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Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–1746
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Journal of Atmospheric and Solar-Terrestrial Physics
1364-68
doi:10.1
n Corr
E-m
alcide.d
claudia.
journal homepage: www.elsevier.com/locate/jastp
Experimental determination of cloud influence on the spectral UV irradianceand implications for biological effects
David Mateos a,n, Alcide di Sarra b, Daniela Meloni b, Claudia Di Biagio b, Damiano M. Sferlazzo c
a Atmosphere and Energy Lab, Applied Physics Department, University of Valladolid, Prado de la Magdalena Street s/n, 47005, Valladolid, Spainb ENEA/UTMEA-TER, Via Anguillarese 301, 00123S. Maria di Galeria, Italyc ENEA/UTMEA-TER, Contrada Capo Grecale, 92010 Lampedusa, Italy
a r t i c l e i n f o
Article history:
Received 27 October 2010
Received in revised form
24 March 2011
Accepted 1 April 2011Available online 9 April 2011
Keywords:
UV spectral radiation
Cloud optical thickness
Spectral cloud modification factor
Radiative transfer
Tropospheric ozone
Biologically effective solar ultraviolet
radiation
26/$ - see front matter & 2011 Elsevier Ltd. A
016/j.jastp.2011.04.003
esponding author. Tel.: þ34 983 42 31 33; fa
ail addresses: [email protected], dmateosvi
[email protected] (A. di Sarra), daniela.meloni@e
[email protected] (C. Di Biagio), damiano.sferla
a b s t r a c t
Measurements of UV spectra, total ozone, cloud cover, and cloud optical thickness, obtained at
Lampedusa (central Mediterranean), are used to investigate the influence of clouds on the spectral
UV irradiance, through the cloud modification factor (CMF), and on five biological processes. The CMF
decreases with cloud optical thickness (COT), from about 0.5 for COT�15 to 0.25 for COT�45, and
decreases with increasing wavelength above 315–320-nm. Observations display an increase in the CMF
from 295 to 320-nm, which is related to enhanced absorption by tropospheric ozone due to the long
photon path lengths under cloudy conditions. The use of a wavelength independent CMF instead of the
experimentally determined spectral curves produces an overestimation of the biological effects of UV
irradiance. The overestimation may be as large as 30% for the DNA damage, 20% for vitamin D synthesis,
12% for plant damage, and 8–10% for phytoplankton inhibition and erythema.
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Ultraviolet (UV) radiation is the most energetic part of thesolar spectrum that reaches the lower atmosphere. It exerts asignificant influence on several processes that take place in theatmosphere and the biosphere. UV radiation affects human health(inducing erythema or sunburn, DNA damage, skin cancer, andcataract; favouring the synthesis of vitamin D); produces materialdegradation (i.e., polymers, pigments, and dyes); induces mod-ifications to the atmospheric chemistry and on biogeochemicalcycles (see e.g., UNEP, 2007); affects plants and terrestrialecosystems (e.g., Heisler et al., 2003) (i.e., impairs photosynthesisin many plant species and increases plants susceptibilityto disease, Caldwell et al., 1998; affects crop growth and devel-opment, Kakani et al., 2003); damages aquatic ecosystems(i.e., reduces the productivity of phytoplankton; induces changesin microbial and invertebrate communities, which may affectnutrient cycling in the soil, Hader et al., 2007).
The UV radiation’s indirect effects are still not well known(Caldwell et al., 2007). The influence of ozone depletion on thebiosphere and its relationship with the increase of UV radiationlevels at surface have been studied in detail, and the cloud
ll rights reserved.
x: þ34 983 42 31 36.
[email protected] (D. Mateos),
nea.it (D. Meloni),
[email protected] (D.M. Sferlazzo).
conditions have resulted in strongly modulating UV radiativefluxes (e.g., Mateos et al., 2010a).
The cloud effect on solar radiation, particularly in the UVrange, is only partially explored due to the lack of measurementsof cloud optical properties, which show a large temporal andspatial variability. The interest in the cloud–UV radiation relation-ships have increased during recent years (Seckmeyer et al., 1996;Schwander et al., 2002; Mateos et al., 2010b; among others).
The cloud modification factor (CMF) is defined as
CMF¼UVcloudy=UVclear ð1Þ
where UVcloudy is the UV irradiance measured under cloudy sky,and UVclear is the UV irradiance simulated or estimated for thesame atmospheric conditions, i.e. solar zenith angle, ozonecolumn, aerosol, pressure, and temperature, under cloudless sky.All the quantities in Eq. (1) are spectrally dependent (Calbo et al.,2005).
Early modelling studies (e.g., Spinhirne and Green, 1978)revealed the existence of a wavelength dependency in the CMF,which was experimentally shown by, for example, Webb (1991),Seckmeyer et al. (1996), Mayer et al. (1998), Schwander et al.(2002), Grant and Gao (2003), Bernhard et al. (2004), Winieckiand Frederick (2005), and Lopez et al. (2009). In some cases, theCMF was found to monotonically decrease for increasing wave-length (e.g., Seckmeyer et al. 1996; Lopez et al., 2009), while itdisplayed a maximum around 315–320-nm and decreased forlonger wavelengths (Schwander et al., 2002; Bernhard et al., 2004).
D. Mateos et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–17461740
As suggested by Lopez et al. (2009), different cloud types seem toproduce different spectral effects, wavelength independent for cirri.Several modelling analyses have shown that the CMF wavelengthdependency is due to the superimposition of different mechanisms.For example, Kylling et al. (1997) showed that the wavelengthdependence of the cloud transmissivity is caused by photons reflectedupward by the cloud itself, and then scattered downward again bythe atmosphere above the cloud. While the molecular scatteringstrongly increases with decreasing wavelength, the transmittance ofthe cloud alone has very small wavelength dependence, and the cloudmodification factor behaviour is determined by the combination ofthese two effects. Bernhard et al. (2004) found a different behaviourin Antarctica for low Sun and high surface albedo. Under suchconditions, clouds do not determine an additional component of thealready large upward flux due to the high surface albedo. In this case,the wavelength dependence is due to the radiance distribution on thecloud top and the dependence of cloud transmittance on the angle ofincidence. Several studies suggested that the reduced CMF at shortwavelengths is produced by enhanced absorption by interstitial ozonemolecules within the cloud layer along the extended photon pathlengths in cloudy conditions (e.g., Mayer et al., 1998; Schwander et al.,2002; Kerr, 2005; Winiecki and Frederick, 2005). Lindfors and Arola(2008) carried out a model study to highlight the different behaviourof the cloud attenuation at 320 and 400-nm and found that shorterwavelengths are, in general, less attenuated than longer ones with astrong dependence on solar zenith angle. In addition, they showedthat both effects suggested by Kylling et al. (1997) and Bernhard et al.(2004) play an important role.
This paper addresses the wavelength dependence of CMFbased on ground based measurements of spectral UV irradiance,total ozone, cloud cover, and cloud optical thickness.
In this study, the cloud modification factor is calculated usingmeasured UV spectra and the dependence on solar zenith angle,cloud optical thickness and tropospheric ozone is investigated.Measurements made at the Mediterranean island of Lampedusaare used in the analysis. Radiative transfer simulations have beenused in order to better understand the cloud-radiation interac-tions. Several studies discuss the dependence of the CMF on cloudoptical thickness; most of these are based on radiative modelanalyses (e.g., Spinhirne and Green, 1978; Schwander et al., 2002;Nichol et al., 2003; Bernhard et al., 2004, and others). In very fewcases simultaneous measurements of the cloud optical thicknessand UV spectra are used (Mayer et al., 1998; Bernhard et al.,2004); however, these are mainly case studies, with a limitednumber of spectra and COT values. In this study, a relatively largeset of combined measurements of COT and UV spectra is used toexperimentally evaluate the CMF behaviour, also allowing inves-tigating the dependency on the solar zenith angle. Moreover, thisis the first study of this kind in the Mediterranean region. In orderto get further in the knowledge of the UV effects on the biosphere,the estimation of horizontal weighted UV irradiance under over-cast conditions has been carried out. To this end, five biologicalspectral weighting functions have been used to quantify therelevance of the results obtained in this study. The selectedspectra take into account the effects on both human health andplants: erythema, DNA damage, vitamin D synthesis, plantdamage, and phytoplankton inhibition.
2. Measurements and methodology
The measurements used in this study were obtained atLampedusa (35.51N, 12.61E, 40 m a.s.l.) during the periodMarch–December 2005. Lampedusa is a small island (22 km2)located in the southern sector of the central Mediterranean Sea.The instrumentation is installed at the Station for Climate
Observations maintained by ENEA on the north-eastern coast ofLampedusa (for more details, see e.g. Meloni et al., 2007). All theconsidered instruments are placed on the roof of the Station andhave the horizon free from obstructions.
Spectral UV irradiance and total ozone column measurementswere performed by the double monochromator Brewer MKIII 123spectrometer. A description of the instrument and of its routinemode of operation is given by Kerr et al. (1985). The doublemonochromator MKIII has a substantial improvement in thequality of UV measurements below 305-nm with respect to thesingle-monochromator instrument, due to the better stray lightsuppression. The UV spectrum is measured in the 286.5–363.0-nmrange at 0.5-nm steps, while the full width at half maximum(FWHM) spectral resolution is 0.56-nm. The 286.5–363-nm intervalis scanned by the Brewer within approximately 8 min. This instru-ment was calibrated for ozone measurements in March 2002 andMarch 2006, through a comparison with travelling Brewer #017. Nochanges in the instrumental calibration constants were necessaryduring this period. The estimated uncertainty on total ozone fromdirect sun measurements is 1%. Spectral UV measurements werecalibrated using a field calibrator described by Early et al. (1998),which uses US National Institute of Standards and Technology(NIST) traceable 1000 W FEL lamps. Four calibrations were per-formed in the period March 2005–February 2006; changes in theinstrumental responsivity were smaller than 7% throughout thisperiod, and were taken into account. Measured spectra werereferred to the mean Sun–Earth distance and corrected for thecosine error of the diffuser. The estimated expanded uncertainty onthe spectral irradiance measurements decreases with wavelength,and is smaller than 10% throughout the Brewer spectral range (seee.g., Bernhard and Seckmeyer, 1999). For further details about theinstrument and the calibration processes see di Sarra et al. (2008).Spectra measured at solar zenith angles (SZA) from 201 to 701 in 101steps are used in this study.
A total sky imager, Yankee Environmental Systems TSI-440, issetup to automatically capture sky images every 5 min duringdaylight. The TSI consists of a digital colour video cameramounted to look down on a curved mirror to provide a horizon-to-horizon view of the sky. This instrument has a solar-ephemerisguided shadow band to block the direct-normal radiation fromthe sun.
The digital image processing provides cloud cover, clouduniformity, cloud type (low or high) and information on sunobstruction by clouds. In this study, only the cloud cover productis used. The fractional sky cover is in some cases overestimateddue to the occurrence of white regions near the sun and/or nearthe horizon for low solar elevation angles in the images. Thisphenomenon is due to strong forward scattering of visible light byaerosol and haze, and required a visual inspection of all theimages to assess the accuracy of the cloud cover evaluated by theautomatic algorithm or to correct the data if there is an over-estimation due to the presence of desert dust particles, amongothers. The uncertainty on the cloud cover determination issmaller than 1 okta.
The cloud optical thickness (COT) is calculated from theshortwave irradiance measurements made with Eppley precisionspectral pyranometers (PSP). In the period March–December 2005three different PSPs were alternately operational at Lampedusa.The PSP measurements were corrected for the thermal offsetsignal and for the cosine response of the instruments (Di Biagioet al., 2009). The PSP measurements were referred to the WorldRadiometric Reference scale defined by the World MeteorologicalOrganisation (WMO) by comparison against a PSP calibrated inAugust–September 2005 at the PMOD World Radiation Centre inDavos, Switzerland. The total estimated uncertainty on the short-wave irradiance measurement is less than 5%. The cloud optical
D. Mateos et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–1746 1741
thickness is calculated applying the formula by Barnard and Long(2004), designed for fully overcast sky. It is difficult to assess theuncertainty associated with the COT determinations. Barnard andLong (2004) found differences smaller than 10% or 2 COT unitswith respect to COT estimated with the algorithm by Min andHarrison (1996) using multi-filter rotating shadowband radio-meter observations.
The LibRadtran radiative transfer model (Mayer and Kylling,2005) has been used to simulate the Brewer UV irradiancemeasurements. The model assumes a plane-parallel atmospherewith standard mid-latitude vertical profiles of temperature,pressure, and ozone. The radiative transfer equation is solvedusing the improved version of the discrete ordinate method ofStamnes et al. (1988). The average spectral aerosol optical depthmeasured at Lampedusa in the period 2001–2003 (Pace et al.,2006) were used in the calculations and a marine aerosol typewas assumed in the 0–2 km layer. The modelled irradiances wereconvoluted with the measured Brewer slit function to betterreproduce the CMF derived from Brewer measurements. Bothcloud-free and cloudy scenarios were simulated at the same solarzenith angles of the Brewer spectra and for values of total ozoneranging from 250 to 400 DU at 5 DU steps.
To simulate the UV irradiance with clouds, overcast conditionsare assumed, with a homogeneous cloud layer between 3 and3.5 km a.s.l. and an effective droplet radius of 10 mm. The liquidwater content (LWC) of the cloud has been calculated using theformula by Ackerman and Stephens (1987). The parameterizationof Hu and Stamnes (1993) is used to translate LWC and effectiveradius into optical properties. Cloudy UV spectra have beensimulated for COT values from 5 to 70 at 5 COT steps.
3. Results and discussion
3.1. CMF dependence on cloud cover and solar zenith angle
According to the WMO classification (http://worldweather.wmo.int/oktas.htm), the cloud cover is grouped in 4 categoriesof cloudiness from 0–2 oktas (cloudless) to 8 oktas (overcast). Inthis study, only the extreme categories are used due to the lownumber of cases which comply with the selection criteria on COTand total ozone.
As a first step in the calculation of the CMF, UV spectra incloud-free conditions are needed. To do so, all the Brewer spectrameasured under 0 oktas are identified and grouped for differentclasses of total ozone column (from 250 to 400 DU, 5 DU steps)and SZA (from 201 to 701, 101 steps); the obtained cloud-free UV
Fig. 1. Spectral dependency of the CMF for 0–2 oktas (left panel), and overcast (8
spectra have been averaged in order to obtain cloudless referencespectra as a function of total ozone column and SZA.
To calculate the CMF, each UV spectrum in cloudy conditions isdivided by the corresponding UV spectrum in cloud-free condi-tions at the same SZA and total ozone amount. Moreover, allspectra displaying a change in cloudiness among three consecu-tive TSI images (i.e., over a 10 min interval) are discarded.
The CMF dependence on SZA for low cloudiness (0–2 oktas),and for overcast skies is shown in Fig. 1. As expected, the CMFvalues for low cloudiness are between 0.97 and 1 for all SZA anddo not significantly depend on wavelength. A relatively largevariability is still present, and is attributed to the limited numberof cases with cloud cover 1 and 2 oktas, which comply with theselection method and the intrinsic high variability of cloud opticalproperties. The overall behaviour is however determined by thedominant role of cloud-free conditions in this class. The absenceof wavelength dependence suggests that the total ozone effect isefficiently removed by the employed methodology.
The CMF appears to depend on SZA and on wavelength underovercast conditions. Also in this case, a large variability can beobserved. The CMF curves at SZA¼301 and 401 are, respectively, thelargest and lowest between 320 and 360-nm. The large variabilityand this solar zenith angle dependence are mostly due to the factthat clouds with different COT and optical properties are includedfor each SZA. At SZA¼301, CMF shows a large wavelength variability,which is mainly due to the limited number of overcast cases fallingin this class, which includes summertime spectra. At low solarzenith angles (30–401) the CMF increases with wavelength up to320-nm, while it increases only up to 305-nm at large solar zenithangles. A possible explanation for this effect is that the enhancementof the photon path length under cloudy conditions, with respect toclear sky, is larger at small solar zenith angles. At shortest wave-lengths, the diffuse component of the irradiance is also dominant forclear sky at large solar zenith angles.
CMF shows, in some cases, a decrease in wavelength above310–320-nm; this decrease seems to be more evident at 601 and701 solar zenith angle. As mentioned above, the variability ofcloud optical thickness strongly limits the reliability of theseresults, and an investigation as a function of COT has been carriedout and is shown in the following section.
3.2. CMF dependence on cloud optical thickness
Only cases of overcast skies were considered to study thedependence of the CMF on the cloud optical thickness. Three COTranges were taken into account: 10–20, 20–30, 40–50. Table 1shows the number of spectra falling in each of the three COT
oktas, right panel) cloud cover as a function of the solar zenith angle (SZA).
D. Mateos et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–17461742
ranges as a function of the solar zenith angle. Despite a largenumber of spectra being used in this study, only few comply withthe selection criteria because of the large temporal variability incloud properties and distribution, and the relatively long timerequired to acquire UV spectra by the Brewer. Fig. 2 (left panels)shows the dependence of CMF on wavelength and on SZA for thethree COT ranges.
The CMF decreases with increasing COT, from about 0.5 for10oCOTo20 to 0.25 for 40oCOTo50. The dependence on SZAis smooth, except for 10oCOTo20, where the CMF appears toincrease with the solar zenith angle. A weak decrease in CMF forincreasing wavelength can be observed above 315–320-nm,which seems to be more evident at large solar zenith angles.
Fig. 2. Measured (a, c, and e) and modelled (b, d, and f) values of CMF versus wavelen
optical thickness. Dashed bold lines in the right panels are relative to simulations with d
(d), and (f).
Table 1Number of UV spectra at 8 oktas for the three ranges of the cloud optical thickness
(COT) as a function of the solar zenith angle.
COT range 301 401 501 601 701
10–20 5 9 7 1 2
20–30 2 5 4 1 6
40–50 2 2 2 1 3
These results are consistent with the mechanism proposed byKylling et al. (1997), which is expected to produce a wavelength-dependent CMF. Part of the observed variability in the CMFbehaviour may also be due to the effects of different aerosoltypes and amount.
With an aim of interpreting the observed results, the spectralCMFs have been calculated from the simulated spectra for thecentral COT value of each interval. The results are also shown inFig. 2 (right panels). Ozone column is the same for measured andmodelled CMFs. Differences between the curves on the left andright hand sides of Fig. 2 can be attributed to the variability ofCOT within the intervals in the measured CMF spectra and, to alesser extent, to the variability of aerosol optical properties.Consistently with observations, the dependence on the solarzenith angle appears stronger for COT¼15, and weaker for largerCOT values. The agreement between model and measurements isgood at longer wavelengths, while differences appear mainlybelow 310-nm. Based on a modelling study, Bruhl and Crutzen(1989) suggested that tropospheric ozone may have a large role inthe modulation of UV-B radiation (280–315-nm) in the presenceof elevated aerosol amounts, due to the enhanced photon pathlengths in the troposphere. This effect is expected to be moreevident when clouds are present, because of their large optical
gth as a function of the solar zenith angle (SZA) for different values of the cloud
oubled amount of tropospheric ozone at 501, 601, and 701 SZA, respectively, in (b),
D. Mateos et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–1746 1743
thickness, as discussed by Mayer et al. (1998), Winiecki andFrederick (2005), and others. The ozone vertical distribution usedin the model calculations is taken from a mid-latitude profile,which is scaled to match the measured total ozone. For thisprofile the tropospheric ozone amount is about 10% of the totalcolumn, and the surface mixing ratio is about 30 ppb. However,the tropospheric ozone contribution may be larger than 10% atNorthern mid- and high-latitudes (e.g., Fishman et al., 2003).
We have modified the ozone vertical profile used in the model,without changing the total ozone column, to verify if the CMFbehaviour for wavelengths shorter than 310-nm is regulated bytropospheric ozone. Model calculations were thus repeated with atropospheric fraction (from 0 to 10 km) two times larger than inthe standard profile (surface ozone mixing ratio of about 60 ppb).The obtained CMFs are reported in Fig. 2 as dash bold lines forSZA¼501 and COT¼15, SZA¼601 and COT¼25, and SZA¼701 andCOT¼45. Lower values of CMF are found below 320-nm, and asteeper increase with wavelength is observed, which betterreproduces, although still overestimating, the measured CMF forall classes of COT. Several studies suggest that elevated ozone iscommon at the surface over the Mediterranean, mainly insummer (e.g., Kalabokas et al., 2008; Nolle et al., 2002; Kulkarniet al., 2011). Elevated ozone levels may be found also throughoutthe troposphere (e.g., Kourtidis et al., 2002). Surface ozonemeasurements made at Lampedusa in spring and summer, notavailable in 2005, show that tropospheric ozone is generallylarger than in the standard mid-latitude profile. A larger reductionof diffuse irradiance at the surface due to enhanced troposphericozone has been observed and explained by Meloni et al. (2003).A reliable description of the ozone vertical profile is thus neededfor an accurate determination of the cloud modification factor bymodel at short wavelengths.
3.3. CMF dependence on cloud altitude
The role of the cloud layer altitude has been investigated bymeans of model simulations using COT¼15 and cloud layersbetween 1 and 1.5 km and between 4.5 and 5 km. The obtainedresults are shown in Fig. 3 for two wavelengths, 305 and 340-nm,the first strongly absorbed by ozone, and the other not influencedby ozone. At 305-nm (left panel) the effect of cloud height showsa dependency on solar zenith angle: for SZA lower than 401 theattenuation of the diffuse component is weaker for the cloud layerat 1–1.5 km altitude, while the opposite occurs for SZA larger than501. The differences among the three CMFs are always lower than5%. At 340-nm (right panel) the CMF decreases with SZA and nodependence on cloud height appears. These results confirm
Fig. 3. Model simulation of the CMF as a function of three clou
modelling results by Spinhirne and Green (1978), Gadhavi et al.(2008) and others, who found that only wavelengths r300-nmshow sensitivity on cloud height.
3.4. UV irradiance estimation under cloudy conditions:
biological effects
An empirical relationship linking the UV fluxes and cloudcover, independent of wavelength, is used in several studies(e.g., Calbo et al., 2005).
In this section we estimate the errors associated with the useof a wavelength independent CMF while calculating the effects ofUV radiation on biological systems. Several action spectra areconsidered; the weighted UV dose rate for the selected effects iscalculated using both a wavelength independent CMF, and theCMF determined from the observations at Lampedusa.
The weighted UV dose rate is calculated as
WUV¼
Z l ¼ 363 nm
l ¼ 286:5 nmEclearðlÞRðlÞCMFðlÞdl ð2Þ
where Eclear(l) is the spectral irradiance under cloud-free condi-tions and R(l) is the action spectrum for a specific process. Fivedifferent action spectra relevant for health and for plants areconsidered in this study: the erythemal action spectrum(McKinlay and Diffey, 1987); the DNA damage (Setlow, 1974),the synthesis of vitamin D (CIE, 2006), the plant damage (Flintand Caldwell, 2003), and the phytoplankton inhibition (Cullenet al., 1992).
The WUV for all 5 action spectra is calculated from measuredcloud-free UV irradiance spectra (i.e., with CMF¼1) with 310 DUof ozone, and for different values of the solar zenith angle. TheWUV under cloudy conditions for different values of SZA and COTis then calculated using both the spectral CMFs displayed in Fig. 2(WUVl), and a wavelength independent CMF (WUVC). The latterCMF value is chosen equal to the CMF at 320-nm in Fig. 2.
Two sets of UV index, UVIl and UVIC, are derived by multi-plying the erythemal WUVl and WUVC, respectively, by 40. It isworth mentioning here that the spectral range used in this studyreaches 363-nm, and the integral is limited to this wavelength.However, the UV index is defined up to 400-nm. In order to takeinto account the missing fraction of the spectrum, which producesa slightly underestimation of the UVI values (e.g., o5% forSZAo501 and TOC¼300 DU), a factor dependent with SZA wasapplied to UVI calculation. This factor was evaluated at differentsolar zenith angles as the ratio between the UVI calculated usingsimulated spectra up to 400-nm, and the UVI obtained frommodelled spectra up to 363-nm.
d heights at 305-nm (left panel) and 340-nm (right panel).
Fig. 4. (a) Dependence of UV index values on cloud optical thickness at two solar zenith angles. Solid lines mean the real dependence showed in this study and the dashed
ones mean the assumption of the CMF constant. (b) Dependence of the UVI differences between both methodologies.
Fig. 5. Dependence of the differences between the assumption of the CMF constant and the real wavelength-dependant in the UV range for four biological action spectra:
(a) DNA damage, (b) vitamin D synthesis, (c) plant damage, and (d) inhibition of phytoplankton.
D. Mateos et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–17461744
Fig. 4 shows the retrieved values of UVIl and UVIC and thepercent difference between the two determinations. The assump-tion of a constant CMF in the whole UV range produces system-atically higher UVI values than those obtained by means of theobserved spectral dependence. UVIC always overestimate UVIl. Atsolar zenith angles of 301, 401, and 501 the difference betweenUVIC and UVIl increases with the cloud optical thickness. At largeCOT, the difference may be as large as 10%. The differences arealways smaller than 5% at SZA¼701.
Fig. 5 shows the percent difference between WUVl and WUVC
using the other action spectra for different solar zenith angles andcloud optical thicknesses. The differences are always positive. Thedifference between WUVl and WUVC is as large as 30% whenconsidering the DNA damage, 20% for vitamin D synthesis, 12% forplant damage, and 8% for phytoplankton inhibition. The differenceobtained for all action spectra display a dependency on the cloud
optical thickness except at SZA¼701, where the differencesbecome smaller and do not appear to depend on COT.
4. Conclusions
Measurements of UV spectra, cloud cover, cloud optical thick-ness, and total ozone column performed at the ENEA Station forClimate Observation in Lampedusa during the period March–December 2005 have been combined in order to study thewavelength dependence of the cloud modification factor (CMF)and the implications for the estimated weighted UV irradiationeffects on biological systems. The main results are
�
On an average, CMF decreases with increasing COT, beingabout 0.5 for COT¼15, 0.37 for COT¼25, and 0.25 for COT¼45.
D. Mateos et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1739–1746 1745
This behaviour is also well-described by radiative transfermodel simulations.
� Dependence of CMF on solar zenith angle in the UV range ismore evident at low–moderate values of COT (10–20 interval).
� CMF decreases with increasing wavelength above 320-nm, asalready shown by previous studies.
� From 295 to 320-nm the CMF increases, the growth beingmore evident at large COT and SZA. This effect is attributed toabsorption of diffuse irradiance by tropospheric ozone, as aresult of the longer photon path lengths in cloudy conditions.These results confirm previous studies and suggest that areliable determination of tropospheric ozone amounts isrequired for a precise characterisation of the CMF at shortwavelengths. This may be particularly relevant for the Medi-terranean, which is characterised by elevated summer tropo-spheric ozone values. Moreover, the cloud layer altitude playsa minor role which is only observed at short wavelengths.
� Use of a wavelength independent CMF instead of the experi-mentally determined spectral curves produces an overestima-tion of the biological weighted UV irradiation effects, whichhas been estimated to be as large as 30% for the DNA damage,20% for vitamin D synthesis, 12% for plant damage, 8% forphytoplankton inhibition, and 14% for the erythema. Theoverestimation is larger at low solar zenith angle and largecloud optical thickness.
Acknowledgments
The authors gratefully acknowledge the financial supportextended by the Valladolid University to the research staff. TheSpanish Innovation and Science Ministry and the AutonomousGovernment of Castile and Leon region collaborate under the Projects:CGL2009-08097(CLI) and REF. GR220-2008, respectively, by means ofDr. Julia Bilbao and Dr. Argimiro de Miguel. The study was partlysupported by the Italian Ministry for University and Research throughthe Aeroclouds Project. Contributions by F. Monteleone, G. Pace, andS. Piacentino are gratefully acknowledged.
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