Derivation of total ozone abundance and cloud effects from spectral irradiance measurements

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<ul><li><p>Derivation of total ozone abundance and cloud effects fromspectral irradiance measurements</p><p>Knut Stamnes, James Slusser, and Melissa Bowen</p><p>We describe a method to infer total ozone abundance and effective cloud transmission from global (diffuseplus direct) spectral irradiance measurements taken at the Earth's surface. The derivation of total ozoneabundance relies on the comparison of measured irradiance ratios at two wavelengths in the UV part of thespectrum with a synthetic chart of this ratio computed for a variety of ozone abundances. One of thesewavelengths should be appreciably absorbed by ozone (e.g., 305 nm) compared with the other one (e.g., 340nm). This synthetic ratio (and therefore also the inferred total ozone abundance) is insensitive to the value ofthe surface albedo used in the model computations. Comparison with independent in situ and remote (fromground and space) determinations of total ozone abundance shows that measurements of global irradiancesprovide a reliable means of inferring the total column ozone amount for clear as well as cloudy sky conditions.Computer simulations are used to demonstrate that the ozone abundance inferred from global irradiancemeasurements is quite insensitive to cloud effects, whereas the use of the scattered irradiance only or thezenith sky intensity (measured routinely in the Dobson network on overcast days) requires substantialcorrections for cloud effects. Effective cloud transmission is estimated from the data by comparing themeasured irradiance at a wavelength where ozone absorption is minimal (e.g., 350 nm) to the clear-sky value.Irradiances generated by a plane-parallel radiation model as a function of cloud optical thickness are used toestimate an equivalent stratified cloud optical depth. These estimates of cloud transmission and opticaldepth are sensitive to ground reflection, implying that the accurate determination of cloud attenuationrequires precise knowledge of the surface albedo.</p><p>IntroductionSince the discovery of the Antarctic ozone hole' andreports indicating declining levels of ozone abundanceon a global scale,2 3 there has been renewed scientificinterest as well as considerable public concern aboutthe possible impact of ozone depletions on the UVradiation reaching the biosphere. While a long-termglobal data set on total ozone abundance exists fromthe Dobson network dating back more than 30 years,relatively few measurements are available of the bio-logically effective UV radiation reaching the ground.</p><p>The penetration of UV solar radiation through theatmosphere depends not only on solar elevation andozone abundance but also on the following geophysicalparameters: (1) stratospheric temperature; (2) alti-tude distribution of ozone; (3) surface albedo; (4) cloud</p><p>The authors are with the Geophysical Institute and Departmentof Physics, University of Alaska Fairbanks, Fairbanks, Alaska99775-0800.</p><p>Received 11 September 1990.0003-6935/91/304418-09$05.00/0.C 1991 Optical Society of America.</p><p>height, optical depth, and morphology. Besides thetotal ozone content, temporal and spatial variation incloud cover may be the most important parameteraffecting UV penetration, and possible systematicchanges in cloudiness might jeopardize attempts toassess trends in ozone abundance.4 Thus, in view ofthe sparsity of measurements providing a direct linkbetween the UV radiation received by the biosphereand the total ozone abundance, one needs to accountfor possible trends in cloud cover as it affects UVpenetration to assess trends in the UV radiation envi-ronment.</p><p>The purpose of this paper is to document and discussa procedure for deriving total ozone abundance andeffective cloud transmission from global spectral irra-diance measurements. This allows us to determinethe link between the biological UV dose, ozoneamount, and cloud attenuation from spectral measure-ments of the UV radiation calibrated in absolute units.Such measurements are currently emerging,5-8 andthey are urgently needed to establish this missing linkand thereby enable us to monitor the time evolution ofthe UV radiation environment. This time history is ofgreat interest to biologists concerned with the impactof potential ozone depletions on living organisms (see,</p><p>4418 APPLIED OPTICS / Vol. 30, No. 30 / 20 October 1991</p></li><li><p>for example, Ref. 9). Application of the techniquesdocumented here to analyze UV radiation measure-ments in Antarctia is reported elsewhere.8</p><p>Method of AnalysisOur analysis is based on the assumption that observedspectral irradiances in the UV region between say 280and 350 nm are available with appropriate resolution(a few nanometers are adequate for our purposes).Such measurements are now available through theU.S. National Science Foundation UV monitoring pro-gram in Antarctica where scanning spectrometers areused to measure the global downward irradiance with aspectral resolution of about 0.5 nm. Our aim here is todemonstrate that spectral measurements such as thesecan be used to make reliable simultaneous assessmentsof biological UV dose, ozone abundance, and effectivecloud transmission.</p><p>The global irradiance (i.e., the sum of the diffuse anddirect irradiance rather than the individual compo-nents) is required to assess the biological effects of UVradiation. It is easier to measure only the global irra-diance, because the separation into diffuse and directcomponents requires tracking of the sun. Moreover,measurement of the direct component is complicatedbecause it requires a correction for scattered lightcaused by forward scattering of atmospheric particu-late matter into the instrument field of view, which isusually considerably larger than the solar disk. In theDobson method (which relies on direct Sun measure-ments whenever the Sun is not obscured by clouds),this forward-scattering problem is dealt with by utiliz-ing the difference between two wavelength pairs for apartial correction of the effects of particulate matter,including the wavelength dependence of aerosol scat-tering. These considerations provide a strong motiva-tion for using global irradiance measurements to infertotal ozone abundance and cloud attenuation as well asa biological UV dose.</p><p>The simultaneous determination of total ozoneabundance and biologically effective UV radiation ispossible by taking ground-based spectral measure-ments (calibrated in absolute units) of both the diffuseand direct components.5 One may then infer the totalozone amount by utilizing the directly attenuated solarirradiance at two different wavelengths in the UV re-gion, one that is appreciably absorbed by ozone (forexample, 305 nm), the other that is not (for example,340 nm). The Dobson method relies essentially onthis principle.' 0 If, however, only the global irradi-ance is measured (which is the quantity of interest forUV dose determination), the scattered radiation mustbe taken into account. To infer column ozone abun-dance and effective cloud transmission from the globalirradiance measurements, we have utilized a UV radia-tion model that includes multiple scattering andground reflection. Global spectral irradiance mea-surements described elsewhere8 will be used to test andverify the procedures presented here for the inferenceof total ozone abundance. These measurements are</p><p>part of the National Science Foundation UV monitor-ing program in Antarctica.</p><p>UV Radiation ModelThe basic model is described in detail elsewhere."Only a brief outline is provided here. The tempera-ture-dependent ozone absorption cross sections aretaken from Ref. 12. We used the values given for 226K; this is close to the 229 K temperature adopted in theinference of ozone abundance from the Dobson net-work. To account for molecular scattering, atmo-spheric density profiles are needed. In this work weutilized the McClatchey et al.1 3 atmospheric densityprofiles for the sub-Arctic summer. The inferredozone abundance and cloud optical depth are, howev-er, quite insensitive to the choice of a clear sky atmo-spheric density profile.</p><p>To solve the radiative transfer equation we used thediscrete ordinate method described in Ref. 14. Thissolution pertains to slab geometry. To account for thelarge solar zenith angles encountered at McMurdo, weadopted a newly developed spherical radiative transfermodel.15 This spherical model takes into account theearth's curvature by converting the spherical radiativetransfer problem into a series of plane-parallel ones.For solar zenith angles of less than 900, it is sufficientto treat the multiple scattering in slab geometry as longas the single-scattering driving term is correctly com-puted using spherical geometry.15</p><p>To illustrate how effective cloud transmission can,in principle, be determined from the irradiance data,we show a comparison in Fig. 1 of the computed clearsky irradiance and the observed irradiance at 349 nmwhere ozone absorption is minimal. Computed clearsky irradiances are shown for three values of surfacealbedo (0.70, 0.75, 0.80) presumed to bracket the valueappropriate for the partially snow-covered surface atArrival Heights, McMurdo, in January. Unfortunate-ly, no measurements of surface albedo at McMurdo are(to our knowledge) available for the period of observa-tions. The measurements shown in Fig. 1 were takendaily in 1989 at 0.00 GMT, which is close to local noonat McMurdo Station. A more detailed description ofthe measurements is provided in Ref. 8. Assumingthat the surface albedo was 0.75, we notice that on twodays there is close agreement between observed andmodel-predicted irradiance. We may therefore for thesake of argument assume that the sky was clear onthese two days and that discrepancies between theobserved irradiance and the predicted clear sky valueon other days are due to cloud effects. Thus, we mayuse this discrepancy to define cloud transmission andan equivalent stratified cloud optical depth as ex-plained below. Possible aerosol effects (expected tobe small in the pristine Antarctic atmosphere) are thusincluded in our definition of cloud. Here we shouldstress that our inferred cloud optical depth is depen-dent on our choice of surface albedo, which shouldhave been determined by a measurement. The lack ofsuch a measurement implies that the effect we ascribe</p><p>20 October 1991 / Vol. 30, No. 30 / APPLIED OPTICS 4419</p></li><li><p>E1.0 31.0 61.0 91.0Day Number</p><p>Fig. 1. Comparison of computed global irradiance at 349 nm forclear sky conditions (solid curves) and observed irradiance (dashedcurve). The computed curves pertain to the surface albedo of 0.7(lower curve), 0.75 (middle curve), and 0.8 (top curve).</p><p>to clouds could partially be due to the incorrect choiceof surface albedo, as illustrated in more detail below.Estimation of Cloud Transmission and Equivalent OpticalDepthIt may be inferred from Fig. 1 that clouds have a stronginfluence on UV attenuation. Thus, changes in tropo-spheric cloud cover can significantly affect UV pene-tration, although variations in cloudiness are expectedto influence UV-B (280-320-nm) and UV-A (320-400-nm) radiation in much the same way because the wave-length dependence of the cloud effect is very weak inthe UV spectral range.'r- 8 The effect of clouds on UVpenetration will, in general, depend on cloud type,height, and morphology. If such information is notavailable from observations, one may nevertheless at-tempt to quantify the influence of clouds on the irradi-ance reaching the biosphere by comparing the mea-sured irradiance to the anticipated clear sky value at awavelength where ozone absorption is minimal. Ourmodel allows the insertion of stratified cloud layers.'9Thus, we may compute the downward irradiance at thesurface as a function of cloud optical depth. Compari-son of model-predicted and observed irradiance willthen allow us to infer the equivalent stratified cloudoptical depth, which makes the computed irradianceagree with the observed value. In Fig. 2 we show anexample of model-predicted irradiance (normalized tothe clear sky value) as a function of optical depth forseveral solar zenith angles.</p><p>Since the real cloud cover may consist of brokenrather than idealized stratified clouds, the inferredoptical depth is to be interpreted as the equivalentstratified cloud optical depth consistent with the ob-servations and the assumed surface albedo in the sensedefined above. In spite of this limitation we believethat the equivalent optical depth inferred from theobservations in this manner may provide a usefulquantitative measure of the influence of clouds on the</p><p>00</p><p>0.0 20.0 40.0 60.0 80.0 100.0Cloud Optical Depth</p><p>Fig. 2. Computed global irradiances as a function of the cloudoptical depth normalized to the clear sky value. The solid curves arefor the surface albedo of 0.4, and the dotted curves are for 0.8.Ratios are shown for several solar zenith angles from top to bottom:10, 30, 50, and 70.</p><p>q</p><p>0C)qa.) 8a</p><p>Ca</p><p>6</p><p>6so</p><p>275.0 280.0 285.0 290.0Day Number</p><p>295.0 300.0</p><p>Fig. 3. Schematic illustration of the synthetic chart used to deter-mine ozone abundance. The computed ratios of irradiance at 340-305 nm are shown as solid curves as a function of the day number(zenith angle) for several values of total ozone abundance. Thesmooth curves are 20 DU apart. The solid curve with squares marksthe corresponding observed irradiance ratios.</p><p>penetration of UV radiation. However, since thecloud optical depth inferred in this manner may bemodel dependent, it may be preferable to describe thecloud effect instead by an effective cloud transmission,which is just the ratio of the true global irradiance atthe ground to the clear-sky value. Accurate knowl-edge of the surface albedo would significantly enhanceour capability to retrieve a meaningful value of cloudtransmission or optical depth.Determination of Column Ozone AbundanceTo infer column ozone content from the observed glob-al irradiances we proceed as follows. The radiativetransfer model is used to generate a synthetic chart ofthe ratio of global irradiances at 340 and 305 nm. This</p><p>4420 APPLIED OPTICS / Vol. 30, No. 30 / 20 October 1991</p></li><li><p>chart was produced for a variety of column ozoneamounts (by scaling the standard profile from themodel atmosphere) and for the actual solar zenithangles occurring throughout the season. The columnozone amount was then derived by matching the ob-served irradiance ratio on any particular day to theappropriate curve in the synthetic chart. This proce-dure is illustrated in Fig. 3. A similar procedure forinferring column ozone amounts from global irradi-ance data is described in Ref. 17.</p><p>Uncertainties in Cloud and Ozone Derivation</p><p>Influence of Surface Albedo on Cloud EstimationThe downward irradiance at the surface depends onthe reflection by the underlying surface as illustratedin Fig. 1. In general, the downward irradiance re-ceived at the surface is expected to increase with thesurface albedo for specified atmospheric and illumina-tion conditions. This implies that the surface albedomust be known to estimate effective cloud transmis-s...</p></li></ul>

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