JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D20, PAGES 26,107-26,113,OCTOBER 27, 1998
Changes in ultraviolet radiation due to stratospheric and tropospheric ozone changes since preindustrial times
All A. Sabziparva. r,1 PiersM. de F. Forester and Keith P. Shine Department of Meteorology, University of Reading, Reading, England
Abstract. This paper estimates global changesin ultraviolet radiation since preindustrial times, resulting from anthropogenicstratospheric and tropospheric ozone changes. A sophisticatedradiative transfer schemei,semployed to calculate changesin the diurnally integrated UVB (defined here as 280-320 nm) and
erythemallyweighted(EER) irradiancereachingthe Earth's surface.Strato,spheric ozone changesare obtained from combining total ozone mapping spectrometer satellite data with Dobson ground-basedinstrument data. Tropospheric ozone changesare estimated from the University of Oslo three-dilnensiona,1chemical transport model. These ozone trend data are combined in three ways to calculate the associated ultraviolet trend and estimate its uncertainty. We show that in low
latitudesa significant(up to 5% for UVB and 9% for EER) decreasein surfaceUV may have occurred, due to tropospheric ozone increases. Large UV increasesare
found at high latitudes,with up to 60% increasesin October and 20% increasesin April. The possibleeffectsof soot and sulphate aerosolchangeson the UV are also briefly examined; increasedaerosol amounts may have decreasedsurface UVB and
EER by tip to 7% locally and about 2% on a global average.
1.
broadband
Introduction
Stratosphericozonedepletionis expectedto increase levelsof l•fV radiationreachingthe Earth's surface[see
UV
decreases have been observed in the
United States [Ju,•tusa•d Mu•'ph,q,1994;Scottoct al., In the absenceof global coverageof measured ultra-
e.g. H/orhlMcteorologt, calO•u,zzation (WMO,), 1994]. violet trends, radiative transfer schemescan be used to
There are relatively good long-term global measurecalculate trends in UV resulting from measuredor esmentsof stratospheri('ozonedepletionbut very fexvrelitimated changes to the input parameters. Br(ihl and ablelong-termmeasurements of the subsequent. changes ("rutzc• [1989]estimatedUV trendsfrom both stratoin ultravioletradiation [WMO, 1994]. Althoughthe spheric and tropospheric ozone •'hangesbetween 1966 quality and quantity of UV measurementshave in- and 1986 from a two dimensional chemical model. Their creasedgreatly in recentyears [e.g. •S'eckme,l/c•' et ul., model contained no heterogeneouschemistry, so under19951,long time seriesmeasure•nents of UV only exestimated lower stratospheric ozone depletion. Madist at, a small number of locations; these few locations ron•ch[1992]estimatedUV increasesfrom total ozone are unableto give a globalpictureof UV trends. Also, mapping spectrometer (TOMS) ozone depletion
owingto the complexityof the instrumentation,highresolutionUV spectrometersare notoriouslydifficult to / Wibll Ldol•lUl calibrate and keeps•:,aole. • '' It is c)111• ---'..... "' ...... c..• 111ablu:.•,.... merit calibration and checking that measurementsat
between 1979 and 1989. However, these were for clear sky and did not account for the variation of ozone pro-
file with latitude (althoughthe ozonecolumnchanged); this study also reported exploratory calculations of the
different, geographical locationscanbe compared[•Ceckeffect of changesin tropospheric ozone, concludingthat me!let et al., 1995]. High-latitude and somemidlatthey were too sinall to offset the effect of stra,tospheric itude sites have detected UV increases [Madronich, changes. 1992;McKenzieet ,l., 1991;Ix'errand McElr'oy,1993]; 1Now at College of Agriculture, University of Bou-Ali Sina, Hamadan, IslamicRepublicof Iran Copyright1998 by the AmericanGeophysicalUnion. Paper number 98JD02277.
0148-0227/ 98/ 98JD-02277509.00
In this paper we use trends in total column ozone since 1964, derived from both ground-basedand satellite instruments, and an estimate of tropospheric ozone increases since 1850, from a, three-dimensional chemical transport model, as inputs to a radiative transfer scheme to calculate
the associated
trend
in UV
irradi-
antes due to changesin atmospheric composition as a result of anthropogenic activity. 26,107
26,108 2.
Data
SABZIPARVAR ET AL.: CHANGES IN UV RADIATION sets and
Method
Using the discrete-ordinate radiative transfer model
with four streams [Forster, 1995], ultraviolet irradiantes a,t the surface were calculated
at 1 nm resolution.
This model takes inputs of the ozone and temperature profile in the atmosphere, as well as the solar zenith angle, surface albedo, aerosol, and cloud information. To obtain a baseline for the UV trend analysis, values of UVB were calculated using climatological val-
1964a,nd 1979 [from WHO, 1994]and addingto these the 1979-1994TOMS version7 trends [HcPctcrs et al., 1996]at 50 latitudinalresolution.The Dobsoninstrumentchangescontainno seasonalinformation,so the annual averagetrend is taken for eachsea, son. At latitudeshigherthan 600 N and 550 S, whereno Dobson trendsare given,the highestlatitude data,point is used. This approximationmay slightly underestimate the ozonedepletion at high latitudes; however,as most of the stratosphericozonedepletion has occurredsince
ues of the model input parameters [A. A. Sabzipar- 1979, its effect will be small. If it is assumedthat there ha.sbeen little or no long-term trend in stra,tospheric
va.r et al. A model-derived climatology of ultraviolet irradiance at the Earth's surface, submitted to Pho-
ozone due to human activity between 1850 and 1964,
tochemistryand Photobiology,1998]. The climatolo- combiningthesetrends will give the total trend since isjustifiedas (1) thereis gical ozone concentrations were based on a combina- 1850. We feel this assumption tion of ozonesonde
and satellite
solar backscattered
ul-
traviolet instrument data; cloud and albedo data were derived from the International
Satellite
Cloud Climato-
no overall in trend in total column ozone from the early
part of the Dobsontotal columnrecord [WMO, 1994] and (2) the probablecausesof anthropogenic strato-
logy Project; and the temperature profiles were based on European Centre for Medium-Range Weather Forecasts analyses. The ozone profiles, used as inputs to the radiative transfer scheme,were modified according to our estimate of the past ozone changes, and subsequentalterations in the UV reachingthe surfacewere
spheric ozone depletion, particularly emissionsof chlorofiorocarbons and halons, were not significant before
then
in the total columnozonetrends [WHO, 1994]. In this
calculated.
1. Tropospheric ozone. Because of increased industrialization and increased biomass burning tropospheric ozone has been increasing since preindustrial times. Although there are a few ground-basedmeasurements of troposphericozone dating back to the last century, tropospheric ozone changescan be quite localized, making it difficult to infer global trends from
a few local measurements [WMO, 1994] or to infer changesin free troposphericozone. Hence chemical transport models have often been employedto estimate the global distribution of troposphericozone changes. This paper uses 1850-1994seasonalanthropogenictroposphericozonechangescalculatedwith the Oslo University three-dimensional chemical transport model [Berntse• ½tal., 1997];at, the time of writing, there have been no publishedstudiesshowingthe evolution of this ozonechangesincepreindustrialtimes. The reader is referred to Berntsen ½t al. [1997]for details of the chemical transport model and evaluation of the
1960.
3. Total ozone trends. Becauseof problems of satellite
ozone
retrieval
beneath
clouds
it is unclear
how
much of any tropospheric ozone changesare recorded paper, three total column ozone change scenariosare used to bracket possibleextremes. 4. STRAT. This a,ssumesthere has been no changein the troposphericozoneand that the stratosphericozone has been depleted by the combined TOMS-Dobson trend
since 1964.
5. STR, AT+TROP. This assumes that the TOMS and Dobson instruments have not recorded any tropo-
spheric ozone increases,so the total column ozonedepletion in STRAT is reduced by the increasesin troposphericozone, since 1850. This assumesthat the total column ozone trends contain no troposphericcomponent, which is more plausiblefor the satellite-derived trends than for those from the Dobson network.
6. TOMS
STRATMOD+TROP.
This assumes that
the
and Dobson trends have recorded the "true"
total column ozone trend and that this trend is made up of a positive pa,rt, due to troposphericozone increases,
and a negativepart, dueto stratospheric depletion.The troposphericchangesare taken since1850, and the stratosphericdepletion is calculatedfrom a residualof the similar confidence limits to ozonesonde trends fi'om LoTOMS-Dobson total column changesand the tropogan [1994],bearingin mind the ozonesonde trendsare spheric increases;this assumesthat all the changein troposphericozone occurred since 1964 and that the from a shortertime period (1970-1991). 2. Stratospheric ozone. Halocarbon-inducedstra- total column ozone trends include all of this tropotospheric ozone depletion is larger and has occurred sphericsignal. This scenariohas the sametotal column more recently than troposphericozoneincreases;it has changeas scenarioSTRAT, but the stratosphereis more been measured by ground-basedinstrument networks heavily depleted. It is expectedthat the TOMS instrumentchangehas and severalsatellite instruments. The zonally averaged total column ozone trends from different instruments not detected all of the troposphericozone changeover modeled ozone trends. The ozone trends are usually within 30% of other model predictions,and agreewithin
agreeto within a few percentof eachother [WMO, the lastfewdecades[McPeters½tal., 1996];also,tropowellbefore1964 [WMO, 1994]. In this paper, seasonalozonetrends are pro- sphericozonebeganincreasing ducedby taking the Dobsoninstrumenttrendsbetween 1994]. This would make the actual ozonechangeand
SABZIPARVAR
ET AL.-
CHANGES
IN UV
RADIATION
26,109
,
1850-1994
Ozone change (%)
January
......
April
i.•x .....
• •';•'"'• x.......
-5
-10
-
.
_
-15
-50
0
50
-50
Latitude(degrees)
July
.......
0
50
Latitude(degrees)
October
/..,,,...
o --•0
-5 -20 -10
-30
-40
-15
-50
0
50
-50
Latitude(degrees)
0
50
Latitude(degrees)
Figure 1. The percentagechangein total columnozonesincepreindustrialtimes as a function
of latitudefor the $TRAT (solidline) and the STRAT+TROP (dashedline) scenarios.The $TRATMOD+ TROP scenariohas identical tota,1column ozone changesto the $TRAT scenario. The four mid-seasonmonths are plotted.
henceUV changelikely to lie somewherein betweenthe
from zero [McPeters et al., 1996] neither are the UV
STRAT+TROP
trends. In January and April, there are between 10 and
and STRATMOD+TROP
scenarios.
Figure 1 showsthese total columnozonechangesas 20% increasesin UVB at high latitudes. The northern a function
of latitude
for the four seasons.
Both
scen-
hemisphere in .July and October shows UVB increases
arioshave large ozonedepletions(up to 34%) a,t high
that are smaller than 10%.
latitudes and smaller changesat low latitudes, with the STRAT+TROP scenario giving low latitude total column ozone increasesof up to 5%.
2. STRAT+TROP. For all seasons, this scenario shows a less positive UV trend than STRAT, and over large parts of the Earth the total [•VB trend is negative.
a.
The largestreductionsreaching5% are at low la,t.itudes and high northern latitudes in July and at low latitudes
Results
in April.
3. STRATMOD+TROP. This scenario has the same Figure 2 showsthe decreases in the daily integrated total column ozone depletion as STRAT, however, the UVB (definedherea,s280-320nm) associatedwith the Berntse• et al. [1997]tropospheric ozoneincreases since depletion of ozone in the stratosphere is greater than 1880. The maximum
UVB
reduction
is seen over Africa
in October(8%); this is due to a,combinationof biomass burning creating large ozone increasesand low
STRAT.
This
redistribution
of ozone
in the
vertical
column will usually lead to a reduction of UVB, except for large solar zenith angles near the terminator
of the solar zenith angles. Reductionsof almost 8% are also (seeBr•.'hland Crutzen [1989]for a discussion seen in the northern midlatitudes in July. There are increasedefficiency of tropospheric ozone in absorbing generally larger reductions in UVB over the land than UV). The UVB trend has generallybeen reducedby the ocean. Antarctica shows reductions of less than 2% 1-2ø70 comparedto STRAT. In the tropics these results are consistentwith Br•'hl
for all seasons.
and Crutzen [1989] who found decreasesin UVB of
Figure 3 gives the expected latitudinal variation of UVB dose changesfor the four seasonsusing the three separate ozone change scenarios.The UV trends from
a few percent. However, due to their simple stratospheric chemistry scheme and shorter time period of
the three
ozonechange(1966-1986)they founda maximumUVB
scenarios
are discussed below.
1. STRAT. This scenarioshowsUVB increasesnearly everywhere, with a 88% increase in the UVB dose at 85øS in October. In the tropics, there is only a very slightly positive UVB trend, but since the total ozone
increaseof only 10% at high latitudes, comparedto our 58%.
The hemispheric differencesare most pronounced in July and October, with the northern hemisphere showchangesin this region are not statistically significant ing very little trend and the southernhemisphereshow-
SABZIPARVAR
26,110
Mox
Mox
=
=
(Januory)
-0,680 Min Contour--0.50
.-0•9 Contour
5
-'
in = =
ET AL.'
CHANGES
--6,184
-7•882
!,00
IN UV RADIATION
Max
Mox
(iA•ri!! Min =
-=..... 0.5 :Contour
=
---8.15!
1.00
(October)
.......... 1.040 C'ontour
Min = 1 O0
Figure 2. The percentagechangein daily integratedsurfaceUVB irradiance, as a, result of increasesin troposphericozonesincepreindusi. rial times [from B(:rntscnet al., 1997]. Hesults are for the four mid-seasonmonths, calculationsare performed at 50 resolution. Th•• climatology contains cloud amount and optical depth a,t three levels, and a continental aerosol of optical depth 0.1 is placedin the model boundarylayer. Contour intervalsare differentfor eachframe.
ing large increasesin UVB. In January and April, there
trend, except in Od,ober at high latitudes where in-
is a much smaller difference between the hemispheres, creasesin EER of up to 68% are seen. For the STRAT
with both hemispheresshowingincreasesin UV. This is due to a com'bination of smaller tropospheric ozone increasesand a larger stratosphericozonedepletionin the southern hemisphere;aerosolchanges,discussedin section4, may also add to this interhemisphericdifference.
Figure4 showsthe erythemallyweighted(EER) UV changesas a functionof latitude for the three ozonedepletion scenarios. The EER action spectrum is taken from Mctfinlo.qand Diffe.q[1987].In general,the EER trend is more negative than the correspondingUVB
+TROP scenario, decreasesin EER of about 5% are seenover most of the northern hemispherein July, and decreasesof up to 9% are seena.t low latitudes in April.
Compared to Ma.dro•,ch [1992], our erythema trends were 5-10% more negativeat. low latitudesand 2-5% more negativeat high latitudes. This wasfound to be due to the increasesof tropospheric ozone and the inclusion of clouds in our model; t.he UV trends of Mad-
ro•.ich[1992]werereproducible to within 1% if troposphericozoneincreasesand cloudswere excludedfrom our model calculations.
When the ozone percentage
SABZIPARVAR
ET AL.' CHANGES
1850-1994
IN UV RADIATION
26,111
UVB change (%)
January
April
2O
.....
-5
'
,
- 50
0
50
o
-50
5O
Latitude(degrees)
Latitude (degrees)
October
July 25
60
'
2O 4O
15
10
2O
5 0
_•
, ,.-,',•-. 0
-5o
-•.• -:o
50
-50
Latitude (degrees)
0
50
Latitude (degrees)
Figure 3. Changes in the dailyintegrated surface UVB irradiance sincepreindustrial timesas a fimctionof latitudefor the STRAT (solidline), STRATMOD+TROP(dottedline) andthe STRAT-/-TROP (dashed line)scenarios. Thefourmid-season monthsareplotted.
changes wereconstant withaltitudesimilarradiation ively. Whereas in this study, for the STRAT+MOD scenario, we find a RAF of 0.5 for the UVB and an amplification factors (RAFs)werefoundto Mad•'onich ctal. [1995]. However, in thiswork,ozone isbeing de- RAF of 0.7 for the Erythema. These RAFs are smaller pleted above thetropopause andincreased below. This than expected because a change in tropospheric ozone will usually have a larger effect on the UV than a, sim-
leads to different RAFs. For example, in January at
midlatitudes, Madron, ichet al. [1995]foundR,AFsof ilar changein stratosphericozone [Brfi'hland C•'utzen, 0.87 and 1.1 for the UVB and Erythema dose,respect- 1989]; thus the UV increasesfrom combinedstrato-
1850-1994
EER change (7o)
January
April
-lO
-
-50
0
50
-50
Latitude (degrees)
0
50
Latitude (degrees)
15 '' iJuly80 October 10
60
5
,,
40
ß
0............... ....... t -lO
-50
0
'
Latitude (degrees)
-2Ol
50
,
-50
0
,
50
Latitude (degrees)
Figure 4. Changes in thedailyintegrated surface EER irradiance sincepreindustrial timesas a functionof latitudefor the STRAT (solidline), STRATMOD-/-TR, OP (dottedline) and the STRAT-/-TROP (dashed line)scenarios. Thefourmid-season monthsareplotted.
26,112
SABZIPARVAR ET AL.: CHANGES IN UV RADIATION
Table 1. Changes in Surface UVB and EER From Aerosol Increases Experiment
January AOD
UVB
July EER
AOD
UVB
Annual Average EER
IPCC Soot. Dust
Sulphate Biomass Total
Langner and Rodhe Northern Hemisphere Southern Hemisphere Extremes
Cooke
+0.03 +0.004 +0.144
-0.26 -0.05 -1.40
-0.28 -0.05 -1..50
+0.02 +0.006 +0.126
-0.11 -0.05 -0.72
-0.12 -0.05 -0.80
+0.019
-6.6
-6.2
+0.011
-3.6
-3.4
AOD
UVB
EER
+0.003 +0.004 +0.019 +0.017 +0.043
-1.10 -0.60 -0.14 -0.13 -1.97
-1.00 -0.55 -0.15 -0.14 -1.84
+0.023 +0.004
-0.17 -0.05
-0.19 -0.05
and Wilson
Extremes
Aerosolchangesare given in units of optical depth at 550 nm (AOD); UVB and EER changesare in percent. The aerosolchangescomefrom IPCC [1995](severalaerosoltypes),Langn½',' and Rodhe[1991](sulphateaerosols),and Cooke and Wilson[1996](soot.aerosols).Sulphateandbiomass burningaerosols are assumed to be ammoniumsulphateat 70% relative humidity. Soot,aerosolsm'efrom fossilfuel combustionand biomassburning. For the radiative transfer calculation, refractiveindicesare taken from World ClimateRe.s•',rchProgram[1986]. The IPCC changesare for the globaland annual
average;the Lungherand Rodhe sulphatedata are calculationsusing a chemicaltransport model with 10ø resolution; seasonaland hemisphericaveragesas well as extreme valuesare shown. Extreme valuesare the changesin UVB and EER found in January and July at the geographicallocation of maximum aerosolchange.
sphereand troposphericchangesare not aslargeasone approximations in the calculation of the UV climatomight expect. The differencebetweenthe changesin UVB and EER
logy were found to have a much smaller effect on the UV irradiancesthan the uncertainty in the ozonetrends
1997]. result from the shape of the EER normalized action themselves[•q'abziparvar, spectrum,which is 1 at wavelengthslessthan 300 nm and decreaseswith wavelengthbut remainssignificant 4. Discussion at wavelengthsgreater than 320 nm. At low latitudes,whereozoneamountsare generallylowest,andthe Owing to uncertainties in the ozone trends a.t latitpathlengthis shortest,the EER doseis dominatedby udes lower than 50øN most of the UVB trends calcuradiation at UVB wavelengths;if the increaseddoseis lated must be treated with some caution. Nevertheless greatestat the shorterUVB wavelengths, to whichEER these trends give a useful indication of how UVB doses is most sensitive, then the percentagechange in EER exceedsthat of UVB. At high latitudes, with generally higher ozone and longer pathlengths,the EER reaching the surface is dominated by higher wavelengths,which are relatively insensitive to ozone. The uncertainty in the ozone changescausesuncer-
may have changed since preindustrial times. Aerosol, cloud, and surfacealbedo changesover the same time period, •vhetherof anthropogenicor natural origin, will
time period of their sourceand their influenceon the UV examined. It was found that in the tropics two standard
Table 1. Sulphate aerosolincreasescan reduce the UVB
have also affected the actual UV trends.
Using estimates of increasesin severaltypes of aero-
sol [La•g•er andRodhe,1991;Cookeand Wilso•, 1996; tainty in the predictedUV trends. WMO [1994]and I•tcrgovemme•tal Papal o• Climate Cha•ge (IPCC), McPctcrs et al. [1996]both quote standarderrorsin 1995], the effect of increasingaerosolconcentrations their ozone trends. These errors were weighted by the were crudely examined; these results are presentedin
errors correspondto roughly a 1-2% UVB trend. This could rise to as much as 20% at the polesbut generally remained less than 5%. Likewise, the 30% uncertainty in the troposphericozone trends, quoted earlier, leads to a corresponding1-2% trend in the UVB. In this study, ozone changesare split into a tropospheric and stratospheric part with a constant percentage change to the ozone mass mixing ratio in each region;the tropopauseis set at 200 mbar. This and other
by up to 1.4%, in certain places,and soot aerosolcan reducethe UVB by up to 6.6%, although the globally averagedchangesare considerablylessthan 0.3%. From the IPCC [1995]estimatesof changesin severalaerosol types, a possibleglobally averagedreductionof 2.0% in the UVB
was calculated.
Locally, aerosolcould have a much larger effect as the aerosol optical depths can be considerablylarger than those usedhere. For example,Liu et al. [1991] found that aerosolcouldreducethe UVB doseby up to
SABZIPARVAR ET AL.: CHANGES IN UV RADIATION
26,113
ited by J. T. Houghton et al., Cambridge Univ. Press, 20%. Aerosolchangescouldalsoadd to the interhemiNew York, 1995. sphericdifferences in UV trends, actingin the same Justus, C. G., and B. B. Murphy, Temporal trends in surface
•;ay as troposphericozone, to decreasethe U\; more irradiance at ultraviolet wavelengths, J. Geophys. ties., in the northern helnispherethan in the southern hemi99, 1389-1394, 1994. sphere.S½ckmcyer et ctl. [1995]presentevidenceof this Kerr, J. B., and C. T. McElroy, Evidence of large upwards trends in ultraviolct-B radiation linked to ozone depiction, althoughsomeof the interhemispheric differencein aerScience, 260, 311-314, 1993. osol may be due to natural rather than anthropogenic Langncr, J., and H. Rodhc, A global three-dimensional causes. Sabzipavvav [1997]has shownthat surfacealmodel of the tropospheric sulphur cycle, J. Atmos. Chem., bedo and cloud coverchangescould alsoproducesigni13, 225-263, 1991. ticant UV trends. However,the actual changesin clouds Liu S.C., S. A. McKccn, and S. Madronich, Effcct,s of anand surface albedo remain
5.
very uncertain. ß
Conclusions
t,hropogcnic aerosols on biologically active ultraviolet radiation, Gcophys. Res. Lctt., 13, 2265-2268, 1991. Logan, J. A., Trends in the vertical distribution of ozone: An analysis of ozonesondc data, J. Gcophys. Res., 99, 25553-25585,
1994.
This paper has producedestimatesof UVB and EER Madronich, S., Implications of recent total atmospheric trends sincepreindustrialtimesfrom both stratospheric ozone measurements for biologically active radiation reaching the Earth's surface, Gcophys. Res. Lctt.. 19, 34ozonedepletion and troposphericozone increases. At 40, 1992. high latitudes, there are significantincreasesin UVB Madronich, S., R. McKenzie, M. M. Caldwell, and L. and EER of greaterthan 20%, with UVB and EER deO. Bjorn, Changes in ultraviolet radiation reaching the creasesof a few percent at low latitudes. These trend Earth's surface, Ambio., 2•, 143-152, 1995. estimates are uncertain, and trends in other paramet-
McKenzie, R. L., W. A. Matthews, and P. V. Johnson, The relationship between crythcmal UV and ozone, derived from spectral irradiance measurements, Coophys. Rc.,. Lctt., 18, 2269-2272, 1991. authors knowledge,the first estimate of UV changes McKinlay, A. F., and B. L. Diffcy, A reference action specsincepreindustrialtimes. This informationis crucialto trum for ultra-violet induced crythcma in human skin, in those interested in the possibleeffectsof UV (:hanges, Huma, Exposure to Ultraviolet Radiation: Risks (•nd Regluatious, Intcruational Congrcs.•Series, vol. 6, pp. 17-22, whether on the chemistry of the atmosphereor on the Elsevier, New York, 1987. Earth's biota. When better estimates of ozone, aerosol, McPeters, R. D., S. M. Hollandsworth, L. E. Flynn, J. R. and cloudchangesbecomeavailable,it oughtto be able Herman, and C. J. Seftor, Long-term ozone trends derived
ers, such as aerosols,may have modified some of these I•V trends due solelyto ozonechanges.It shows,t.othe
to refine this study still further. The study also points the way to predictingpossibleUV changesin the future which would allow direct comparison to trends measured by [•V instruments;it elnpasizesthat changesin stratosphericozoneare not the solecauseof UV trends. Acknowledgments. Thanks to Terje Berntsenfor the troposphericozone data and Stacey Hollandsworth for sup-
plyingthe Version7 TOMS trends. AAS wouldlike to thank the Iranian Ministry of Culture and Higher Education for their sponsorship.PMF was funded by a NERC Grant. We thank
the reviewers
for useful comments.
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A. A. Sabziparvar, College of Agriculture, University of Bou-Ali Sina, Hamadan, Islamic Rebublic of Iran. P.M. de F. Forster and K. P. Shine, Department of
Meteorology,University of Reading, Whiteknights, Reading RG6 6BB, United Kingdom. (e-mail: p.m.forster@read-
ing.ac.uk;
[email protected])
mate Change1994,RadiativeForcingof Climate(]hange (ReceivedJanuary 12, 1998; revisedJune30, 1998; and an Evaluationof IPCC lS92 EmissionScenqrios, ed- acceptedJuly 7, 1998.)
