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Spatio-temporal aerosol optical characteristics over the Arabian Sea during the pre monsoon season D. G. Kaskaoutis, M. C. R. Kalapureddy, P. C. S. Devara, P. G. Kosmopoulos, P. T. Nastos, K. Krishna Moorthy, and H. D. Kambezidis Atmospheric Research Team, Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Lofos Nymphon, P.O. Box 20048, 11810 Athens, Greece Physical Meteorology and Aerology Div., Indian Institute of Tropical Meteorology, Pahan, Pune 411008, India Department of Geology and Geoenvironment, University of Athens, University campus 15784 Athens, Greece Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India


Introduction
The characterization of aerosol particles and the spectral dependence of their extinctions are very important as they strongly influence the radiative properties (single scattering albedo, asymmetry factor, refractive index) in the atmosphere (El-Metwally et al., 2008).The Arabian Sea region has a unique weather pattern in view of the Indian monsoon and the associated winds that reverse direction seasonally.It is a location where the pristine air masses from the Southern Indian Ocean (IO) and the polluted air from Asia meet during winter and spring season, providing a very interesting area for aerosol studies (Ramanathan et al., 2001).Moreover, the investigation of the aerosols over oceans is important from the standpoint of understanding their anthropogenic and natural impacts as well as in estimating their contribution to radiative forcing (e.g.Haywood et al., 1999;Satheesh et al., 2006a).However, the high spatio-temporal variability and limited measurements of the aerosol physical and optical properties over oceanic regions makes it difficult to understand their impact on weather modification and hence climate change.Due to these reasons the oceanic regions, surrounding India, have always been the subject of importance and investigating extensively by various experimental campaigns.Despite the airmass origin and the local and regional meteorology, the aerosol load and spatial distribution around the AS region are highly variable from day to interanual scales and strongly affected by the El Nino-La Nina patterns and the location of the ITCZ (e.g.Moorthy and Satheesh, 2000;Ramachandran and Jayaraman, 2002;Li and Ramanathan, 2002).It has been noticed higher AODs and steeper gradients over the northern part of the AS which is mainly attributed to the advected mineral dust from west Asia and also shown a large effect on the radiative forcing in the region (e.g.Satheesh et al., 2006a).Larger sea-salt contribution to the AOD found over AS, especially over the southern AS, during the summer monsoon season which is directly associated with the stronger sea-surface winds (Satheesh et al., 2006b).The chemical species that contribute significantly to the AODs over the AS noticed to be sulfate, sea salt and nitrate, potassium, organics, Black Carbon (BC), dust, fly ash and ammonium (Savoie et al., 1987;Krishnamurti et al., 1998;Nair et al., 2004;Kumar et al., 2008).
Large reduction in BC mass fraction over the AS region found from winter to summer significantly affect the seasonal radiative forcing (Babu et al., 2004).Several earlier estimates of the anthropogenic contribution to mean AOD over the AS and northern Indian Ocean showed large variations from 65% to >90% (Satheesh et al., 1999;Ramachandran 2004b).The total aerosol-mass concentration decreased from as high as about 80 µgm −3 near the Indian coast to very low values of only a few tenths of µgm −3 over the most distant oceanic regions.The large increase in the small-particle concentration near the coast was also consistent with the corresponding large increase in the Ångstr öm exponent, which increased from 0.2 over the Indian Ocean to about 1.4 near Introduction

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Full the Indian coast (Krishnamurti et al., 1998).In the following years we might expect aerosol concentrations in this region to increase in concern with the rapid economic development that is taking place in this region, especially in the Indian subcontinent.Therefore, it needs continuous and systematic efforts to monitor the aerosol field and properties over this region since the knowledge of their effects on the marine environment and in our changing planet is a real challenge.
The Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) (Moorthy et al., 2008) was conducted in the pre-monsoon season of 2006 over the Bay of Bengal (BoB), IO and AS.In the present study, we use the ICARB campaign ocean segment data over AS during the pre-monsoon season (April-May) of 2006 focusing on the AOD and Ångstr öm exponent spectral variation and curvature, which can constitute the basis for aerosol type discrimination over the area.The results are examined against total and size-resolved aerosol-mass concentration measured simultaneously in the marine atmospheric boundary layer (MABL).In addition, we also discuss the errors and the uncertainties revealed in the estimation of the curvature of the lnAOD vs lnλ line over oceanic regions, where the AOD is relatively low.The inclusion of two wavelength bands for characterizing the aerosol optical properties reveals the effects of the wavelength in the aerosol microphysical properties, which can be significant over oceanic regions with a very inhomogeneous aerosol field.These investigations are the very first showing the spatial distribution of the AOD curvature over the AS region.

Data collection
The whole set of oceanic segment measurements during the ICARB campaign was conducted onboard the Oceanographic Research Vessel Sagar Kanya.AS region was covered in the second leg (SK223B) of the campaign started from the port of Kochi (on the west coast of India, Fig. 1) on 18 April and ended after 24 days at Goa port (15.4 • N, 73.8 • E) on 11 May.It reached its farthest west point ( 14• N, 58 possible marine regions in the AS to be covered during the period of observations (see Fig. 1).The points on the track show the position of the ship at 05:30 UTC on each day obtained from a GPS receiver; the dates being identified beside with suffix A for April and My for May.The intense field phase of ICARB in the AS region covered the longitudinal sector 58 • E-76 • E with a latitudinal coverage from 8 • N to 22 • N.
High temporal resolution (∼10 min) observations of direct-beam solar radiation were made using the hand-held sunphotometer (Microtops-II, Solar Light Company, USA).This sunphotometer provides the AOD at 6 channels (340,440,500,675,870, and 1020 nm with a FWHM of 5 to 10 nm), columnar water vapor and ozone derived from instantaneous solar flux measurements using its internal calibration.The field-of-view (FOV) of the Microtops-II is 2.5 • .A typical error in the AOD estimation from the Microtops-II measurements is ±0.03 (Morys et al., 2001).
It should be noted that Shaw (1980) reported anomalous weak absorption at 1010 nm, possibly from water vapor, since the extinction at this wavelength increased when column water vapor (PW) increased.Eck et al. (2001) showed, using water vapor line spectrographic parameters as given by Giver et al. (2000), that at 1020 nm computed water vapor optical depth of ∼0.009 for a tropical atmosphere with 4.0 cm of PW.There is another possible uncertainty associated with the 1020 nm channel due to the temperature-sensitivity of silicon detector at this wavelength (∼0.002/ • C) (Holben et al., 1998).This remains a source of uncertainty at AOD 1020 that may not exist for the other wavelengths where the detector temperature sensitivity is insignificant.Unfortunately, the Microtops II algorithm does not take into account the temperature sensitivity of the silicon photodiodes.This is probably a problem only during very clear conditions when the AOD is very small.However this uncertainty of AOD 1020 has minimized by keeping the Microtops II in shaded area after each observation to maintain it with the near same environmental temperature.In the present study, PW is observed to be 2.22±0.44 cm (Kalapureddy et al., 2008).So, the possible water vapor absorption effects at 1020 nm have not been considered in this study.
During the campaign three sets of measurements were collected in quick succession Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion (∼1 min) at every 10-min time interval, to avoid any possible errors due to sun pointing on a moving platform.If the three measurements were not close in magnitude, thus producing large differences (above 5%) in the AODs, the dataset was rejected from further analysis.Great attention was given to choose which of the three measurements in each 10-min interval should be retained for further analysis.Thus, they were retained for further analysis that AOD estimation of the three derived, which gave the best second-order polynomial fit to the lnτ α vs lnλ data points (Eck et al., 1999;Kaskaoutis et al., 2007a); the other two AODs were discarded.The fit was controlled by the highest R 2 associated with it.In case the errors were close to each other, additional criteria were followed, such as the AOD values not to change significantly from the previous measurements or the spectral AOD variation to have as less spectral curvature as possible.Furthermore, data recorded around cloud passage or near the FOV of the instrument were not considered for analysis (Kalapureddy and Devara, 2008).After this procedure 1268 out of 4443 spectral measurements remained for further analysis.Thus, depending on the sky conditions the number of available data varies from 23, on 19 April to 69 on 1 May.

Climatology of the area
The climatology of the area is mainly dictated by the presence of monsoon, which divides the year into four periods.The mean winds are weak and predominantly from the north/northeastern direction during the winter monsoon season (December to March) where advection from the Indian continent is important.In the summer monsoon season (June to September) the mean winds are strong (10 to 15 ms −1 ) and mainly westerly/southwesterly favoring the advection from oceanic regions and western land areas.
During March-June, it is hot and dry in the Africa, Arabian Peninsula and the Iranian desert regions, while the westerly winds carry significant amount of dust particles over Northern AS (Moorthy and Satheesh, 2000;Li and Ramanathan, 2002;Moorthy et al., 2005).A complete analysis of the wind pattern and rainfall over AS has been reported in Moorthy and Satheesh (2000).During the ICARB campaign, winds over the AS are mainly from the northwestern sector with an average speed of about 6-9 ms −1 (Kalapureddy and Devara, 2008).Precipitation was almost absent throughout the cruise period.A low level anticyclone centered over the central AS was the cause for the northwesterly winds.The mean synoptic wind pattern (derived from the NCEP-NCAR reanalysis data) at 850 hPa over AS during ICARB comprised of strong westerlies over the northern part AS and weaker northerly/northeasterly over the southern part of AS (Nair et al., 2008).

Theoretical background and methodology
The spectral dependence of AOD is frequently parameterized by the Ångstr öm exponent α, which is computed from the Ångstr öm's (1961) empirical formula: where β represents the aerosol load in the atmosphere and is equal to the AOD at 1 µm, while α contains information about the nature of the aerosol particles present in the atmosphere and is a good indicator of their size or of the fraction of the fine-mode aerosols in the atmosphere (Schuster et al., 2006).The validity of the Ångstr öm formula assumes the validity of the Junge power law (Junge, 1955) for the aerosol-size distribution, especially in range from 0.05 to 10 µm, where significant extinction takes place and the spectral variation of the refractive index does not impose significant variations on the Mie extinction factor.The Junge power law introduces a lognormal aerosol-size distribution, which is far away from the bimodal distribution of the aerosols in the atmosphere (Eck et al., 1999).Consequently the Ångstr öm formula is not always valid for all environments and especially in an extended wavelength range.Therefore, the Ångstr öm formula is, in general, well fitted to the AOD values at the shorter or longer wavelengths; when the spectral range is extended, significant uncertainties may reveal (Kaskaoutis and Kambezidis, 2006).This Introduction

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Full fact leads to the departure from linearity when the spectral AOD is plotted in logarithmic coordinates (Eck et al., 1999;Kaskaoutis et al., 2007a).Due to this discrepancy, very differing α values can be obtained for different wavelength ranges (Reid et al., 1999;Kaskaoutis et al., 2007a).Under these conditions, a curvature is observed in the lnτ α vs lnλ data points, which contains useful information about the aerosol type and size distribution (King and Byrne, 1976;Eck et al., 1999;O' Neill et al., 2001).Therefore, a second order polynomial fit to the lnτ α vs lnλ data can give a greater accuracy: where the coefficient a 2 accounts for the curvature often observed in sunphotometric measurements (Eck et al., 1999;Kaskaoutis et al., 2007a).
The AOD values at each wavelength are obtained from the direct-beam irradiance measurements via the Bouguer-Beer law.Extensive analysis of the errors retrieved via this methodology is described in Kaskaoutis and Kambezidis (2006).The ozone optical depth was omitted in the derivation of the AOD, since its contribution to the total atmospheric optical depth can be significant under low turbidities.However, no correction was taken into account due to the trace-gases optical depth assuming it negligible over the oceanic regions (Kaskaoutis et al., 2007b).The Ångstr öm parameters, α and β, were calculated via the least-squares method in the spectral ranges 340-1020 nm and 340-870 nm.According to the analysis performed by Kaskaoutis and Kambezidis (2008), this method is the least imprecise and wavelength dependent for the calculation of the Ångstr öm parameters.The second-order polynomial fit (Eq.2) was also applied to the AOD values at six wavelengths (340, 440, 500, 675, 870 and 1020 nm), although three of them were just sufficient for this computation.The same fit was also applied excluding the 1020-nm wavelength in order to investigate the differences in the retrieved parameters.Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion

Estimation of errors and uncertainties
Before analyzing the temporal and spatial variation of the aerosol optical properties over AS a detailed analysis is given regarding the errors and the uncertainties of the 2nd order fits to the spectral AOD data.An account of the typical errors in the estimation of the a 1 and a 2 (Eq.2) as well as some relationships between the computed coefficients, are provided in this section.The second-order polynomial fit to the lnτ α vs lnλ data has been shown to provide good agreement with measured AODs, involving differences of the order of the uncertainty in the measurements, opposite to the linear fit to the lnτ α vs lnλ, which yields significant differences with measured AODs (Eck et al., 2001;Kaskaoutis and Kambezidis, 2006).However, this is not always valid over oceanic regions with relative low AODs.Figure 2 shows the correlation of the typical errors in a 1 and a 2 computed in the two spectral bands, 340-1020 nm and 340-870 nm with the AOD at 500 nm (AOD 500 ).Large errors in the computed coefficients are seen for low AOD 500 values (<0.2) also exhibiting large variation.Nevertheless, as the atmospheric turbidity increases (AOD 500 >0.3) the error in the estimates decreases dramatically.This feature is common in sunphotometric measurements, since has been observed in many studies (see Kaskaoutis et al., 2006b and references therein) and is attributed to the higher scatter of the spectral AODs under low turbidity conditions.The day-to-day variation of the errors in a 1 and a 2 is presented in Fig. 3. Large errors occurred in a 1 and a 2 in the period from 21 to 24 April and, especially, from 1 to 4 May, closely associated with low AOD values as will be seen in the following since the ship was cruising at the far AS.On the other hand, low errors are presented in the period from 25 to 27 April, the ship was near to Indian coast and from 6 to 10 May (ship in the northern AS), both closely associated with high AODs.When the spectral band 340-1020 nm was used for the polynomial fit, the errors were significantly lower in almost all cases, exhibiting mean values of erra 1 =0.295±0.182and erra 2 =0.273±0.166against those of erra 1 =0.495±0.284and erra 2 =0.380±0.229computed when the 340-870 nm was considered.It was found that the highest errors in both a 1 and a 2 are found on Introduction

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Full days or cases where the curvature (a 2 ) in the the lnτ α vs lnλ relation was high and, more specifically negative, indicative of the presence of fine-mode aerosols.As will be seen in the following, on the days of high errors (21-24 April and 1-4 May), when the ship was cruising in the far AS, the α values were the highest; on the other hand significant presence of coarse-mode aerosols in the northern part of the AS (29 April and 6-10 May) results in a bimodal aerosol size distribution where the linear and the second order polynomial fits present higher accuracy.To this respect, the correlations between the errors in a 1 and a 2 with the Ångstr öm exponent α for each of the two spectral bands used are strongly positive.The following relations hold: These results are in general agreement with those reported by previous studies (Eck et al., 1999;Kaskaoutis et al., 2007a) that showed that the 2nd order polynomial fit presents larger uncertainties for large fine-mode aerosol fraction, while the fit seems to be more accurate for bimodal aerosol size distributions with a large coarse-mode fraction.In general, Fig. 3 showed that the use of the 340-1020 nm band, instead of the 340-870 nm one, seems to be more appropriate for the aerosol retrievals.This fact also indicates the great effort and attention spent on the accuracy of the AOD 1020 .
Through the Ångstr öm's formula (Eq. 1) both coefficients α and β are assumed independent from each other and from the wavelength interval used for their determination.However, it is well known from numerous publications (e.g. the recent study by Kaskaoutis and Kambezidis, 2008) that both parameters depend on wavelength.When their computation takes place at only shorter or longer wavelengths large uncertainties in the retrievals may occur; these standard errors are reduced when the spectral range Introduction

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Full includes both shorter and longer wavelengths.In Fig. 4, α and β calculated in the spectral bands 340-1020 nm and 340-870 nm are correlated.As regards β, the correlation seems to be excellent, since the R 2 value is 0.99, the slope and the internship near to 1 and 0, respectively.As a result, the mean β values are differing in the third decimal.On the other hand, greater uncertainties associated with higher scatter in the correlation plot are shown regarding the α values (R 2 =0.85).In some cases the data points are far away from the y=x line exhibiting large differences.The mean α value is larger at the 340-870 nm band, while for higher α values those retrieved at 340-1020 nm band overestimate those at 340-870 nm band.Similar correlations between the α values computed in similar wavelength ranges are presented over four AERONET sites (Kaskaoutis et al., 2007a).However, the study showed that the R 2 value was lower (0.81) in the clean oceanic environment of Nauru compared to that over land sites (R 2 ranged from 0.89 to 0.96).The larger dependence of α values on wavelength is attributed to the fact that this parameter is more sensitive to the spectral AOD distribution (the decreasing rate of AOD with respect to wavelength), while β expresses the AOD at 1 µm by definition.In Fig. 5, the correlation between α and a 1 (Eqs.1, 2) is displayed for all the period of measurements using the two spectral intervals.In both figures, as expected, there is an anti-correlation between α and a 1 , exhibiting a significant scatter.The scatter is more intense regarding the calculations in the 340-870-nm spectral range where the R 2 value is 0.61.A little higher R 2 value is obtained in the spectral band 340-1020 nm, thus implying better agreement between α and a 1 .This fact indicates that the curvature is not so significant and the AOD spectral distribution in logarithmic coordinates approaches linearity (α=−a 1 ).In a previous study (Kaskaoutis et al., 2007a) it was found that the coefficient a 1 is significantly different from α, especially in the maritime and desert aerosol environments of Nauru and Solar Village, respectively.Considering the scatter of the α vs. a 1 data points the role of the curvature in the spectral AOD becomes significant.Consequently, the inclusion of the spectral curvature in the analysis enhances the knowledge about the volume fraction and effective radius of the Introduction

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Full  et al., 2006).From Fig. 5 it is concluded that the use of the 340-1020-nm spectral band for the calculation of the α, a 1 and a 2 values leads to the least imprecise results and with the lowest curvature.This will be further established by the lowest a 2 values in the 340-1020-nm region in the following section.
According to Schuster et al. (2006), the Ångstr öm exponent α is equal to the difference a 2 -a 1 to a first approximation.An attempt to verify the validity of this statement is given in Fig. 6.When the curvature is negligible (a 2 ∼0), then α=-a 1 .The correlations between the Ångstr öm exponent α and the differences a 2 -a 1 are shown in Fig. 6 for the spectral ranges 340-1020 nm and 340-870 nm.The two parameters are strongly correlated, as indicated by the R 2 values of 0.96 (340-870 nm) and 0.99 (340-1020 nm).Data points lying onto the y=x line or closely around it indicate the validity of the relation a 2 -a 1 = α.According to our analysis, the scattered points (340-870 nm case) correspond to cases where the second-order polynomial fit does not provide high accuracy.These findings are in agreement with those by Schuster et al. (2006) who concluded that the relation α=a 2 -a 1 can be considered valid for bimodal size distributions with equivalent contributions from fine-and coarse-mode particles.An overestimation of the a 2 -a 1 against α is observed for high α values (upper panel).Schuster et al. (2006) reported that the condition a 2 -a 1 ≥2 corresponds to size distributions dominated by fine-mode aerosols and the a 2 -a 1 ≤1 to size distributions dominated by coarse-mode particles, whereas intermediate values of a 2 -a 1 to a wide range of fine-mode fractions.Values greater than 2 are absent in our study, while a significant number of values <1.0 is observed in both graphs.This is expected since the measurements were performed over an oceanic environment where coarse-mode aerosols have a significant fraction.Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion 6 Results

Daily variation of the aerosol properties
Figure 7 presents the frequency distributions of AOD 500 and α 340−1020 .The AOD 500 varies significantly from 0.07 to 0.706 thus implying large variability in aerosol load over the entire AS during the ICARB period.The mean AOD 500 in AS was found to be 0.246±0.114,which is significantly higher than that observed over open oceanic regions in Pacific and Atlantic oceans (Smirnov et al., 2002).However, the AOD 500 value over AS is much lower than that observed over BoB (0.36±0.12) during ICARB (Kalapureddy and Devara, 2008), or from previous studies (e.g.Ramachandran, 2004a).The highest frequency of occurrence is depicted at AOD 500 =0.15, while a significant fraction of the AOD 500 values is above 0.3.The AOD 500 frequency distribution appears rather lognormal, with the lower values belonging to the far AS region and the higher ones near the west coast of India and in the northern part of AS, where the effect of the nearby deserts becomes important.The frequency distribution of the α 340−1020 approaches the Gaussian distribution as was also observed in the Athens area in May 2005 (Kaskaoutis et al., 2006).This fact implies simultaneous presence of fine-and coarse-mode particles since the α 340−1020 varies widely from 0.186 to 1.558, exhibiting a mean value of 0.904±0.186.This value agrees with that of 0.86±0.20 over AS and is significantly lower than that (1.21±0.11)over BoB reported by Kalapureddy and Devara (2008) during ICARB.The highest frequency is depicted in the interval 0.8-0.9, while similar frequency of occurrences for high (α>1.the Bermuda and Ascension islands, the mean α values were found to be 0.93 and 0.62, respectively (Smirnov et al., 2002).These values are, in general, lower than that over the AS (0.90) calculated in the present study.This indicates the higher influence of the continental anthropogenic aerosols of the Indian subcontinent that dominate the aerosol load over this area.All the studies conducted over the oceanic regions surrounding India (BoB, IO and AS) agree with the higher α values in comparison with those over open oceans.Figure 8 presents the spectral AOD variation for all the days of the ICARB campaign in the AS region.The AOD at 340 nm (0.315±0.136) is significantly higher than that at longer wavelengths, 0.149±0.075at 870 nm.On the first days of the cruise, when the ship was nearer to the Indian coast, the spectral AOD variation is larger due to the proximity to the urbanized coast.It is known that aerosols of smaller size contribute more to the AOD at shorter wavelengths and the condensation growth and the coagulation mechanism of submicron aerosols are more efficient in producing accumulationmode aerosols (Moorthy et al., 1999).These processes are more efficient over coastal regions due to larger amount of hygroscopic components in the coastal aerosol composition.As a direct consequence of the anthropogenic emissions in the industrialized ports of the Indian west coast, the AODs present the highest values in the period from 25 to 27 April, when the ship approaches the coast (Fig. 1).Another significant aspect is the low AODs from 1 to 4 May, when the ship was far away from land.It is seen that over the entire study area the AODs at the shorter wavelengths are ∼3 times higher than those measured at the longer ones, indicating the dominance of smaller size aerosols over AS.The western coastal belt of India is densely populated, urbanized and industrialized.It has 5 major ports (from north to south Kandla, Mumbai, Goa, Mangalore and Kochi, marked in Fig. 1) besides several medium and small fishing harbors.More than 40% of the Indian population inhabits the coastal belt of >7500 km length.The effluents from the anthropogenic activities from these regions can contribute to fine-and accumulation-mode aerosols through secondary production mechanisms (condensation, coagulation, and gas-to-particle conversion) in the warm Introduction

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Full and humid tropical environment (Moorthy et al., 2005, Babu et al., 2008).On the other hand, on 29 April and on the last days of the cruise the AODs at longer wavelengths are comparable in magnitude with those of 25-27 April.However, the shorter wavelengths AODs are much lower, thus indicating aerosol presence of different origin and composition than the previous ones.As will be shown in the following these aerosols are mainly of desert origin.
The day-to-day variation of the α values computed in the spectral bands 340-1020 nm and 340-870 nm is shown in Fig. 9.The two α values exhibit similar day-today variability and their mean values and standard deviations are 0.904±0.186for 340-1020 and 0.924±0.179for 340-870.Satheesh et al. (2006a) also found lower α values determined in the spectral band 380-1025 nm compared to those in 380-870 nm.This means that as the wavelength region used for α determination shifts towards shorter wavelengths higher α values are observed, a fact that implies a convex-type curve in the lnτ α vs lnλ relationship, characteristic of aerosols with significant fraction of coarsemode particles (Eck et al., 1999;Schuster et al., 2006;Kaskaoutis et al., 2007a).This is more pronounced on 29 April and from 6 to 10 May, where the α values are lower, indicative of the presence of coarse-mode aerosols.In contrast, from 1 to 4 May (large α values) the α 340−1020 seems to be larger than that at 340-870 nm, implying a concavetype curve, where the α values become larger as the wavelength interval includes longer wavelengths (Eck et al., 1999;Schuster et al., 2006;Kaskaoutis et al., 2007a).
Due to the bimodal aerosol-size distribution, very different values of α were calculated in different wavelength intervals due to the significant wavelength dependence of the Ångstr öm exponent.Almost a three-fold increase (from 0.5 to 1.5) in the value of α was observed over AS when the wavelength interval changed from shorter (340-440 nm) to longer (675-870 nm) wavelengths (Kalapureddy and Devara, 2008).The t-statistic test between the two sets of α values showed that the mean values are statistically insignificant at the 95% confidence level.As previously noted on the first few days of the cruise when the ship was near the coast, the α values were high (above 1.0).The same feature was repeated between 1 and 4 May when the ship was in the far AS.It is Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion rather strange to observe such high α values over an oceanic environment, but it was found that on those days the air masses came from continental India and the winds were rather low (below 5 ms −1 ) not favoring the production of marine aerosols.Therefore, with the absence of intense sea-surface winds and the absence of air masses originating from the arid landscapes surrounding AS from north and west, the aerosol loading over the area is mainly dominated by small anthropogenic components, thus explaining the relative high α values.In contrast, on 29 April and in the period 6-10 May, the α values were rather low indicating significant contribution of coarse-mode particles in the aerosol burden.On these days, the research vessel was in the northern part of the AS and under the continental influence of the nearby arid landscapes.
The day-to-day variability in α values over AS could be due to both variations in the aerosol emissions from the Indian subcontinent and the variability in the meteorological conditions (winds and relative humidity).It should be mentioned that the monsoon surface winds may produce large sea-spray aerosols over the adjoining sea surface; the marine aerosols so produced may have large-size dominance.
The coefficients a 1 and a 2 obtained by the polynomial fit (Eq.2) in the AOD values using the spectral bands 340-1020 nm and 340-870 nm are shown in Fig. 10.Significant daily variability was observed in both a 1 and a 2 values (Fig. 10) with large peaks and gaps.Significant differences can also be observed even in a few (10-20) minute time interval, since the a 1 and a 2 values are very sensitive to any change in the AOD value at any wavelength.In both figures the a 2 values are negative in the majority of the cases (65% for 340-1020 nm and 81% for 340-870 nm) indicating a concave type curve in the lnτ α vs lnλ relation.This type of curve implies the presence of significant amount of fine-mode aerosols in the atmosphere (Eck et al., 1999).Similar negative curvatures are also found over a tropical coastal Indian station, Visakhapatnam, during the summer monsoon period (Madhavan et al., 2008).A significant curvature is observed in the measured AOD spectra almost for all days of the cruise over the AS region.This curvature can be shown by the a 2 values, which can be very large in certain occasions.Since the measurements were conducted on a moving ship, a large Introduction

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Full intra-daily and a day-to-day variability in the curvature (a 2 values) and also in a 1 values are expected; these variabilities could be due to the change in the measurement position.Systematic presence of negative a 2 values, indicative of fine-mode dominating aerosols, is observed on certain days (21 to 24 April and 1 to 4 May), when the research vessel was in the far AS region.Furthermore, both figures show the presence of a 2 values close to 0 on days when the ship was close to the west Indian coast (18-19 and 25-27 April), thus indicating a bimodal aerosol size distribution with nearly similar contribution of fine and coarse particles.On the other hand, both figures reveal the presence of positive a 2 values on 29 April and on the last days of the cruise when the ship was in the northern AS region indicative of the presence of significant fraction of coarse desert-dust aerosols.Previous studies using measurements (Eck et al., 1999;Kaskaoutis et al., 2007a) or theoretical approaches (Schuster et al., 2006) have shown that the curvature is larger (more negative a 2 values) in cases with significant fraction of fine-mode aerosols, becoming negligible (close to zero or even positive a 2 values) in atmospheres dominated by bimodal aerosol size distributions or coarse-mode particles.These results seem to be in close agreement with those observed over the AS region.The sensitivity of the a 1 and a 2 values to the AOD spectral distribution and the small variability of the AOD values is verified from the large range in both a 1 and a 2 .Thus, the a 1 varies widely from −3.529 to 0.139 (340-1020 nm) and from −2.596 to 0.459 (340-870 nm).The respective a 2 values range from −2.303 to 0.927 and from −1.176 to 0.769.In general, the mean a 2 values (−0.127±0.315)are lower in the spectral range 340-1020 nm thus implying less curvature, on average, compared to those computed without the AOD 1020 value (0.198±0.272).Assuming negligible the curvature for −0.1<a 2 <0.1 it was found that the 38.8% of the cases lie within this limit when the 340-1020 nm band was used, while the fraction is lower (30.8%) for the band 340-870 nm.These cases are mostly observed in the period 25-27 April and on the last days of the cruise when bimodal aerosol size distributions with intermediate values of α dominate.In these cases (492 for 340-1020 nm) the errors in a 1 and a 2 computations Introduction

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Full are much lower (0.232 and 0.215, respectively) than those presented above, Fig. 3. Furthermore, the α 340−1020 value is 0.844, too close to that of the absolute a 1 =−0.849.
As regards the 340-870 nm band, the errors in a 1 and a 2 computations assuming the curvature negligible (391 cases) are 0.373 and 0.302, respectively, much lower that the values presented in Fig. 3.The mean α 340−870 value is 0.816, close to that of the absolute a 1 =−0.847.From the above results we can conclude that the use of the 340-1020 nm band seems to be more appropriate since results in lower curvatures, lower uncertainties and better correlations.A direct comparison of these values with those reported by Kaskaoutis et al. (2007a) for four AERONET sites should be avoided due to the large standard deviations in the present analysis.These researchers found a positive mean a 2 at Nauru and Solar Village indicative of the presence of coarse-mode aerosols, i.e. maritime and desert particles, respectively.Furthermore, a 2 values near to zero associated with large standard deviations were found in Alta Floresta and Ispra where both fine-and coarse-mode aerosols dominate depending on the season.Regarding a 1 , almost all of its values, except very few cases, are negative, as expected, since a 1 =−α on cases without curvature.However, on certain cases, and especially in the period 1 to 4 May (far AS region), some "unreal" positive a 1 values are observed (Fig. 10).On those days the AODs were the lowest (Fig. 8), while the α values the highest, implying low columnar abundance and predominance of accumulation-mode particles.It is interesting to note that for the central part of AS, around the same area as above, from measurements of total and size-segregated mass concentrations of aerosols in the MABL, Nair et al. (2008) have reported very low aerosol-mass concentration and a relatively high (>0.5)accumulation fraction.They had attributed it to faster subsidence (settling down) of aerosols, particularly of the coarse mode, caused by the strong descent of air masses associated with the strong low-level anticyclone prevailing over the area.The number of positive a 1 values was larger when they were computed in the 340-870-nm band (7 cases) against 5 in the band of 340-1020 nm.However, a significant fraction of positive a 1 values was observed in the clean marine environment of Nauru (Kaskaoutis et al., 2007a).From a detailed analysis it was established that Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion these (positive) values are obtained in cases that AOD 1020 >AOD 870 or when the AODs at the shorter wavelengths are much higher compared to those at the longer ones, thus implying the high α values.The positive a 1 values in the band of 340-870 are mainly attributed to the larger AOD 870 values instead of the AOD 675 values, a rather common characteristic in clean marine environments (Kaskaoutis et al., 2007a).However, these cases are very few over AS, a fact that differentiates this environment of a background clean marine one.Also, the nearly absence of "unreal" positive a 1 values shows the accuracy of the AOD retrievals over a moving platform in an oceanic area.
During the pre-monsoon season the dry Indian landscapes along with the anthropogenic activity can drive an injection of significant amount of particulate matter in the atmosphere.These particles can be carried over the oceanic environment by the prevailing northeasterly winds within the boundary layer mainly.The northeast monsoonal low-level flow can transport sulfates, mineral dust and other aerosols from land areas being upwind of AS.This transport may also include fine dust from the Arabian desert whose dust size belonging to the accumulation size regime (Hess et al., 1998).The results showed a relatively complicated aerosol mixture of both anthropogenic pollution and mineral dust along with local maritime aerosols in the AS region.In order to understand the connection between continental emissions and impacts over the AS and IO, it is necessary to focus on the role of the northeast monsoon in the large-scale atmospheric circulation.

Air-mass trajectories
Air-mass back trajectories ending at the ship location each day on 12:00 UTC at three altitudes, 500, 1500 and 3000 m a.s.l., were calculated by the HYSPLIT model (Draxler and Rolph, 2003) (Fig. 11).The back-trajectory analysis provides a three dimensional (latitude, longitude and height) description of the pathways followed by air masses as a function of time by using the National Center for Environmental Prediction (NCEP) reanalysis wind as input to the model.These trajectories are important to identify the source regions and the transport pathways of the aerosols before they reach the 22241 Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion measurement site.Hence they are helpful in investigation of the aerosol properties and aerosol types.The 7-day air-mass back trajectories for all the days of the measurements in the central part of AS indicate absence of any significant long-range transport from the adjoining continents, as the air masses seem to come from oceanic origin mainly for the 500-m back trajectories during the cruise days, because the air masses originated and remained within the AS region (Fig. 11a-b).The easterly low-level (1500 m) flow from the Indian subcontinent into the AS can carry pollutants and continental aerosols.It has been seen that sulfates and other continental aerosols are transported thousands of kilometers over AS and IO during the northeast monsoon season (Ramachandran and Jayaraman, 2002).It was found that on the days with relative high α-Ångstr öm values (1-4 May), when the ship was in the far AS, the air masses originated mainly from continental India and some times from Arabia (see Fig. 11c-f).The upper-level flow from the Arabian Peninsula precludes the transport of desert dust to the AS at this time of the year.The transport patterns can be quite different for pollutants and aerosols that penetrate the boundary layer to the free troposphere.The westerly flow in the mid-to-upper troposphere north of about 15 • N can transport mineral dust from the Arabian Peninsula to the AS.Aerosol transport from the arid landscapes at 3000 m or 1500 m was detected on 29 April and 5-6 May, when the AOD were high and the α low.High AODs appeared on the last days of the cruise, when the ship was in the northeastern part of the AS; they rather consisted of mixed particles since the air trajectories show the transport of both anthropogenic and natural aerosols.Meteorological studies showed that high aerosol concentrations could be linked to transport from the Indian subcontinent and also from sources in the Middle East and Arabian Peninsula (Moorthy et al., 2005).Although heavier aerosols may reside (via dry deposition) there still remains a strong possibility for the aerosols from Arabia and the other arid and semi-arid regions to make its way to the AS region.The high AOD values presented in the northern part of the AS provide a strong support to this assertion.Furthermore, mineral dust was found to be a substantial component in most of the aerosol samples collected Introduction

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Full in AS (Savoie et al., 1987;Nair et al., 2004;Kumar et al., 2008).Analyzing spectral AODs from a network of observatories over Indian mainland during ICARB, Beegum et al. (2008) showed the significant role of transported dust in enhancing AOD and decreasing α over the Indian subcontinent during April and over the northern part of India and the Indo-Gangetic Plains in May.

Spatial distribution of the aerosol properties
In this section the spatial distribution of the aerosol load and the optical properties over the AS region are presented.Similar AOD and α maps have already been published by Kalapureddy and Devara (2008) for the whole ICARB campaign, including BoB, IO and AS.In the present study these graphs are presented over AS in more detail, while the a 1 and a 2 spatial distributions over the area are presented for the first time.
Figure 12 shows the AOD 500 spatial distribution over the AS.A strong southwest-tonortheast gradient in the AOD 500 values is revealed since the aerosol load over the northern part of the AS is larger than that of the southern.However, the lowest AOD 500 values are depicted in the central part of AS (around 13-14 • N) on 30 April and 1 May.
A similar strong southeast-to-northwest gradient is observed since the aerosol field is larger near the coastal India.Additionally to the latitudinal variation, a significant longitudinal one is observed, since there is a strong gradient of decreasing AOD 500 values from around the Indian coast towards the far AS.The high AOD 500 close to the coast arises mainly from the anthropogenic activities over the coastal regions.It is known that the west coastal regions of India are highly urbanized and industrialized.As the anthropogenic aerosols are generally in the submicron size and, for this reason, have longer residence times, they get transported to greater distances over the ocean.Such cases of aerosol and pollution transport from urbanized regions and mineral dust from the arid areas of northwestern India, Iran and Arabia during the premonsoon season have been reported from the back-trajectory analysis (Fig. 11).As the distance from the coast increases these aerosols undergo changes in size due to coagulation and condensation; they also extinct over oceanic areas due to the sedi-22243 Introduction

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Full Screen / Esc Printer-friendly Version Interactive Discussion mentation and precipitation processes.These processes lead to a decrease in aerosol load and thus AOD becomes lower in the far AS.Ramachandran (2004b) has indicated via the term "scaling distance" the importance of wind speed over the ocean surfaces in altering the aerosol spectral characteristics and the effect it has on the gradients seen in terms of the continental influence.Close to the Indian subcontinent aerosol-mass concentrations were found to range between 21 and 53 µg m −3 (Nair et al., 2004) and were significantly larger than those near the Maldives (∼19 µg m −3 ) or at the Kaashidhoo Climate Observatory (Ramanathan et al., 2001).As the Sagar Kanya approached the Asian continent in the north AS region where the synoptic winds were mainly northerly/northwesterly originating from the arid landscapes in Iran and Pakistan (Fig. 11), this caused an enhancement in AOD on 7-10 May.It was found that this enhancement could be attributed to the advection of aerosol-burdened continental air over the northern part of the AS.In a previous study, the AOD was found to increase with latitude between equator and 12 • N, while over northern AS did not show significant latitudinal variations (Satheesh et al., 2006a).These researchers reported mean AOD values of 0.29±0.12during the winter monsoon season (November to March) and 0.47±0.14during the summer monsoon season (April to September).The respective α values were 0.7±0.12 and 0.3±0.08.The differences in AOD spatial distribution between this study and the results of those researchers may mainly be due to the differences in the period and measurements obtained and the fact that the present study does not include data southern than 8 • N, where the AOD was significantly lower without strong continental influence (Satheesh et al., 2006a).
The spatial distribution of the α 340−1020 values over the entire AS region is shown in strong south-to-north gradient in α 340−1020 values around 15-16 • N, with α 340−1020 values above 0.9 in the south and below 0.8 in the north.However, the most common situation is the intermediate values of α 340−1020 (near to 0.9), indicating a mixed aerosol field over AS from both anthropogenic and natural emissions.This mixed aerosol is also the case near to the Indian coast due to both influences of anthropogenic and natural sources.The anthropogenic emissions seem to play a significant role in the α 340−1020 values over AS, which are higher than other marine environments in the Pacific and Atlantic.Unlike the other marine regions, where sea-salt aerosols contribute mostly to the total aerosol-mass concentrations, the aerosol chemical composition analysis performed over IO and Kaashidhoo during INDOEX 1999 found that the sea salt contributes only 11% and 17% to the total aerosol loading, respectively, while the anthropogenic aerosol transported from continental India contributes the most (Ramanathan et al., 2001;Satheesh et al., 2002).Furthermore, Satheesh et al. (2006b) found that the sea-salt contribution to the AOD was about 20-30% in the northern AS and 30-40% in the southern.This fact can partly explain the relative high α values over the far AS.The anthropogenic, as well as the dust contribution, is even higher near the coast of AS and in its northern part as has been shown in Nair et al. (2004).It is well known that the northern AS is influenced by the Indian subcontinent and West Asian arid regions, whereas the southern AS and northern IO are influenced by several source regions where the ITCZ has a great impact (Nair et al., 2004;Satheesh et al., 2006a).As a result, the temporal and spatial variations in the spectral dependence of the AOD are indicative of the changes in the aerosol-size distribution mainly due to the different aerosol types and air masses.Extensive measurements over AS from the Minicoy Island (Moorthy and Satheesh, 2000) have shown that the AOD is higher during the summer monsoon season, whereas its spectral variation is higher during winter monsoon season.Thus, the AOD, as well as its spectral dependence, show large temporal variation over specific regions of AS.
The new results provided in the present study are focused on the a 1 and a 2 values obtained from the polynomial fit (Eq.2) in the two spectral bands.In Fig. 14  right panels) the spatial distribution of the a 1 values over the AS region is shown for the spectral bands of 340-1020 nm and 340-870,nm, respectively.The maps were constructed utilizing the whole dataset, in order the spatial distribution of the values as well as the differences arisen in either spectral band to be captured.In general, the two maps providing the a 1 values agree to each other.More specifically, large negative a 1 values are depicted in the far and central AS.In the northern part of the AS and along the Indian coast the a 1 values are less negative than those in the far AS.A general disagreement between the a 1 values shown in Fig. 14 is detected in the south AS, where a 1 for 340-1020 nm is less negative than those of 340-870 nm.One should bear in mind that this area is associated with very low AODs and, therefore, the estimated errors may be high.Thus, the inclusion of the AOD 1020 value in the polynomial fit may produce large differences in the a 1 values.The correlation of the a 1 values in the two spectral bands exhibits large scatter (R 2 =0.35), further highlighting the great importance of the wavelength interval used for such computations.
The most important parameter of the polynomial fit is the a 2 value, which presents the curvature of the AOD spectral distribution in logarithmic coordinates and can be used as an additional tool for the aerosol-type discrimination (Kaskaoutis et al., 2007a).
The spatial distribution of both a 2 values is shown in Fig. 15 (left and right panels) for the 340-1020 and 340-870 nm, respectively.Positive values of a 2 are indicative of convex-type curves closely associated with coarse-mode particles, while negative curvatures with fine-mode aerosols.The majority of the area is covered with negative a 2 values (in both figures) over most of the southern and central parts of AS, as well as along the Indian coast, closely associated with aerosols having a large fine-mode fraction.On certain occasions, positive a 2 values were also found in the southern part of AS, especially in Fig. 16a.Furthermore, the inclusion of AOD 1020 in the a 2 retrievals results in larger fraction of positive a 2 values (35%) against 19% for the 340-870 nm case, a scenario that seems to be more likely over an oceanic environment.These positive a 2 values from the 340-1020 nm band against those (negative) from the 340-870 nm are mainly observed near to the Indian coast and in the south AS region. .The coarse-mode aerosols (radii>1.0µm) contributed, on average, by 58% to the total mass even in small regions where the accumulation mode was dominant.Furthermore, the fine-mode fraction was 0.42; for 62% of the measurements, it layed within 0.35 and 0.55.In 4% of the total observations this fraction was >0.75 mainly over the regions with negative a 2 values.More specifically, in the central AS, bounded by 65-70 • E and 10-15 • N, Nair et al. (2008) found that, although the aerosol-mass concentration was quite low (<15 µgm −3 ), the contribution of accumulation aerosols to the total mass was greater than 50%.This conclusion is close to ours, since in the same area the AOD is low (Fig. 12), the α-Ångstr öm high (Fig. 13) and the a 2 negative (Fig. 15).Nair et al. (2008) also found high values of mass concentration (∼30 µgm −3 ) in the northern AS (64-68 • E, 19-22 • N), which are mainly attributed to aerosol transport from arid regions of West Asia.The presence of large aerosol load associated with desert particles over this area can be also highlighted in Figs. 12, 13 and 15.Finally, relative high aerosol-mass concentrations (20±7 µgm −3 ) were measured along the West Indian coast, mainly composed of anthropogenic emissions (Nair et al., 2008).The present results also show increased AOD 500 values over this region associated with urban/industrial pollution transport over AS.A direct correlation of the results obtained by Nair et al. (2008)  the vertical column focusing on the agreement in the spatial distribution of the aerosol optical properties and size distribution constitutes a real challenge.Aerosols found over the oceans are mainly sea-salt particles, produced by the evaporating water droplets injected into the atmosphere, and wind-blown dust, transported from the surrounding landmasses.Over the pristine oceanic regions the major contribution to the AOD is from sea salt and sulfate from natural sources.The emission strength of these aerosols is dependent strongly on the surface wind speed.Over the northern IO it has been seen that sea salt and sulfate from natural sources contribute less than 20% to the AODs (Ramanathan et al., 2001;Satheesh et al., 2002), while BC, fly ash, dust, organics and anthropogenic sulfate the rest.As mineral dust particles can be transported over long distances, they can contribute significantly to the aerosol mass in the marine atmosphere (Quinn et al., 2002).This contribution is much higher in the northern parts of the AS where the desert-dust aerosol type (high AOD, low α-Ångstr öm, positive a 2 ) seems to dominate.Even though it is possible that the oceanic environment is less heterogeneous compared to the land one, several studies have shown that the aerosol properties show significant variations, even over reasonably small spatial scales and the spatial changes are distinctly different in the different seasons (Moorthy and Satheesh, 2000;Smirnov et al., 2002).This is mainly due to changes in air-mass types, the impact of advection and transport, and the proximity to continental areas over which the aerosol properties vary.This is true even over the AS and the tropical Indian Ocean (Kalapureddy and Devara, 2008).band of 340-1020 nm for the polynomial fit gave more accurate results than the respective of 340-870, where the typical errors can be high.The spectral AOD and α-Ångstr öm values exhibited significant day-to-day variations along the ship cruise mainly attributed to the distance of the ship from the main land, the air-mass origin and atmospheric dynamics.The mean AOD 500 was found to be 0.246±0.114and the α 340−1020 0.904±0.186.In the majority of the cases the coefficient a 2 was negative, thus implying a concave-type curve indicative of the higher presence of fine-mode aerosols.The observed spatial and temporal variation of the AOD and aerosol optical properties indicated the importance of the role of the transport of aerosols that originate from different source regions (air-mass regimes) pertain to AS.
The data presented a strong latitudinal and longitudinal gradient in the AOD and Ångstr öm exponent.The AODs were found to exhibit a steep gradient as the ship moved from the Indian coast towards the pristine oceanic region, with higher values near the coast and nothern part of AS.Very low AOD values were observed in the central part of the AS region.The α-Ångstr öm also exhibited significant spatial variation, with the higher values to be detected near the Indian coast and in the far AS.The anthropogenic and urban activities along the densely populated western coast of India could be attributed to the former; the later arose from the strong vertical descent of rather clean air, associated with low-level mid-ocean anticyclone which lead to faster depletion of coarse-mode particle concentration.Low values of α are associated with coarse-mode arid aerosols were found in north AS.The present study was the very first one providing the spatial distribution of the coefficients a 1 and a 2 over AS even in two spectral bands.A general agreement was found in the spatial distribution of each coefficient determined in different wavelength bands, despite some differences observed in the south AS region.Positive values of a 2 , (indicative of coarse-mode aerosols) were mainly observed in the northern AS.Negative a 2 values (fine-mode aerosol abundance) were observed in the central and far AS.In the study region, in addition to the transport of dust from the arid West Asian regions, convex curvature could also arise from the presence of sea-salt particles as the measurements were Introduction

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Full conducted over oceanic regions and from well-mixed aerosols with significant contribution of coarse-mode.
The results indicated a relatively complicated aerosol mixture of both industrial pollution and mineral dust.This dictates the need for careful measurements and analysis to discriminate the dominance of specific aerosol types (e.g.anthropogenic aerosols, maritime particles, mineral dust) in the AS.The combination of such particle types is highly variable and makes attribution of the aerosol radiative properties to specific aerosol types difficult to be detected without detailed and precise measurements.Introduction

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Full aerosols at intermediate values of the α (Schuster 1) and low (α<0.7)values is observed.During the 5-year period 1996-2000 the α was 1.50±0.05over AS, while the β was 0.10 (Ramachandran, 2004a).The large difference in the α values presented in Ramachandran (2004a) may be attributed to different time intervals and the different spatial coverage of the AS region.However, the mean α value in April for the period 1996-2000 was much lower, 1.27±0.07(Ramachandran, 2004a).The mean α values over Lanai, Nauru and Tahiti in the Pacific Ocean were found to be 0.76, 0.43 and 0.74, respectively.In the Atlantic Ocean on 22235 Fig. 13.The α 340−1020 values are high (above 1.0) near the coast (18 April) and in the central part of the AS closely associated with the low AOD 500 values (Fig. 12).It is worth noting that in the far AS the α 340−1020 values remained high, without showing strong influence from the arid regions of the Arabian Peninsula.In contrast, low α 340−1020 values are observed in the north part of the AS region, thus indicating the influence of desert-dust particles transported over this region.It is characteristic the sunphotometric measurements of AOD and size segregated mass concentration in the MABL over AS were performed during the pre-monsoon season (April-May 2006) in order to shed light in the spatio-temporal heterogeneity.The present study provides an extensive analysis regarding the errors and the uncertainties in computing the aerosol optical properties.It was found that the use of the spectral

Fig. 1 .
Fig. 1.The cruise legs (solid lines) of the Sagar Kanya 223B in the Arabian Sea in the period 18 April-10 May 2006 with indications of the daily position of the ship at 05:30 UTC denoted by the circles.The major ports and coastal urban centers on the mainland adjoining the cruise track are also identified.The date at each position also indicated (A=April, My=May).
is concluded that the spectral band used for the a 2 retrievals strongly differentiate the results, as the coefficient of determination between the two sets of a 2 values is low, R 2 =0.23.Positive values of the a 2 (in both figures) are depicted in the northern AS, where the high AODs are dominated by coarse-mode aerosols.
at sea level with this study of Introduction