, the aerosol load in the atmosphere can be raised for a short time by one or two orders of magnitude after major volcanic eruptions, enhancing heterogeneous reactions which facilitate ozone destruction by reactive chlorine. For instance, ozone concentrations in the 3 to 6 months following the Pinatubo eruption were reduced by as much as 15 to 20 per cent in the altitude range 24 to 25 km, where the largest volcanic aerosol loading was observed (Grant et al, 1992).

Addition of NO_x to the atmosphere is expected to decrease ozone in the stratosphere and increase ozone in the troposphere. The observed increase in tropospheric ozone at lower altitudes in the northern hemisphere (see Chapters 4 and 32) cannot compensate for the ozone depletion in the stratosphere, since tropospheric ozone represents only about 10 per cent of the overall ozone column.

• the disturbance of the thermal structure of the atmosphere, probably resulting in changes in atmospheric circulation;
• reduction of the ozone greenhouse effect (see Chapter 27);
• changes in the tropospheric ozone budget and, consequently, in the oxidising capacity of the troposphere (see Chapter 32); and
• increase of UV-B radiation at ground level.

Other effects might also occur. An unwanted surprise could be enhanced ozone depletion above the Arctic in connection with global stratospheric cooling, a feature known to accompany tropospheric greenhouse warming. Furthermore, even with the progressive phase-out of ozone-damaging chemicals, the high chlorine/bromine levels in the stratosphere, combined with lower temperatures, could enhance ozone depletion during the next 50 years. Moreover, this sequence can be exacerbated by the ozone loss itself, since the absorbing capacity of ozone has, so far, maintained the relatively high stratospheric temperatures.

All other factors being constant, there is no scientific doubt that stratospheric ozone depletion will increase UV-B radiation at ground level. Increased UV-B radiation could have effects on human health, ecosystems, materials, and physico-chemical processes in the lower atmosphere. Although more knowledge is needed to draw firm conclusions about several potentially important effects, the recognised deleterious effects are serious enough to justify action by the world's nations (UNEP, 1991).

Calculations based on the measured ozone trends indicate that they should be accompanied by increased UV-B over large areas of the Earth (see Figure 28.3), particularly at latitudes poleward of 30š. Evidence from direct observations is now emerging to confirm increased levels of UV radiation during episodes of ozone depletion in Antarctica (see Chapter 4). Smaller episodic increases have been measured in Australia. Observations from the high-altitude observatory of Jungfraujoch in Switzerland indicate an increasing UV trend larger than expected (see Figure 4.8). Other factors may blur or even compensate increasing UV trends, such as cloud variability and increases in aerosol extinction and tropospheric ozone. This may be an explanation for decreasing UV trends observed in the USA.

The key areas of uncertainty in assessing the consequences of ozone depletion are:

• clarification and quantification of influences on human health, especially the immune system, and occurrences of melanomas and cataracts;
• effects on terrestrial and aquatic biota of increased UV radiation during enhanced ozone depletion episodes;
• quantification of the primary long-term effects on food production and quality, on forestry, and on natural ecosystems;
• the absence of a worldwide network of UV-B monitoring stations with suitable spatial and measurement resolution to support scientific and health assessments.

In the northern hemisphere, the biologically active UV radiation (measured by the annual DNA-damage weighted dose) is estimated to have increased by 5 per cent per decade at 30šN and about 10 per cent per decade in the polar region. A few hours' exposure to UV radiation may result in sunburn and 'snow blindness' and hours to days of pain and irritation. Continuous exposure could result in skin ageing, skin cancer, a depression of the immune system and corneal cataracts. Irradiation of the skin with UV-B can suppress the immunological defence of the skin, and not only the exposed skin. Besides lowering resistance to infectious diseases, this could also reduce the effectiveness of vaccination programmes.

Skin cancer is probably one of the most common forms of cancer among white people. Most cases (90 per cent) are skin carcinoma. The most life-threatening form of skin cancer is melanoma (cancer of the pigment cells). It is estimated that a long-term reduction in the ozone concentration of 10 per cent will result in an increase of 26 per cent in the cases of non-melanoma skin cancer. Skin cancer deaths due to excess UV-B radiation at mid-latitudes are expected to rise to 2 per million inhabitants by 2030. The increase in visits to sunny areas may have contributed to the worldwide rise in melanoma. Exposure to sunlight has now been associated with another cancer: that of the salivary gland. These findings suggest the possibility of a systemic effect of UV-B in humans, since the salivary gland is rarely, if ever, exposed.

As with skin carcinoma, it is the cumulative dose which determines the harmful effects of UV radiation on the eyes, for example, glaucoma and cataract. It is estimated that each 1 per cent depletion of the ozone column will increase occurrence of these afflictions by between 0.6 and 0.8 per cent. Worldwide, this would cause blindness in another 100 000 to 150 000 cases. Such concerns are not limited to fair-skinned populations but are also observed in deeply pigmented individuals, where pigment may be constitutive or acquired. On the basis that children spend considerably more time out of doors and have longer to live under a depleted ozone layer than adults, calculations for Britain show that a child's lifetime risk of developing non-melanoma skin cancer, under current ozone depletion rates, is 10 to 15 per cent higher than the risk under an intact ozone layer (Diffey, 1992). Under these conditions, an adult's risk is increased by less than 5 per cent. These predicted increases in risk can be minimised by changing behaviour during the summer months, for example spending less time outdoors, wearing wide-brimmed hats, applying tropical sunscreens, or a combination of these.

Effects on ecosystems due to an increase in UV radiation are difficult to quantify but are probably more important than those on human health (see Chapter 16). Prevention of these effects is the only possible way to avoid them, as protection of natural ecosystems from UV-B radiation is not possible. As with health effects, effects on ecosystems usually appear only after a delay. Through fisheries and crops these effects are also connected with the food supply and climate changes.

Marine phytoplankton produce at least as much biomass as all terrestrial ecosystems combined. Increases in UV-B radiation adversely affect the nitrogen metabolism, photosynthesis and vitally important adaptive functions such as orientation and mobility. As a result the survival and productivity of phytoplankton are threatened. Recent results (Smith et al, 1992) indicate a reduction by at least 6 to 12 per cent in phytoplankton productivity inside and outside the Antarctic ozone-hole region during the spring months in 1990. One consequence of losses in phytoplankton is reduced biomass production, which is propagated throughout the whole food web. According to a rough estimate, a long-term 16 per cent reduction in the ozone concentration may lead to a 5 per cent reduction in primary production and a reduction of fish stocks of 6 to 9 per cent. Phytoplankton fix a large proportion of the carbon dioxide in the oceans and are therefore important in relation to the greenhouse effect (see Chapter 27). A hypothetical loss of 10 per cent of the marine phytoplankton would reduce the oceanic annual uptake of carbon dioxide by about 5 gigatonnes (an amount equivalent to the annual global emissions of carbon from fossil fuel combustion). Also, phytoplankton production of dimethylsulphide (DMS), which acts as a precursor of cloud nucleation, would be reduced, with potential effects on the global climate.

There are wide differences in sensitivity to UV radiation between terrestrial species and varieties within species. In extensive field studies of crops, half showed a lower yield after exposure to increased UV-B radiation. A large number of the tested plants showed reduced growth, photosynthetic activity and flowering. Ecological equilibria will continue to be altered if depletion of the ozone layer continues.

Damage to materials from increased UV radiation has not been much studied, with the exception of plastics, particularly PVC, which show more rapid yellowing and degradation. UV-B radiation particularly degrades wood and plastic products, leading to discoloration and loss of strength. These effects may result in increased costs of using higher levels of conventional light stabilisers, possible design of new stabilisers, and faster replacement of the affected products.

Chemical reactivity in the troposphere is expected to increase in response to increased UV radiation. Tropospheric ozone concentrations could rise in moderately or heavily polluted areas, but should decrease in unpolluted regions (with low levels of nitrogen oxides), as recently confirmed by measurements in Antarctica. Other potentially harmful substances (hydrogen peroxide, acids and aerosols) are expected to increase in all regions of the troposphere due to enhanced chemical activity. These changes could exacerbate problems of human health and welfare, increase damage to the biosphere, and might make current air quality goals more difficult and expensive to attain.

Effects of future ozone depletion on climate change and global circulation cannot currently be predicted with reasonable certainty. However, results published in 1993 (Wang et al, 1993) indicate that the combined effect of stratospheric ozone decline and tropospheric ozone increase will significantly enhance radiative forcing in the northern hemisphere and contribute to surface warming.

All leaks or emissions of substances causing stratospheric ozone depletion such as CFCs and halons which do not occur naturally in the environment should be avoided. First, a mix of compounds with allowed release rates and termination dates could be formulated, with the goal of minimising both short-term and long-term ozone losses. Additional important factors to consider include the feasibility, the economic and technological aspects of any measure, and the potential impact of the emitted compounds on greenhouse warming.

An effects-based approach could be to reduce the concentration of chlorine and bromine in the atmosphere to 1.5 to 2 parts per billion by volume (ppbv) chlorine equivalent (WMO/UNEP, 1989), the level measured in 1975 just before the hole in the ozone layer was discovered. In fact, it will be almost impossible to reach 1.5 to 2 ppbv before 2080 due to the long atmospheric lifetime of the CFCs. Even if global consumption and emissions of CFCs and halons had been completely eliminated before 1995, the depletion of the ozone layer would still continue up to 2050 (see Figure 28.4). This is the earliest date at which chlorine concentrations previous to the discovery of the Antarctic ozone-layer 'hole' can be reached. Every five years' delay will lead to 0.5 ppbv more chlorine in the stratosphere and an 18-year extension of the depletion. The pre-industrial level of 0.6 ppbv could be a target for the centuries thereafter.

Another option is to establish a link with negligible or acceptable health hazards. At present it is not possible to draw up a standard for sustainability based on environmental and health impacts, due to scientific uncertainties. In the longer term it should be possible to formulate such a standard based on potential health hazards. It is clear, however, that stabilisation at the present concentration of 3.5 ppbv will result in unacceptable risks to human health and the health of ecosystems (RIVM, 1991).

Various indices have been developed for comparing the impact of one halocarbon source gas against others as scientific guidance to public and industrial policy makers when assessing the relative contribution of ozone-depleting emissions (Table 28.2). The simplest of these is the chlorine loading potential (CLP) representing the amount of total chlorine delivered from emission to the stratosphere, relative to that of the same mass of CFC-11. Ozone depletion is not only dependent on the amount of chlorine or bromine in the stratosphere but also on other physico-chemical processes and the atmospheric lifetime of the various molecules. Thus, the ozone depletion potential (ODP) represents the amount of ozone destroyed by emission of a gas over its entire atmospheric lifetime relative to that due to emissions of the same mass of CFC-11. As a relative measure, the ODP does reveal the trade-offs associated with the use of one compound rather than another. These indices have become increasingly important as the search for substitutes for the most damaging species has intensified, and concerns about increased short-term ozone losses have grown. Most emissions of CFCs reach the atmosphere some time after their production, given the way in which these products are used.

The 1989 Montreal Protocol to the Vienna Convention for the protection of the ozone layer (signed in 1987) requires the phase-out of fully halogenated CFCs. These will be replaced mostly with new chemicals not yet released into the environment, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). However, any replacement compound should be scrutinised with respect to its possible detrimental effect on other environmental processes, such as global warming, photochemical pollution or other atmospheric changes (see also Chapter 27). The toxicity of these new chemicals and their degradation products need to be characterised not only by their effects on the ozone layer (expected to be less detrimental), but also by their possible effects on humans, fauna and flora (see Chapter 38).

Preliminary results based on a limited set of data (US EPA, 1990) on HFCs and HCFCs concluded that these chemicals could be used in a manner consistent with safety for humans and other biota. However, there are suggestions that HCFC-123 has induced an increase in non-malignant tumours in rats.

A drawback with substitute products for CFCs is that they often have a significant global warming potential, comparable with that of CFCs (see Table 27.1). Used in comparable quantities, their contribution to global warming will be substantial. Therefore use of HCFCs and HFCs is not a permanent solution, and their emissions and unnecessary releases should at least be restrained. In order to integrate these new constraints, the halocarbon global warming potential (HGWP) index was introduced to compare the effects of the emissions of a given gas with the same emissions of CFC-11 (Fisher et al, 1990a, b).

A complementary goal is to minimise the risks of exceptional ozone depletion events from unforecastable physico-chemical processes (eg, in connection with unprecedentedly large halocarbon abundances or their coupling to unpredictable influences such as volcanic eruptions).

Prompt international actions for release reduction have been implemented as interim measures before the complete stop in use or production of compounds which lead to stratospheric ozone depletion. The Montreal Protocol states that the production of CFCs should be reduced to 50 per cent of the 1986 production level before 1999. It has been ratified by 32 European countries. The protocol was tightened further in London in June 1990, while provisions were also made to facilitate the participation of developing countries. This is particularly important as forecast increases in CFC consumption, especially for refrigerators, in China and India would otherwise counteract all measures taken in the industrialised countries. According to the revised protocol, the production and use of all fully halogenated CFCs, and of the three most important halons and carbon tetrachloride (CCl₄), will be reduced to zero before 2000. For methyl chloroform (CH₃CCl₃), a 70 per cent reduction is aimed for by 2000 and 100 per cent by 2005. It was also decided to consider the contribution of CFCs and their alternatives to the greenhouse effect. Following the publication of new scientific evidence on enhanced ozone depletion, the EC decided to anticipate the London amendment with an 85 per cent cut by the end of 1993 and complete phase-out by the end of 1995 for CFCs and halons, as well as for carbon tetrachloride and methyl chloroform.

In 1992 the countries involved (65 signatory countries) considered the progress made, and agreed more stringent measures in the Copenhagen amendments to the Montreal Protocol. These will put a stop to global emissions of CFCs, carbon tetrachloride and methyl chloroform before 1996. The Copenhagen amendments regulate a limited consumption of HCFC, leading to phase-out before 2030. Production of halons (notably for fire extinguishers) was phased out in 1994. However, because of the long lifetime of these compounds, consequent ozone depletion will continue well into the next century and beyond.

Figure 28.5 shows future CFC production and atmospheric chlorine concentrations according to four scenarios: an incomplete fulfilment of the Montreal Protocol, a full implementation of the Montreal Protocol, and the London and Copenhagen amendments. If the Montreal Protocol is not fully implemented, the atmospheric concentrations of chlorine will rise to more than 10 ppbv. In this event, a reduction of the ozone layer in the tropics of up to 4 per cent (and at higher latitudes of at least 4 to 12 per cent) is expected, particularly in winter. It is likely that the ozone depletion will be dramatically greater than this; the current models explain only half of the ozone depletion at mid- and high latitudes as measured by satellite observations. It will take at least 70 years before chlorine concentrations in the stratosphere decrease below 2 ppbv, at which no further depletion of the ozone layer is expected, even if the London amendment is strictly implemented. The unlimited production and use of CFC substitutes could of course prolong this 70-year period.

These calculations have shown that the rapid phase-out of CFCs, methyl chloroform, carbon tetrachloride and halons is the main factor in reducing ozone destruction over both short and long time horizons. If it is accepted that they are needed, substitute or better 'transitional substances' can help towards that goal. However, substitute products do still contain chlorine or bromine atoms and moreover have an elevated global warming potential, and thus should themselves be replaced. The next 20 years are likely to encompass the highest atmospheric halocarbon abundances that the Earth has presumably ever experienced, and therefore increased ozone depletion is to be expected.

Since the Montreal Protocol, there has been an intense search for substitutes for the CFCs harmful to the ozone layer. There are many alternatives to CFCs used in applications such as aerosol propellants, foam-blowing agents and cleaning of electronic equipment. However, in refrigerating, freezing and air conditioning equipment, the replacement will have to be made in two stages: replacement of the refrigerating CFC gases in existing equipment with minor modifications, and new equipment which can use non-halogenated gases or gas mixtures. Substitution products for the first stage are halocarbons with shorter tropospheric lifetimes and fewer chlorine atoms, for example, CFC-22.

Before the phase-out of compounds with large ODPs in the long term (ie, for chronic effects like the ozone hole), and reaching the time limit set on the widespread use of substitutes for the same reason, policy makers would probably consider how best to minimise the growing ozone losses that will occur during this transition (Solomon and Albritton, 1992). Therefore the critical issues involve identifying which substances are needed, for how long, and in what quantities.

Short term ODPs for the currently uncontrolled compounds could be a principal factor in those decisions. In those applications for which multiple substitutes exist (such as HCFC-141b and HCFC-123), industry may choose to invest in the compounds with the smaller short-term ODPs. Similarly, policy makers may set low ceilings on the production of compounds with larger projected ODPs in the next few decades, and higher ceilings on those with smaller short-term ODPs. The Copenhagen agreement set a ceiling for CFCs of 3.1 per cent of the 1989 levels, and 100 per cent of 1989 levels for HCFCs, expressed in ODP tonnes. A phase-out by 2030 was also decided.

The high global warming potential of HCFCs means that a shift to environmentally safer technologies should be implemented as soon as possible. The introduction of mandatory recycling for minor uses of CFCs and halons which may be deemed essential, and of legally binding schemes to recover CFCs and other ozone-depleting compounds held in current equipment (equivalent to several years' production), could also be envisaged as effective counteracting measures.

Finally, proposals for global environmental engineering have been made to help resolve the problem of stratospheric ozone depletion and attenuate its effects ­ for example, the injection of thousands of tonnes of alkanes (such as ethane or propane) into the polar stratosphere to prevent ozone depletion, and for climate change, iron fertilisation of the sea to enhance the oceanic fixation of atmospheric carbon. In order to avoid inherent risks involved in any large-scale intervention, including risks of unanticipated side-effects, proper scientific assessments of such proposals are required. First assessments using simple models show that the above proposals are not technically possible and may anyway not achieve their goals (Cicerone et al, 1991). Performing such assessments is clearly essential for a responsible exploration of the feasibility of such proposals. Indeed, pursuing such ideas can contribute to an improved understanding of environmental systems by exposing the weaknesses of the understanding of global systems and stimulating new research (Cicerone et al, 1992).