Whatis
Ozone
Depletion?
The ozone layer protects the Earth from the ultraviolet rays sent down by the sun. If the ozone layer is depleted by human action, the effects on the planet could be catastrophic.
Ozone is present in the stratosphere. The stratosphere reaches 30 miles above the Earth, and at the very top it contains ozone. The suns rays are absorbed by the ozone in the stratosphere and thus do not reach the Earth.
Ozone is a bluish gas that is formed by three atoms of oxygen. The form of oxygen that humans breathe in consists of two oxygen atoms, O2. When found on the surface of the planet, ozone is considered a dangerous pollutant and is one substance responsible for producing the greenhouse effect.
The highest regions of the stratosphere contain about 90% of all ozone.
In recent years, the ozone layer has been the subject of much discussion. And rightly so, because the ozone layer protects both plant and animal life on the planet.
The fact that the ozone layer was being depleted was discovered in the mid-1980s. The main cause of this is the release of CFCs, chlorofluorocarbons.
Antarctica was an early victim of ozone destruction. A massive hole in the ozone layer right above Antarctica now threatens not only that continent, but many others that could be the victims of Antarctica's melting icecaps. In the future, the ozone problem will have to be solved so that the protective layer can be conserved.
Causes of Ozone Depletion
Cl + O3 ClO + O2
ClO + O Cl + O2
Ozone is converted to oxygen, leaving the chlorine atom free to repeat the process up to 100,000 times, resulting in a reduced level of ozone. Bromine compounds, or halons, can also destroy stratospheric ozone. Compounds containing chlorine and bromine from man-made compounds are known as industrial halocarbons.
Emissions of CFCs have accounted for roughly 80% of total stratospheric ozone depletion. Thankfully, the developed world has phased out the use of CFCs in response to international agreements to protect the ozone layer. However, because CFCs remain in the atmosphere so long, the ozone layer will not fully repair itself until at least the middle of the 21st century. Naturally occurring chlorine has the same effect on the ozone layer, but has a shorter life span in the atmosphere.
Effects of Ozone Depletion
Health Effects
Ozone is present in the stratosphere. The stratosphere reaches 30 miles above the Earth, and at the very top it contains ozone. The suns rays are absorbed by the ozone in the stratosphere and thus do not reach the Earth.
Ozone is a bluish gas that is formed by three atoms of oxygen. The form of oxygen that humans breathe in consists of two oxygen atoms, O2. When found on the surface of the planet, ozone is considered a dangerous pollutant and is one substance responsible for producing the greenhouse effect.
The highest regions of the stratosphere contain about 90% of all ozone.
In recent years, the ozone layer has been the subject of much discussion. And rightly so, because the ozone layer protects both plant and animal life on the planet.
The fact that the ozone layer was being depleted was discovered in the mid-1980s. The main cause of this is the release of CFCs, chlorofluorocarbons.
Antarctica was an early victim of ozone destruction. A massive hole in the ozone layer right above Antarctica now threatens not only that continent, but many others that could be the victims of Antarctica's melting icecaps. In the future, the ozone problem will have to be solved so that the protective layer can be conserved.
Causes of Ozone Depletion
Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction. Although natural phenomena can cause temporary ozone loss, chlorine and bromine released from man-made compounds such as CFCs are now accepted as the main cause of this depletion.
It was first suggested by Drs. M. Molina and S. Rowland in 1974 that a man-made group of compounds known as the chlorofluorocarbons (CFCs) were likely to be the main
source of ozone depletion. However, this idea was not taken seriously until the discovery of the ozone hole over Antarctica in 1985 by the British Antarctic Survey.
Chlorofluorocarbons are not "washed" back to Earth by rain or destroyed in reactions with other chemicals. They simply do not break down in the lower atmosphere and they can remain in the atmosphere from 20 to 120 years or more. As a consequence of their relative stability, CFCs are instead transported into the stratosphere where they are eventually broken down by ultraviolet (UV) rays from the Sun, releasing free chlorine. The chlorine becomes actively involved in the process of destruction of ozone. The net result is that two molecules of ozone are replaced by three of molecular oxygen, leaving the chlorine free to repeat the process: It was first suggested by Drs. M. Molina and S. Rowland in 1974 that a man-made group of compounds known as the chlorofluorocarbons (CFCs) were likely to be the main
source of ozone depletion. However, this idea was not taken seriously until the discovery of the ozone hole over Antarctica in 1985 by the British Antarctic Survey.Cl + O3 ClO + O2
ClO + O Cl + O2
Ozone is converted to oxygen, leaving the chlorine atom free to repeat the process up to 100,000 times, resulting in a reduced level of ozone. Bromine compounds, or halons, can also destroy stratospheric ozone. Compounds containing chlorine and bromine from man-made compounds are known as industrial halocarbons.
Emissions of CFCs have accounted for roughly 80% of total stratospheric ozone depletion. Thankfully, the developed world has phased out the use of CFCs in response to international agreements to protect the ozone layer. However, because CFCs remain in the atmosphere so long, the ozone layer will not fully repair itself until at least the middle of the 21st century. Naturally occurring chlorine has the same effect on the ozone layer, but has a shorter life span in the atmosphere.
Effects of Ozone Depletion
Health Effects
The principal impacts of UV on health are mediated through two organ systems, the skin and the eyes, which receive all of the exposure. These systems are generally either well adapted to (the skin) or well protected from (the eyes) such exposures; humans benefit from UV exposures through the initiation of Vitamin D3 production, and even detrimental effects such as DNA damage, are normally corrected by efficient repair mechanisms. Our average day-to-day exposure to UV is generally sufficient to ensure appropriate levels of Vitamin D. There is some evidence in tissue culture that Vitamin D has inhibitory effects on tumor cell growth. Such effects have been suggested as the mechanism underlying an observed increase in breast and colon cancers at higher latitudes (where solar exposures are less). This linkage is still conjectural, however. The production of the biologically active vitamin is self-limiting so that excessive exposures are not likely to be associated with any benefit.
New findings with regard to the molecular events underlying UV-induced nonmelanoma skin cancer extend the earlier observations that these tumors often have alterations (mutations) in a particular (p53) tumor suppressor gene that are typical of UV-B radiation. The recent findings indicate that similar UV-B alterations in a second tumor suppressor gene (ptc) are associated with spontaneous (non-hereditary) basal cell carcinomas (BCC). These data provide the most direct evidence that UV-B radiation contributes to the development of these skin tumors.
Additional studies on the involvement of p53 indicate that the frequency of such typical UV-B alterations in the skin tumors may decrease with lower dose levels and spectral shifts toward the longwave UV-A. An additional finding in UV-induced carcinogenesis comes from recent epidemiologic studies, which indicate that BCC is not principally related to cumulative UV dose, but as with melanoma, appears to be more related to childhood and intermittent over-exposures.
New epidemiologic studies on melanoma for the most part confirm earlier work. Several studies have suggested that sunscreen use may not be protective for melanoma and may even be associated with increased risk.
A possible explanation offered to explain these observations is that wearing sunscreens, while reducing UV-B exposure, provides little protection from UV-A exposures and that such exposures are important to melanoma risk. Although such an interpretation is in line with the spectral response of melanoma induction in a fish model, such epidemiologic findings may be biased by linkages among susceptibility, exposure and sunscreen use. New work in the opossum model for melanoma has shown that short-term neonatal exposures to broadband UV-B/UV-A radiation (of approximately a week) result in highly aggressive (metastatic) melanomas.
It is now clear that there are at least three mechanisms by which UV-B exposures may suppress cellular immunity: DNA damage, isomerization of urocanic acid, and through the active metabolite of vitamin D. Immune effects can occur both locally at the skin and systemically throughout the body. The local effects have long been considered important to the development of skin cancer, but there has been little evidence found with regard to the importance of systemic effects. Recent observations of an increase in non-Hodgkin's lymphoma with increasing solar radiation have led to the suggestion that UV-B-induced systemic immunosuppression contributes to cancer development in this system. The implications of either local or systemic immune effects for human infectious diseases are still unknown. Epidemiological studies are still required to explore these issues.
There is very little new information on UV effects on the eye. A critical lack is an action spectrum for eye effects associated with chronic exposures, e.g., cataract.
Effects on Terrestrial Ecosystems
Since the last report in 1994, there has been progress on understanding the mechanisms of UV action on organisms. There has also been an increasing emphasis on outdoor studies of nonagricultural vegetation. In such experimentation, the importance of maintaining a realistic spectral balance between UV and longer wavelength radiation during experiments has received more attention. This is important because longer wavelength radiation may have ameliorating effects. The UV-A can also have effects in some respects similar to that of UV-B. In addition to UV-B supplementation studies designed to simulate the consequences of ozone reduction on sunlight, other research of late has involved manipulation of natural solar radiation with filters to show the effectiveness of both UV-B and UV-A on organisms.
Many of the effects of UV-B may not involve damage, but instead be a matter of the plant using this type of radiation as a signal for altering growth form and some physiological processes. One example of such a change is the commonly observed increase in UV-absorbing protective compounds in plant leaves following exposure to UV-B. This may explain, at least in part, many of the seemingly paradoxical results of recent and earlier studies suggesting stimulating effects in plants by UV-B. The UV-B responsiveness is often very specific to species, or even races of a given species, and depends on other environmental factors such as mineral nutrition, drought, local air pollutants, etc. These differential effects on species will likely lead to changes in species interactions and on ecosystem dynamics. Various indirect effects of elevated UV-B radiation are receiving attention, including effects of UV-B on decomposition of plant litter, which may influence cycling of nutrients.
Other indirect effects include influence on plant pathogen susceptibility and plant attractiveness to herbivores and pollinators. Many of these indirect effects may be UV-B action mediated through changes in plant structure, secondary chemistry and timing of life cycle events. Nevertheless, these effects may ultimately be the most important on ecosystems. This is not to dismiss more direct, apparently detrimental effects of UV-B on organisms, including microbes and lower plants such as mosses. The peat-forming moss, Sphagnum, appears to be sensitive which has implications for some processes in biogeochemical cycles such as decreased carbon storage. Trace gas emission from vegetation is also potentially important. One recent study, however, showed no effect of elevated UV-B on emission of isoprene, an important precursor compound for tropospheric ozone formation.
Recent studies suggest that potentially detrimental effects of UV-B radiation in evergreen woody plants may accumulate from year to year as had been indicated in earlier work. Another example of accumulating effects has recently been shown for subsequent generations of native annual plants. With exposure of each generation, the manifestations of adverse effects were intensified.
The interacting effects of elevated UV-B with other factors important in global change such as elevated CO2 and temperature are receiving more attention. While elevated CO2 and temperature usually have more pronounced effects than elevated UV-B, there are often interactions among these factors such that the effects may counteract each other or sometimes they may be additive.
Effects on Aquatic Ecosystems
In the recent past, scientific and public interest has focused on marine primary producers and aquatic ecosystems, which resulted in a multitude of studies indicating mostly detrimental effects of UV-B radiation on aquatic organisms. The interest has expanded to include effects in individual species as well as specific responses and has concentrated on ecologically significant groups and major biomass producers using mesocosm studies, emphasizing species interactions. In addition, light penetration into the water column was investigated by several research groups.
Macroalgae and seagrasses are major biomass producers in aquatic ecosystems. In contrast to phytoplankton most of these organisms are sessile and can thus not avoid the exposure to solar radiation at their growth site. Recent investigations showed a pronounced sensitivity to solar UV-B, and effects have been found throughout the top 10 - 15 m of the water column. Mechanisms of protection and repair are being investigated.
Controversy still exists regarding the interpretation of data on UV-B effects in Antarctic phytoplankton. Estimates of the decreases in overall biomass productivity by different authors range from 0 - 12%. Most recently, shifts in phytoplankton community structure have been demonstrated, which may have consequences for the food web.
Bacteria play a vital role in mineralization of organic matter and provide a trophic link to higher organisms. Recently, the mechanism of nitrogen fixation by cyanobacteria has been shown to be affected by UV-B stress. Wetlands constitute important ecosystems both in the tropics and at temperate latitudes. In these areas cyanobacteria form major constituents in microbial mats. The organisms optimize their position in the community by vertical migration in the mat which is controlled by both visible radiation and UV-B. Cyanobacteria are also important in tropical and subtropical rice paddy fields where they contribute significantly to the availability of nitrogen. Growth, development and several physiological responses of these organisms are affected by solar UV.
Dissolved organic carbon (DOC) and particulate organic carbon (POC) are degradation products of living organisms. These substances are of importance in the cycling of carbon in aquatic ecosystems. UV-B radiation has been found to break down high molecular weight substances and make them available to bacterial degradation. In addition, DOC is responsible for short wavelength absorption in the water column. Especially in coastal areas and freshwater ecosystems, penetration of solar radiation is limited by high concentrations of dissolved and particulate matter. On the other hand, climate warming and acidification result in faster degradation of these substances and thus enhance the penetration of UV radiation into the water column.
UV effects on aquatic animals have found increased interest. In addition to effects on larval stages in amphibia, sea urchins were found to be affected by solar UV-B radiation despite the fact that they have partial protection from mycosporine amino acids, which they take up with their food. Corals have been known to be directly affected by solar UV; in addition, photosynthesis in their symbiotic algae is impaired, resulting in reduced organic carbon supply.
All of the previous studies on UV penetration into the water column were based on occasional measurements. This will be corrected by a recent project involving the development of a monitoring system (ELDONET) for solar radiation in Europe using three-channel dosimeters (UV-A, UV-B, PAR), which are being installed from Abisko (North Sweden, 68° N, 19° E) to Tenerife (Canary Islands, 27° N, 17° W). Some of the instruments will be installed in the water column (North Sea, Baltic Sea, Kattegat, East and Western Mediterranean, North Atlantic), establishing the first network of underwater dosimeters for continuous monitoring.
Effects on Biogeochemical Cycles
Terrestrial studies are continuing to examine the influence of enhanced UV-B radiation on microbial decomposition of plant litter to carbon dioxide and nutrients. Additional studies have confirmed that litter from plants grown under enhanced UV-B is enriched in lignin and decomposed to CO2 more slowly by soil microorganisms.
The fate and transport of trifluoroacetate (TFA), a persistent substance derived from the oxidation of certain CFC replacements (HFC-123, HFC-124, HFC-134a), was investigated in a temperate forest of North America. The study indicates that the added TFA was retained in vegetation and soil, especially in the case of wetlands with organic soils. In addition, biological accumulation was observed as was a lack of microbial consumption.
Studies of photodegradation of dissolved organic carbon (DOC) by UV radiation are continuing and expanding. Several studies published over the past year have provided additional evidence that exposure to UV radiation enhances the degradation of DOC to CO2 and ammonium. In addition, low- molecular-weight organic products are produced and are readily assimilated by microorganisms.
Volatile compounds are produced by aquatic DOC photodegradation and additional studies of the photoproduction of carbon monoxide have appeared. CO is an important trace gas that strongly influences biogeochemical cycles through its effects on chemical reactions in the atmosphere. Two new studies of marine CO photoproduction have resulted in disparate estimates of global oceanic sea-to-air emissions. These two global flux estimates are two orders of magnitude apart: 10-15 Tg CO year-1 and 1000 Tg year-1 (1 Tg = 1012 g). Total annual emissions of CO from all sources, by comparison, are about 2000-2500 Tg year-1 primarily from fossil fuel combustion and CH4 oxidation.
New modeling approaches are being developed to predict the interactions and feedbacks between climate change and UV-B induced changes in marine biogeochemical cycles. Important links exist between oceanic dimethylsulfide (DMS) production, sea-to-air DMS flux and subsequent changes in the sulfate aerosol-related atmospheric radiation balance. These alterations indicate possible consequences with regard to climate and climate prediction, because DMS is an important source of cloud condensation nuclei over parts of the ocean.
Effects on Air Quality
The photo-dissociation of ozone by atmospheric UV-B radiation is a key reaction that controls urban and regional oxidants, the self-cleaning capacity of the troposphere, and the atmospheric lifetimes of many natural and anthropogenic gases. In the last year, published evidence from theoretical, laboratory, and field studies shows that UV-A wavelengths also contribute to this reaction, and therefore some revision of current tropospheric chemistry models is required. Although quantitative estimates of the implications of this new finding have not been carried out yet, impacts are likely for predicted geographical and seasonal distributions of gases such as methane (CH4) and carbon monoxide (CO), and the sensitivity of tropospheric chemistry to stratospheric ozone depletion.
Further evidence for the linkage between stratospheric UV transmission and tropospheric composition was found in the records of surface CH4 and CO concentrations. Temporary increases in tropical CH4 and CO concentrations were observed in the second half of 1991, in phase with high levels of stratospheric sulfur dioxide and sulfate aerosols that resulted from the June 1991 eruption of Mt. Pinatubo. The reduced stratospheric UV transmission is believed to have lowered the rate of CH4 and CO removal, due to decreased photo-dissociation of tropospheric ozone, and therefore decreased hydroxyl radical (OH) production.
The atmospheric chemistry of CFC substitutes and the possible build-up of their breakdown products (particularly trifluoroacetic acid, or TFA) has attracted more attention. The background atmospheric concentrations of HFC-134a (a principal precursor of TFA) are increasing rapidly, according to measurements at Cape Grim, Tasmania (41° S) between 1978 and 1995, and Mace Head, Ireland (53° N) between July 1994 and May 1995. Model estimates suggest that HFC-134a emissions have risen rapidly from ca 0.25x106 kg in 1991 to ca 8x106 kg in 1994. TFA concentrations in air and water samples collected in Germany, Switzerland and Israel in 1995 contained surprisingly high levels of TFA, comparable to concentrations predicted by models for the year 2010. This suggests that an additional yet unknown source of TFA may be present.
Modeling studies give different estimates for future concentrations of TFA in rainwater, e.g., tropical maxima of 0.114 µg l-1 predicted for the year 2020, and a global mean of 0.160 µg l-1 by the year 2010 predicted by another study. However, other modeling studies have concluded that a number of factors can enhance local concentrations of TFA. The TFA concentrations in the precipitation of arid and semi-arid regions may be 2-4 times greater than the global mean. Furthermore, in urban air where high OH and HFC levels are present, the rate of formation of TFA may be enhanced by an order-of-magnitude or more. Therefore, local TFA concentration in rainwater as high as 2-20 µg l-1 or more appears plausible. Water bodies characterized by little or no outflow and which have high evaporation rates may be susceptible to accumulation of rain-borne TFA, leading to significant concentration enhancements. Model calculations predict that such wetlands could experience concentrations of up to 100 µg l-1 within 30 years, even when seepage is as much as 10%.
Materials Damage
Efforts to refine the wavelength sensitivity data for common polymers have continued, with the commodity polymers polyethylene and polypropylene receiving the most attention. Results from several research groups include data on the relative photodamage to polyolefins, acrylics, polystyrene, and other polymers, caused by exposure to different regions of a solar-simulated source spectrum. With polyethylene, the wavelength range 300-330 nm in simulated sunlight was reported to be most effective in reducing the tensile extensibility of the material, a result in agreement with previous findings. These types of data, referred to here as activation spectra, are useful in the estimation of damage to polymers exposed to UV-enhanced solar radiation that results from ozone depletion.
Recent findings have shown that while solar radiation is principally responsible for outdoor degradation of polymers including polyethylene, factors such as temperature and exposure to moisture also have an impact on the useful lifetime of materials outdoors. Several studies have reported on the acceleration of weathering of polyethylene films under harsh desert conditions, where the radiation dose and the temperatures are particularly high.
Research was recently carried out on polycarbonate photodegradation aimed at generating action spectra for the yellowing and molecular-level breakdown of this polymer used in glazing applications. This research also studied the irradiance dependency of the degradation process, an important consideration, since the construction of action spectra and their derivation from polychromatic exposure experiments assume the damage process to be irradiance-independent within the range of irradiance encountered in the experiments. The findings facilitate the intercomparison of previously reported wavelength sensitivity data for polycarbonate polymers. An on- going study is aimed at separating out the contributions of solar radiation and temperature, to the damage suffered by polyethylene polymers during outdoor exposure. The data will allow a better understanding of the role of temperature in modifying the UV-induced photodegradation of polymers exposed to UV-enhanced sunlight resulting from ozone depletion.
Future Effects Studies
The global environment is experiencing a wide range of perturbations. Increased greenhouse gases, sulfate and dust aerosols and increased UV-B from ozone depletion are all occurring simultaneously. Assessment of the potential risks from such changes needs to consider the full context in which these changes are taking place and must take into account the many interactions that may occur. This will require a long-term integrated, interdisciplinary research program designed to include studies which evaluate not only UV-B but also the other processes which are contributing to global environmental change.
New findings with regard to the molecular events underlying UV-induced nonmelanoma skin cancer extend the earlier observations that these tumors often have alterations (mutations) in a particular (p53) tumor suppressor gene that are typical of UV-B radiation. The recent findings indicate that similar UV-B alterations in a second tumor suppressor gene (ptc) are associated with spontaneous (non-hereditary) basal cell carcinomas (BCC). These data provide the most direct evidence that UV-B radiation contributes to the development of these skin tumors.
Additional studies on the involvement of p53 indicate that the frequency of such typical UV-B alterations in the skin tumors may decrease with lower dose levels and spectral shifts toward the longwave UV-A. An additional finding in UV-induced carcinogenesis comes from recent epidemiologic studies, which indicate that BCC is not principally related to cumulative UV dose, but as with melanoma, appears to be more related to childhood and intermittent over-exposures.
New epidemiologic studies on melanoma for the most part confirm earlier work. Several studies have suggested that sunscreen use may not be protective for melanoma and may even be associated with increased risk.
A possible explanation offered to explain these observations is that wearing sunscreens, while reducing UV-B exposure, provides little protection from UV-A exposures and that such exposures are important to melanoma risk. Although such an interpretation is in line with the spectral response of melanoma induction in a fish model, such epidemiologic findings may be biased by linkages among susceptibility, exposure and sunscreen use. New work in the opossum model for melanoma has shown that short-term neonatal exposures to broadband UV-B/UV-A radiation (of approximately a week) result in highly aggressive (metastatic) melanomas.
It is now clear that there are at least three mechanisms by which UV-B exposures may suppress cellular immunity: DNA damage, isomerization of urocanic acid, and through the active metabolite of vitamin D. Immune effects can occur both locally at the skin and systemically throughout the body. The local effects have long been considered important to the development of skin cancer, but there has been little evidence found with regard to the importance of systemic effects. Recent observations of an increase in non-Hodgkin's lymphoma with increasing solar radiation have led to the suggestion that UV-B-induced systemic immunosuppression contributes to cancer development in this system. The implications of either local or systemic immune effects for human infectious diseases are still unknown. Epidemiological studies are still required to explore these issues.
There is very little new information on UV effects on the eye. A critical lack is an action spectrum for eye effects associated with chronic exposures, e.g., cataract.
Effects on Terrestrial Ecosystems
Since the last report in 1994, there has been progress on understanding the mechanisms of UV action on organisms. There has also been an increasing emphasis on outdoor studies of nonagricultural vegetation. In such experimentation, the importance of maintaining a realistic spectral balance between UV and longer wavelength radiation during experiments has received more attention. This is important because longer wavelength radiation may have ameliorating effects. The UV-A can also have effects in some respects similar to that of UV-B. In addition to UV-B supplementation studies designed to simulate the consequences of ozone reduction on sunlight, other research of late has involved manipulation of natural solar radiation with filters to show the effectiveness of both UV-B and UV-A on organisms.
Many of the effects of UV-B may not involve damage, but instead be a matter of the plant using this type of radiation as a signal for altering growth form and some physiological processes. One example of such a change is the commonly observed increase in UV-absorbing protective compounds in plant leaves following exposure to UV-B. This may explain, at least in part, many of the seemingly paradoxical results of recent and earlier studies suggesting stimulating effects in plants by UV-B. The UV-B responsiveness is often very specific to species, or even races of a given species, and depends on other environmental factors such as mineral nutrition, drought, local air pollutants, etc. These differential effects on species will likely lead to changes in species interactions and on ecosystem dynamics. Various indirect effects of elevated UV-B radiation are receiving attention, including effects of UV-B on decomposition of plant litter, which may influence cycling of nutrients.
Other indirect effects include influence on plant pathogen susceptibility and plant attractiveness to herbivores and pollinators. Many of these indirect effects may be UV-B action mediated through changes in plant structure, secondary chemistry and timing of life cycle events. Nevertheless, these effects may ultimately be the most important on ecosystems. This is not to dismiss more direct, apparently detrimental effects of UV-B on organisms, including microbes and lower plants such as mosses. The peat-forming moss, Sphagnum, appears to be sensitive which has implications for some processes in biogeochemical cycles such as decreased carbon storage. Trace gas emission from vegetation is also potentially important. One recent study, however, showed no effect of elevated UV-B on emission of isoprene, an important precursor compound for tropospheric ozone formation.
Recent studies suggest that potentially detrimental effects of UV-B radiation in evergreen woody plants may accumulate from year to year as had been indicated in earlier work. Another example of accumulating effects has recently been shown for subsequent generations of native annual plants. With exposure of each generation, the manifestations of adverse effects were intensified.
The interacting effects of elevated UV-B with other factors important in global change such as elevated CO2 and temperature are receiving more attention. While elevated CO2 and temperature usually have more pronounced effects than elevated UV-B, there are often interactions among these factors such that the effects may counteract each other or sometimes they may be additive.
Effects on Aquatic Ecosystems
In the recent past, scientific and public interest has focused on marine primary producers and aquatic ecosystems, which resulted in a multitude of studies indicating mostly detrimental effects of UV-B radiation on aquatic organisms. The interest has expanded to include effects in individual species as well as specific responses and has concentrated on ecologically significant groups and major biomass producers using mesocosm studies, emphasizing species interactions. In addition, light penetration into the water column was investigated by several research groups.
Macroalgae and seagrasses are major biomass producers in aquatic ecosystems. In contrast to phytoplankton most of these organisms are sessile and can thus not avoid the exposure to solar radiation at their growth site. Recent investigations showed a pronounced sensitivity to solar UV-B, and effects have been found throughout the top 10 - 15 m of the water column. Mechanisms of protection and repair are being investigated.
Controversy still exists regarding the interpretation of data on UV-B effects in Antarctic phytoplankton. Estimates of the decreases in overall biomass productivity by different authors range from 0 - 12%. Most recently, shifts in phytoplankton community structure have been demonstrated, which may have consequences for the food web.
Bacteria play a vital role in mineralization of organic matter and provide a trophic link to higher organisms. Recently, the mechanism of nitrogen fixation by cyanobacteria has been shown to be affected by UV-B stress. Wetlands constitute important ecosystems both in the tropics and at temperate latitudes. In these areas cyanobacteria form major constituents in microbial mats. The organisms optimize their position in the community by vertical migration in the mat which is controlled by both visible radiation and UV-B. Cyanobacteria are also important in tropical and subtropical rice paddy fields where they contribute significantly to the availability of nitrogen. Growth, development and several physiological responses of these organisms are affected by solar UV.
Dissolved organic carbon (DOC) and particulate organic carbon (POC) are degradation products of living organisms. These substances are of importance in the cycling of carbon in aquatic ecosystems. UV-B radiation has been found to break down high molecular weight substances and make them available to bacterial degradation. In addition, DOC is responsible for short wavelength absorption in the water column. Especially in coastal areas and freshwater ecosystems, penetration of solar radiation is limited by high concentrations of dissolved and particulate matter. On the other hand, climate warming and acidification result in faster degradation of these substances and thus enhance the penetration of UV radiation into the water column.
UV effects on aquatic animals have found increased interest. In addition to effects on larval stages in amphibia, sea urchins were found to be affected by solar UV-B radiation despite the fact that they have partial protection from mycosporine amino acids, which they take up with their food. Corals have been known to be directly affected by solar UV; in addition, photosynthesis in their symbiotic algae is impaired, resulting in reduced organic carbon supply.
All of the previous studies on UV penetration into the water column were based on occasional measurements. This will be corrected by a recent project involving the development of a monitoring system (ELDONET) for solar radiation in Europe using three-channel dosimeters (UV-A, UV-B, PAR), which are being installed from Abisko (North Sweden, 68° N, 19° E) to Tenerife (Canary Islands, 27° N, 17° W). Some of the instruments will be installed in the water column (North Sea, Baltic Sea, Kattegat, East and Western Mediterranean, North Atlantic), establishing the first network of underwater dosimeters for continuous monitoring.
Effects on Biogeochemical Cycles
Terrestrial studies are continuing to examine the influence of enhanced UV-B radiation on microbial decomposition of plant litter to carbon dioxide and nutrients. Additional studies have confirmed that litter from plants grown under enhanced UV-B is enriched in lignin and decomposed to CO2 more slowly by soil microorganisms.
The fate and transport of trifluoroacetate (TFA), a persistent substance derived from the oxidation of certain CFC replacements (HFC-123, HFC-124, HFC-134a), was investigated in a temperate forest of North America. The study indicates that the added TFA was retained in vegetation and soil, especially in the case of wetlands with organic soils. In addition, biological accumulation was observed as was a lack of microbial consumption.
Studies of photodegradation of dissolved organic carbon (DOC) by UV radiation are continuing and expanding. Several studies published over the past year have provided additional evidence that exposure to UV radiation enhances the degradation of DOC to CO2 and ammonium. In addition, low- molecular-weight organic products are produced and are readily assimilated by microorganisms.
Volatile compounds are produced by aquatic DOC photodegradation and additional studies of the photoproduction of carbon monoxide have appeared. CO is an important trace gas that strongly influences biogeochemical cycles through its effects on chemical reactions in the atmosphere. Two new studies of marine CO photoproduction have resulted in disparate estimates of global oceanic sea-to-air emissions. These two global flux estimates are two orders of magnitude apart: 10-15 Tg CO year-1 and 1000 Tg year-1 (1 Tg = 1012 g). Total annual emissions of CO from all sources, by comparison, are about 2000-2500 Tg year-1 primarily from fossil fuel combustion and CH4 oxidation.
New modeling approaches are being developed to predict the interactions and feedbacks between climate change and UV-B induced changes in marine biogeochemical cycles. Important links exist between oceanic dimethylsulfide (DMS) production, sea-to-air DMS flux and subsequent changes in the sulfate aerosol-related atmospheric radiation balance. These alterations indicate possible consequences with regard to climate and climate prediction, because DMS is an important source of cloud condensation nuclei over parts of the ocean.
Effects on Air Quality
The photo-dissociation of ozone by atmospheric UV-B radiation is a key reaction that controls urban and regional oxidants, the self-cleaning capacity of the troposphere, and the atmospheric lifetimes of many natural and anthropogenic gases. In the last year, published evidence from theoretical, laboratory, and field studies shows that UV-A wavelengths also contribute to this reaction, and therefore some revision of current tropospheric chemistry models is required. Although quantitative estimates of the implications of this new finding have not been carried out yet, impacts are likely for predicted geographical and seasonal distributions of gases such as methane (CH4) and carbon monoxide (CO), and the sensitivity of tropospheric chemistry to stratospheric ozone depletion.
Further evidence for the linkage between stratospheric UV transmission and tropospheric composition was found in the records of surface CH4 and CO concentrations. Temporary increases in tropical CH4 and CO concentrations were observed in the second half of 1991, in phase with high levels of stratospheric sulfur dioxide and sulfate aerosols that resulted from the June 1991 eruption of Mt. Pinatubo. The reduced stratospheric UV transmission is believed to have lowered the rate of CH4 and CO removal, due to decreased photo-dissociation of tropospheric ozone, and therefore decreased hydroxyl radical (OH) production.
The atmospheric chemistry of CFC substitutes and the possible build-up of their breakdown products (particularly trifluoroacetic acid, or TFA) has attracted more attention. The background atmospheric concentrations of HFC-134a (a principal precursor of TFA) are increasing rapidly, according to measurements at Cape Grim, Tasmania (41° S) between 1978 and 1995, and Mace Head, Ireland (53° N) between July 1994 and May 1995. Model estimates suggest that HFC-134a emissions have risen rapidly from ca 0.25x106 kg in 1991 to ca 8x106 kg in 1994. TFA concentrations in air and water samples collected in Germany, Switzerland and Israel in 1995 contained surprisingly high levels of TFA, comparable to concentrations predicted by models for the year 2010. This suggests that an additional yet unknown source of TFA may be present.
Modeling studies give different estimates for future concentrations of TFA in rainwater, e.g., tropical maxima of 0.114 µg l-1 predicted for the year 2020, and a global mean of 0.160 µg l-1 by the year 2010 predicted by another study. However, other modeling studies have concluded that a number of factors can enhance local concentrations of TFA. The TFA concentrations in the precipitation of arid and semi-arid regions may be 2-4 times greater than the global mean. Furthermore, in urban air where high OH and HFC levels are present, the rate of formation of TFA may be enhanced by an order-of-magnitude or more. Therefore, local TFA concentration in rainwater as high as 2-20 µg l-1 or more appears plausible. Water bodies characterized by little or no outflow and which have high evaporation rates may be susceptible to accumulation of rain-borne TFA, leading to significant concentration enhancements. Model calculations predict that such wetlands could experience concentrations of up to 100 µg l-1 within 30 years, even when seepage is as much as 10%.
Materials Damage
Efforts to refine the wavelength sensitivity data for common polymers have continued, with the commodity polymers polyethylene and polypropylene receiving the most attention. Results from several research groups include data on the relative photodamage to polyolefins, acrylics, polystyrene, and other polymers, caused by exposure to different regions of a solar-simulated source spectrum. With polyethylene, the wavelength range 300-330 nm in simulated sunlight was reported to be most effective in reducing the tensile extensibility of the material, a result in agreement with previous findings. These types of data, referred to here as activation spectra, are useful in the estimation of damage to polymers exposed to UV-enhanced solar radiation that results from ozone depletion.
Recent findings have shown that while solar radiation is principally responsible for outdoor degradation of polymers including polyethylene, factors such as temperature and exposure to moisture also have an impact on the useful lifetime of materials outdoors. Several studies have reported on the acceleration of weathering of polyethylene films under harsh desert conditions, where the radiation dose and the temperatures are particularly high.
Research was recently carried out on polycarbonate photodegradation aimed at generating action spectra for the yellowing and molecular-level breakdown of this polymer used in glazing applications. This research also studied the irradiance dependency of the degradation process, an important consideration, since the construction of action spectra and their derivation from polychromatic exposure experiments assume the damage process to be irradiance-independent within the range of irradiance encountered in the experiments. The findings facilitate the intercomparison of previously reported wavelength sensitivity data for polycarbonate polymers. An on- going study is aimed at separating out the contributions of solar radiation and temperature, to the damage suffered by polyethylene polymers during outdoor exposure. The data will allow a better understanding of the role of temperature in modifying the UV-induced photodegradation of polymers exposed to UV-enhanced sunlight resulting from ozone depletion.
Future Effects Studies
The global environment is experiencing a wide range of perturbations. Increased greenhouse gases, sulfate and dust aerosols and increased UV-B from ozone depletion are all occurring simultaneously. Assessment of the potential risks from such changes needs to consider the full context in which these changes are taking place and must take into account the many interactions that may occur. This will require a long-term integrated, interdisciplinary research program designed to include studies which evaluate not only UV-B but also the other processes which are contributing to global environmental change.
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