Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2008

Abstract

After the enthusiastic celebration of the 20^th Anniversary of the Montreal Protocol on Substances that Deplete the Ozone Layer in 2007, the work for the protection of the ozone layer continues. The Environmental Effects Assessment Panel is one of the three expert panels within the Montreal Protocol. This “EEAP” deals with the increase of the UV irradiance on the Earth's surface and its effects on human health, animals, plants, biogeochemistry, air quality and materials. For the past few years, interactions of ozone depletion with climate change have also been considered. It has become clear that the environmental problems will be long-lasting. In spite of the fact that the worldwide production of ozone depleting chemicals has already been reduced by 95%, the environmental disturbances are expected to persist for about the next half a century, even if the protective work is actively continued, and completed. The latest full report was published in Photochem. Photobiol. Sci., 2007, 6, 201–332, and the last progress report in Photochem. Photobiol. Sci., 2008, 7, 15–27. The next full report on environmental effects is scheduled for the year 2010. The present progress report 2008 is one of the short interim reports, appearing annually.

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Ozone and changes in biologically active UV radiation reaching the Earth's surface

• The Montreal Protocol continues to be effective in protecting the stratospheric ozone layer. However, the timing of the return to pre-1980 ozone and UV values cannot yet be predicted precisely and continuation of monitoring and improvements of models are necessary

Evidence for the success of the Montreal Protocol continues to mount. The concentrations of ozone depleting substances are decreasing, and the concentrations of replacement chemicals are increasing.^1,2 Stratospheric ozone also continues to recover. A European study showed that, at mid-latitudes in the northern hemisphere and in the Arctic, there was an almost monotonic negative trend from the late 1970s to the mid 1990s followed by an increase, as expected from the changes in ozone depleting substances, which peaked in 1997. However, changes in atmospheric circulation are also playing a key role in the observed turnaround.³ Improved models that include better estimates of atmospheric circulation predict a slightly faster ozone layer recovery at mid latitudes, and a slightly slower recovery at high latitudes compared with earlier model results.⁴ Unfortunately, few high-quality long-term measurements are available to confirm the ongoing success of the Montreal Protocol in terms of UV radiation received at the Earth's surface.

• The effectiveness of the Montreal Protocol in countering global warming has been further established

As reported last year, the climate protection already achieved by the Montreal Protocol alone is larger than the reduction target of the first commitment period of the Kyoto Protocol (ending 2012).⁵ This climate protection is achieved by reductions in the concentrations of ozone-depleting substances that are also greenhouse gases.⁶ Furthermore, without the Montreal Protocol the more extensive ozone depletion would have led to changes in present-day surface temperatures similar in magnitude to those expected by 2025 due to increasing greenhouse gases. At polar latitudes in both hemispheres, ozone depletion would have led to a warmer surface on average (see Fig. 1). However, there are substantial regional differences: some regions would have warmed, while others would have cooled.⁷

Fig. 1 Mean difference in temperature, in K, between the simulation with 9 ppbv chlorine (no Montreal Protocol) and 3.5 ppbv chlorine (present day). From O. Morgenstern, P. Braesicke, M. M. Hurwitz, F. M. O'Connor, A. C. Bushell, C. E. Johnson and J. A. Pyle, The World Avoided by the Montreal Protocol, Geophys. Res. Lett., 35, L16811, DOI:10.1029/2008GL034590 (ref. 7). Reproduced by permission of the American Geophysical Union.

• Changes in ozone and aerosols affect climate. These forcing agents should be included in future climate models

Interactions between ozone depletion and climate change are more complex than previously thought. For accurate prediction of future changes in climate, all forcing agents must be included, rather than the principal greenhouse gases alone. These forcing agents should include changes in ozone with altitude⁸ and longitude,⁹ changes in ozone depleting substances,¹⁰ and changes in aerosol.¹¹ For example, climate models that include stratospheric chemistry predict that the observed increase in westerly winds at southern high latitudes will not continue as previously thought, but will decrease in the next few decades as ozone recovers¹²—see Tropospheric air quality, below.

• UV radiation at high latitudes will be affected by the reduction in sea ice due to global warming

The extent of sea-ice in the Arctic is decreasing rapidly due to global warming and models suggest that the summertime ice cover will disappear within the next few decades.^13,14 Thus organisms originally living below the ice will be exposed to increased UV doses, but organisms above the surface will receive decreased UV doses due to the reduced reflectivity—see Biogeochemical cycles, below.

• The atmosphere is a complex system and any deliberate interventions should be treated with great care as they may have unanticipated adverse effects

It has been suggested that climate change can be counteracted by injection of sulfur compounds directly into the stratosphere to produce aerosols that reflect incoming solar radiation. However, this geo-engineering strategy would increase Arctic ozone depletion during the 21^st century and delay Antarctic ozone recovery by 30 to 70 years.¹⁵

Human health: Effects of solar UV radiation and interactions with climate change

• Climate change can amplify several damaging effects of solar UV-B radiation on human health

The impacts of temperature and UV-B dose on the incidence of non-melanoma skin cancer in subjects living in 10 regions of the USA were estimated separately. It was concluded that for the same UV dose, each one degree C increase in temperature would result in estimated increases in the incidences of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) of 3% and 6%, respectively.¹⁶ High temperatures and humidity, as experienced in the tropics, may increase the deleterious effects of UV-B radiation on human health, including suppression of immunity to infectious diseases and skin cancers.¹⁷

• The incidence of melanoma in young Caucasian adults is increasing in some parts of the world but not in others (where sun protection campaigns appear to be working). Population-based studies in various countries continue to indicate that sun exposure is an important factor in the induction of melanoma

The annual age-adjusted incidence of cutaneous melanoma (CM) in Caucasian women, aged 15–39 years, living in nine areas of the USA, has increased over the last 20 years.¹⁸ This increased incidence correlates with reported increases in risk-related behaviours associated with increased exposure to solar UV radiation, the primary environmental cause of CM. The prevalence of sunburn in recent years has increased significantly amongst adults in 20 states of the USA.¹⁹ The number of days per year spent in outdoor activity by individuals less than 15 or more than 30 years of age is associated with increased CM risk,²⁰ and the majority of the USA population engage in multiple risk-related behaviours.²¹ In contrast to the results from the USA, the incidence of CM in younger people in Queensland, Australia is stabilising currently—a situation attributed to public health campaigns directed at reducing harmful solar exposure.²² This outcome clearly shows that such campaigns can be effective at reducing the incidence of CM.

• Trend analyses conducted in several countries demonstrate an increasing incidence of the non-melanoma skin cancers, squamous cell carcinoma and basal cell carcinoma, in which sun exposure is the major risk factor

For example, in the UK, there has been an annual increase in the incidence of basal cell carcinoma (BCC) over the past ten years, with the largest rise in the 30–39 year age group.²³ In Sweden, the incidence of squamous cell carcinoma (SCC) has become four times greater in both men and women over the past 40 years; it was concluded that this rise may reflect behavioural and societal changes, particularly intentional tanning.²⁴ In subtropical Australia, the annual incidence of primary BCCs in Caucasians was 1.5% in 2006.²⁵ This is ten times higher than that observed in equivalent populations living in temperate climates. Overall, these results indicate that changes in human behaviour are still needed with regard to sun exposure in order to reduce the incidences of BCC and SCC.

• Evidence continues to accumulate indicating that sun exposure can also have beneficial effects in protecting humans against several internal cancers and autoimmune diseases

Patients with BCC or SCC who had the highest accumulated sun exposure were shown to have a markedly decreased risk of developing colorectal cancer.²⁶ In addition, exposure to sunlight was associated with a reduced risk of advanced breast cancer in women with fair skin,²⁷ and sun exposure in early life was associated with a lower risk of prostate cancer.²⁸ A study of the relationship between the solar UV-B irradiance at latitudes 46–52° N in Canada and prevalence of the autoimmune disease, multiple sclerosis (MS), confirmed earlier results that the incidence of MS increased with increasing latitude.²⁹ Uniquely, this report showed that low exposure to solar UV-B radiation in early life, particularly in the first year, can be an important risk factor for MS in later life. A common suggestion in these reports is that the observed protective effects are due to the action of vitamin D, synthesized in the skin in response to UV-B radiation. Vitamin D has multiple effects on the immune system, cellular differentiation and cell death that help to control some tumours and autoimmune diseases. Public health agencies and individuals need to be aware that exposures to UV-B radiation are associated with beneficial and detrimental effects, and act accordingly.

• The need to protect the eye from cataract and pterygium (two adverse consequences of ocular exposure to UV-B radiation) has been strengthened by results from additional populations and environments

Comparison of the numbers of cases of pterygium and cataract in four indigenous populations in the Amazonian rainforest revealed that differences in the amount of sun exposure are largely responsible for the observed differences in the prevalence of these two diseases.³⁰ A study in a Croatian island population found a prevalence of pterygium of 23% amongst farmers and fishermen compared with none in subjects residing in urban areas.³¹ In a Tibetan population living at high altitude in China, those individuals who seldom used sunglasses had a five-fold increase in the risk of pterygium compared to the rest of the population.³² Recurrence of pterygium following surgery was reduced in those exposed to less sunlight.³³ The public health need for eye protection (sunglasses) was emphasised in all of these studies.

• A number of natural products, when given orally, can protect humans from some of the adverse consequences of UV-B radiation

Examples of such products include probiotic bacteria (such as those found in yoghurt),³⁴ vitamin B3,³⁵ carotenoids and flavonoids,^36,37 and green tea polyphenols.³⁸

• Widespread uses of some new pharmaceuticals have the potential to adversely affect human responses to UV-B radiation

An example is infliximab (Remicade®, a monoclonal antibody against tumour necrosis factor-α) that is frequently used to treat a variety of autoimmune diseases (e.g., rheumatoid arthritis and Crohn's disease). Animals treated with infliximab have impaired DNA repair of UV-B-induced precancerous lesions in the skin. In humans, a similar effect could increase the risk of skin carcinogenesis.³⁹

Terrestrial ecosystems: Effects of solar UV radiation and interactions with climate change

• New studies confirm that UV-B radiation interacts with other climate factors and affects plant responses to several environmental stressors

Two studies^40,41 combined substantial UV-B radiation stress and water deprivation. Overall, the UV-B and water stress reduced growth considerably, but less than would be predicted as the additive effect of both stressors. Experiments that manipulated CO₂ concentration and UV-B radiation showed that increased CO₂ stimulates growth and may compensate for some negative effects of high UV-B radiation^42,43 as shown in earlier research. Elevated UV-B radiation with four weed species reduced the efficacy of a common herbicide.⁴⁴ Earlier studies also showed that herbicide efficacy can be reduced by UV-B radiation.

• Solar UV-B radiation can be a stress for plants in high-latitude ecosystems

Solar UV-B radiation exclusion studies in northern Greenland showed that ambient UV-B radiation decreased photosynthesis in bog blueberry plants despite increases in protective UV-absorbing pigments. Belowground biomass was decreased in response to solar UV-B, and soil microbial communities were altered.⁴⁵ Thus, present day solar UV-B radiation may have pronounced effects on plants in this environment.

• Systemic responses of plants to UV-B radiation continue to be documented

These responses in plant parts not directly exposed to the UV-B radiation can also have further biological consequences. For example, supplemental UV-B radiation applied to cotton plants did not alter plant growth but did increase content of phenolic substances in roots which, in turn, reduced numbers of nematodes (roundworms) and nematode eggs.⁴⁶ Thus, the UV-B exposure of the shoot would potentially reduce belowground pest damage. Plants in a peat bog exposed to supplemental UV-B radiation had higher amounts of phenolic substances in roots, reduced carbohydrates in leaves, and increased carbohydrates in belowground storage organs. These changes may be responsible for the observed lower bacterial growth rates and changes in bacterial community composition within the peat where no sunlight penetrates.⁴⁷ Bacterial activity in peat bogs is important for ecosystem function such as carbon sequestration.

• Increasing evidence from arid and semiarid ecosystems suggests a direct role of UV-B radiation in photodegradation of dead plant material

Attenuation of solar UV-B radiation modestly reduced decomposition of twigs and senescent leaves of a desert shrub (Larrea tridentata) in the Mojave Desert⁴⁸ as also shown to a greater degree in earlier studies in a semiarid steppe.⁴⁹ Total exclusion of UV (UV-B and UV-A) significantly reduced decomposition of grass litter from the Colorado shortgrass steppe under drought conditions,⁵⁰ and in a California annual grassland.⁵¹ Indirect evidence from studies in arid and semiarid ecosystems supports the idea that loss of litter mass is faster in bare soil patches exposed to high UV, than under the shade of shrubs.^52,53 Increased aridity in response to climate change in some regions is likely to increase the importance of litter photodegradation as a driver of carbon emissions and nutrient cycling—see Biogeochemical cycles, below.

• UV-B radiation and elevated temperature stimulate emission of methane in plants

Recently, methane (CH₄) emissions from terrestrial ecosystems were reported to occur in the presence of oxygen rather than only under conditions of low or no oxygen, as had been documented previously. High temperatures stimulate the CH₄ emission rates under solar radiation and especially when the UV radiation is enhanced.⁵⁴ Experiments conducted without elevated temperatures and UV radiation resulted in little or no CH₄ emission.^55,56 The CH₄ emissions are derived from degradation of plant cell wall components.^57–59 McLeod and co-workers concluded that UV-B radiation is more effective in CH₄ production than longer wavelengths.⁵⁹ These plant-mediated effects of UV radiation may increase CH₄ in the atmosphere—see Biogeochemical cycles, below.

• Changes in solar UV-B radiation can have strong effects on insect herbivores and plant pathogens

A recent study⁶⁰ found that the diamondback moth (Plutella xylostella), which is an important pest of crops, laid their eggs preferentially on plants grown in low levels of UV-B radiation, confirming previous work.⁶¹ Foggo and co-workers⁶⁰ also showed that plant exposure to UV-B radiation resulted in reduced growth of the P. xylostella caterpillars, and an increased likelihood that the caterpillars would be attacked by parasitic wasps. In another study, enhanced UV-B radiation appeared to reduce pathogen infection in rice.⁶² Studies involving mammalian herbivores and several species of forages, on the other hand, have found little or no effects of UV-B radiation on plant nutritional value.^63,64 Overall, it appears that UV-B radiation can affect many biotic interactions, but the consequences of changes in UV-B irradiance for the functioning of terrestrial food webs are difficult to predict. Recent work has provided additional insights into the mechanisms whereby UV-B radiation interacts with molecular components of plant defense system.^65–67 This knowledge can be useful for breeding crop varieties resistant to multiple stresses.

• Some animals can detect and react specifically to UV-B in their behavior

Although animal responses to UV radiation have historically been considered to be responses to the UV-A component, recent work has provided additional evidence that a specific response to UV-B occurs. For example, females of a jumping spider species choose a mate based on sex-specific UV-B reflectance patterns.^68,69 This finding corroborates earlier reports of behavioral response to UV-B radiation for leaf thrips^70,71 and frogs.⁷² Flight activity of hornets correlated better with daily patterns of UV-B radiation than with other weather variables.⁷³ The finding that some animals can react specifically to UV-B under natural conditions⁷¹ raises the possibility that changes in UV-B irradiance may directly affect animal behavior. Thus, ozone column changes could potentially affect animal behavior with important consequences for insect reproduction, herbivory, and predation.

Aquatic systems: Effects of solar UV radiation and interactions with climate change

• Increases in UV-B radiation due to human-induced ozone depletion combined with global climate change can negatively impact many aquatic species and ecosystems

These effects have implications for marine and freshwater ecosystems, and the economic and social systems that depend on them. Many new studies, especially from East Asia, India and South America, investigated the effects of solar UV radiation, climate change and their possible interactions, on scales ranging from whole communities to individual organisms at the cellular, biochemical and genetic level. Covering new geographical locations and species, these most recent studies largely confirm previous findings.⁷⁴

• Interactions among stressors are important when considering environmental and climate change on single species, as well as the effects on entire communities⁷⁵

For example, declines in amphibian populations are likely due to many factors, including habitat loss, disease, contaminants, introduced species and UV-B radiation. The effect of UV-B varies widely among species and can vary within populations of the same species or at different life-history stages. This variation often leads to opposing conclusions about how UV-B affects amphibians. Larvae are more susceptible to damage from UV-B radiation compared with embryos. Salamanders are more susceptible compared with frogs and toads. Furthermore, UV-B radiation interacts synergistically with other environmental stressors in amphibia, including those resulting from climate change, and results in greater than additive effects on survival when two stressors are present.⁷⁵ Another example, from shallow water corals exposed to high UV radiation, shows that photoinduced toxicity of polycyclic aromatic hydrocarbon is a synergistic stress factor.^76–78

• Freshwater resources for the rapidly growing human population are being affected by ozone depletion and global climate change

For example, a recent Australian study revealed⁷⁹ increased solar UV radiation, temperature, and evaporation of water plus changes in precipitation and surface runoff, as some of the outcomes of climate change and ozone depletion. Such changes have an impact upon the aquatic ecosystems in lakes and rivers, and alter the character of dissolved organic matter and, consequently, have the potential to affect the quality (e.g., pathogens), quantity and treatability of freshwater resources.

• UV radiation may penetrate to greater depths in the clearest waters than previously thought

New estimates for the absorption coefficients of chemically pure water in the UV region have been shown to be roughly half the values previously accepted. These findings are important for estimating solar UV radiation penetrating into the water column in the clearest natural waters with possible consequences for estimating ozone-depletion effects on phytoplankton productivity, aquatic photochemistry, photobiology and biogeochemical cycles.⁸⁰

• Algorithms have been developed for the retrieval of attenuation coefficients for diffuse UV-A/visible radiation from satellite sensors

A new algorithm permits retrieval of the diffuse attenuation coefficient at key wavelengths from multispectral remote-sensing reflectances. This opens the opportunity for global scale observations in the fields of marine photochemistry and photobiology that will complement in situ studies of the effects of enhanced UV radiation on aquatic systems.⁸¹

• Fish are an important component of the human food supply and are negatively affected by increased UV-B

Estimating the impact of exposure to increased ambient UV-B levels on marine and freshwater fish in nature is complex. Recent studies in large flow-through tanks, supported by earlier laboratory studies, suggest that increased ambient UV-B exposure has negative impacts on growth of marine and freshwater fish and their resistance to disease.^82,83 Any shortfall in fish supplies is likely to affect developing nations more than developed nations.^84–86

Biogeochemical cycles: Effects of solar UV radiation and interactions with climate change

• Scientific information continues to mount that interactions between climate and ozone changes are affecting the biogeochemical cycles that are vital to the Earth system

These cycles include the biological, chemical, and physical processes that control the exchanges of materials between land, freshwaters, oceans and the atmosphere, which in turn influence climate and the health of ecosystems. These biogeochemical cycles include those of carbon, nitrogen, sulfur, halogens, and metals.

• Climate change and solar UV-B radiation can act together to amplify the increase in atmospheric CO₂

In aquatic systems, increases in atmospheric CO₂ caused by human activities result in acidification of the oceans,⁸⁷ decreased mixing,^88,89 and increased inputs of material from land into freshwaters and the ocean.^90–92 These physical and chemical changes can have negative effects on carbon capture by many aquatic organisms,^93,94 resulting in decreases in oceanic uptake of atmospheric CO₂. In addition, these changes may influence UV-induced decomposition of dissolved organic matter (DOM),⁹⁵ resulting in increased production of CO₂. In terrestrial systems, recent evidence confirms that direct photodegradation by sunlight, primarily UV-B,^49,89 plays an important role in the decomposition of dead plant material in low rainfall/high sunlight ecosystems^50,51—see Terrestrial ecosystems, above. Future increases in temperature and changes in the distribution of rainfall are expected to lead to increases in the area covered by these low rainfall/high sunlight ecosystems. Modeling the balance of CO₂ uptake and release from terrestrial ecosystems will need to take account of this increased direct role of UV-B in decomposition.

• UV-induced cycling of halogen compounds affects atmospheric ozone concentrations and thus solar UV radiation and climate

Naturally-produced halogen compounds (CHBr₃, CH₂Br₂, CH₃I, CH₃Cl, and CH₃Br) influence atmospheric ozone depletion (also see discussion in sections on changes in ozone and UV radiation and on tropospheric air quality, composition, and processes). Marine ecosystems are important sources of these halogen compounds.^96,97 Methyl chloride (CH₃Cl) has been increasing over the South Pole in response to climate change⁹⁸ and possibly to UV-induced photoreactions involving chloride and colored dissolved organic matter.⁹⁹

• Changes in UV-B radiation influence concentrations of many trace gases that have wide-ranging effects on atmospheric chemistry and climate change

The newly discovered process of methane production from some terrestrial plants under non-waterlogged conditions is stimulated by UV-B radiation (also see Terrestrial ecosystems).^59,100 However, the contribution of this process to the global production of this highly important greenhouse gas (IPCC 2007) remains unclear and controversial.^101–104 Increased UV-B over a sub-arctic ecosystem caused a significant increase in the release of isoprene,¹⁰⁵ a volatile organic compound which is important in the chemistry of tropospheric ozone. Atmospheric radiation and temperature are influenced by changes in aerosols derived from naturally-produced dimethylsulfide. New models of the biogeochemical cycling of dimethylsulfide^106–108 have taken into account the range of processes that can be influenced by solar UV radiation, including UV-induced stress of phytoplankton.¹⁰⁹ These updates on trace gas biogeochemical cycles should be taken into account in global change models.

• UV-B radiation may increase the biological availability of trace nutrients such as iron and the toxicity of metals such as mercury and copper with consequences for aquatic ecosystems and human health

Rainwater is an important source of trace metals (e.g. iron) to aquatic systems.^110,111 Solar UV radiation modifies the chemical form of iron in rainwater⁸⁹ and stabilization of this form by DOM increases its biological availability to aquatic organisms.¹¹⁰ UV induces the methylation and demethylation of mercury,¹¹² and methylmercury may adversely affect human health through bioaccumulation in aquatic food webs.

Tropospheric air quality, composition, and processes: Effects of solar UV radiation and interactions with climate change

• The impact of ozone depleting substances and climate change on the chemistry of the stratosphere can change weather patterns and, as a result, regional air quality

The upper troposphere/lower stratosphere has been difficult to characterize in models, but understanding of this region is crucial for modeling the movement of air. New models of global circulation that include tropospheric and stratospheric chemistry better account for movement of ozone and other gasses between the troposphere and lower stratosphere.¹¹³ Simulations are now showing that changes in stratospheric chemistry can increase the severity of storms (e.g.¹¹⁴).

• Long-term records in Antarctica reveal a small upward trend in the concentration of ground-level ozone¹¹⁵

In Antarctica, atmospheric chemistry can be directly affected by changes in solar radiation and alterations in stratospheric/tropospheric ozone exchange due to the ozone hole. Ground-level ozone is produced photochemically by the reaction of UV-radiation and nitrogen oxides (NOx) released (also by UV-radiation) from the snow,¹¹⁶ and not because of the transport of ozone from the stratosphere. The seasonal changes in ozone concentration at ground level result primarily from increases in the summer. The trend (∼0.2% per year) could be driven by increased transport of reactive nitrogen from mid-latitudes (pollution) or increased photolysis of NOx, or both. This observation challenges the concept that Antarctic air is pristine and unaffected by human activity but the small changes make it difficult to identify the causes.

• Regional climate and hence tropospheric air quality can be influenced by both changes in stratospheric ozone and the effects of greenhouse gases

Ozone depleting substances and greenhouse gases can contribute to alterations in global circulation¹²—see section above Ozone and changes in biologically active UV radiation. Changes in these large-scale atmospheric circulation patterns have been associated with changes in regional climate, for example a reduction in rainfall in SW Australia.¹¹⁷ Such changes will also affect air quality through changes in local climate.

• Regional air quality could be affected by stratospheric ozone recovery and climate change

Ozone concentrations at ground level are a function of local, regional, and global sources.¹¹⁸ A variety of modeling studies estimating the impacts of future global warming on tropospheric air quality^119–121 have highlighted significant increases in concentration of tropospheric ozone at the global scale. Small increases in background ozone concentrations can result in significantly longer periods when air-quality regulatory standards are exceeded. Current air quality models have yet to explicitly include changes in ozone depleting substances.¹¹⁹

• The measurement of alternate chemical reaction rates involving nitrogen oxides challenges the importance of UV-B as the driver of ground-level atmospheric quality

UV-B radiation drives the production of a key atmospheric reactive species (hydroxyl radical, ˙OH) through a well-established process involving tropospheric ozone and water vapour. ˙OH is the primary reactant in the destruction of organic pollutants in the atmosphere. An alternate reaction pathway for production of ˙OH involving nitrogen dioxide (NO₂) and water suggests that this pathway plays an important role in the atmosphere.¹²² However, this conflicts with earlier measurements of the rates.^123,124 Another reaction involving nitric oxide (NO) is calculated to lead to a 5–12% reduction in ozone in the troposphere,¹²⁵ but decreases the agreement between the modeled and measured values. Clearly, there are still significant uncertainties in our understanding of the ˙OH production and its importance in the destruction of air pollutants.

• Natural sources of reactive halogens from the oceans have been proposed as having a significant role in the destruction of ozone in the lower atmosphere

Additional sources of reactive forms of bromine and iodine from productive (tropical) oceans have been better inferred from measurements in the Atlantic.¹²⁶An understanding of the contribution made by such open ocean sources is required to adequately model the balance between production and destruction of tropospheric ozone.¹²⁷ However, limited satellite measurements suggest that reactive iodine chemistry is confined to small regions near the equator and the poles.¹²⁸ The consequences of these sources of halogens on the predicted future ozone levels in the troposphere have yet to be evaluated.

Materials: Effects of solar UV radiation and interactions with climate change

• Composite plastic materials with wood-powder filler, popularly used in building construction, shown to have good UV stability

Both high-density polyethylene (HDPE)¹²⁹ and polypropylene (PP)¹³⁰ plastics work particularly well in this application. Solar UV-B induced damage to the materials based on HDPE (58–59% wood) is reported to be markedly lower than those based on PP. The discoloration was lower by ca. 34% while the chemical resistance was lower by several hundred percent, depending on the exposure time.¹³¹ These results help in designing wood-plastic composites that are more durable under exposure to solar radiation.

• An emerging trend in stabilizing plastics materials against UV-B damage is the use of nanoparticle fillers

Conventional oxide pigments (ultrafine grades) are used as whiteners or fillers in numerous plastic formulations. Nanoscale oxides such as zinc oxide¹³² and titanium dioxide¹³³ are successfully used as high-grade stabilizers in plastics. For instance, commercial nano-titania appears to be several-fold more efficient compared to the conventional ultrafine grades of the pigment in controlling discoloration in polypropylene. The higher surface area per unit mass of filler in the nanoscale material is responsible for the increased efficiency of stabilization. However, depending on the choice of surface chemical composition, the light-stabilizer effectiveness can vary widely. This was demonstrated for composites of 5 wt% Bohemite mineral nanopowder in PP where the photooxidation rates increased by up to 41%,¹³⁴ depending on the nature of surface treatment.

• Surface modification of even the conventional grades of titania can alter their stabilizer effectiveness against UV-B

Surface modification of titania can not only stabilize but can also degrade the plastic component depending on the surface chemistry used. Studies on the mechanisms of photoreactions of surface-treated titania with the plastic material as well as additives^135,136 have been reported. The findings provide insights into the design of new grades of fillers. Surface modification can also improve dispersion of the pigments, thereby enhancing their efficiency in stabilizing the plastic against UV-B damage.¹³⁶

• The presence of colorants in the plastic can affect the stability of the formulations exposed to solar UV-B

Recent reports on polycarbonates (PC) (used in glazing) and polypropylene (PP) plastics (used outdoors in construction) suggest that diazo-type colorants used in the formulation can improve their stability under laboratory exposure to solar UV-B wavelengths.¹³⁷ This mechanism can contribute to additional stability even in plastics already stabilized with conventional light stabilizers such as the most-used HALS.¹³⁸

• In general, the UV-induced degradation rates of plastics as well as biomaterials increase with increasing ambient temperature

Increase in average temperatures due to climate change can exacerbate the degradation of plastics exposed to solar UV-B, further reducing their service life outdoors.

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Footnotes

† This publication should be cited as follows: Andrady et al., Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2008, United Nations Environment Programme, Environmental Effects Assessment Panel, Photochem. Photobiol. Sci., 8, 2009, DOI:10.1039/b820432m

‡ List of contributing authors in alphabetical order: Anthony Andrady, Pieter J. Aucamp, Alkiviadis F. Bais, Carlos L. Ballaré, Lars Olof Björn, Janet F. Bornman (Co-Chair), Martyn Caldwell, Anthony P. Cullen, David J. Erickson, Frank R. de Gruijl, Donat-P. Häder, Mohammad Ilyas, G. Kulandaivelu, H. D. Kumar, Janice Longstreth, Richard L. McKenzie, Mary Norval, Nigel Paul, Halim Hamid Redhwi, Raymond C. Smith, Keith R. Solomon (Secretary), Barbara Sulzberger, Yukio Takizawa, Xiaoyan Tang (Co-Chair), Alan H. Teramura, Ayako Torikai, Jan C. van der Leun (Co-Chair), Stephen R. Wilson, Robert C. Worrest and Richard G. Zepp.

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