Changes in biologically active ultraviolet radiation reaching

1011-1344/98/$ - see front matter q 1998 UNEP. Published by Elsevier Science S.A. All rights reserved.PII S1011- 1344 (98 )00182 -1

Thursday Dec 03 01:33 PM StyleTag -- Journal: JPB (Journal of Photochemistry and Photobiology B: Biology) Article: 7735

Journal of Photochemistry and Photobiology B: Biology 46 (1998) 5–19

Changes in biologically active ultraviolet radiation reachingthe Earth’s surface

S. Madronich a,*, R.L. McKenzie b, L.O. Bjorn c, M.M. Caldwell dä National Center for Atmospheric Research, Atmospheric Chemistry Division, PO Box 3000, 1850 Table Mesa Drive, Boulder, CO 80307-3000, USA

b National Institute of Water and Atmospheric Research, NIWA, Lauder, Central Otago 9182, New Zealandc Plant Physiology, Lund University, Box 117, S-221 00 Lund, Swedend Ecology Center, Utah State University, Logan, UT 84322-5230, USA

Abstract

Stratospheric ozone levels are near their lowest point since measurements began, so current ultraviolet-B (UV-B) radiation levels are thought to be close totheir maximum. Total stratospheric content of ozone-depleting substances is expected to reach a maximum before the year 2000. All other things being equal,the current ozone losses and related UV-B increases should be close to their maximum. Increases in surface erythemal (sunburning) UV radiation relative tothe values in the 1970s are estimated to be: about 7% at Northern Hemisphere mid-latitudes in winter/spring; about 4% at Northern Hemisphere mid-latitudesin summer/fall; about 6% at Southern Hemisphere mid-latitudes on a year-round basis; about 130% in the Antarctic in spring; and about 22% in the Arctic inspring. Reductions in atmospheric ozone are expected to result in higher amounts of UV-B radiation reaching the Earth’s surface. The expected correlationbetween increases in surface UV-B radiation and decreases in overhead ozone has been further demonstrated and quantified by ground-based instruments undera wide range of conditions. Improved measurements of UV-B radiation are now providing better geographical and temporal coverage. Surface UV-B radiationlevels are highly variable because of cloud cover, and also because of local effects including pollutants and surface reflections. These factors usually decreaseatmospheric transmission and therefore the surface irradiances at UV-B as well as other wavelengths. Occasional cloud-induced increases have also beenreported.

With a few exceptions, the direct detection of UV-B trends at low- and mid-latitudes remains problematic due to this high natural variability, the relativelysmall ozone changes, and the practical difficulties of maintaining long-term stability in networks of UV-measuring instruments. Few reliable UV-B radiationmeasurements are available from pre-ozone-depletion days. Satellite-based observations of atmospheric ozone and clouds are being used, together with modelsof atmospheric transmission, to provide global coverage and long-term estimates of surface UV-B radiation. Estimates of long-term (1979–1992) trends inzonally averaged UV irradiances that include cloud effects are nearly identical to those for clear-sky estimates, providing evidence that clouds have notinfluenced the UV-B trends. However, the limitations of satellite-derived UV estimates should be recognized. To assess uncertainties inherent in this approach,additional validations involving comparisons with ground-based observations are required. Direct comparisons of ground-based UV-B radiation measurementsbetween a few mid-latitude sites in the Northern and Southern Hemispheres have shown larger differences than those estimated using satellite data. Ground-based measurements show that summertime erythemal UV irradiances in the Southern Hemisphere exceed those at comparable latitudes of the NorthernHemisphere by up to 40%, whereas corresponding satellite-based estimates yield only 10–15% differences. Atmospheric pollution may be a factor in thisdiscrepancy between ground-based measurements and satellite-derived estimates. UV-B measurements at more sites are required to determine whether thelarger observed differences are globally representative. High levels of UV-B radiation continue to be observed in Antarctica during the recurrent spring-timeozone hole. For example, during ozone-hole episodes, measured biologically damaging radiation at Palmer Station, Antarctica (648S) has been found toapproach and occasionally even exceed maximum summer values at San Diego, CA, USA (328N).

Long-term predictions of future UV-B levels are difficult and uncertain. Nevertheless, current best estimates suggest that a slow recovery to pre-ozonedepletion levels may be expected during the next half-century. Although the maximum ozone depletion, and hence maximum UV-B increase, is likely to occurin the current decade, the ozone layer will continue to be in its most vulnerable state into the next century. The peak depletion and the recovery phase could bedelayed by decades because of interactions with other long-term atmospheric changes, e.g., increasing concentrations of greenhouse gases. Other factors thatcould influence the recovery include non-ratification and/or non-compliance with the Montreal Protocol and its Amendments and Adjustments, and futurevolcanic eruptions. The recovery phase for surface UV-B irradiances will probably not be detectable until many years after the ozone minimum. q 1998UNEP. Published by Elsevier Science S.A. All rights reserved.

Keywords: Ozone depletion; Ultraviolet-B radiation; Action spectra; Cloud cover; Radiation; Amplification factor; Aerosol; Stratosphere

1. Introduction

An important consequence of stratospheric ozone deple-tion is the increased transmission of solar ultraviolet (UV)

* Corresponding author. Tel.: q1-303-497-1430; Fax: q1-303-497-1400;E-mail: [email protected]

radiation to the Earth’s lower atmosphere and surface. UVradiation is known to affect many biological and chemicalprocesses, and is largely detrimental to individual organisms.Specific concerns include increases in the incidence of skincancer, ocular damage, and other health effects in humansand animals; damage to terrestrial and oceanic vegetation;damage to some outdoor materials; changes in the chemistry

S. Madronich et al. / Journal of Photochemistry and Photobiology B: Biology 46 (1998) 5–196


Fig. 1. Biologically active UV radiation. The overlap between the spectralirradiance F(l) and the erythemal action spectrum B(l) given by McKinlayand Diffey [6] shows the spectrum of biologically active radiation,F(l)B(l). The area under the product function F(l)B(l) is the biologi-cally active dose rate. Thick lines are for a total ozone column of 348 DU,thin lines for 250 DU (one Dobson Unit, or DU, is defined as the height inmillicentimeters that pure gaseous ozone would occupy if compressed to1013 hPa at 08C, and thus equals 2.69=1016 molecules cmy2) (from Mad-ronich and Flocke [7]).

of the lower atmosphere (the troposphere), e.g., photochem-ical smog formation; and alterations of the biogeochemicalcycles of non-living organic and inorganic matter whose deg-radation depends on the exposure to ambient solar radiation[1–3].

Environmental UV radiation is highly variable. Some ofthese variations are easily quantified, such as those due tochanges in the solar elevation with latitude, time of day, andseason. Variations in the atmospheric ozone column amountare of direct importance to surface UV radiation. Continuousozone observations have been available from ground-basedstations since the 1950s for a number of locations, and near-global observations have been available from satellite-basedinstruments for most of the period since 1979. Other factors,such as clouds, are much less predictable and their spatial andtemporal distributions are still poorly characterized, espe-cially on local scales and for short-term fluctuations. Addi-tional localized perturbations may stem from surfaceelevation and reflections, and from variable atmospheric tur-bidity associated with air pollution.

In this report, the dependence of UV radiation on atmos-pheric ozone is emphasized. Systematic reductions in ozonehave been observed in the last two decades, a likely result ofhuman activities and in particular the emission of halogen-containing compounds of mostly anthropogenic origin [4,5].Observations confirm that ozone reductions lead to increasedUV radiation levels at the Earth’s surface, if all other factorsthat influence atmospheric transmission (e.g., clouds, pollut-ants) are constant. However, it is also necessary to view theseozone-related UV increases in the context of the natural UVvariability, and to consider the possibility of long-termchanges in other factors such as cloud cover and air pollution.

2. Biologically active UV radiation

The solar radiation at the top of the Earth’s atmospherecontains a significant amount of radiation of wavelength (l)shorter, and therefore more energetic, than that of visible light(400–700 nm). Wavelengths in the range 100–400 nm con-stitute the ultraviolet (UV) spectral region. The shortest ofthese wavelengths (UV-C, 100–280 nm) are essentiallycom-pletely blocked (absorbed) by atmospheric oxygen (O2) andozone (O3). Wavelengths in the UV-B range (280–315nm 1) are absorbed efficiently though not completely by O3,while UV-A wavelengths (315–400 nm) are absorbed onlyweakly by O3 and are therefore more easily transmitted to theEarth’s surface.

Fig. 1 illustrates the wavelength dependence of UVspectralirradiance at the Earth’s surface. The strong decrease belowabout 330 nm is due to absorption by atmospheric ozone.Reductions in ozone lead to an increase at these wavelengths,as shown in the Figure, with the largest fractional (percent-

1 Some workers use 320 nm rather than 315 nm (C.I.E. definition) as theboundary between UV-A and UV-B.

age) increase occurring at progressively shorter wavelengthsin the UV-A and UV-B ranges. Although UV-B irradiancesare much smaller than those in the UV-A region, many bio-logical responses to UV exposure are far greater at the shorterwavelengths. Thus even relatively small increments of UV-B radiation can lead to substantial biological effects.

To estimate the biological impacts of ozone-related UVincreases, the wavelength dependence of the sensitivity toUV exposure must be known at least approximately. Spectralsensitivity functions (action spectra) have been determinedin laboratory and field studies for a number of biologicalendpoints. Such action spectra allow the estimation of theeffect of simultaneously changing radiation at differentwave-lengths by different amounts, as happens when ozone reduc-tions occur. Fig. 1 shows the action spectrum for erythema(skin-reddening) induction by UV radiation, and the spectraloverlap between significant sensitivity (at shorter wave-lengths) and significant spectral irradiance (at longer wave-lengths). For this particular action, the overlap is greatest inthe range 300–320 nm, and is quite sensitive to ozoneamounts as shown in Fig. 1. A useful measure of this overlapis the biologically active UV irradiance, or exposure UVbio,defined as the area under the spectral overlap function,

UV s F(l)B(l) dlbio |

S. Madronich et al. / Journal of Photochemistry and Photobiology B: Biology 46 (1998) 5–19 7


where F(l) is the spectral irradiance, B(l) is the actionspectrum for a particular biological effect, and the integral iscarried out over all UV wavelengths.

The sensitivity of UVbio to atmospheric ozone is frequentlyexpressed with the radiation amplification factor (RAF),defined as the percentage increase in UVbio that would resultfrom a 1% decrease in the column amount of atmosphericozone. The RAFs are given in Table 1 for a number of dif-ferent known effects. The RAFs can generally be used onlyto estimate effects of small ozone changes, e.g., of a fewpercent, because the relationship between ozone and UVbio

becomes non-linear for larger ozone changes. For actionspec-tra that decrease approximately exponentially with increasingwavelength over 300–330 nm, the biologically active irradi-ances scale with larger ozone changes according to a powerrelationship [8,14,15]:

yRAFUV ;(Ozone)bio

The RAFs presented in Table 1 have been computed witha model of the propagation of spectral UV radiation throughthe atmosphere, combined with the appropriate action spectrafor the different effects. RAFs may also be derived fromspectral UV measurements made at the Earth’s surface, whenthese are combined (numerically) with the appropriateactionspectra. Generally good agreement is found between thesetwo methods, within the combined uncertainties of the meas-urements and the models [8,9,16,17].

RAFs are useful indicators of the sensitivity of a particulareffect (i.e., a particular action spectrum) to ozone changes.Large RAF values indicate that the radiation associated witha particular effect is strongly sensitive to changes in atmos-pheric ozone, while small RAF values indicate that the rele-vant UVbio is less sensitive to ozone changes. Values ofRAF;0 mean that the UVbio for that particular effect is notdependent on ozone, as occurs in cases when an action spec-trum shows strong sensitivity to longer UV-A and visiblewavelengths, but not to UV-B radiation.

In many cases the full spectral sensitivity is not wellknown, and only estimates of the RAF value can be made. Aparticularly important consideration is the potential role oflonger (UV-A and visible) wavelengths, where even rela-tively low sensitivity (per photon) may be of importancebecause the ambient radiation increases strongly with increas-ing wavelength (see curve marked F(l) in Fig. 1). To showthis sensitivity to longer wavelengths, the RAFs given inTable 1 were also calculated by extrapolating the measuredaction spectra to 400 nm, and, for cases where such extrap-olation leads to significant changes in the RAF, the re-calculated values are shown in square brackets. The RAFscalculated from extrapolated action spectra are not necessar-ily more accurate than those without extrapolation, but rathershow that such RAFs are quite uncertain, and more detailedmeasurements of the action spectra are needed to assess thesensitivity to ozone.

It should be cautioned, however, that: (i) neither the actionspectra nor the resulting weighted irradiances (UVbio) give

a measure of the absolute damage to any particular organism;(ii) weighted irradiances computed from different actionspectra cannot be compared directly to one another, becausethe action spectra usually specify only the spectral shape ofthe sensitivity, not the absolute value; (iii) even for a singleaction spectrum, increases in UVbio do not necessarily implya proportionate increase in effect, if dose–response relationsfor that effect are non-linear; (iv) any damage to a specificorganism must be viewed in the context of its entire ecosys-tem including consideration of other stresses (e.g., nutrientavailability, temperature) and interactions with other organ-isms (e.g., species competition); (v) action spectra areusually determined from short-term laboratory or field exper-iments, while the effects of environmental UV increases maybe felt on longer time scales; and (vi) considerable uncer-tainties are inherent in the experimental determinations ofaction spectra.

3. Measurements of UV radiation

The last decade has seen a great increase in the numberand general quality of solar UV measurements. Many newcommercial and research-grade UV detectors have beendeveloped, calibration procedures have been improved, andseveral national and international intercomparisonshavebeencarried out [56–62]. Agreement among similarly calibratedspectroradiometers is typically 5% or better in the UV-Arange, and 5–10% in the UV-B range. Comparisons betweendifferent types of instruments (e.g., spectroradiometers,broad-band meters, filter radiometers) are more difficult,because of the need to put the different measured quantitieson a similar basis, for example, through the use of modelinterpolations (e.g., [63–65]).

Direct measurements of surface UV radiation confirm to alarge extent the theoretical expectations, if allowances aremade for local conditions [8,12,66–71]. However, the anal-ysis, interpretation, and utilization of the measurements stilllag behind the growing data archives. Some general patternsof temporal and geographical variations are also being iden-tified [9,11,72–78]. For example, ground-based measure-ments show that summertime erythemal UV irradiances inthe Southern Hemisphere exceed those at comparable lati-tudes of the Northern Hemisphere by up to 40% [72],whereas corresponding satellite-based estimates yield only10–15% differences [5]. Atmospheric pollution may be afactor in this discrepancy between ground-based measure-ments and satellite-derived estimates. UV-B measurementsat more sites are required to determine whether the largerobserved differences are globally representative.

3.1. Ozone-related UV radiation changes

The evidence is overwhelming that under cloud-free skiesUV-B radiation is controlled largely by ozone [8,9,12,79–94]. The response of UV-B radiation to ozone changes is



Table 1Radiation amplification factors (RAFs) at 308N

Effect RAF Reference

January (290 DU) July (305 DU)

SkinErythema 1.7 1.7 [18]Erythema reference 1.1 1.2 [6]ErythemaU 1.6 1.5 [19]Skin cancer in SKH-1 hairless mice (Utrecht) 1.5 1.4 [20]SKH-1 corrected for human skin transmission 1.2 1.1 [21]Elastosis 1.1 1.2 [22]Photocarcinogenesis, skin edema 1.6 1.5 [23]Photocarcinogenesis (based on STSL) 1.5 1.4 [24]Photocarcinogenesis (based on PTR) 1.6 1.5 [24]Melanogenesis 1.7 1.6 [18]Melanoma in fish 0.1 0.1 [25]DNA relatedGeneralized DNA damageU 2.2 2.1 [26]Mutagenicity and fibroblast killing 2.2 [1.7] 2.0 [2.7] [27,28]Fibroblast killing 0.3 0.6 [29]Cyclobutane pyrimidine dimer formation 2.4 [2.0] 2.3 [2.1] [30](6–4) photoproduct formation 2.7 [2.3] 2.5 [2.3] [30]HIV-1 activation 4.4 [0.1] 3.3 [0.1] [31]EyesDamage to cornea 1.2 1.1 [32]Damage to lens (cataract) 0.8 0.7 [32]Other effects on animal cellsOccupational exposure limit 1.4 1.5 [33]Immune suppression 1.0 [0.4] 0.8 [0.4] [34]Cell mortality in Chinese hamster 1.3 1.2 [35]Substrate binding in Chinese hamster 0.4 0.4 [35]Membrane damageMembrane-bound Kq-stimulated ATPase inactivation 2.1 [0.3] 1.6 [0.3] [36]PlantsIsoflavonoid formation in bean 2.7 [0.1] 2.3 [0.1] [37]Photosynthetic electron transport 0.2 0.1 [38]Overall photosynthesis in leaf of Rumex patientia 0.2 0.3 [39]DNA damage in Alfalfa 0.5 0.6 [40]PhytoplanktonInhibition of motility (Euglena gracilis) 1.9 1.5 [41]Inhibition of photosynthesis (Phaeodactylum sp.) 0.2 0.3 [42]Inhibition of photosynthesis (Prorocentrum micans) 0.3 0.4 [42]Inhibition of photosynthesis, in Antarctic community 0.8 0.8 [43]Inhibition of photosynthesis (Nodularia spumigena, cyanobacterium) 0.2 0.2 [44]Tropospheric photolysisO3qhn™O(1D)qO2

U 1.7 1.5 [45]O3qhn™O(3P)qO2 0.1 0.1 [45]H2O2qhn™OHqOH 0.4 0.4 [45]HNO3qhn™OHqNO2 1.1 1.0 [45]NO2qhn™O(3P)qNO 0.0 0.0 [45]HCHOqhn™HqCHO 0.5 0.5 [45]HCHOqhn™H2qCO 0.2 0.2 [45]Aqueous photochemistryCO production (Suwannee River) 0.3 0.3 [46]CO production (Pacific Ocean)U 0.3 0.3 [47]COS production (Gulf of Mexico) 0.2 0.2 [48]COS production (North Sea) 0.6 0.6 [48]Photodegradation of nitrate ions 1.1 1.0 [49]Photodegradation of HCHO (Biscayne Bay) 1.3 1.1 [50]Photoproduction of H2O2 in freshwater 0.1 0.1 [51]Materials damageYellowness induction in polyvinyl chloride 0.2 0.2 [52]Yellowness induction in polycarbonate 0.4 0.4 [53]

(continued)



Table 1 (continued)

Effect RAF Reference

January (290 DU) July (305 DU)

Other weighting functionsTemple U. Robertson–Berger meter 0.8 0.7 [54]Solar Light Robertson–Berger meter (model 501) 1.2 1.1 [55]Ozone cross section (273 K) 0.8 0.8 [45]UV-A (315–400 nm) 0.03 0.02UV-B (280–315 nm) 1.25 0.99UV-B9 (280–320 nm) 0.87 0.71Exponential decay, one decade per 14 nm 1.00 1.00

Updated from Ref. [2] with new entries denoted by (U). RAFs are computed on basis of daily integrals. Listed references are for the original action spectraused in the calculation of the RAFs. Measurements of action spectra often do not cover the full UV-A wavelength range. For such cases, the RAF calculationswere repeated by extrapolating the action spectra to 400 nm; values in square brackets show the RAF estimated from the extrapolated action spectra, for caseswhere the RAF changed by at least 0.2 units, and are indicative of the highly uncertain UV-A contribution.

Fig. 2. Dependence of erythemal ultraviolet (UV) radiation at the Earth’ssurface on atmospheric ozone, measured on cloud-free days at various loca-tions, at fixed solar zenith angles. Legend: South Pole [8]; Mauna Loa,Hawaii [9]; Lauder, New Zealand [10]; Thessaloniki, Greece (updatedfrom Ref. [11]); Garmisch, Germany [12]; and Toronto, Canada (updatedfrom Ref. [13]). Solid curve shows model prediction with a power ruleusing RAFs1.10.

strongly dependent on wavelength because of the rapidincrease of the ozone absorption cross section toward shorterwavelengths, with greater sensitivity at short wavelengthsand low sun, where the slant ozone optical depth is greater(see, for example, the review by Madronich et al. [95], themore recent measurements by Fioletov and Evans [13], andreferences therein). For biologically weighted radiation,measurements under cloud-free skies also show the theoret-ically expected dependence on ozone. Fig. 2 shows the sen-sitivity of erythemal (skin-reddening) UV radiation to theozone column amount, as measured at a number of differentlocations and for different solar zenith angles. When

expressed on a relative (percentage) basis, the increases inerythemal UV radiation are seen to correlate closely withozone reductions, whether the latter stem from natural fluc-tuations and seasonal cycles, or from systematic long-termdepletion. The largest percentage UV increases areassociatedwith the largest percentage reductions in atmospheric ozone.

Current losses of stratospheric ozone are discussed in Ref.[5]. Relative to the values in the 1970s, these are estimatedto be about 50% in the Antarctic spring (the ozone hole),about 15% in the Arctic spring, about 6% at Northern Hem-isphere mid-latitudes in winter/spring, about 3% at NorthernHemisphere mid-latitudes in summer/fall, and about 5% atSouthern Hemisphere mid-latitudes year-round. The corre-sponding increases in erythemal UV radiation are estimatedto be 130, 22, 7, 4, and 6%, respectively. No significant ozonetrend has been found in Equatorial regions. The geographicalextent and severity of the Antarctic ozone hole have remainedessentially unchanged since the early 1990s. Relatively littlechange in the mid-latitude ozone losses has been observed inthe last half-decade.

High levels of UV-B radiation have been observed directlyin association with the Antarctic ozone hole [72,96–98], andon occasion the measured DNA-damaging radiation at Pal-mer Station, Antarctica (648S) has been found to exceedmaximum summer values at San Diego, USA (328N) [97].It should be noted that monitoring of UV-B irradiances inAntarctica began only in 1989, well after the appearance ofthe ozone hole, so that the UV-B levels in pre-ozone-holeyears can be only estimated.

The smaller increases of UV-B radiation at mid-latitudes,while expected, have not yet been detected unambiguously.The record of mid-latitude UV-B measurements is not suffi-cient for the derivation of statistically significant trends.Littleor no reliable historical information on the climatology ofUV radiation is available from pre-ozone depletion days(e.g., pre-1980). The few available long-term UV measure-ment records have been hampered by the difficulty in main-taining stability of UV-measuring outdoor instruments overperiods of decades, and by changes in atmospheric turbidity



associated with local pollution. For example, measurementsobtained with Robertson–Berger meters over the period1974–1985 suggested a decrease in UV radiation at 14 USAlocations [99]; however, a recent re-analysis of these datahas identified calibration shifts which, when removed, indi-cate that no significant trend can be derived from the datarecord [100]. Furthermore, increases in UV due to strato-spheric ozone reductions may have been masked in someurban areas experiencing increasing levels of local air pol-lution (e.g., [101]). Pronounced ozone losses have occurredfor shorter periods of time, e.g., in the few years after the1991 eruption of Mount Pinatubo (Philippines) [102] andover the Arctic during six of the past nine winters [103–105],with correspondingly higher measured UV-B radiation levels(e.g., [13,80]).

Tropospheric ozone is also an effective absorber of UV-Bradiation [106]. In urban and industrialized regions, tropo-spheric ozone is formed by the photochemical reactions ofsome pollutants (nitrogen oxides and hydrocarbons), whilein remote regions it stems from both downward transportfrom the stratosphere, and from in situ photochemical pro-duction by both natural and anthropogenic precursor com-pounds transported from source regions [4]. Model-basedestimates suggest that for industrialized regions of theNorthern Hemisphere, the increases in tropospheric ozonesince pre-industrial times may have reduced DNA-damagingUV radiation by 3–15% [106–110]. Comparisons betweenspectral UV measurements in Germany and New Zealandalso suggest that the lower UV radiation levels observed inGermany may be explained partly by higher troposphericozone levels [111]. Recent changes in tropospheric ozoneare estimated to be much smaller than those since pre-indus-trial days, with both positive and negative trends reported fordifferent geographic locations [4,5]. Their contributions tothe trend in the total ozone column are much smaller thanthose from changes in stratospheric ozone over the same timeperiod (e.g., 1980 to present). Other gases such as sulfurdioxide (SO2) and nitrogen dioxide (NO2) can also reduceatmospheric UV transmission; however, significant effectsare limited to some extremely polluted urban environments.

3.2. Cloud-related UV radiation changes

Clouds generally reduce surface UV irradiances, althoughthe magnitude of this effect is highly variable depending oncloud amount and coverage, cloud cell morphology, particlesize distributions and phase (water droplets and ice crystals),and possible in-cloud absorbers (especially troposphericozone). It is useful to note that under some conditions, UVirradiances can be higher than for clear sky, as for examplewhen both direct sunlight and light scattered by clouds (e.g.,the sides of bright broken clouds) reach the observer[112,113].

Numerous statistical correlations between UV transmis-sion and cloud cover have been carried out (e.g., [87,114–121]), but because of the high spatial and temporal variabil-

ities of clouds, no single value can be given for their effectson surface UV levels. For example, analysis of the Robert-son–Berger meter data record shows that monthly averageUV levels are reduced by 10–50%, depending on season andlocation in the USA [108,122]. An important aspect ofclouds is that, by introducing strong variability in the UVintensities reaching the Earth’s surface, they complicate thedetection of long-term trends [123–125].

Cloud transmission depends somewhat on wavelength. Inthe UV-A region, transmission increases slightly towardshorter wavelengths due to increased multiple reflectionsbetween cloud and the surrounding air molecules[112,126,127]. At shorter wavelengths, in the UV-B range,long photon path lengths in clouds can increase absorptionby tropospheric ozone, resulting in a sharp decrease in effec-tive transmission [128,129].

3.3. Aerosol-related UV radiation changes

Small particles suspended in air (aerosols) can have asignificant effect on the transmission of UV-B radiation tothe Earth’s surface. The magnitude of the effect is highlyvariable, depending on the number of particles and their phys-ical and chemical make-up (e.g., sulfate haze, soot, dust, sea-salt aerosols). Such particles are frequently found in thelowest part of the troposphere (the boundary layer), and areoften associated with pollution.

Liu et al. [130] estimated that anthropogenic sulfateaerosols (associated primarily with fossil fuel combustion)have decreased surface UV-B irradiances by 5–18% in indus-trialized regions of the Northern Hemisphere. Additional evi-dence for the role of aerosols comes from simultaneousmonitoring of UV irradiances and atmospheric turbidity inrelatively polluted environments [101,119,131,132], fromdifferences between locations in the Northern (more pol-luted) and Southern (less polluted) hemispheres [72,111],and from the increases in UV irradiances with increasingsurface elevation, in excess of those expected from pollution-free conditions [95,133–135]. The measured effects on UVradiation are highly variable and specific to the various loca-tions (e.g., [136]).

An important consideration is whether the aerosol particlesare highly absorbing (e.g., soot) or simply scatter (re-direct)the incident radiation (e.g., sulfate aerosols). All particlestend to reduce the UV irradiance (defined as the radiationincident on a horizontal surface). However, scattering bynon-absorbing aerosols can actually increase the UV expo-sure on non-horizontal surfaces due to the additional radiationincident from low angles [135,137,138]. The net effects onbiota from such changes in direction of incidence are not wellunderstood.

Stratospheric aerosols are usually too sparse to have anyeffect on atmospheric UV transmission. An exception arisesfollowing a major volcanic eruption, such as that of MountPinatubo in June 1991, which injected large amounts of ashand sulfur dioxide (SO2) into the stratosphere. The heavier



Fig. 3. Changes in daily surface spectral irradiances at 310 nm, computed for cloud-free conditions from satellite-based ozone observations (TOMS, version7, [148], monthly averages over different latitude bands indicated in each panel). Values given are deviations from the 1979–1993 means. Solid curves giveabsolute deviations (left scale), while dotted curves show percentage changes (right scale). Note change of scales for different latitude bands.

ash sedimented out of the stratosphere relatively quickly andits optical effects were of limited geographical extent. Gase-ous SO2, on the other hand, was removed from the strato-sphere mainly by chemical reactions to form H2SO4

molecules, which then readily nucleated into sulfate aerosolparticles. Higher stratospheric sulfate aerosol loadings wereobserved for several years after the eruption, during whichtime these particles were distributed on global scales. Cal-culations indicate that the effects on biologically weightedUV irradiances were quite small, of the order of a few percent[107,139,140], with even some possible enhancements atvery short wavelengths and low sun when aerosols scattersome photons directly downward, thus allowing a shortercrossing of the stratospheric ozone layer [141,142]. Ground-based measurements of UV irradiance after the Mount Pina-tubo eruption confirm the small decreases and also show astrong increase in diffuse/direct radiation at all wavelengths,in good agreement with theoretical models [143–145]. Aless direct but more important UV-related consequence ofstratospheric aerosols is their effect on stratospheric ozone

itself. Significant destruction of stratospheric ozone by het-erogeneous chemical processes involving the aerosols waspredicted [146,147] and observed for several years after theMount Pinatubo eruption [4,102].

4. Model-derived surface UV radiation

In view of the high spatial and temporal variability ofsurface UV radiation, and the difficulty of maintaining cali-bration within networks of instruments, it is unlikely thateither a satisfactory global UV climatology or representativelong-term UV trends can be derived from ground-based mon-itoring stations alone. Satellite-based observations of theatmosphere, on the other hand, provide the spatial (global ornearly global) coverage required for climatology develop-ment, as well as nearly continuous long-term monitoring. Forexample, the development of a climatology of UV radiationincident on the oceans will necessarily be based on suchsatellite-derived estimates. However, the derivation of sur-



Fig. 4. Satellite-derived erythemal spectral irradiance at the Earth’s surface (a) for January 1992, (b) for July 1992. From Ref. [155].



Table 2Trends in biologically active radiation (weighted with the erythemal actionspectrum of McKinlay and Diffey [6] derived from total ozone and cloudreflectivity measurements from the Total Ozone Mapping Spectrometer(TOMS, version 7) over 1979–1992. Adapted from Herman et al. [150]

Latitude band Trend a Uncertainty b

degrees % decadey1 "2s

608S–708S 4.5 5.5608S–508S 5.5 4.5508S–408S 3.5 3.5408S–308S 1.5 2.5308S–208S 2.0 2.5208S–108S 1.5 2.5108S–08 1.5 3.008–108N 1.0 3.5108N–208N 0.0 2.5208N–308N 1.0 3.0308N–408N 3.0 3.0408N–508N 3.5 3.0508N–608N 5.0 4.0608N–708N 4.0 4.0

a Zonally averaged trend over given latitude band, values rounded to nearesthalf percent.b As corrected by Herman et al. [155] and includes combined instrumentalerror and variability of UV radiances.

face UV irradiance from satellite-based observations isindirect, because satellite instruments see radiation reflectedby the atmosphere and surface of the Earth. The determinationof radiation transmitted to the surface requires the use ofradiative transfer models to relate transmission, reflection,and atmospheric absorption.

Fig. 3 shows the changes in UV radiation (at 310 nm)reaching the surface, computed for clear skies using satellite-based ozone measurements between 1979 and 1993. Asexpected from ozone trends [5], UV trends are not significantin the tropics, but increase pole-ward in both hemispheres.The largest changes (percentage and absolute) are seen inthe Southern Hemisphere polar regions, but significant inter-annual and shorter variability should be noted at all latitudes,even after considering monthly and zonal averages. Patternsof long-term changes also differ between hemispheres, e.g.,with largest changes occurring in the early 1980s at southernmid-latitudes, while northern mid-latitudes show a more per-sistent long-term trend.

Significant progress has been made in recent years in util-izing satellite-based measurements of cloud cover as well asatmospheric ozone, to derive estimates of surface UV radia-tion levels [149–151]. Recent work also suggests that it maybe possible to derive tropospheric aerosol distributions fromsatellite-based observations [152–154]. Fig. 4 shows thetype of coverage and geographical detail currently possiblewith the satellite-based approach. Long-term trends in cloudcover have partly offset or augmented UV trends resultingfrom ozone changes in some regions, but have been shownby Herman et al. [150] to have little effect on long-termchanges when averaged over large geographical scales(zonalmeans). This type of analysis represents a considerableimprovement over earlier analyses of satellite data that con-sidered only ozone changes, with no consideration of clouds(e.g., [156]). Table 2 shows the trends in surface UV radi-ation (erythemal weighting) over 1979–1992, derived frommeasurements of ozone and cloud reflectivity by the TotalOzone Mapping Spectrometer (TOMS, version 7) aboardthe Nimbus 7 satellite. Positive trends are statistically signif-icant at the two-standard-deviation level over much of themid-latitudes of both hemispheres. Extension to more recentyears is complicated by the use of different instrumentsaboard different satellites, and analysis is still underway [5].

The limitations of these satellite-derived surface UV esti-mates should be recognized. The ozone and cloud determi-nations at any specific location are based on a single satelliteoverpass per day, and are estimated for other times by inter-polation or, more simply, by assuming constancy over thespecific day. Therefore it is essential that comparisons bemade between ground-based UV monitoring and the satellite-derived UV levels, in order to have a more complete assess-ment of the uncertainties inherent in this method. Preliminaryresults of such comparisons are encouraging (e.g., [149]),but more ground-based validation is needed over longer per-iods of time and different geographical locations. Even so,comparisons to ground-based UV observations will not be

able to account fully for some location-specific biases. Forexample, optical instruments borne by satellites have diffi-culty seeing the lower atmosphere (especially in the presenceof clouds) so local conditions (e.g., pollution) are not sam-pled accurately. Additional local factors, such as surfacereflections and elevation gradients, are also problematic.Other promising approaches combine, as above, satellite datawith radiative transfer calculations, but also include someground-based observations by other instruments such as vis-ible and total solar radiation detectors which are moreaccurate and much more widely deployed than UV detectors[157,158].

5. Future UV radiation levels

The prediction of future UV radiation levels must be con-sidered according to the time scales of interest. On short timescales, of the order of a few days or a week, UV radiationforecasts incur all of the difficulties of forecasting weather(especially clouds); of estimating atmospheric profiles ofozone and other gases and particles, some anthropogenic; andof accounting for a variety of other possible local factorsincluding surfaces (elevation, orientation, reflectivity).These factors make accurate UV forecasts impracticalbeyonda few days. Next-day forecasts based on meteorological anal-yses are now being made with some success in a number ofcountries. In most cases, the results are disseminated to thepublic, with UV radiation levels expressed as a dimensionlessUV index. International standardization was reached[159,160] on the method of calculation of the index, which



Fig. 5. Scenario for future changes in ozone and erythemally weighted UV radiation at the Earth’s surface, at 458N and 458S. UV radiation changes are estimatedfrom ozone changes, which in turn are estimated from changes in atmospheric amounts of ozone-destroying substances (halocarbons). All other factors areassumed constant. Future scenarios shown are based on current control measures (Montreal 1997 Amendments), with scenario A1 (baseline, solid curves)accounting for the fact that production of some ozone-depleting substances is currently already below the allowed maximum, while in scenario A3 (dashedcurves) production is at the maximum allowed level. Dotted curves are the zero-emission limit (starting in the year 2000) and only illustrate the minimumdelay time imposed by atmospheric processes (from Ref. [162]).

is defined as the UV irradiance, in units of W my2, weightedby the erythemal action spectrum of McKinlay and Diffey[6], then multiplied by 40. Using this scale, a UV index of10 or more may be considered ‘extreme’.

Long-term UV predictions (years, decades, or longer) areexceedingly difficult and uncertain, and therefore only appro-priate in a statistical sense of averages, variabilities, andbroadgeographical patterns. Even then, many assumptions must bemade not only about the future state of the ozone layer, butalso about possible long-term changes in clouds, troposphericpollutants, and changes in surface albedo. In consideringfuture biological effects of UV changes, it is also necessaryto allow for uncertain long-term changes in ecosystem sizeand composition and — specifically for humans — changesin behavior, migration, and demographics.

Predictions of future ozone amounts are in themselves alsovery difficult. Natural perturbations such as major volcaniceruptions are unpredictable, though their importance to strat-

ospheric ozone was clearly demonstrated in the aftermath ofthe 1991 Mount Pinatubo eruption. Large uncertainties existconcerning the interactions of stratospheric chemistry withexpanding human activities, e.g., the increasing emissions ofso-called greenhouse gases and the associated changes inglobal climate, the effluents from growing fleets of subsonicand supersonic aircraft, and the changes in tropospheric airquality and self-cleaning (oxidizing) capacity. Their inter-actions with stratospheric ozone are current subjects of activeresearch and are still not well quantified [5]. A recent study,for example, suggests that the recovery of the ozone layermay be delayed significantly by interactions with increasinggreenhouse-gas concentrations [161].

With a clear understanding of these uncertainties, it isnevertheless of interest to examine the implications of currentinternational regulations to the future of the ozone layer, andconsequently to the future of UV radiation. The 1987 Mon-treal Protocol and its subsequent Adjustments and Amend-



ments limit the production and emission of ozone-destroyingsubstances, primarily halocarbons. The atmospheric concen-trations of these chemicals had been increasing throughoutthe 1970s and 1980s, but observations in the last few years(e.g., [163]) show a marked slowing of growth and evendecreases in many of these compounds as a result of imple-mentation of the Protocol [5]. Fig. 5 shows the temporalchange of ozone and surface UV radiation (at 458N and 458S)computed in correspondence to the halocarbon loading of theatmosphere. This calculation assumes that changes in UVradiation are due solely to ozone changes, which in turn areassumed to respond only to atmospheric halocarbon loading.The quantitative relation between ozone and halocarbonchanges is based on the measured changes in both quantitiesthrough the 1980s [164]. The future scenarios shown in theFigure are based on current control measures (Montreal 1997Amendments), with scenario A1 accounting for the fact thatproduction of some ozone-depleting substances is currentlyalready below the allowed maximum, while under scenarioA3 production is at the maximum allowed level. In eithercase the UV radiation is expected to return to normal (pre-1980) levels by the middle of the next century. Scenario A2shows the ozone/UV recovery if there is no emission afterthe year 2000; while this scenario is obviously unrealistic, itillustrates the natural time scale for the removal of the halo-carbons already present in the atmosphere, and is therefore afundamental limit to the rate of recovery.

Given the numerous uncertainties listed above, it isunlikely that future UV radiation changes will follow pre-cisely any scenario presented in Fig. 5. Two features of thisFigure are nonetheless noteworthy. First, the return to pre-ozone depletion levels will take several decades even underthe most optimistic scenarios of compliance with interna-tional regulations of ozone-depleting substances. Secondly,and perhaps more important, is to note that in the presenthalf-decade (1995–2000) ozone reductions are the largestsince ozone observations began. The observed slowing andeven turnover of the rate of growth of some atmospherichalocarbons is highly significant, but large uncertainties,stemming from both future human activities and the imperfectunderstanding of the complexity of the atmosphere, leaveopen the question of the extent and timing of the return tonatural levels of stratospheric ozone and surface UVradiation.

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Changes in biologically active ultraviolet radiation reaching