Causes and Environmental Implications of Increased UV-B Radiation

By R. E. Hester, R. M. Harrison

The Royal Society of Chemistry

Copyright © 2000 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-265-4

Contents

Stratospheric Ozone Depletion: a Discussion of Our Present Understanding J. A. Pyle, 1,
1 Introduction, 1,
2 Background, 1,
3 Detection of Ozone Loss, 4,
4 Recent Observations of Ozone Loss, 7,
5 The Future, 12,
6 Conclusion, 5,
7 Acknowledgements, 16,
Ozone Depletion and Changes in Environmental UV-B Radiation Ann R. Webb, 17,
1 Introduction, 17,
2 Historical Interest in UV-B, 19,
3 Determinants of UV at the Ground, 20,
4 Changing Factors in Transmission, 23,
5 UV Radiation at the Ground, 26,
6 Observations of UV Radiation, 27,
7 Longer-term Assessments of UV Irradiances, 34,
8 UV Forecasting, 35,
9 Conclusion, 36,
10 Acknowledgements, 36,
Marine Photochemistry and UV Radiation Robert F. Whitehead and Stephen de Mora, 37,
1 Introduction, 37,
2 Basics of Marine Photochemistry, 38,
3 Marine Photoreactants, Products and Processes, 48,
4 UV-B Radiation and Global Significance for Marine Biogeochemical Cycles, 56,
5 Summary and Conclusions, 60,
Assessing Biological and Chemical Effects of UV in the Marine Environment: Spectral Weighting Functions Patrick J. Neale and David J. Kieber, 61,
1 Introduction, 61,
2 Chemical Action Spectra, 64,
3 Biological Weighting Functions, 67,
4 Comparative Spectroscopy of Weighting Functions, 72,
5 Assessment of UV Effects, 78,
6 Summary and Conclusions, 82,
Effects of Solar UV-B Radiation on Terrestrial Biota Jelte Rozema, 85,
1 Evolution of Terrestrial Biota and the Stratospheric Ozone Layer, 85,
2 Solar UV-B, Polyphenolics, the Pool of Organic Carbon in Terrestrial Environments, and the Balance between Oxygen and Carbon Dioxide in the Earth’s Atmosphere, 90,
3 Current Stratospheric Ozone Depletion: Increased Solar UV-B Radiation Reaching the Earth, 90,
4 Effects of Enhanced Solar UV-B Radiation on Terrestrial Plants, Adaptations of Terrestrial Plants to Solar UV-B: Evidence from Physiological Studies, 92,
5 Methodologies for the Study of UV-B Effects on Plants of Terrestrial Biota, 95,
6 Direct and Indirect UV-B Effects on Terrestrial Ecosystem Processes and Feedbacks, Autotrophic and Heterotrophic Relationships, 97,
7 Conclusions and Outlook, 103,
8 Acknowledgements, 104,
Sunlight, Skin Cancer and Ozone Depletion Brian L. Difley, 107,
1 Introduction, 107,
2 Trends in Atmospheric Ozone and Ambient Ultraviolet Radiation, 108,
3 Human Exposure to Solar Ultraviolet Radiation, 109,
4 Effects of Ultraviolet Radiation on Skin, 113,
5 Risk Analysis of Human Skin Cancer, 115,
Subject Index, 121,

CHAPTER 1

Stratospheric Ozone Depletion: a Discussion of Our Present Understanding

J. A. PYLE

1 Introduction

Ozone is an important stratospheric constituent. It absorbs solar radiation strongly at wavelengths around 300 nm, protecting the biosphere from harmful radiation. Ozone is also an important climate gas. The absorption of solar radiation heats the atmosphere and is responsible for the increase of temperature with altitude through the stratosphere. Ozone is also a greenhouse gas, absorbing and emitting in the infrared.

Depletion of ozone was first detected in the Antarctic stratosphere in the mid-1980s. That the depletion is global has since been determined using both satellite and ground-based ozone measurements. This anthropogenic depletion is expected to have important consequences, including leading to enhanced UV at the surface.

In this chapter in Section 2 we will first consider the background to the problem of ozone depletion. The role of ozone is briefly reviewed and the theory of stratospheric ozone depletion is introduced. Ozone loss was first detected in southern polar latitudes. Intensive observational studies then confirmed that the loss was also occurring in middle latitudes and the Arctic. In Section 3 these first observations of ozone depletion in both polar and middle latitudes are discussed, along with the consequent changes in theoretical understanding. More recent measurements of ozone loss up to the end of the 1990s are considered in Section 4. Section 5 speculates about the future state of the ozone layer into the 21st century.

2 Background

The Role of Ozone

Ozone is present in the atmosphere in trace amounts. In the troposphere below around 10 km the ozone mixing ratio is about 50 ppbv (parts per billion by volume). Mixing ratios are much higher in the stratosphere and reach a peak at around 10 ppmv (parts per million by volume) in the region known as the ozone layer. The highest concentrations occur in the low stratosphere, between about 15 and 30 km, depending on latitude.

A frequently used measure of ozone is its integrated column amount, the sum of the ozone concentration between the surface and the top of the atmosphere. Values typically range from a little over 200 m atm cm (or Dobson units, DU) in the tropics to greater than 400 DU in the high latitude spring. Figure 1 shows the latitude and seasonal distribution of column ozone. This variation with space and time has been broadly know for about 70 years, following the pioneering measurements of column ozone by Dobson and his co-workers.

The importance of ozone has been recognised for a long time. Ozone plays several important roles. It is toxic and high concentrations at the Earth’s surface have implications for the health of both humans and plants. In the stratosphere, ozone absorbs solar ultraviolet radiation strongly, especially at wavelengths less than about 310 nm in the UV-A and UV-B parts of the spectrum. Ozone thus acts as a filter, preventing potentially harmful UV radiation from reaching the surface.

The penetration of radiation around 300 nm depends critically on the ozone column amount and the precise wavelength of the radiation (since the efficiency of absorption by ozone varies strongly with wavelength around 300 nm). Changes in the ozone column can have a significant impact in changing the penetration of UV to the surface, and for this reason any depletion of the stratosphere ozone column is a cause for concern.

The absorption of solar radiation by ozone also plays a very important role in determining atmospheric structure (ozone is an important climate gas). For example, the rise in temperature with altitude within the stratosphere is a result of the absorption of solar energy by ozone molecules. Absorption of infrared radiation by ozone is particularly important in the lower stratosphere, where changes in ozone are predicted to have a significant impact on surface temperature.

Theory of Stratospheric Ozone

Research in the early 1970s established a good description of the chemical processes responsible for the observed distribution of stratospheric ozone. Before that it had been thought that a sequence of reactions proposed by Chapman could explain the observations of ozone. In Chapman’s theory, ozone is produced following the photolysis of molecular oxygen by solar UV radiation at wavelengths less than about 240 nm (equation 1). Two reactions (equations 2 and 3) rapidly interconvert O and O3 (so that these two species can be thought of together as ‘odd oxygen’). Finally, ozone (or ‘odd oxygen’) is destroyed by the reaction of O and O3 (equation 4):

O2 + hv [right arrow] λ <242nm (1)

O + O2 + M [right arrow] O3 + M (2)

O3 + hv [right arrow] O2 + O λ <1100nm (3)

O + O3 + [right arrow] 2O2 (4)

where M is any third body (usually N2 or O2).

During the 1960s it became apparent that these reactions overpredict the observed ozone. The resolution of this discrepancy came through the suggestion that a series of catalytic reactions of the following form could remove ozone:

X + O3 [right arrow] XO + O2 (5)

XO + O [right arrow] X + O2 (6)

net: O + O3 [right arrow] 2O2

Thus, these reactions effectively catalyse reaction (4), thereby destroying ozone. The reactive radical species, X, is reformed in the reaction sequence so that the reactions may cycle many times until X is removed by another process. Note that if X is present in the parts per billion range, then one sequence through reactions (5) and (6) will only remove about a part per billion of ozone, very small compared to the abundance of ozone. However, when the cycle operates many thousands of times, it can then exert a controlling influence on stratospheric ozone.

X can be, for example, OH, NO or Cl. These are all reactive radical species which are present in the stratosphere following the breakdown of the source gases H2O, N2O and the CFCs (the chlorofluorocarbons).

With the understanding of the role of these catalytic cycles came the realisation that the ozone layer could be perturbed by the introduction of enhanced concentrations of the radical species, X. For. example, it was proposed that oxides of nitrogen, emitted directly into the stratosphere by supersonic aircraft, could lead to a large, additional ozone destruction and hence to a reduction in the stratosphere ozone column. Similarly, Molina and Rowland showed that the large build-up in the atmosphere of CFCs, then widely used as aerosol propellants, in refrigeration systems and for foam blowing, could also lead to a depletion of the stratospheric ozone layer. They showed that although the CFCs are inert in the troposphere, if they are carried high enough into the stratosphere they can be broken down by ultraviolet radiation to liberate Cl, leading to ozone loss.

During the 1970s, large reductions in ozone were predicted for a proposed fleet of supersonic aircraft or following the large growth in the production of CFCs. A particular concern with the CFCs was that the compounds being produced in largest quantities, CFCl3, and CF2Cl2, had atmospheric lifetimes of many decades. Once emitted into the atmosphere, they could therefore remain in the atmosphere for many years as potential ozone-depleters. In the event, only a small fleet of supersonic aircraft was produced, although the topic of ozone depletion by aircraft emissions has re-emerged quite recently as an important issue. However, CFC production and usage continued to grow rapidly throughout the 1970s and the early part of the 1980s. In the next section we present the first evidence that these CFC emissions did indeed have a damaging effect on the stratosphere.

3 Detection of Ozone Loss

The Antarctic ‘Ozone Hole’

The first evidence of anthropogenic ozone depletion came dramatically with the discovery by Farman et al. of the Antarctic ozone hole. They showed, from observations of column ozone made at Halley Bay (76°S) beginning in 1957, that there had been a rapid decline in the average October column amounts during the late 1970s and early 1980s. Ozone had decreased from values of around 300 DU during the 1960s to about 150 DU in the early 1980s. These measurements created massive scientific interest. The catalytic theories, discussed in Section 2, indicated that the largest ozone depletion should occur in the upper stratosphere. The reported ozone decline was occurring in the lowest part of the stratosphere (see Figure 2) and in springtime, when theory indicated that little loss should occur. A new theoretical understanding was required.

Within two years, the huge research effort had produced a broadly consistent picture of the processes leading to ozone loss (see, for example, the recent review by Solomon). It was shown that the loss of polar ozone was rapid, occurring over just several weeks from late August to mid-October. The loss was largest between about 12 and 20 km. Over part of this altitude range, essentially complete removal of ozone occurred (see Figure 2). Other measurements demonstrated that the loss of ozone was primarily due to a large build-up of active (ozone-destroying) chlorine species in polar latitudes. Elevated ClO concentrations were measured in the Antarctic lower stratosphere from NASA’s ER-2 high-flying research aircraft by Anderson and colleagues. Observations showed that the springtime increase in ClO was clearly anticorrelated with the observed decline in ozone. Furthermore, the ClO arose mainly from the breakdown in the stratosphere of the CFCs (the contribution of natural sources to the stratospheric chlorine budget is small, being a little less than 20%). Thus, these measurements confirmed that anthropogenic depletion of ozone was occurring and led in 1987 to the adoption of the Montreal Protocol, designed to phase-out production of ozone-depleting substances.

Why the depletion occurred in the springtime Antarctic lower stratosphere was an intriguing question. In particular, why was the concentration of ClO so high when the current theories had suggested that the most abundant forms of chlorine would be HCl and ClONO2, two species which do not destroy ozone? The answer in part arises from the particular meteorological conditions over Antarctica. During the winter, strong westerly winds (the ‘polar vortex’) circulate around the Antarctic lower stratosphere, isolating the air over Antarctica where the temperatures fall to below 190K. At these low temperatures, polar stratospheric clouds (PSCs) form, as a co-condensate of water and nitric acid. It was realised that reactions on the surface of the PSCs could turn chlorine into active forms:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

The Cl2 is easily photolysed to liberate chlorine atoms.

It was also appreciated that additional catalytic chlorine cycles, somewhat different to those discussed in the previous section, could be important. In particular, Molina and Molina showed that a cycle involving the chlorine monoxide dimer, Cl2O2, was particularly efficient at low temperatures:

ClO + ClO + M [right arrow] Cl2O2 + M (8)

Cl2O2 + hv [right arrow] Cl + ClO2 + M (9)

ClO2 + M [right arrow] + Cl + O2 + M (10)

2Cl + 2O3 [right arrow] 2ClO + 2O2 (11)

net: 2O3 + hv [right arrow] 3O2 (12)

This, and a cycle involving ClO and BrO, are now believed to explain the majority of the polar loss.

Global Ozone Loss

With the confirmation that the Antarctic ozone loss was indeed due to chemical destruction involving anthropogenic halogen species, attention turned to ozone levels globally. A detailed study of ozone data sets from satellites and from the ground-based network was carried out under the auspices of the World Meteorological Organisation by the International Ozone Trends Panel and by many individual scientists.

Their studies confirmed that ozone depletion was a global phenomenon. Thus, in 1991, the Stratospheric Ozone Review Group (SORG) reported that the analysis of satellite ozone data showed a global loss of 3% between 1979 and 1990 and that, over the same period, the decline in northern mid-latitudes in early spring was greater than 8% (these figures can be contrasted with the decline in Antarctic springtime ozone of about 50%). The data also suggested that there had been a decline in ozone over the Arctic. Outside the polar regions, the annual-mean losses of ozone averaged over middle latitudes were comparable in the two hemispheres. No statistically significant loss was detected in the tropics. Figure 3 shows the estimated decadal trend in ozone estimated from the Total Ozone Mapping Spectrometer (TOMS) satellite data. The Antarctic decline in springtime is the most obvious feature, but significant losses are seen elsewhere.

4 Recent Observations of Ozone Loss

Following the adoption of the Montreal Protocol in 1987, the Protocol was strengthened during the 1990s in a series of amendments in line with the increasing evidence of the impact on the stratosphere of ozone-depleting substances. Since the beginning of 1996 (1995 in the European Union), production of CFCs and carbon tetrachloride has been phased out in developed countries and schedules for the phase out of replacements are in place. Similarly, production of halons (bromine-containing species used in fire extinguishers, etc.) is now phased out and some controls on CH3Br have been agreed. The impact of this action is that the abundance of chlorine in the stratosphere is now at its peak and should begin to fall slowly (Figure 4). Chlorine in the troposphere is definitely declining. The loading of bromine species in the stratosphere is expected to peak early in the 21st century. In the context of international regulation of ozone-depleting substances, we report in this section the observations of ozone loss during the 1990s, while in the next section the possible future changes of ozone during the 21st century are considered.

Polar Ozone

The depletion of Antarctic ozone in the spring continues unabated. Every spring, the ozone column inside the polar vortex is depleted and minimum values of around 100 DU are typically reported. Almost complete removal of ozone occurs in the very low stratosphere between about 12 and 20 km. There is some small interannual variability depending on meteorological conditions, so that records in particular measures of ozone depletion (e.g. the area covered by the ‘hole’) are frequently reported. In some ways, these records are misleading. Essentially, the magnitude of the Antarctic ozone depletion should have reached its peak, but will remain close to this peak for decades. While it is difficult to imagine how the polar column loss can get significantly larger (since near complete depletion already occurs in the lower stratosphere), recovery of ozone levels is not expected for many years (see Section 5). The ‘ozone hole’ is expected to be a feature of observations for at least the first half of the 21st century.

(Continues…)Excerpted from Causes and Environmental Implications of Increased UV-B Radiation by R. E. Hester, R. M. Harrison. Copyright © 2000 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.