4.1. Aerosol Optical Depth and Angstrom Coefficients
 The spatial distribution of AOD at 500 nm over the cruise area is shown in Figure 2 (left). The implicit assumption in generating the map is that the temporal variations during the campaign period were far less significant compared to the spatial variations, the validity of which has been ascertained by examining the temporal variations of AOD from the island station Port Blair (PBR in Figure 1) in the BoB. These temporal variations (shown in Figure 2, right) showed that the AOD fluctuated in the range 0.35–0.55 with a mean of 0.43 ± 0.085 with no perceptible trends. The horizontal line in Figure 2 (right) depicts the monthly mean AOD at (500 nm) at Port Blair. The spatial distribution in Figure 2 (left) reveals significant heterogeneity. On the basis of the mean pattern and geographical distinctiveness, the study area has been divided into seven regions, R1 to R7 (as shown in Figure 2), within which the AOD was fairly homogenous but differed significantly between the regions, as will be seen subsequently. The details and criteria followed for these are given in Table 1. Upon examination, while region R1, close to eastern costal India and likely to be most influenced by the Indian mainland, was characterized by moderately high AODs (>0.4), and the highest AODs (going as high as 0.8 at 500 with an average of >0.5) were observed over region R2, in the northern or head BoB. The peculiar nature of this high that is rather detached from the mainland is quite similar to that seen in the earlier ICARB campaign of 2006 [Moorthy et al., 2008; Nair et al., 2008a]. In contrast, the lowest AODs (∼0.1) prevailed in the northeastern R3 region, east of R2. Moderately high values of AOD (∼0.5) prevailed over the entire R4 region, with a small patch of higher AOD toward the southern end, and close to the Malacca strait, a region that has hitherto remained unexplored. This high AOD is attributed to the higher winds that prevailed in this region during the campaign, which would have been conducive for the production of sea-salt aerosols. Moderate AODs prevailed over central BoB (R5), while lower values (<0.3) were observed over the southern BoB and equatorial Indian Ocean (R6) as well as over the coastal regions of peninsular India (R7).
Figure 2. (left) Spatial map of AOD (at 500 nm) during W-ICARB. The different regions are also marked and the (right) day-to-day variations of AOD (at 500 nm) observed at the island station Port Blair during the campaign period. The horizontal line represents the mean value during the period. The costal and island stations with simultaneous measurements are also marked (TVM, Trivandrum; CHN, Chennai; VSK, Visakapatnam; BBR, Bhubaneswar; PBR, Port Blair).
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Table 1. Grouping of the Oceanic Region and the Mean Values of AOD, α, and ν for Each Group
|Region||Description||Mean AOD||Mean α||Mean ν|
|R1||Eastern coastal region of the Indian subcontinent bound by the oceanic region ∼13°N–20°N and 80°E–87°E.||0.50 ± 0.05||1.30 ± 0.06||4.10 ± 0.06|
|R2||Northern BoB, bound by the oceanic region of 15°N–21°N and 87°E–92°E.||0.56 ± 0.02||1.33 ± 0.07||4.06 ± 0.05|
|R3||Northeastern BoB, the coastal region off Bangladesh and Myanmar, bound by the oceanic region of 14°N–21°N and 92°E–96°E.||0.19 ± 0.03||1.43 ± 0.08||4.12 ± 0.03|
|R4||Eastern BoB, between the islands of Andaman Nicobar and East Asia, bound between the oceanic region of 6°N–14°N and 92°E–97°E. This region has remained unexplained until W-ICARB.||0.35 ± 0.02||1.33 ± 0.10||4.25 ± 0.04|
|R5||Central BoB, bound by 6°N–14°N and 84°E–92°E||0.30 ± 0.02||1.09 ± 0.12||3.90 ± 0.04|
|R6||Southernmost BoB and northern Indian Ocean bound between 3°N–6°N and 83°E–92°E.||0.27 ± 0.03||0.80 ± 0.17||4.04 ± 0.03|
|R7||Coastal oceanic region near off-peninsular India and Sri Lanka bound between 3°N–9°N and 76°E–81°E.||0.34 ± 0.03||0.64 ± 0.20||4.06 ± 0.05|
 The mean AOD spectra, representative of these seven regions, obtained by averaging the AODs within each region over the given period are shown in Figure 3 (left). Measurements of spectral AODs were also carried out continuously over the mainland coastal locations Trivandrum (TVM), Chennai (CHN), Visakhapatnam (VSK), Bhubaneswar (BBR), and Port Blair (PBR; Figure 1) concurrently with the cruise and the average AOD spectra (for the cruise period) for these regions are shown in Figure 3 (right). Five-channel Microtops instruments (compared with the one used on board) were used at Bhubaneswar and Chennai, and 10-channel multiwavelength radiometers (MWRs) were used at Trivandrum, Visakhapatnam, and Port Blair. The MWR provided mean AOD for forenoon and afternoon parts of the day at 10 channels (380 to −1025 nm) following Langley plot techniques, the details of which are available in several earlier papers [Saha et al., 2005; Gogoi et al., 2009]. The MWR-derived AODs agreed well with the mean AOD estimated from the concurrent Microtops measurements for the same station, as ensured during comparison. During the initial period of the campaign, an average AOD of 0.47 ± 0.03 (at 500 nm) was observed at Chennai, which was comparable to those observed over the nearby oceanic regions (Figure 2, left). At Visakhapatnam, the averaged AOD at 500 nm for the first half of the campaign was 0.73 ± 0.06. Examination of the cruise data over R1 (Figure 2, left) revealed a similar increasing trend in AOD from off the coast of Chennai to off the coast of Visakhapatnam. The mean AODs over the coastal location of Trivandrum (0.34 ± 0.01) and the island station Port Blair (0.38 ± 0.04) were also comparable to those measured over the nearby oceanic regions during the campaign period.
 Examining the average spectral AODs for the seven ocean regions (Figure 3), we note that even though high AODs occurred over R1 and R2, the steepest spectra were observed over R3, followed by R2, R4, and R1, implying significant contribution of accumulation mode aerosols over the eastern BoB, followed by northern-head BoB, eastern, and then western coastal regions of BoB. At the mid-ocean regions (R6 and R5) and at coastal Trivandrum (R7), the spectra tend to become flatter and depict high values of AOD at longer wavelengths. The spectral steepness over regions R1, R2, and R7 are comparable to that of the nearby coastal and island stations. By performing a regression analysis of the spectral AODs (in log-log scale) with the Angstrom relation τ = βλ−α, the Angstrom wavelength exponent α and turbidity coefficient β were estimated for each cruise-based measurement as well as for the data from the land stations.
 The spatial variation of α and β are shown in Figure 4. The regional mean values of AOD at 500 nm and α are given in Table 1. Over the entire oceanic regions that have proximity to the mainland or islands (regions including R1, R2, R3, and R4), α remained high (>1.2), with the highest values (as high as 1.5) occurring over R3 where, interestingly, the AOD values were very low. Another region of high α was observed at R1, off Bhubaneswar, where AODs were also high. Even over the mid-ocean regions of BoB, frequent occurrence of α > 1.0 were observed, except at a few patches centered at R5 and R6. The smallest values of α are observed over R7, except at coastal Trivandrum. Upon examination of the coastal landmass measurements, it is noticed that high values of α prevailed over Visakhapatnam, where α was higher than that seen over the adjacent oceanic region R1, whereas at Bhubaneswar the values of α over the mainland were lower than those prevailing over the adjacent oceanic region R2, showing mixing of different types of aerosols in these regions. On the other hand, at Port Blair and Trivandrum, the mean α values were comparable to those seen over the nearby oceanic region. The Angstrom turbidity coefficient β followed a pattern similar to that of AOD (Figure 4, right). Values of β remained generally high (∼0.2) for the entire R1 and R2 regions and the highest values (∼0.3) occurred in R2, indicating high columnar loading over the region. The advected coarse dust from northwestern India across the Indo-Gangetic plains (Figure 5) would enhance the coarse mode aerosol dominance over R1 and R2. It (β) also remained high partly over R5, R4, and R7; these patches of highs are found to be associated with regions of high wind speed. A similar pattern of isolated highs is seen also in the QCM measured coarse mode mass concentrations in the MABL (Figure 6, top). The smallest values of β occurred throughout region R3 (∼0.05), where the columnar loading was low and mainly by fine and accumulation mode aerosols, resulting in steep AOD spectra.
Figure 5. Mean trajectory clusters arriving at different regions during the cruise period. The regional mean values of AOD (500 nm), α, and β are also shown.
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 With a view to delineating the role of transported (long-range) aerosols toward the observed spatial pattern, we examined the 5 day isentropic airmass back trajectories arriving at each of the seven regions at three height levels: 500 m (within the MABL), 1500 m (above MABL), and 3500 m (free troposphere). The most dominant pathways have been identified following a cluster analysis [Hafner et al., 2007] and the results are shown in Figure 5. The corresponding mean values of AOD, α, and β are also shown in Figure 5. The analysis brings out the following:
 1. The trajectory clusters arriving at R1 have very long continental overpass through the arid regions of western Asia and northwestern India before arriving at R1, across central India. These trajectories would thus be conducive for the significant advection of transported dust aerosols to the measurement region, which would significantly add to the existing aerosol abundance in the region (with significant anthropogenic components owing to the local human activities discussed earlier). On the basis of the observations from several land stations over the Indian subcontinent as well as over the island locations of Port Blair and Minicoy, Beegum et al.  have shown that the advected mineral dust aerosols from the arid western Asian regions lead to significant enhancement in the column AOD over the Indo-Gangetic plain (IGP). Furthermore, because the transported mineral dust is known to have a dominant submicron mode at ∼0.5 μm [Hess et al., 1998], it would contribute toward the accumulation mode aerosols also and thereby to the higher values of α. The region R1 adjoins the highly urbanized and industrialized eastern coastal India with three major harbors, Chennai, Visakhapatnam, and Paradweep. Consequently, significantly high anthropogenic aerosol concentration would prevail over these regions. The observed high values of AOD and moderately high α at R1 is the consequence of these factors.
 2. At R2, where the highest values of AODs are observed, the trajectories were found to have a wide spread over the Indian subcontinent, covering all of central India and the IGP and providing conduits for transport of continental aerosols, mostly of mixed type (anthropogenic + natural), to the measurement location, which would contribute significantly to the observed high values of AOD. The IGP, accounting for ∼21% of the land area of the Indian subcontinent, is a densely populated region with a high degree of anthropogenic activities [Nair et al., 2007] such as coal-based power plants, industries, transport, mining, and urban and agricultural activities. Several investigators have also reported the persistence of high AOD and large concentrations of aerosols over this region during winter months [Jethva et al., 2005; Ramana et al., 2004; Singh et al., 2004; Prasad et al., 2006]. The prevailing westerly winds, combined with the orography of the IGP that spatially confines the aerosols into a rather narrow channel and leads to the outflow into the northern and head BoB, results in the large loading of aerosols in that region [Girolamo et al., 2004; Nair et al., 2007; Niranjan et al., 2007].
 3. The trajectories coming to R3, where the lowest AOD and highest values of α were observed (during the cruise period), arrived here mainly from eastern coastal India and would be mostly devoid of significant natural components. In addition, the eastern coastal belt of India, comprising several ports and industries, is a hot spot of accumulation mode aerosols [Moorthy et al., 2005; Niranjan et al., 2005]. This is indicated by the very high α values observed over Visakhapatnam and Bhubaneswar during W-ICARB. Because the trajectories arriving at R3 have a long history of more than 4 days (∼1000 km) over the ocean, there would be significant reduction in the aerosol abundance by subsidence and dispersion, leading to a decrease in the loading and an increase in the spectral steepness.
 4. Even though moderate values of AOD are observed at R4, in the eastern BoB, α remained high (comparable to R2 and R3) and the trajectory analysis revealed significant advection from the continental locations of East Asia and south China. It suggests that fine and accumulation mode particles advected from these regions contributed significantly to the aerosols over R4. Several earlier studies from the BoB have demonstrated the role of advection from East Asia and south China in the enhancement of the AOD and its spectral steepness, as well as black carbon mass concentrations in the MABL over Port Blair [Moorthy et al., 2003; Moorthy and Babu, 2006; Nair et al., 2009; Vinoj et al., 2009]. Streets et al.  have reported that the anthropogenic aerosol emissions have increased in recent decades in East Asia because of the increase in urban activities, and this finding corroborates our inference.
 5. The trajectories reaching R5 were found to have originated from the western coastal regions of East Asia, which might transport anthropogenic aerosols along with the coarse sea-salt aerosols, because these trajectories have traversed across the vast oceanic regions where the wind speeds were moderately high (Figure 1). This would result in a mixed type of aerosols (coarse mode sea salt + accumulation mode anthropogenic aerosols) and explains, at least qualitatively, the moderate values of AOD and fairly low values of α.
 6. The trajectories arriving at R6 and R7 were mostly oceanic in nature. They would be mostly characterized by a smaller amount of accumulation mode aerosols and a larger amount of coarse mode sea salt because the marine aerosols generally have a coarse mode associated with sea spray. This would result in the moderate values of AOD and low values of α. As the southern BoB opens to the vast Indian Ocean, anthropogenic aerosol concentration would be insignificant.
4.4. Vertical Heterogeneity
 The dynamics of the MABL plays an important role in the vertical distribution of aerosols over any given region even though it is weaker over the ocean than over the landmass. If the dynamics associated with the regional-scale weather occurs over a large spatial extent, it could cause the columnar and MABL aerosols to behave similarly, leading to a vertical homogeneity. On the other hand, short-scale weather phenomena, long-range transport of aerosols above the MABL, and stratified turbulences would lead to vertical heterogeneity in the aerosol profile. With a view to examining these, we plotted the temporal variations of mass concentrations in the MABL (MT) with the column AOD over different regions, and the results are shown in Figure 11. Over R1 and R2, both the parameters were found to correlate poorly, with correlation coefficients of 0.42 and 0.37, respectively. This clearly indicates that, over these regions, the MABL and the column properties differed significantly and hence there exists vertical heterogeneity in aerosol properties. Earlier lidar observations off the coast of Bhubaneswar (during the premonsoon season of 2006) have clearly shown the presence of elevated aerosol layers in the altitude region of ∼2–3 km, extending ≥150 km offshore from the coastline [Babu et al., 2008; Satheesh et al., 2009] with the elevated layers contributing as much as 50%–60% of the mean to the column AOD. In light of the preceding observations, it appears that the vertical heterogeneity over these regions is a persistent phenomenon. However, interestingly, over R3 and R4 we have observed excellent linear dependence between MT and AOD with correlation coefficients of 0.94 and 0.98, implying a rather homogenous vertical distribution. We have calculated the exception p values using statistical methods [Fisher, 1970] for the correlation coefficients over distinct regions. For regions R1, R3, R4, and R5, p was better than <0.0001 (that is, the correlation coefficients are strongly significant with better than 99.9% confidence) and for R2, the p = 0.02 and is significant at the 98% confidence level. This finding, along with the low values of AOD and the vertical structure of convective mixing in the ABL in these regions as revealed from concurrent GPS-aided radiosonde ascents from the ship, suggested the absence of any distinct elevated layers over these regions; the aerosols were mostly confined within the ABL with very little abundance above. The observed lowest values of AOD, high α, as well as high fine mode fractions FF over R3 also corroborate this finding. Over R4, the observed MT values were high, reaching up to ∼60 μg m−3, and thus contributed significantly to the observed moderate values of AOD over that region. The regression analysis over the mid-ocean regions of R5, R6, and R7 together revealed a statistically significant linear dependence with a correlation coefficient of 0.63 between MT and AOD, implying a fairly good vertical heterogeneity. To vindicate these inferences based on concurrent vertical profiles of aerosols over BoB, during the study period, we examined the CALIPSO data.
 CALIPSO, one of the satellites in the A-train constellation [Stephens et al., 2002], provides the vertical distribution of aerosol backscatter and extinction. The extinction coefficients at 532 nm were estimated from the altitude profiles of extinction coefficients by weighting the integrated extinction coefficient with the corresponding AOD measured using Microtops (interpolated to 532 nm using the Angstrom equation) for each of the ship locations, and the area-averaged normalized extinction coefficient profiles for all seven regions were generated. These are shown in Figure 12 for the altitude regions of 0 to 5 km, where the features are distinct. It is interesting to note from Figure 12 that the profiles representing regions R1 and R2 are far separated in absolute magnitudes as well as in the vertical distribution from the profiles for the rest of the regions. Over R2, a prominent aerosol layer is observed at ∼0.5 km and another less prominent one at ∼2 km. Similarly, over R1, two distinct layer structures are noticed: one at ∼1 km and the other at ∼2 km, both being equally prominent. These are identified by the horizontal arrow marks in Figure 12. This feature is quite consistent with the aircraft-based observations taken off the east coast of India by Satheesh et al.  during ICARB 2006. Over the regions R3 and R4, the profiles did not indicate the presence of any elevated layer structure and the values of extinction remained nearly steady up to ∼2 km, vindicating the earlier conclusion drawn from Figure 10 of rather homogeneous vertical distributions there, compared to other regions of the BoB. Over the region of R5, even though the extinction coefficient values remained constant up to 2 km, layered structures were visible at higher altitudes (>3 km). These layer structures, examined in light of the back trajectory cultures (Figure 5), would be composed of different types of aerosols, characteristic of the region from which they are transported, thereby adding to the heterogeneity. These findings ascertain the role of elevated aerosol layers in bringing in heterogeneity in aerosol spatial distribution over the BoB.
Figure 12. Altitude variations of CALIPSO-derived normalized extinction coefficient averaged over different regions.
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4.5. Comparison With Earlier Observations Over the BoB
 As mentioned in the beginning of this paper, W-ICARB focused on the winter season, with a view to examining the seasonal distinctiveness by comparing it with the premonsoon observations of ICARB 2006. Such delineation did not exist for the BoB except perhaps for the observations from the island location of Port Blair [Moorthy et al., 2003]. The spatial variation of AOD at 500 nm during the premonsoon season of 2006 (ICARB 2006) is shown in Figure 13. It revealed the persistence of extremely high values of AOD in the northern BoB, especially due north of ∼15°N; the occurrence of a blob of very high AOD (with values as high as 1.0) at ∼17°N and 87°E and ∼2° away from the eastern coastal India; moderate values of AOD over the entire coastal region and central region of the BoB; and comparatively lower values over the southern and eastern BoB, all salient features. With a view to examining the temporal changes in aerosol properties from winter to premonsoon, we have examined the spatial variation of the difference in AOD (ΔAOD = AOD during W-ICARB − AOD during ICARB) and the similar change in the Angstrom exponent, Δα, between the two seasons. The results are shown in Figure 14 (left and right, respectively). Examining the 3 years of AOD (2006–2008) data from the island location of Port Blair (Figure 15), it is seen that, despite a gradual increase in AOD from 2006 to 2008, the mean AODs for winter seasons (December to February) are significantly lower than values for the premonsoon seasons (March to May). However, examination of Figure 13 (left) shows that a similar feature was noted during W-ICARB only in the northern (head) BoB and in the southern parts, ∼5°N. Over a large region at and south of PBR, the AODs were higher during W-ICARB than during ICARB. A small pocket of higher AOD occurred off Bhubaneswar during W-ICARB as well. Over the rest of the major portion of the BoB, the AOD during W-ICARB was nearly the same as that during ICARB of the premonsoon season of 2006. Positive values of ΔAOD (Figure 14, left) are observed in the southern region, especially near the Andaman Nicobar Islands, with a clear longitudinal gradient with an increasing trend toward the east. This is an indication of the advected accumulation mode aerosols from East Asia. High negative anomalies are observed off the Myanmar coast (over R3) as well as over part of R2 and R5. The size distribution, inferred from α, also showed large deviation from winter to premonsoon seasons, as evident from the spatial pattern of Δα (Figure 14, right). The spectra were much steeper during W-ICARB over the entire oceanic region east of 85°E, with a peak value of Δα of ∼0.8 near the Andaman Nicobar Islands and with a distinct longitudinal variation depicting a definite increasing trend toward the east. Over the entire eastern BoB, positive values with Δα > 0.3 are observed with more and more positive values toward south. This is quite in line with the earlier observations by Moorthy et al. , who reported an increase in the steepness of the AOD spectrum at Port Blair during winter, when the station was under the influence of increased advection from East Asia and south China regions. Satheesh et al.  also reported that the major source of aerosols over the northern BoB is the eastern coast of India and central India, whereas the transported aerosols from East Asia during winter and from the Arabian Sea during summer were found to modulate the aerosol properties over the southern BoB and Port Blair. Several investigators have reported that the aerosol properties over Port Blair are mostly influenced by the advection from East Asia, the Indian subcontinent, as well as from the Arabian Sea and the equatorial Indian Ocean [Moorthy et al., 2003; Satheesh et al., 2006; Moorthy and Babu, 2006]. During the winter season, anthropogenic aerosol dominance is observed over the region (high α) as the increased advection from the East Asian region dominates, whereas during the premonsoon season, mixed types of aerosol prevail because the trajectories were mainly from the Indian subcontinent and the Arabian Sea and equatorial Indian Ocean, leading to relative dominance of coarse mode aerosols. These cause comparatively lower values of α and higher AOD values.
Figure 14. Spatial variation of the difference in AOD (at 500 nm) between the W-ICARB and ICARB of 2006 and that of the Angstrom wavelength exponent.
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 On the other hand, the western BoB, which is under the influence of advection mainly from the Indian mainland, showed lower values of α during winter, suggesting that this airmass transported more coarse, natural aerosols. Thus, the BoB is under the strong influence of concentrating source impacts on its eastern and western regions, which also undergo several changes. A comparison of the present observations with earlier reports is given in Table 3.
Table 3. Comparison of the Present Observations With Earlier Reports
|Campaign and Cruise Number||Region||Period of Campaign||Summary of Results|
|SK 161 B||NW Bay of Begnal (BoB)||Mar 2001||Mean AOD of ∼0.6 over the western coastal BoB and decreases toward the central region to reach ∼0.2. High values of α (as high as 1.8) at the coastal region with a mean value of ∼1.20 [Satheesh et al., 2002].|
|SK 188||NW BoB||Feb 2003||AODs in the range 0.3–0.6 (500 nm), with an average value of 0.41. The average AOD for the western coastal BoB was 0.54 and was found to decrease toward the mid-BoB (0.30). The average value of α at the coastal region was ∼1.12 and at the mid-BoB was 1.10 [Vinoj et al., 2004].|
|SK 197||NW BoB||Oct 2003||The values of AOD (at 500 nm) were in the range 0.2–0.7 with a mean value of 0.43. The average value of α was 0.93.|
|ICARB||Entire BoB and northern Indian Ocean||Mar–Apr 2006||AODs (at 500 nm) in the northern BoB were in the range 0.3–1.2, with a mean value of 0.49 ± 0.01; over the south, these are in the range 0.1–0.9 with a mean value of 0.29, and over northern Indian Ocean values lie between 0.1 and 0.6 with a mean value of 0.28. AODs were extremely high at a large region of approximately 3° × 3° in size and centered about 17.4°N, 87.1°E with a mean value of 0.87. In general, the entire BoB exhibits moderate (≥1.0) values of α AOD except in the southeast BoB (0.5). The highest values (1.3) were observed over the eastern coastal region of India [Nair et al., 2009].|
|W-ICARB (Present Study)||Entire BoB including eastern BoB||Dec 2008 to Jan 2009||High values of AOD were observed over the northern and northwestern part of BoB with “two detached highs” in which AODs were as high as ∼0.8, while the lowest AOD values (∼0.1) were observed over the northeast BoB (Myanmar and Bangladesh coast). In the same region, the Angstrom wavelength exponent α showed the highest values of ∼1.5, even though generally high values prevailed over the eastern as well as northern coastal regions of India (present study).|