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doi:10.1016/j.ja

�CorrespondE-mail addr

(K.V.S. Badari

Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 589–599

www.elsevier.com/locate/jastp

Influence of atmospheric aerosols on solar spectralirradiance in an urban area

K.V.S. Badarinatha,�, Shailesh Kumar Kharola,D.G. Kaskaoutisb,c, H.D. Kambezidisb

aForestry and Ecology Division, National Remote Sensing Agency (Department of Space—Government of India) Balanagar,

Hyderabad 500 037, IndiabAtmospheric Research Team, Institute for Environmental Research and Sustainable Development,

National Observatory of Athens, Lofos Nymphon, P.O. Box 20048, GR-11810 Athens, GreececUniversity of Ioannina, Department of Physics, Laboratory of Meteorology, GR-45110 Ioannina, Greece

Received 11 May 2006; received in revised form 25 September 2006; accepted 20 October 2006

Available online 14 December 2006

Abstract

Solar radiation reaching the earth’s surface at different wavelengths has been extensively discussed during the last

decades. Great emphasis has been placed on the potential increase in surface UV radiation due to the depletion of

stratospheric ozone. The present study reports the variation of solar spectral irradiance and its relation with aerosols over a

typical urban environment in India. Synchronous measurements of aerosol optical depth, UV irradiance, aerosol-particle

size, black carbon (BC) concentration and solar irradiance have been carried out at the urban station of Hyderabad located

in central India. Considerable reduction in the UV intensity has been observed during periods of high aerosol loading.

A comparison of the erythemal UV (UVery) intensities on normal day with those of high aerosol loading suggested a �24%

decrease in the UVery reaching the ground. Satellite observations showed forest fire occurrence over the region. PAR and

diffuse-to-direct-beam ratio of solar irradiance showed marked differences under varying aerosol-loading conditions.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Aerosol; Atmospheric turbidity; Solar spectral radiation; Erythemal UV; Diffuse-to-direct-beam ratio

1. Introduction

Atmospheric aerosols are fine particles that canscatter and absorb the incident solar radiationcontributing to cooling of the earth’s surface anda simultaneous warming of the lower atmosphere(Keil and Haywood, 2003; Pace et al., 2006). Besides

e front matter r 2006 Elsevier Ltd. All rights reserved

stp.2006.10.010

ing author.

ess: badrinath_kvs@nrsa.gov.in

nath).

this direct radiative effect, aerosols act as condensa-tion nuclei in the formation of clouds modifyingtheir microphysical properties. The aerosol-numberdensity, chemical composition and size distributioncan influence the albedo and lifetime of clouds aswell as the rate and the amounts of precipitation(Abel et al., 2005; Lohmann and Feichter, 2005).Aerosols degrade the air quality in urban areas andreduce visibility. Continental aerosols are mainlywind-blown mineral dust as well as carbonaceousand sulfate particles produced by forest fires, land

.

ARTICLE IN PRESSK.V.S. Badarinath et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 589–599590

use and industrial activities, while marine aerosolsare mainly sea-salt particles produced by wave-breaking and sulfate particles produced by theoxidation of dimethyl sulfide released by phyto-plankton. As the oceans cover more than 70% ofthe earth’s surface, they consist one of the largestsources of natural aerosols. Being hygroscopic innature, marine aerosols are crucial in cloud forma-tion in the marine boundary layer and are alsoimportant in the radiative coupling between oceanand atmosphere. Continental aerosols can be bothscattering and absorbing, while marine aerosols aremostly of scattering type (Dubovik et al., 2002);thus, there can be an argument about the planetaryalbedo. Aerosols in urban environments are physi-cally and chemically different from aerosol inremote areas with the most obvious differencesbeing the high concentration of sulfur and heavymetals in urban aerosols (Latha and Badarinath,2004). The variety of sources, natural and anthro-pogenic, the short lifetimes of aerosols and theirinfluence by the meteorological parameters, espe-cially by relative humidity (Day et al., 2000; Hanel,1976; Horvath, 1996), result in a spatially andtemporally heterogeneous aerosol field, makingaerosol characterization and modeling a real chal-lenge (Smirnov et al., 2002).

On the other hand, the amount of solar ultravio-let (UV) radiation penetrating the earth’s surface iscritically important for the health of biologicalsystems (Feister and Grasnick, 1992; Nemeth et al.,1996; Sutherland et al., 1991); practically no solarradiation reaches the ground at wavelengths shorterthan 290 nm due to its strong absorption bystratospheric ozone. The biologically harmful UV-B radiation lies in the spectral range 280–320 nm,while erythemal response of the human skin ismaximum at about 297 nm. Erythema, which isdefined as a reddening of human skin in response tosolar radiation, extends through both UVery andUV-A (315–400 nm) (Herman et al., 1996). Auto-correlation between total column ozone and surfaceUV radiation is a complex function of manyvariables, including solar zenith angle, altitude,cloud cover, aerosol loading, surface albedo andvertical profile of ozone. The effect of aerosols onthe UV radiation constitutes a great scientific issue.Therefore, many studies have been carried out (Liuet al., 1991; Kylling et al., 1998); these researchershave reported that high loading of the absorbingparticles could cause reduction of UV flux at thesurface by more than 50%. In the last decades, a

continuous increase in the biological active solarUV-B radiation due to a decrease in the ozoneamount emerges at a global scale (Zerefos et al.,1995). Nevertheless, at regional scales even adecrease of the ozone amount of 50DU incombination with an increase in the aerosol loadingcan lead to a decrease in the UV radiation (Balis etal., 2002; Papayannis et al., 1998). Therefore,continuous ground-based observations play animportant role in improving the understanding ofsome of these effects (Madronich and Flocke, 1997).

The precise determination role of all the aboveparameters in quantifying and modifying the UVery

levels is very difficult to be distinguishable due tothe combined involvement of all these parameters inthe radiation processes in the atmosphere. There-fore, the use of solar radiation models (e.g.SMARTS) is necessary for the improvement of theknowledge at the role of each parameter. Indeed,systematic investigations on the temporal variationof UVery radiation and its influencing parametersare still sparse (Gueymard, 1995). This paperprovides a case study of changes in ground-levelsolar irradiance, diffuse-to-direct-beam ratio andUVery as well as their relationship with the aerosolsunder different turbidity conditions over the tropi-cal urban area of Hyderabad, India, using simulta-neous measurements. Such measurements are verylimited all over India.

2. Study area

Fig. 1 shows the map of the study area. The studyarea of Hyderabad is located between 171100 and171500N latitude and 781100 and 781500E longitude.Hyderabad is the fifth largest city in India; itspopulation is 3449.878 inhabitants according to thecensus of 2001, a purely urbanized area. The climateof the region is semi-arid with a total rainfallamount of �700mm occurring mostly during themonsoon season in the period June–October. Theminimum and maximum temperatures during Jan-uary 2006 were 10 and 33 1C, respectively, with clearsky conditions. The relative humidity values inJanuary are normally high during nighttime (90%),while during daytime the values lie in the range30%–40%. The measurements for the case studywere carried out in the premises of the NationalRemote Sensing Agency (NRSA) campus located atBalanagar (171280N and 781260E) located wellwithin the urban center under clear sky conditions.

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Fig. 1. Location map of the study area.

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3. Data sets and methodology

The attenuation of the solar radiation in theatmosphere is given via the Lambert–Bouguer–Beerlaw:

Ebl ¼ E0l expð�tlmÞ. (1)

From this exponential decrease, the total atmo-spheric optical depth can be derived. The relativeoptical air mass, m, computed via Kasten andYoung’s (1989) formula was corrected for pressurevariations; t(l) is the wavelength-dependent totaloptical depth. For a variation of 10% in pressure,the optical air mass leads to a variation in t(l) ofabout 0.7–0.8% at 500 nm. These uncertainties seemto reach 1% in the UV spectrum. It is assumed, as inUtrillas et al. (2000), that the optical air mass leadsto error values lower than 0.1% for zenith anglessmaller than 851.

The aerosol optical depths (AODs) were obtainedfrom direct-beam irradiance measurements at sev-eral wavelengths (380, 440, 500, 675, 870 and1020 nm) using a MICROTOPS-II sunphotometerwith an instrumental accuracy of 72%. Thedetector consists of a silicon photodiode mountedbehind a set of continuously variable interferencefilters. The AOD, ta(l), was retrieved from the totaloptical depth after subtracting the Rayleigh scatter-

ing, tR(l), and the contribution of gas absorbers:

taðlÞ ¼ tðlÞ � tRðlÞ � tO3ðlÞ � tNO2

ðlÞ

� tmgðlÞ � tH2OðlÞ. ð2Þ

The two last components, which are due to themixed-gases and water-vapor absorption, have noinfluence in the specific wavelengths of the MICRO-TOPS II instrument and, therefore, were omitted fromEq. (2). The Rayleigh scattering has been calculated by the formula tR(l) ¼ (P/P0)� 0.008735�l�4.08 (Leckner, 1978). In this formula, P is the actualair pressure in hPa and P0 ¼ 1013.25hPa. Eck et al.(1999) reported a maximum error in computed ta(l) at340nm of �0.021, 0.013 at 380 s and 0.007 at 440nmassuming a 3% maximum departure from the meansurface pressure. Therefore, as pressure measurementsare quite accurate, further errors in the ta(l)determination caused by errors in tR(l) values arenegligible. For the calculation of the ozone opticaldepth, its spectral absorption coefficients provided byMODTRAN were used, while the total columnarozone amount was measured using a MICROTOPS-II Ozonometer. Eck et al. (1999) reported thatdepartures from the climatological ozone values ashigh as 50% resulted in additional uncertainty incomputing ta(l) of only �0.0036 at 340nm, �0.0045at 500nm and �0.0063 at 675nm. Therefore, theseerrors in calculating ta(l) are almost negligible.Therefore, the errors in the determination of ta(l)arise from the errors in the measured direct-beamirradiances. The largest sources of error in ta(l) fromany instrument are the direct-beam irradiance mea-surements and the determination of the extraterrestrialirradiance using the Langley calibration, since theerrors by subtracting the other components are anorder of magnitude lower. It was found, as inKaskaoutis et al. (2006c), that the errors in computingta(l) are higher under low turbidity conditions due toinstrumental uncertainties. This means that on dayswith high turbidity, the relative error is lower than 5%in most cases, while under low turbidity conditions therelative error is usually over 10%. A UV-B radiometerfrom Solar Light Co. (Gayatri and Prasad, 1993)located in the NRSA campus was used to measureUVery in the range 280–315nm. The cosine responseof the instrument is 75% with a resolution of0.01MEDh�1 (Devara et al., 1996; Niranjan et al.,1995). Moreover, a multi-filter rotating shadow bandradiometer (MFRSR) was used for this study. Atnominal wavelengths of 415.9, 496.6, 622.4, 670.2,868.3, and 938.5nm (FWHM�10nm), the MFRSR

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AOD variation at 1100hrs

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Fig. 2. (a) Diurnal variation of AOD at 500 nm on different days.

(b) Aerosol optical depth (AOD) variation at 1100h (LST) on

different days.

K.V.S. Badarinath et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 589–599592

takes measurements of total, diffuse horizontal, anddirect-beam irradiances. The instrument’s sensors areshaded at periodic intervals (�5min) by a rotatingshadow band, so the diffuse radiation is measured.The shadow band then moves, and a measurement oftotal downward radiation is performed. The differenceof these two quantities is the downward component ofthe direct-beam solar irradiance. The major advantagein using this technique is that each measurement(total, diffuse, and direct-beam) is calibrated identi-cally, thus reducing errors that would be arisen usingindependent instruments. The Ultraviolet MFRSR(UVMFR-7) is an instrument that measures diffuseand total global irradiance, and computes directirradiance at four or seven narrow-bandwidth wave-lengths in the UV-B and UV-A regions. Continuousand concurrent measurements of the mass concentra-tion of black carbon (BC) were also carried out usingan aethalometer; model AE-21 of Magee Scientific.The instrument aspirates ambient air from an altitudeof �3m above the ground using its inlet tube and itspump. The BC mass concentration is estimated bymeasuring the change in the transmittance of a quartz-filter tape, on to which the particles impinge. Theinstrument has been operated at a time base of 5min,continuously on the experiment days with a flow rateof 3 lmin�1. The instrument has been factorycalibrated and errors in the measurements are�72%. In addition, continuous measurements ofPM grain-size distribution were performed with aGRIMM 1108 laser spectrometer (Le Canut et al.,1996). This dust monitor determines the particlematter level in 15 different grain-size channels from0.3 to 420mm.

4. Results and discussion

Using the methodology described above the AODwas derived using the MICROTOPS II on twospecific days representative of relatively clear(normal) and turbid atmospheres. Fig. 2a showsthe diurnal variation of AOD at 500 nm (AOD500)on normal and turbid days as obtained throughMICROTOPS II measurements. The AOD500 va-lues are markedly higher on turbid compared tonormal day. On 18 January 2006, the AOD500

values were about 0.6, remaining very high for thewhole day with unimportant diurnal variation.Little higher values were derived at both earlymorning and noon hours as a consequence of theenhanced local activities in the city. Also, fromFig. 3 the higher BC concentrations at the early

morning hours on 18 January are obvious, whichhave an influence in the enhancement of theAOD500. In a previous study in the same area(Latha and Badarinath, 2005), it was found that asignificant correlation between BC concentrationand aerosol loading exists. Unfortunately, irradi-ance data after 16:00 LST, where the BC concen-trations are the highest, were not available due tosunset. These high AOD500 values are representativeof urban environments as they are of the samemagnitude with high AOD500 values reported forAthens (Kaskaoutis et al., 2006c). Also, the AOD500

values estimated in Hyderabad on the turbid day issufficiently higher than those reported by Duboviket al. (2002) for urban sites, while are comparablewith the AODs for regions that are affected bybiomass burning episodes (Dubovik et al., 2002).Therefore, it is concluded that this high aerosolloading on 18 January is not only of urban-industrial origin. On the other hand, 20 Januarycan be characterized as a relatively clear day for thisurban area, although the AOD500 values are muchhigher than background-aerosol conditions (Smir-nov et al., 2003). Nevertheless, in the early morningthe AOD500 exhibits relatively high values, about0.4, strongly correlated with the BC concentrations,Fig. 3. In the rest of this day the decrease of the BC

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concentration in combination with favorable me-teorological conditions, e.g. higher air temperaturesleads to the ventilation of the atmosphere overHyderabad.

Fig. 2b shows the AOD estimated at differentwavelengths 380, 440, 500, 675, 870, and 1020 nmusing the sunphotometer on each experimental day.In this figure, the AODs exhibit different opticalcharacteristics. The wavelength dependence of theAOD differs significantly from day-to-day. On 18January, the distinct feature of the spectral AODdistribution is its gradual and discernible decreasewith wavelength. It is worth to be noted that thisfeature was evident throughout the day. At theshorter wavelengths, the AOD increases signifi-cantly implying more curvature in its spectraldistribution. This is attributed to the presence of ahigher fine-to-coarse mode ratio on this day. Sincethe aerosols are mainly of anthropogenic origin(fine-mode particles, Eck et al., 1999), the spectraldistribution of the AOD is stronger (Cachorro et al.,2000). The presence of a higher concentration of thefine-mode particles, which are selective scatters,enhances the irradiance scattering and, therefore,the AOD values with a greater level at the shorterwavelengths. The fine-mode particles have a muchgreater effect on the AOD at the visible wavelengthsthan at the near infrared wavelengths. Likewise, thecoarse-mode particles provide similar contributionsto the AOD at both wavelengths (Schuster et al.,2006). Moreover, as reported by Molnar andMeszaros (2001), the fine particles are responsiblefor the 82% of the scattering in the atmosphere.

On the other hand, the AODs on 20 Januaryappear to have little or much weaker wavelengthdependence, representative of rural environments(Cachorro et al., 2000). Such a weak wavelengthdependence was also evident for desert dust aerosolsin the Persian Gulf (Smirnov et al., 2002).

The high occurrence of fine-mode particles isreflected in the high values of the Angstromexponent a (Schuster et al., 2006) with a simulta-neous increase in the turbidity parameter b. TheAngstrom parameters were estimated from theAOD values at 380 and 870 nm, using the Volzmethod. On the normal day the mean a and b were1.126 and 0.142, respectively, while on the turbidday these values were increased to 1.334 and 0.229,respectively. The high b values are representative ofurban environments and are of the same magnitudewith those reported for Athens (Jacovides et al.,2005; Kaskaoutis et al., 2006c). The a values arealso similar with those reported for the Athens arearevealing similar atmospheric conditions over thetwo urban areas. The correlation between a andAOD can reveal the aerosol type under specificcircumstances. For example, moderate to small avalues associated with maritime aerosols generallycorrespond to small AODs, whereas aerosol frombiomass burning show large AODs and large avalues and desert dust aerosols show large AODsand small a values (Cachorro et al., 2001). However,the a values depend strongly on the spectral intervalused for their determination (Cachorro et al., 2001;Jacovides et al., 2005). Hence, the informationcontained in the a-AOD scatter plot becomes moredifficult to interpret and, therefore, this diagram isnot presented here. On the other hand, detailedspectral information given by the determination of ain different spectral ranges, together with the AODat different wavelengths, constitutes a useful tool forthe determination and discrimination of the aerosoltype (Cachorro et al., 2001; Reid et al., 1999;Schuster et al., 2006).

The AOD showed slightly higher values at 870 nmcompared to that at 675 nm implying negative avalues for this narrow spectral interval. The samefeature was reported by Cachorro et al. (2001), sincethey estimated negative a values in the spectralrange 765–865 nm in Huelva, Spain. Nevertheless,this weak wavelength dependence is far from the‘‘anomalous extinction’’ reported by Weller et al.(2000) and Adeyewa and Balogun (2003).

BC concentration values have been observed tobe high on 18 January compared to those on 20January (Fig. 3) with an average concentration 1.5times larger than that on normal day values. Onboth days, BC concentrations exhibits a distinctdiurnal variation with the higher values taking placeat the evening hours, after 19:00 LST due to trafficdensity and decreased boundary layer height (Latha

ARTICLE IN PRESSK.V.S. Badarinath et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 589–599594

et al., 2004). High BC concentrations also occur inearly morning due to increased human activities inthe urban area. At noon, the enhanced height of themixed layer leads to the dispersion of the pollutantsand the ventilation of the air. The higher BCconcentrations at early morning hours on both dayshave a great influence in the diurnal variation of theAODs (Fig. 1a).

In order to understand the possible sources ofsuch a high aerosol loading on 18 January, DMSP-OLS nighttime satellite processed data sets forforest fires were analyzed. Fig. 4 shows the night-time fires image generated from DMSP-OLS,NOAA-AVHRR false color composite (FCC)nighttime data of 17 January 2006 overlaid on 18January. In the NOAA-AVHRR FCC, fire burntscars can be seen in forest regions around the studysite of Hyderabad. It can be seen from Fig. 4 thatthe majority of the forest fires were prevalent mainlynorthwest of the study area.

A useful tool for the data interpretation is theHYbrid Single-Particle Langrangian IntegratedTrajectories (HYSPLIT) code (Draxler and Rolph,2003). This program allows for the calculation ofthe air masses back-trajectories once the trajectorylevels, the day and the time are fixed (Espozito et al.,2004). In the present study, the back-trajectoriesreaching at Hyderabad have been estimated over the5 days preceding 18 January at three levels in theatmosphere, at 500, 1500 and 2000m a.s.l. TheHYSPLIT model (Fig. 5) suggested back-trajec-tories coming from north on 18 January (00.00 h),directly influencing the study area with a significantamount of biomass burning aerosols. The highAODs of this aerosol type (Dubovik et al., 2002) inaddition with the anthropogenic and industrialactivities in the urban area suggest the sufficientlyhigh AODs on 18 January. It is also apparent thatfor the 5 days, the air masses do not move at largedistances, but remain relatively constant abovecontinental India, which in this dry season is quitesusceptible to forest fires. Moreover, the biomass-burning aerosols, and especially the fresh smokeparticles are very absorbing, also exhibiting a strongwavelength dependence on their AODs (Reid et al.,1999). This feature was obvious in the AODwavelength dependence in Fig. 2b.

The different aerosol loading and their differentproperties on the 2 days examined have a greatinfluence on the amounts of solar radiation reachingthe ground. A marked reduction in UVery (Fig. 6)implying a �24% mean reduction of UVery at

ground level was observed on 18 January 2006compared to the normal day. Such a reduction inUVery due to an increase in aerosol loading has alsobeen reported in the literature (Krzyscin andPuchalski, 1998; Reuder and Schwander, 1999).The diurnal variation of the UVery follows solarzenith angle, exhibiting higher values at noon,where solar irradiance is more intense. It is alsoobvious that at 10:00 LST the UVery amounts aresimilar on both days, since at this time the AODsexhibited their least difference, Fig. 2a. For thesame reason, the differences in the UVery on the 2days are higher in the afternoon, where the atmo-sphere on 20 January is clean. The aerosol-particle-density number showed higher values on 18 Januarywith an additional increase in fine-mode particles.The sources of such fine-mode particles are theanthropogenic and industrial activities, the gasesfrom automobile exhausts and also the biomass-burning particles reaching Hyderabad from theneighboring fires of the previous days. As thevehicular traffic is continuous in the urban area ofHyderabad during the period of measurements, theadditional source of the presence of higher fine-mode particle concentration on 18 January could bedue to the biomass-burning aerosols, as forest fireswere observed in DMSP-OLS nighttime satellitedata. Earlier reports on chemical analysis of aerosolsamples in urban areas of Hyderabad suggested aK+ concentration implying the possible sources ofbiomass-burning emissions (Kulshrestha et al.,2004).

Similar to the UVery amounts, the spectralirradiances exhibit a significant attenuation on theturbid day. In Fig. 7, the solar spectral globalirradiances measured using both the MFRSR andUVMFRSR instruments at ten wavelengths areplotted. This figure corresponds to the same LST(11:00) on both days (SZA ¼ 401). Consequently,any effect of the solar zenith angle on the irradiancevalues is negligible and, therefore, the differencesbetween the 2 days are attributed to the differentaerosol type and loading. A significant reduction inglobal spectral irradiance is obvious on the turbidday (18 January) compared to the normal day(20 January). This attenuation exhibits a clearwavelength dependence with the higher differencestaking place in the UV and VIS spectrum. Theglobal spectral distribution exhibits the samepattern on both days, and practically no irradiancereaches the ground for wavelengths below 305 nm,due to its strong absorption by the stratospheric

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Fig. 4. DMSP-OLS nighttime fire image of 17 January 2006 overlaid on NOAA-AVHRR false color composite of 18 January 2006

showing fire locations towards north of the study area.

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ozone and the strong attenuation (scattering andabsorption) caused by the aerosol layer. The higherrelative differences in global irradiances on the 2days are depicted in the UV band, a reduction ofabout 34% is estimated at 317.1 nm, whichdecreases at 496.6 nm (21%) and practicallyvanishes at 938.5 nm. The mean spectral differenceat all wavelengths of UVMFRSR and MFRSR in

global irradiance was estimated to be 28% betweennormal and polluted days. Therefore, the presenceof the fine-mode particles on the turbid day has agreat influence on solar irradiance attenuation at theshorter wavelengths, as suggested in other studies(Reid et al., 1999), too. On the other hand, in theNIR spectrum the differences in irradiance valuesdue to the differences in aerosol loading are very

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Fig. 5. NOAA HYSPLIT model back trajectory map of 18 January 2006.

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small, since the irradiance attenuation in the atmo-sphere continuously decreases with wavelength(Iqbal, 1983). The very small differences in the

NIR are also attributed to the high presence of fine-mode particles, which exhibit a selective attenuationof the solar irradiance. In contrast, a higher

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percentage of coarse-mode particles on the turbidday would decrease the irradiance differences at theshorter wavelengths, while it would increase them atthe longer wavelengths.

The high concentration of fine-mode particlesseems to also have modified the diffuse-to-direct-beam irradiance ratio (DDR) in different spectralbands as shown in Fig. 8. This figure refers to thesame SZA (401) for the 2 days, since the spectralmeasurements correspond to 11:00 LST. Therefore,the significant differences in the curves are attrib-uted to the aerosol loading and their differentoptical properties. The DDR ratio at a specificwavelength as a function of the AOD has alreadybeen used for the discrimination of biomass burningparticles in the Mediterranean, as the more absorb-ing aerosols exhibit lower DDR values (Meloniet al., 2005). It is obvious that the DDR for theturbid day suggested a marked deviation with veryhigh values especially at the shorter wavelengths.The changes (increase) in DDR at shorter wave-lengths are more intense than those at longer due toenhanced values of the diffuse radiation. Theimportant role of scattering is verified at thesewavelengths, where the depletion of the direct-beamirradiance by scattering processes is regained asincreasing diffuse irradiance, leading to large ratiovalues. It is noted that at longer wavelengths theratio appears to be of the same order of magnitudefor both days. Exponential fits can describe theDDR–AOD correlations with a great accuracy, asthe coefficient of determination, R2, is very high onboth days. The good fitting as well as the constantvalues of the curves are a general characteristic ofthe correlation between spectral DDR and AOD asit has been established in recent studies (Latha andBadarinath, 2005; Kaskaoutis et al., 2006a, b). It is

also apparent that the dependence of the ratio onspectral AOD is much smoother under highly turbidair, implying lower exponent values, 3.14 against7.66 for low turbidity. On the other hand, theconstant value is higher for the turbid day, 0.089against 0.021 for the low turbidity case. Thedifferent constant values of the exponential fitsreported by Kaskaoutis et al. (2006b) are attributedto the different spectra used, since the UV region isnot included in the present study. Nevertheless, thesmoother ratio dependence under high turbid air isalso apparent in Athens (Kaskaoutis et al.,2006a, b).

The higher concentration of fine-mode particlesalso reflected in the aerosol-particle size measure-ments using GRIMM on turbid day (Fig. 9). Theanalysis of GRIMM aerosol-particle analyzer datasets suggested that the number of particles reveals ahigher load of PM in the range of 1–3 mm on 18January compared to 20 January. Such high fine-mode particles could result from forest fires thatwere observed in the north of the study area withfavorable wind direction. This is correlated with theDMSP-OLS nighttime fires observed towards northof the measurement site and is also reflected to thehigher a values and the higher levels of AOD500 on18 January 2006. The results of the study provide anaccount of influence of anthropogenic disturbanceson atmospheric aerosol loading inferred fromground-based observations and satellite data.

5. Conclusions

The present study reports the variation of aerosolloading and solar spectral irradiance over thetropical urban area of Hyderabad, India, undervariable atmospheric conditions. It constitutes acase study of a very turbid-polluted day andinvestigates on the influence of the aerosols on

ARTICLE IN PRESSK.V.S. Badarinath et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 589–599598

solar radiation components. The main conclusionscan be summarized as follows:

1.

The tropospheric aerosol loading has a signifi-cant impact on the solar irradiance reachingurban environments in the tropics.

2.

BC concentration on turbid day was observed tobe 1.5 times higher than that on normal daysuggesting additional aerosol particle sources,such as fires as confirmed from satellite observa-tion to the northwest of the study area. Theconcentrations of fine-mode particles were ob-served to be high on the turbid day and areattributed to both anthropogenic activities andforest fires broken out in the north of the studyarea.

3.

The comparison of UVery intensities on a normalday and on a day with high aerosol loadingsuggests a �24% decrease in UVery amounts onthe day with high aerosol loading.

4.

The DDR ratio showed a significant modifica-tion under high aerosol loading conditions.

5.

The concentration of sub-micron range particles,PMo0.5 mm, has been found to be five timeshigher on turbid day compared to the normalone.

Acknowledgements

The authors thank Director of NRSA and Dy.Director (RS&GIS-AA) for necessary help atvarious stages and ISRO-GBP for funding theproject.

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