Influence of atmospheric aerosols on solar spectral irradiance in an urban area

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Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 589–599 www.elsevier.com/locate/jastp

Influence of atmospheric aerosols on solar spectral irradiance in an urban area K.V.S. Badarinatha,, Shailesh Kumar Kharola, D.G. Kaskaoutisb,c, H.D. Kambezidisb a

Forestry and Ecology Division, National Remote Sensing Agency (Department of Space—Government of India) Balanagar, Hyderabad 500 037, India b Atmospheric Research Team, Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Lofos Nymphon, P.O. Box 20048, GR-11810 Athens, Greece c University 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 can scatter and absorb the incident solar radiation contributing to cooling of the earth’s surface and a simultaneous warming of the lower atmosphere (Keil and Haywood, 2003; Pace et al., 2006). Besides Corresponding author.

E-mail address: [email protected] (K.V.S. Badarinath). 1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.10.010

this direct radiative effect, aerosols act as condensation nuclei in the formation of clouds modifying their microphysical properties. The aerosol-number density, chemical composition and size distribution can influence the albedo and lifetime of clouds as well as the rate and the amounts of precipitation (Abel et al., 2005; Lohmann and Feichter, 2005). Aerosols degrade the air quality in urban areas and reduce visibility. Continental aerosols are mainly wind-blown mineral dust as well as carbonaceous and sulfate particles produced by forest fires, land

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use and industrial activities, while marine aerosols are mainly sea-salt particles produced by wavebreaking and sulfate particles produced by the oxidation of dimethyl sulfide released by phytoplankton. As the oceans cover more than 70% of the earth’s surface, they consist one of the largest sources of natural aerosols. Being hygroscopic in nature, marine aerosols are crucial in cloud formation in the marine boundary layer and are also important in the radiative coupling between ocean and atmosphere. Continental aerosols can be both scattering and absorbing, while marine aerosols are mostly of scattering type (Dubovik et al., 2002); thus, there can be an argument about the planetary albedo. Aerosols in urban environments are physically and chemically different from aerosol in remote areas with the most obvious differences being the high concentration of sulfur and heavy metals in urban aerosols (Latha and Badarinath, 2004). The variety of sources, natural and anthropogenic, the short lifetimes of aerosols and their influence by the meteorological parameters, especially by relative humidity (Day et al., 2000; Ha¨nel, 1976; Horvath, 1996), result in a spatially and temporally heterogeneous aerosol field, making aerosol characterization and modeling a real challenge (Smirnov et al., 2002). On the other hand, the amount of solar ultraviolet (UV) radiation penetrating the earth’s surface is critically important for the health of biological systems (Feister and Grasnick, 1992; Ne´meth et al., 1996; Sutherland et al., 1991); practically no solar radiation reaches the ground at wavelengths shorter than 290 nm due to its strong absorption by stratospheric ozone. The biologically harmful UVB radiation lies in the spectral range 280–320 nm, while erythemal response of the human skin is maximum at about 297 nm. Erythema, which is defined as a reddening of human skin in response to solar radiation, extends through both UVery and UV-A (315–400 nm) (Herman et al., 1996). Autocorrelation between total column ozone and surface UV radiation is a complex function of many variables, including solar zenith angle, altitude, cloud cover, aerosol loading, surface albedo and vertical profile of ozone. The effect of aerosols on the UV radiation constitutes a great scientific issue. Therefore, many studies have been carried out (Liu et al., 1991; Kylling et al., 1998); these researchers have reported that high loading of the absorbing particles could cause reduction of UV flux at the surface by more than 50%. In the last decades, a

continuous increase in the biological active solar UV-B radiation due to a decrease in the ozone amount emerges at a global scale (Zerefos et al., 1995). Nevertheless, at regional scales even a decrease of the ozone amount of 50 DU in combination with an increase in the aerosol loading can lead to a decrease in the UV radiation (Balis et al., 2002; Papayannis et al., 1998). Therefore, continuous ground-based observations play an important role in improving the understanding of some of these effects (Madronich and Flocke, 1997). The precise determination role of all the above parameters in quantifying and modifying the UVery levels is very difficult to be distinguishable due to the combined involvement of all these parameters in the radiation processes in the atmosphere. Therefore, the use of solar radiation models (e.g. SMARTS) is necessary for the improvement of the knowledge at the role of each parameter. Indeed, systematic investigations on the temporal variation of UVery radiation and its influencing parameters are still sparse (Gueymard, 1995). This paper provides a case study of changes in ground-level solar irradiance, diffuse-to-direct-beam ratio and UVery as well as their relationship with the aerosols under different turbidity conditions over the tropical urban area of Hyderabad, India, using simultaneous measurements. Such measurements are very limited all over India.

2. Study area Fig. 1 shows the map of the study area. The study area of Hyderabad is located between 171100 and 171500 N latitude and 781100 and 781500 E longitude. Hyderabad is the fifth largest city in India; its population is 3449.878 inhabitants according to the census of 2001, a purely urbanized area. The climate of the region is semi-arid with a total rainfall amount of 700 mm occurring mostly during the monsoon season in the period June–October. The minimum and maximum temperatures during January 2006 were 10 and 33 1C, respectively, with clear sky conditions. The relative humidity values in January are normally high during nighttime (90%), while during daytime the values lie in the range 30%–40%. The measurements for the case study were carried out in the premises of the National Remote Sensing Agency (NRSA) campus located at Balanagar (171280 N and 781260 E) located well within the urban center under clear sky conditions.

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ing, tR(l), and the contribution of gas absorbers: ta ðlÞ ¼ tðlÞ  tR ðlÞ  tO3 ðlÞ  tNO2 ðlÞ  tmg ðlÞ  tH2 O ðlÞ.

Fig. 1. Location map of the study area.

3. Data sets and methodology The attenuation of the solar radiation in the atmosphere is given via the Lambert–Bouguer–Beer law: E bl ¼ E 0l expðtl mÞ.

(1)

From this exponential decrease, the total atmospheric optical depth can be derived. The relative optical air mass, m, computed via Kasten and Young’s (1989) formula was corrected for pressure variations; t(l) is the wavelength-dependent total optical depth. For a variation of 10% in pressure, the optical air mass leads to a variation in t(l) of about 0.7–0.8% at 500 nm. These uncertainties seem to reach 1% in the UV spectrum. It is assumed, as in Utrillas et al. (2000), that the optical air mass leads to error values lower than 0.1% for zenith angles smaller than 851. The aerosol optical depths (AODs) were obtained from direct-beam irradiance measurements at several wavelengths (380, 440, 500, 675, 870 and 1020 nm) using a MICROTOPS-II sunphotometer with an instrumental accuracy of 72%. The detector consists of a silicon photodiode mounted behind a set of continuously variable interference filters. The AOD, ta(l), was retrieved from the total optical depth after subtracting the Rayleigh scatter-

ð2Þ

The two last components, which are due to the mixed-gases and water-vapor absorption, have no influence in the specific wavelengths of the MICROTOPS II instrument and, therefore, were omitted from Eq. (2). The Rayleigh scattering has been calcu lated by the formula tR(l) ¼ (P/P0)  0.008735  l4.08 (Leckner, 1978). In this formula, P is the actual air pressure in hPa and P0 ¼ 1013.25 hPa. Eck et al. (1999) reported a maximum error in computed ta(l) at 340 nm of 0.021, 0.013 at 380 s and 0.007 at 440 nm assuming a 3% maximum departure from the mean surface pressure. Therefore, as pressure measurements are quite accurate, further errors in the ta(l) determination caused by errors in tR(l) values are negligible. For the calculation of the ozone optical depth, its spectral absorption coefficients provided by MODTRAN were used, while the total columnar ozone amount was measured using a MICROTOPSII Ozonometer. Eck et al. (1999) reported that departures from the climatological ozone values as high as 50% resulted in additional uncertainty in computing ta(l) of only 0.0036 at 340 nm, 0.0045 at 500 nm and 0.0063 at 675 nm. Therefore, these errors in calculating ta(l) are almost negligible. Therefore, the errors in the determination of ta(l) arise from the errors in the measured direct-beam irradiances. The largest sources of error in ta(l) from any instrument are the direct-beam irradiance measurements and the determination of the extraterrestrial irradiance using the Langley calibration, since the errors by subtracting the other components are an order of magnitude lower. It was found, as in Kaskaoutis et al. (2006c), that the errors in computing ta(l) are higher under low turbidity conditions due to instrumental uncertainties. This means that on days with high turbidity, the relative error is lower than 5% in most cases, while under low turbidity conditions the relative error is usually over 10%. A UV-B radiometer from Solar Light Co. (Gayatri and Prasad, 1993) located in the NRSA campus was used to measure UVery in the range 280–315 nm. The cosine response of the instrument is 75% with a resolution of 0.01 MED h1 (Devara et al., 1996; Niranjan et al., 1995). Moreover, a multi-filter rotating shadow band radiometer (MFRSR) was used for this study. At nominal wavelengths of 415.9, 496.6, 622.4, 670.2, 868.3, and 938.5 nm (FWHM10 nm), the MFRSR

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4. Results and discussion Using the methodology described above the AOD was derived using the MICROTOPS II on two specific days representative of relatively clear (normal) and turbid atmospheres. Fig. 2a shows the diurnal variation of AOD at 500 nm (AOD500) on normal and turbid days as obtained through MICROTOPS II measurements. The AOD500 values are markedly higher on turbid compared to normal day. On 18 January 2006, the AOD500 values were about 0.6, remaining very high for the whole day with unimportant diurnal variation. Little higher values were derived at both early morning and noon hours as a consequence of the enhanced local activities in the city. Also, from Fig. 3 the higher BC concentrations at the early

a AOD (500nm)

takes measurements of total, diffuse horizontal, and direct-beam irradiances. The instrument’s sensors are shaded at periodic intervals (5 min) by a rotating shadow band, so the diffuse radiation is measured. The shadow band then moves, and a measurement of total downward radiation is performed. The difference of these two quantities is the downward component of the direct-beam solar irradiance. The major advantage in using this technique is that each measurement (total, diffuse, and direct-beam) is calibrated identically, thus reducing errors that would be arisen using independent instruments. The Ultraviolet MFRSR (UVMFR-7) is an instrument that measures diffuse and total global irradiance, and computes direct irradiance at four or seven narrow-bandwidth wavelengths in the UV-B and UV-A regions. Continuous and concurrent measurements of the mass concentration of black carbon (BC) were also carried out using an aethalometer; model AE-21 of Magee Scientific. The instrument aspirates ambient air from an altitude of 3 m above the ground using its inlet tube and its pump. The BC mass concentration is estimated by measuring the change in the transmittance of a quartzfilter tape, on to which the particles impinge. The instrument has been operated at a time base of 5 min, continuously on the experiment days with a flow rate of 3 l min1. The instrument has been factory calibrated and errors in the measurements are 72%. In addition, continuous measurements of PM grain-size distribution were performed with a GRIMM 1108 laser spectrometer (Le Canut et al., 1996). This dust monitor determines the particle matter level in 15 different grain-size channels from 0.3 to 420 mm.

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Fig. 2. (a) Diurnal variation of AOD at 500 nm on different days. (b) Aerosol optical depth (AOD) variation at 1100 h (LST) on different days.

morning hours on 18 January are obvious, which have an influence in the enhancement of the AOD500. In a previous study in the same area (Latha and Badarinath, 2005), it was found that a significant correlation between BC concentration and aerosol loading exists. Unfortunately, irradiance data after 16:00 LST, where the BC concentrations are the highest, were not available due to sunset. These high AOD500 values are representative of urban environments as they are of the same magnitude with high AOD500 values reported for Athens (Kaskaoutis et al., 2006c). Also, the AOD500 values estimated in Hyderabad on the turbid day is sufficiently higher than those reported by Dubovik et al. (2002) for urban sites, while are comparable with the AODs for regions that are affected by biomass burning episodes (Dubovik et al., 2002). Therefore, it is concluded that this high aerosol loading on 18 January is not only of urbanindustrial origin. On the other hand, 20 January can be characterized as a relatively clear day for this urban area, although the AOD500 values are much higher than background-aerosol conditions (Smirnov et al., 2003). Nevertheless, in the early morning the AOD500 exhibits relatively high values, about 0.4, strongly correlated with the BC concentrations, Fig. 3. In the rest of this day the decrease of the BC

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Fig. 3. Black carbon aerosol mass concentrations on different days measured by an aethalometer.

concentration in combination with favorable meteorological conditions, e.g. higher air temperatures leads to the ventilation of the atmosphere over Hyderabad. Fig. 2b shows the AOD estimated at different wavelengths 380, 440, 500, 675, 870, and 1020 nm using the sunphotometer on each experimental day. In this figure, the AODs exhibit different optical characteristics. The wavelength dependence of the AOD differs significantly from day-to-day. On 18 January, the distinct feature of the spectral AOD distribution is its gradual and discernible decrease with wavelength. It is worth to be noted that this feature was evident throughout the day. At the shorter wavelengths, the AOD increases significantly implying more curvature in its spectral distribution. This is attributed to the presence of a higher fine-to-coarse mode ratio on this day. Since the aerosols are mainly of anthropogenic origin (fine-mode particles, Eck et al., 1999), the spectral distribution of the AOD is stronger (Cachorro et al., 2000). The presence of a higher concentration of the fine-mode particles, which are selective scatters, enhances the irradiance scattering and, therefore, the AOD values with a greater level at the shorter wavelengths. The fine-mode particles have a much greater effect on the AOD at the visible wavelengths than at the near infrared wavelengths. Likewise, the coarse-mode particles provide similar contributions to the AOD at both wavelengths (Schuster et al., 2006). Moreover, as reported by Molna`r and Me´sza´ros (2001), the fine particles are responsible for the 82% of the scattering in the atmosphere. On the other hand, the AODs on 20 January appear to have little or much weaker wavelength dependence, representative of rural environments (Cachorro et al., 2000). Such a weak wavelength dependence was also evident for desert dust aerosols in the Persian Gulf (Smirnov et al., 2002).

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The high occurrence of fine-mode particles is reflected in the high values of the A˚ngstro¨m exponent a (Schuster et al., 2006) with a simultaneous increase in the turbidity parameter b. The A˚ngstro¨m parameters were estimated from the AOD values at 380 and 870 nm, using the Volz method. On the normal day the mean a and b were 1.126 and 0.142, respectively, while on the turbid day these values were increased to 1.334 and 0.229, respectively. The high b values are representative of urban environments and are of the same magnitude with those reported for Athens (Jacovides et al., 2005; Kaskaoutis et al., 2006c). The a values are also similar with those reported for the Athens area revealing similar atmospheric conditions over the two urban areas. The correlation between a and AOD can reveal the aerosol type under specific circumstances. For example, moderate to small a values associated with maritime aerosols generally correspond to small AODs, whereas aerosol from biomass burning show large AODs and large a values and desert dust aerosols show large AODs and small a values (Cachorro et al., 2001). However, the a values depend strongly on the spectral interval used for their determination (Cachorro et al., 2001; Jacovides et al., 2005). Hence, the information contained in the a-AOD scatter plot becomes more difficult to interpret and, therefore, this diagram is not presented here. On the other hand, detailed spectral information given by the determination of a in different spectral ranges, together with the AOD at different wavelengths, constitutes a useful tool for the determination and discrimination of the aerosol type (Cachorro et al., 2001; Reid et al., 1999; Schuster et al., 2006). The AOD showed slightly higher values at 870 nm compared to that at 675 nm implying negative a values for this narrow spectral interval. The same feature was reported by Cachorro et al. (2001), since they estimated negative a values in the spectral range 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 to be high on 18 January compared to those on 20 January (Fig. 3) with an average concentration 1.5 times larger than that on normal day values. On both days, BC concentrations exhibits a distinct diurnal variation with the higher values taking place at the evening hours, after 19:00 LST due to traffic density and decreased boundary layer height (Latha

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et al., 2004). High BC concentrations also occur in early morning due to increased human activities in the urban area. At noon, the enhanced height of the mixed layer leads to the dispersion of the pollutants and the ventilation of the air. The higher BC concentrations at early morning hours on both days have a great influence in the diurnal variation of the AODs (Fig. 1a). In order to understand the possible sources of such a high aerosol loading on 18 January, DMSPOLS nighttime satellite processed data sets for forest fires were analyzed. Fig. 4 shows the nighttime fires image generated from DMSP-OLS, NOAA-AVHRR false color composite (FCC) nighttime data of 17 January 2006 overlaid on 18 January. In the NOAA-AVHRR FCC, fire burnt scars can be seen in forest regions around the study site of Hyderabad. It can be seen from Fig. 4 that the majority of the forest fires were prevalent mainly northwest of the study area. A useful tool for the data interpretation is the HYbrid Single-Particle Langrangian Integrated Trajectories (HYSPLIT) code (Draxler and Rolph, 2003). This program allows for the calculation of the air masses back-trajectories once the trajectory levels, the day and the time are fixed (Espozito et al., 2004). In the present study, the back-trajectories reaching at Hyderabad have been estimated over the 5 days preceding 18 January at three levels in the atmosphere, at 500, 1500 and 2000 m a.s.l. The HYSPLIT model (Fig. 5) suggested back-trajectories coming from north on 18 January (00.00 h), directly influencing the study area with a significant amount of biomass burning aerosols. The high AODs of this aerosol type (Dubovik et al., 2002) in addition with the anthropogenic and industrial activities in the urban area suggest the sufficiently high AODs on 18 January. It is also apparent that for the 5 days, the air masses do not move at large distances, but remain relatively constant above continental India, which in this dry season is quite susceptible to forest fires. Moreover, the biomassburning aerosols, and especially the fresh smoke particles are very absorbing, also exhibiting a strong wavelength dependence on their AODs (Reid et al., 1999). This feature was obvious in the AOD wavelength dependence in Fig. 2b. The different aerosol loading and their different properties on the 2 days examined have a great influence on the amounts of solar radiation reaching the ground. A marked reduction in UVery (Fig. 6) implying a 24% mean reduction of UVery at

ground level was observed on 18 January 2006 compared to the normal day. Such a reduction in UVery due to an increase in aerosol loading has also been reported in the literature (Krzyscin and Puchalski, 1998; Reuder and Schwander, 1999). The diurnal variation of the UVery follows solar zenith angle, exhibiting higher values at noon, where solar irradiance is more intense. It is also obvious that at 10:00 LST the UVery amounts are similar on both days, since at this time the AODs exhibited their least difference, Fig. 2a. For the same reason, the differences in the UVery on the 2 days are higher in the afternoon, where the atmosphere on 20 January is clean. The aerosol-particledensity number showed higher values on 18 January with an additional increase in fine-mode particles. The sources of such fine-mode particles are the anthropogenic and industrial activities, the gases from automobile exhausts and also the biomassburning particles reaching Hyderabad from the neighboring fires of the previous days. As the vehicular traffic is continuous in the urban area of Hyderabad during the period of measurements, the additional source of the presence of higher finemode particle concentration on 18 January could be due to the biomass-burning aerosols, as forest fires were observed in DMSP-OLS nighttime satellite data. Earlier reports on chemical analysis of aerosol samples in urban areas of Hyderabad suggested a K+ concentration implying the possible sources of biomass-burning emissions (Kulshrestha et al., 2004). Similar to the UVery amounts, the spectral irradiances exhibit a significant attenuation on the turbid day. In Fig. 7, the solar spectral global irradiances measured using both the MFRSR and UVMFRSR instruments at ten wavelengths are plotted. 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 irradiance values is negligible and, therefore, the differences between the 2 days are attributed to the different aerosol type and loading. A significant reduction in global spectral irradiance is obvious on the turbid day (18 January) compared to the normal day (20 January). This attenuation exhibits a clear wavelength dependence with the higher differences taking place in the UV and VIS spectrum. The global spectral distribution exhibits the same pattern on both days, and practically no irradiance reaches 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.

ozone and the strong attenuation (scattering and absorption) caused by the aerosol layer. The higher relative differences in global irradiances on the 2 days are depicted in the UV band, a reduction of about 34% is estimated at 317.1 nm, which decreases at 496.6 nm (21%) and practically vanishes at 938.5 nm. The mean spectral difference at all wavelengths of UVMFRSR and MFRSR in

global irradiance was estimated to be 28% between normal and polluted days. Therefore, the presence of the fine-mode particles on the turbid day has a great influence on solar irradiance attenuation at the shorter wavelengths, as suggested in other studies (Reid et al., 1999), too. On the other hand, in the NIR spectrum the differences in irradiance values due to the differences in aerosol loading are very

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Fig. 6. Variation in UVery on normal and high aerosol loading condition.

Fig. 7. Variation of total solar irradiance in different spectral bands measured using UVMFRSR and MFRSR for different days.

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

NIR are also attributed to the high presence of finemode particles, which exhibit a selective attenuation of the solar irradiance. In contrast, a higher

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Fig. 8. Variation of diffuse to direct-beam radiation in MFRSR bands with aerosol optical depth in different bands measured using MICROPTOPS-II on different days at 1100 h.

percentage of coarse-mode particles on the turbid day would decrease the irradiance differences at the shorter wavelengths, while it would increase them at the longer wavelengths. The high concentration of fine-mode particles seems to also have modified the diffuse-to-directbeam irradiance ratio (DDR) in different spectral bands as shown in Fig. 8. This figure refers to the same SZA (401) for the 2 days, since the spectral measurements correspond to 11:00 LST. Therefore, the significant differences in the curves are attributed to the aerosol loading and their different optical properties. The DDR ratio at a specific wavelength as a function of the AOD has already been used for the discrimination of biomass burning particles in the Mediterranean, as the more absorbing aerosols exhibit lower DDR values (Meloni et al., 2005). It is obvious that the DDR for the turbid day suggested a marked deviation with very high values especially at the shorter wavelengths. The changes (increase) in DDR at shorter wavelengths are more intense than those at longer due to enhanced values of the diffuse radiation. The important role of scattering is verified at these wavelengths, where the depletion of the direct-beam irradiance by scattering processes is regained as increasing diffuse irradiance, leading to large ratio values. It is noted that at longer wavelengths the ratio appears to be of the same order of magnitude for both days. Exponential fits can describe the DDR–AOD correlations with a great accuracy, as the coefficient of determination, R2, is very high on both days. The good fitting as well as the constant values of the curves are a general characteristic of the correlation between spectral DDR and AOD as it has been established in recent studies (Latha and Badarinath, 2005; Kaskaoutis et al., 2006a, b). It is

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also apparent that the dependence of the ratio on spectral AOD is much smoother under highly turbid air, implying lower exponent values, 3.14 against 7.66 for low turbidity. On the other hand, the constant value is higher for the turbid day, 0.089 against 0.021 for the low turbidity case. The different constant values of the exponential fits reported by Kaskaoutis et al. (2006b) are attributed to the different spectra used, since the UV region is not included in the present study. Nevertheless, the smoother ratio dependence under high turbid air is also apparent in Athens (Kaskaoutis et al., 2006a, b). The higher concentration of fine-mode particles also reflected in the aerosol-particle size measurements using GRIMM on turbid day (Fig. 9). The analysis of GRIMM aerosol-particle analyzer data sets suggested that the number of particles reveals a higher load of PM in the range of 1–3 mm on 18 January compared to 20 January. Such high finemode particles could result from forest fires that were observed in the north of the study area with favorable wind direction. This is correlated with the DMSP-OLS nighttime fires observed towards north of the measurement site and is also reflected to the higher a values and the higher levels of AOD500 on 18 January 2006. The results of the study provide an account of influence of anthropogenic disturbances on atmospheric aerosol loading inferred from ground-based observations and satellite data. 5. Conclusions The present study reports the variation of aerosol loading and solar spectral irradiance over the tropical urban area of Hyderabad, India, under variable atmospheric conditions. It constitutes a case study of a very turbid-polluted day and investigates on the influence of the aerosols on

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solar radiation components. The main conclusions can be summarized as follows: 1. The tropospheric aerosol loading has a significant impact on the solar irradiance reaching urban environments in the tropics. 2. BC concentration on turbid day was observed to be 1.5 times higher than that on normal day suggesting additional aerosol particle sources, such as fires as confirmed from satellite observation to the northwest of the study area. The concentrations of fine-mode particles were observed to be high on the turbid day and are attributed to both anthropogenic activities and forest fires broken out in the north of the study area. 3. The comparison of UVery intensities on a normal day and on a day with high aerosol loading suggests a 24% decrease in UVery amounts on the day with high aerosol loading. 4. The DDR ratio showed a significant modification under high aerosol loading conditions. 5. The concentration of sub-micron range particles, PMo0.5 mm, has been found to be five times higher on turbid day compared to the normal one. Acknowledgements The authors thank Director of NRSA and Dy. Director (RS&GIS-AA) for necessary help at various stages and ISRO-GBP for funding the project.

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