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    Cruise track of SK223A over the BoB and NIO during ICARB. The points on the lines correspond to the ships position at 0530 UTC on the day identified below, with the number standing for the day of month and M, A, and My indicating March, April, and May, respectively. Major ports and urban conglomerates on the mainland close to the cruise track are marked in the figure.

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    Time series of AOD and BC concentration during the period of ICARB, from the island station Port Blair in the Bay of Bengal.

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    Spatial distribution of scattering coefficients at 550 nm.

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    Mean spectral variations of scattering coefficients over the northern (dotted line with filled circle) and southern (dotted line with open circle) BoB.

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    Spatial distribution of å at 550 nm over BoB.

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    Frequency of occurrence of (top) σ550 and (bottom) å550 over the northern (filled circles) and southern BoB (open circles).

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    (top) A comparison of the number size distributions of aerosols over the head BoB (northern BoB) with that over northern Indian Ocean. (bottom) The percentage decrease in the concentration of latter from the former as a function of the particle radius.

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    Scatterplot of scattering coefficients measured using the nephelometer and scattering coefficients estimated from the corresponding number–size distributions from the QCM measurements.

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    Scatterplot of scattering coefficient and total mass concentration.

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    Latitudinal variation of (top left) σ550 (left ordinate) and β550 (right ordinate); (bottom left) MB (left ordinate) and MT (right ordinate); (top right) å550; and (bottom right) FBC (left ordinate) and FMA (right ordinate).

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    Longitudinal variations of scattering coefficient over the northern (solid line, filled circles) and southern (dotted line, open circles) BoB.

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    Latitudinal variations of the (top) aerosol optical depth at 500 nm (τ500), (middle) Angstrom exponent (α), and (bottom) turbidity parameter (β).

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    Frequency of occurrence of α (open bars) over the BoB. Dotted lines show the Gaussian distribution fit and regression parameters are given in the figure.

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    Latitudinal variations of Angstrom exponents estimated from the spectral measurements of ambient scattering coefficients (dotted line with open circles) and column-integrated extinction coefficients (solid line with filled circles).

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    (left) Vertical profiles of total mass concentration (MT, solid filled circles) and accumulation mode mass concentration) (MA, dotted line with open circles) over the northern BoB; horizontal lines show the standard deviation of data. (right) Altitude distribution of normalized aerosol backscatter derived from airborne micropulse lidar measurements over the same region. Note the elevated layer of enhanced aerosol concentration and extinction between 2- and 3-km altitude.

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    Cluster plots of 7-day airmass back trajectories reaching 2250-, 2500-, and 2750-m altitude over the ship’s mean position over (a) head-BoB, (b) northern-central BoB, (c) southern-central BoB, and (d) southern BoB. The regions where the clusters originated are written in the respective panels.

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Optical and Physical Properties of Atmospheric Aerosols over the Bay of Bengal during ICARB

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  • 1 Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India
  • | 2 Centre for Atmospheric and Oceanic Sciences, Indian Institute of Sciences, Bangalore, India
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Abstract

Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.

Corresponding author address: K. Krishna Moorthy, Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum-695 022, India. Email: krishnamoorthy_k@vssc.org

Abstract

Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.

Corresponding author address: K. Krishna Moorthy, Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum-695 022, India. Email: krishnamoorthy_k@vssc.org

1. Introduction

The optical properties of atmospheric aerosols such as the aerosol optical depth (AOD) and scattering (σ) and extinction coefficients, as well as information on their spectral dependencies, are quite important in estimating the radiative impacts and regional climate forcing of aerosols (Houghton et al. 1995; Solomon et al. 2007). Aerosols are known to perturb the energy budget of the earth–atmosphere system, both directly by the scattering and absorption of radiation (mainly solar) and indirectly by modifying cloud properties such as cloud albedo, lifetime, and drop size distribution (e.g., Satheesh and Moorthy 2005; Hansen et al. 2005). Although AOD is the column-integrated aerosol extinction (due to scattering and absorption) and is the parameter with the greatest influence on the aerosol direct radiative forcing, the single scattering albedo (SSA; the ratio of aerosol scattering to extinction) decides the sign of the radiative forcing and thus is critical in determining whether the aerosol radiative effect leads to a net cooling or warming [depending on the surface albedo (e.g., Haywood and Shine 1997)]. Viewed in this perspective, knowledge of the scattering and absorption coefficients of aerosols and their spectral variation assumes immense importance. These aerosol optical properties vary with the wavelength of the radiation in a form usually represented by the Angstrom relation, with high values of the exponent indicating dominance of accumulation mode particles and low values representing dominance of coarse mode particles. The highly distributed nature of the sources and sinks of these particles, the microphysical processes occurring during their atmospheric lifetime and the synoptic-scale circulations all lead to significant heterogeneity in characteristics on both the spatial and temporal scale. This will be particularly important over marine environments impacted by the surrounding densely populated continental landmass, under the influence of changing air masses. The Bay of Bengal (BoB) constitutes an ideal laboratory for such investigations.

The BoB, confined between 80° and 100°E in longitude and 5° and 22°N in latitude, is a relatively small oceanic region, surrounded on more than three sides by densely populated continental landmasses having diverse geographical features, habitat, and industrial activities. Nevertheless, the BoB plays a major role in Asian monsoons and the precipitation pattern over India. The outflow of pollutants from the densely populated and industrialized Indo-Gangetic Plain (IGP), aided by the prevailing westerly winds through the natural channel created by the orography of the high Himalaya in the north and the Vindhya-Satpura ranges to the south, results in significant loading of natural and anthropogenic aerosols (mixed type) in the northern BoB (Girolamo et al. 2004; Nair et al. 2007; Niranjan et al. 2007). Similarly, significant advection of aerosols over to the BoB occurs from East Asian regions (e.g., Moorthy et al. 2003). On the other hand, the southern BoB opens to the vast Indian Ocean with insignificant continental impact and thus is dominated by marine aerosols. This results not only in large gradients in aerosol concentration but also in significant heterogeneity in their optical and physical properties, depending on which of the above components dominates at a given time. Despite these special features, characterization of the microphysical and optical properties (especially the scattering properties) of BoB aerosols is virtually nonexistent. The few field experiments conducted in recent years were limited to cruises along the east coast of India and coastal northern BoB (Satheesh 2002; Quinn et al. 2002a; Sumanth et al. 2004; Vinoj et al. 2004; Ganguly et al. 2005b; Satheesh et al. 2006) and focused only on columnar AOD and ambient total mass concentration (MT) measurements. The long-term measurements from the island station (Port Blair) were limited to spectral AODs and black carbon (BC) mass concentrations (Moorthy et al. 2003; Moorthy and Babu 2006). There were no spatially resolved measurements of aerosol parameters covering the entire BoB region, particularly of the scattering properties, size distributions (mass and number), mass concentrations, and spectral AODs. The Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB) of the Geosphere–Biosphere Program of the Indian Space Research Organization was aimed at addressing this concern (Moorthy et al. 2008).

During the first leg of the ocean segment of ICARB, the dedicated cruise SK223A of the oceanographic research vessel (ORV) Sagar Kanya (SK) made an extensive survey of aerosol properties over the entire BoB and the northern Indian Ocean (NIO) bound between 5° and 21°N and going as far as 93°E from the east coast of India within a span of 27 days from 18 March to 13 April 2006. Collocated measurements of scattering coefficients (total and hemispheric backscattered), the mass size distributions of ambient aerosols, and columnar spectral AODs made onboard the ORV are used in this investigation to understand the spatial and spectral variation of the optical and physical properties of aerosols over the BoB.

2. Experimental setup and database

During the cruise SK223A, the ORV sailed off from Chennai (13.1°N, 80.29°E) on 18 March 2006 and, after the measurements, called at Kochi (9.96°N, 76.27°E) on 13 April 2006 along the track shown in Fig. 1. The points on the track show the daily position of the ship at 0530 UTC, with the day of the month beside it; M indicates March and A indicates April. During the same period, continuous measurements of these aerosol parameters were also made from an aerosol observatory located at Port Blair (11.6°N; 92.7°E) in the BoB (Fig. 1). These time series data were used to quantify any systematic temporal changes in the aerosol properties during the period of the cruise measurements. The particular configuration of the cruise track enabled measurements on the coastal waters adjoining the anthropogenically dominated mainland and far off oceanic regions in rapid succession and with an approximate latitudinal resolution of 2° in the region 5.5°–21°N. In the longitudinal scale, measurements were made from the east coast of India to as far as 93°E. This provided a nearly homogeneous, spatially gridded aerosol dataset within a time span of less than a month, during which the aerosol characteristics are considered to be statistically invariant. This was also corroborated by the prevailing meteorology, which was devoid of any major synoptic weather systems such as cyclones, depressions, or extensive cloud cover during the period of study. The airmass type also remained the same. The continuous time series data from the island station Port Blair (shown in Fig. 2 for BC and AOD at 500 nm) reveal that despite the fair amount of fluctuation about the seasonal mean values, which is typical for this season, neither AOD nor BC showed any trend during the campaign period, thereby conforming that the basic aerosol type did not change drastically during the cruise period (as would happen if the type of the prevailing air mass had changed), thereby vindicating the above assumption (Nair et al. 2008a).

The onboard aerosol measurements were made from a specially designed laboratory on the top deck of the ORV (>10 m above the seawater level). All the sampling instruments aspirated the ambient air through an isokinetic community air inlet pipe fixed to the port side of the ship, which sampled the incoming air as the ship moved forward. The community air inlet was made up of a 2-m-long stainless steel tube having an internal diameter of 25 mm. Nearly laminar airflow through the tube was ensured by a suction system at the other end of the tube projecting into the laboratory, which maintained an airflow of nearly 3 m s−1 through the pipe, so that the airspeed through the pipe nearly matched the sailing speed of the ship. Laboratory calibration of this inlet was done before and after the campaign, revealing near-isokinetic conditions for particles within the size range 0.01 to 4 μm. At the larger sizes, there was some particle loss leading to underestimation in the mass concentration by up to ∼10% for particles in the size range 4–10 μm. The ship always cruised at its full speed except for planned stoppages of ∼1-h duration on each day to enable launching of meteorological balloons (Moorthy et al. 2008), and even during this period the ship was positioned such that the winds came from the port side into the sampling inlet. A global positioning receiver system (GPS) onboard provided continuous information of the spatial coordinate of the ORV, at every minute.

Continuous and near-real-time measurements of total scattering (7°–170°) and hemispheric backscattering (90°–170°) coefficients (respectively σλ and βλ) at three wavelengths (λ = 450, 550, and 700 nm) were carried out using an integrating nephelometer (model 3563, TSI, USA); a state of the art instrument, used extensively worldwide for measuring spectral scattering coefficients of aerosols. Design and technical details of the instrument are available in several earlier papers (e.g., Anderson et al. 1996; Heintzenberg and Charlson 1996). The accuracy of the nephelometer measurements mainly depends on its calibration, and as such, the instrument was calibrated before the campaign using CO2 as a high-span gas (gas having high scattering coefficient) and air as a low-span gas (gas having low scattering coefficient) and the stability of calibration was ensured after the campaign. Relative humidity (RH) and temperature (T) of the sampled air were continuously monitored using the sensors in the instrument. Major uncertainties in the nephelometer measurements are due to the nonidealities in the angular and wavelength responses (Anderson et al. 1996). The angular truncation error is the intrinsic inability of the instrument to integrate and/or measure above 170° and below 7°. This caused a reduction, by a mean factor of 1.5 times, in the measured scattering coefficient for supermicrometer particles; this underestimation was corrected using the spectral information of the scattering coefficient as described by Anderson and Ogren (1998). Following this, the correction factor (C) for the total scattering coefficient (defined as the ratio of the true scattering coefficient to the nephelometer measured one) is linearly dependent on the spectral steepness (å) of the scattering coefficient (estimated from measurements at multiple wavelength) as C = a + , where a and b are constants. We used the values of a and b from Anderson and Ogren (1998) because their study covered a wide spectrum of aerosol species. In the case of submicron aerosols, measured and calculated scattering coefficients showed good agreement with a correction factor of 1.07 (e.g., Kleefeld et al. 2002; Andrews et al. 2006). On average, the total scattering coefficient measurements made using nephelometer have an uncertainty of nearly 10% (Clarke et al. 2002). The nephelometer was operated continuously during the campaign, and the data are available at nearly 5-min intervals throughout; each set of measurements provided σ and β at three wavelengths, 450, 550, and 700 nm.

Size-segregated mass concentrations of composite (total) aerosols were measured using a 10-stage quartz crystal microbalance (QCM) cascade impactor (model PC-2, California Instruments) having a 50% lower size cutoff at each of its 10 size bins at > 25, 12.5, 6.4, 3.2, 1.6, 0.8, 0.4, 0.2, 0.1, and 0.05 μm, respectively, for size bins 1 to 10. It sampled the ambient air through the community inlet at a constant flow rate of 0.24 L (min)−1 during the sampling interval, which was typically 300 s. Measurements were carried out at regular intervals of 45 min, almost round the clock, and as such an average of 25 measurements were available per day. Measurements were restricted to the periods of RH < 78% at the deck level, in view of the sensitivity of the quartz crystal to changes in RH (for high RH). Generally, the uncertainties in measured MT values are in the range of 10% to 20% at very low mass concentrations (∼10 μg m−3) and the error reduces to <10% for the high mass concentrations >30 μg m−3. In estimating the mass concentration and standard deviation, channel 1 of QCM was not considered, particularly in view of the subisokinetic nature of the community inlet at supermicron sizes (>4 μm). In the remaining size range, the error due to the community inlet was smaller compared to other statistical uncertainties associated with the QCM measurements. The consistency of QCM-measured total mass concentrations (MT) with those estimated using high volume samplers have been examined on several earlier field experiments and found to show very good agreement (Moorthy et al. 2005; Nair et al. 2007).

Continuous measurements of instantaneous spectral AODs were made onboard using a Microtops sunphotometer (Solar Light Co.) at five wavelengths (340, 440, 500, 675, and 870 nm) at an average interval of 30 min during clear and cloud-free periods. Latitude, longitude, and time information were fed to the instrument using a GPS. During each measurement, after carefully pointing the instrument toward the sun, three scans were made in quick succession and the smallest of these AODs was taken as the AOD for that particular time. Extreme care was taken to correctly point the instrument toward the sun and ensure the alignment (Porter et al. 2001; Ichoku et al. 2002). Generally, uncertainty in the measured AODs at 500 nm was around ±0.02 or less (Ichoku et al. 2002). As the sky was mostly clear and cloud free during the entire campaign period due to the prevalence of a low-level anticyclone over northern BoB, AODs were available on all the days except on 30 March when the sky was overcast throughout the day and there was a brief spell of rainfall (9 mm), the only event of rain during the entire cruise.

Mass concentrations (MB) of aerosol BC were measured continuously using an aethalometer (AE 31 of Magee Scientific), operated at a time base of 5 min and flow rate of 5 L (min)−1. The details of BC measurements during ICARB using the aethalometer and the uncertainties in the inferred MB are discussed in the earlier papers (e.g., Nair et al. 2007, 2008a) and as such are not repeated. The aethalometer also drew its input from the community inlet and in general, BC being in submicrometer size range, no sampling loss is expected. This was also verified by operating the aethalometer with and without the community air inlet. In recent years, the measurement of BC using aethalometers and the inherent advantages, limitations, precautions, and corrections factors have been extensively studied and intercompared under different ambient and sampling conditions (e.g., Weingartner et al. 2003; Schmid et al. 2006; Hitzenberger et al. 2006; Corrigan et al. 2006). These included aerosols that are purely absorbing, purely scattering, and mixed types, both in nascent form (fresh, shortly after emission) and after aging and mixing with other aerosol types. For aged aerosol systems in which BC is well mixed with other species, Hitzenberger et al. (2006) observed a general agreement (within the respective standard deviations) of the different methods, whereas Schmid et al. (2006) found significant differences, many of which are site specific. In several of these experiments, the aethalometer technique was compared with other methods such as the thermal, integrating sphere (e.g., Hitzenberger et al. 2006), PSAP (Schmid et al. 2006), etc., and correction factors were recommended Weingartner et al. (2003), while measuring different types of (externally) mixed aerosols in varying concentrations, have suggested two correction factors: 1) a “C” factor that arises from the amplification of the attenuation due to multiple scattering of light that passes through the filter tape matrix and 2) an “R” factor arising because of “shadowing” by the particles that load the filter tape while sampling, resulting in a decrease in the optical path in the filter and thus an underestimation of BC at higher particle loads. While the former tends to overestimate BC, the latter somewhat underestimates it and in a way partly compensates for the effect of the former. On the basis of detailed analysis and several experiments, Weingartner et al. (2003) found that the shadowing effect and R factor are quite significant for “pure” soot particles but are almost negligible for aged atmospheric aerosols (a mixture). For the mixed aerosol type (i.e., a mixture of different species), they estimated the C factor to be ∼2.1. In our measurements we used an instrument factor 16.6 m2 g−1, which accounts for a C factor (of 1.9) and an R factor derived from comparison with other techniques so as to have an effective instrument factor of 16.6 m2 g−1 at 880 nm. This is considered to be reasonable in view of the fact that our sampling in the marine environment involved aged aerosols, far from potential source regions over the land, that are well mixed with other aerosol species. Moreover, BC constituted on an average less than 3% of the total mass, so it is not a BC-dominated aerosol system. In the above conditions, the typical uncertainty in the measured BC was in the range of 50–75 ng m−3. We did consider only externally mixed aerosols. Internal mixing (with a BC core enveloped by scattering shell) leads to amplification of absorption (e.g., Chandra et al. 2004) and is a more complex scenario.

In addition to the shipboard measurements, airborne measurements of the altitude profiles of aerosols were also made during ICARB, from several coastal bases, nearly concurrent with the shipboard measurements around these regions (Moorthy et al. 2008; Babu et al. 2008). The measurements were executed using the aircraft of the National Remote Sensing Centre (NRSC) at Hyderabad (India), which dedicated one of its propeller aircraft (Beechcraft 20) to ICARB. The sorties were carried out from five bases: two each on the east [Bhubaneswar (20.23°N, 85.82°E) and Chennai (13.2°N, 80.03°E)] and west coasts [Trivandrum (8.5°N, 77°E) and Goa (15.5°N, 73.8°E)] of India and one from the interior continent [Hyderabad (18.05°N, 78.3°E)]. In all, 26 aircraft sorties were made (from 23 March to 3 May 2006). Vertical profiles of the size-segregated mass concentrations [M*(r)] of composite aerosols having diameters between 0.23 and 20 μm were made using a 16-channel optical particle counter (OPC; model 1.108 of Grimm Aerosol Tech. GmbH; Grimm and Eatough 2009) operated in the mass mode, in the aircraft. The OPC measurements were carried out during daytime, with the cabin of the aircraft kept unpressurised. A micropulse lidar (MPL 1000 of Science and Engineering Services, Inc.) was mounted in a down-looking mode in the aircraft cabin and profiling was done from an altitude of 8 km during early nighttime hours (of the same day or the succeeding day) by sending the laser through the large optical flat window at the floor of the aircraft cabin. From the MPL data the extinction profiles were obtained up to an altitude of 8 km as a function of the aircraft position, following the details given in earlier papers (e.g., Satheesh et al. 2008, 2009). Of the 26 sorties, five were dedicated for the airborne MPL measurements, one sortie for each base (Satheesh et al. 2008). During the other sorties, the aircraft flew unpressurized at lower altitudes and made vertical profiling at high resolution using in situ measuring instruments (Moorthy et al. 2008) at a nominal sampling speed of 300 km s−1.

3. Results and discussion

a. Spatial variation of total scattering coefficient

The spatial composite of aerosol total scattering coefficient at 550 nm (σ550) over the BoB and NIO, constructed using the individual measurements of the nephelometer, is shown in Fig. 3. Extremely high values (∼200 Mm−1; 1 Mm = 106 m) are seen over the northern BoB [especially at the head (20°–22°N) of the BoB], which decreased continuously to reach values as low as ∼25 Mm−1 over the southeastern BoB (decreasing by a factor of 8). In the northern BoB, the high values of σ550 extended more southward at the eastern longitudes (close to the Bangladesh and Myanmar coast) than close to the Indian landmass. Oceanic regions off the highly industrialized ports along the Indian coast (Chennai and Bhubaneswar; Fig. 1) also showed high values of σ550, in the range from ∼100 to 150 Mm−1, probably associated with the urban activities, but the offshore spatial extent was limited. The very low values (∼25 Mm−1) observed south of Port Blair were comparable to those reported close to pristine Antarctic environments (Tomasi et al. 2007). This rapid southward decrease in σ550 from the head-BoB continued up to 8°N, to the south of which the values increased gradually to as high as 75 Mm−1 at 5.5°N.

The high values of σ550 over the northern BoB appear to be associated with high concentrations of fine aerosols resulting from the outflow of pollutants through the channel across the Indo-Gangetic Plain (Niranjan et al. 2007; Nair et al. 2007), which is one of the most aerosol-laden regions of South Asia (Girolamo et al. 2004; Jethva et al. 2005). Extremely high aerosol loading and dust episodes are common over this region during the premonsoon periods of March–April (see, e.g., Dey et al. 2004). In addition to this, the westward-tilted anticyclone prevailing over the BoB during this period favors the advection of East Asian aerosols over the BoB (Wang et al. 2007). Analysis of the aerosol vertical profiles along with concurrent GPS sonde balloon ascents in the northern BoB have shown the occurrence of narrow regions of convectively stable layers of the atmosphere at around 1.5 to 2 km above the ground, sandwiched between convectively unstable layers below and above (Babu et al. 2008; Satheesh et al. 2009). These stable layers (occurring between two unstable layers) can act as conduits for long-range transport of aerosols. The role of such long-range transport in enhancing the AOD at short wavelengths and mass concentrations of BC in the eastern BoB during this season has also been reported from long-term observations by Moorthy et al. (2003) and Moorthy and Babu (2006). Being downwind of these two major source regions (IGP and East Asia), the northern BoB experiences a very high concentration of fine-mode aerosols from the continental anthropogenic sources.

Prior to ICARB, measurements of scattering coefficients over the BoB were limited to the 1999 Indian Ocean Experiment (INDOEX; day numbers 87.5 to 89.45 of 1999) made onboard the research vessel Ronald H. Brown over the southern BoB (5°–10°N, 85°E). The average value of σ obtained during those measurements (48 ± 9 Mm−1) is comparable to the mean value (55.9 ± 23 Mm−1) from our measurements over that region. The mean value of σ for the whole of BoB during ICARB was 94 ± 47 Mm−1; this is, in general, higher than the mean values reported for the larger oceanic regions of Atlantic and Pacific (Quinn et al. 2002b; Fujitani et al. 2007). The high values of σ550 (129 ± 44 Mm−1) over the northern BoB (north of 13°N) are comparable to those reported for continental urban regions of India (∼110 Mm−1; Ganguly et al. 2005a) or rural coastal regions (Mace Head) by Kleefeld et al. (2002).

However, very low values are observed over the southeastern BoB, which are comparable to the values reported for pristine environments (Tomasi et al. 2007). This shows the presence of steep gradients in σ over a relatively short spatial scale over the BoB. Measurements over the Arabian Sea (4°–19°N, 67°–75°E) during 1999 have also shown large spatial variation in σ550 in the range 110 to 20 Mm−1; however, the average (61 ± 28 Mm−1) was higher than the mean value over southern BoB seen in our studies. We have summarized several reported values over distinct oceanic regions in different seasons in Table 1, in which we also have kept our values. Examined in the light of these previous studies, our results show that although the southern BoB exhibits near-pristine characteristics, very high scattering coefficients are encountered in the northern BoB, particularly toward the eastern parts. Such large heterogeneity over relatively small oceanic regions has not been reported earlier and we believe that this are of great significance in developing aerosol models (of scattering and extinction) for radiative forcing calculations.

b. Wavelength dependence of σ

The wavelength dependence of σ strongly depends on the number–size distribution (NSD) of the aerosol particles, the Mie scattering efficiency parameter Qsc (x, m*) being a strong function of the particle size parameter (x = 2πr/λ) (where r is the radius of the particle) and m*, the complex refractive index of the scatterers. As such, the spectral scattering coefficient can be used to delineate, with a fair degree of confidence, the anthropogenic and natural aerosols, if we consider that the fine- and accumulation-mode particles (<1.0 μm) are mostly of anthropogenic origin and the coarse-mode (>1.0 μm) aerosols are of natural origin. In view of the distinctiveness of aerosol type over the BoB, discussed in the previous sections, the wavelength dependence of the σ averaged separately for the northern BoB (solid line joining filled circles) and the southern BoB (dotted line joining open circles), with the two regions demarcated about 13°N latitude, is examined in Fig. 4. The figure shows clearly that in addition to being higher by factor of >2 at all the wavelengths, σ over the northern BoB shows stronger wavelength dependence than that over the southern BoB. With a view to quantifying this difference and examining its spatial variability, the wavelength exponents of the total scattering coefficient were estimated for each of the nephelometer measurements following the Angstrom relation, σλλå, where å is the wavelength exponent of σ. Accordingly, å at 550 nm was determined as
i1520-0469-66-9-2640-e1
Equation (1) corresponds to the broadband spectral behavior of σ, with the implicit assumption that å550 is constant within the wavelength range of 450 to 700 nm. The spatial variation of å550, shown in Fig. 5, differs significantly in several aspects from that of σ550 (Fig. 3), despite the broad similarities. (A comparison of the wavelength dependency of σ with that of AOD is made later in section 3f.) For example, even though a higher value of σ occurred around 20°N, the strongest spectral dependence (highest value of å > 1.6) occurred much below, ∼16°N along the coastal regions adjacent to the Indian mainland. The mean value of å550 for the entire northern BoB was 1.45 ± 0.14 (the values appearing after the ± symbol being the respective ensemble standard deviations). From the above standpoints and the observations in Fig. 5, it is borne out that (i) in general, accumulation-mode particles dominated over the entire northern BoB and (ii) their dominance is higher close to the industrialized coastal regions. Such high values of σ550 and å550, respectively 135 ± 23 Mm−1 and 1.26 ± 0.2, were reported during the Asian Aerosol Characterization Experiment (ACE-Asia) measurements onboard Ronald H. Brown in the East Asian outflow over the Pacific during March–April 2001 (Doherty et al. 2005), showing significant influence of submicron, anthropogenic aerosols. In contrast, as we moved to the southern BoB, å550 decreased dramatically, indicating a large reduction in the accumulation fraction even though it is still quite substantial. Very low values (<0.5) of å550 occurred in the region 7°N, 93°E. Considering the entire BoB, although the mean value of å550 was 1.27 ± 0.2, it varied over a wide range from as low as 0.3 to as high as 1.6. This large spatial heterogeneity has not been considered in any of the earlier aerosol radiative impact assessments over this region (Vinoj et al. 2004; Satheesh et al. 2006; Ganguly et al. 2005b).

From Figs. 3, 4, and 5 and the foregoing discussions, it emerges that the optical properties of BoB aerosols not only varied over a wide range but were distinctly different as well in the northern and southern parts of BoB. The number concentrations, size distributions, and probably the refractive index also changed remarkably between these two regions (though we did not have any measurements of the refractive index). In Fig. 6 we examine the distribution of the frequency of occurrence (in percentage) of (top) σ550 and (bottom) å550 separately for northern (filled circles) and southern BoB (open circles). Despite the wide spread of values, two distinct distributions emerge. The southern BoB is characterized by low σ550 (94% of the values lying below 100 Mm−1), with two rather narrow peaks, one at 45 Mm−1 and the other at 79 Mm−1. In contrast, over the northern BoB, more than 65% of the values of σ550 exceeded 100 Mm−1 and the distribution of σ550 shows two peaks that are far separated, with the less prominent mode occurring around 70 Mm−1 (close to the prominent mode in southern BoB) and the dominant and broad mode at 160 Mm−1, extending as far as 200 Mm−1. It is also interesting to note that although over the southern BoB, σ550 never exceeded 130 Mm−1 (except very close to the Chennai port), over the northern BoB it went as high as 230 Mm−1. On the lower side, the values over the northern BoB never went below 50 Mm−1, whereas in the southern BoB, ∼5% of the total measurements were as low as 25 Mm−1. The distinctiveness is well discernible in å550 also (Fig. 6, bottom). The values of å550 over the southern BoB showed a broad peak with >70% occurring in the range 0.9 to 1.2 (with a peak centered at 1.0, implying a rather shallow spectral dependence) followed by a sharp peak at 1.3; over the northern BoB, å550 showed a sharp and prominent peak centered at 1.5, with 70%–75% of the values lying in the range from 1.4 to 1.6, characterizing steep spectral dependence. This clearly indicates the possibility of at least two different aerosol types over the BoB with distinct physical and optical properties. The coarse-mode natural aerosols (sea salt and/or advected dust) coexist with the fine-mode anthropogenic particles; their relative dominance is distinctly different between the northern and southern parts of the Bay. This aspect is examined in the following sections.

c. Association with measured size distributions

At this juncture, we examine the association of the scattering properties with other physical properties of aerosols, deduced from the collocated measurements. Using the size-segregated mass concentration measurements made with the QCM, the number–size distributions have been estimated by assuming spherical aerosols having a mean density of 2 g cm−3 (following Pruppacher and Klett 1978). The NSDs showed a substantial reduction in the concentration of accumulation mode particles (geometric mean radius between 0.1 and 1.0 μm) as we move from northern BoB to the southern regions as illustrated in Fig. 7. The top of Fig. 7 shows a comparison of the NSD for the northern BoB (north of 13°N; shown by solid lines joining filled circles) with that over the southern BoB/NIO (south of 7°N). The percentage decrease between the concentration of latter and the former is shown in the bottom of Fig. 7 as a function of particle radius. Although there is a significant reduction at all the sizes, which is in line with the features of the scattering coefficient and total mass concentration, a significant decrease (by a factor of ∼5) occurred consistently throughout the accumulation mode regime (over the diameter range 0.06 to 0.6 μm) and also at D = 2.0 μm. Because the accumulation size range contributes significantly to the scattering at the visible wavelengths, the above observation corroborates the inferences drawn earlier from the spatial variation of σ and å.

Using the daily mean size distribution [nQCM(r)] of aerosols retrieved from the QCM measurements, the scattering coefficients σQCM at 550 nm were estimated using the direct Mie equation
i1520-0469-66-9-2640-e2
where Qsca is the scattering efficiency parameter, which is a function of complex refractive index m* and size parameter x. For the BoB, a value of 1.45–0.0015i was used for the complex refractive index based on the aerosol models retrieved by Babu (2005); it was corrected for the changes in the ambient humidity using Hess et al. (1998). In Fig. 8, we show the scatterplot of σQCM at 550 nm against the daily mean values of σ550 from the nephelometer measurements. The regression line drawn through the points reveals a good agreement with a slope of 0.92 ± 0.05 and correlation coefficient of 0.84 (R2 = 0.709, significant at p < 0.0001 for the 27 pairs of data), showing good consistency between the optical and physical properties of aerosols over the BoB.

d. Mass scattering efficiency

The mass scattering efficiency (MSE) of composite aerosols is a measure of the aerosol light scattering per unit mass and is an important and convenient parameter in aerosol radiative forcing studies, particularly when the experimental constraints facilitate only measurements of aerosol mass concentrations. Generally, chemical transport models derive the aerosol optical properties using the MSE or mass absorption efficiency of chemical species and the mass concentrations (e.g., Chin et al. 2002). Globally there exist large uncertainties in the reported values of mass scattering efficiency due to the differences in measurement techniques and size regimes considered (Hand and Malm 2007). The third assessment report of the IPCC has highlighted the importance of MSE in aerosol radiative forcing estimates and pointed out that uncertainties in the current values of MSE are among the major sources of uncertainty in the estimations of direct radiative forcing, especially for the fossil fuel aerosols (Houghton et al. 1995). While sulfate aerosols are reported to have a MSE of 5.0 ± 1.6 m2 g−1 (Quinn et al. 1995), anthropogenic aerosols generated from fossil fuel burning and industrial activities are reported to have a wide range of MSE from 2.3 to 4.7 with a mean value of 3.5 m2 g−1 (Houghton et al. 1995). Based on extensive measurements over different oceanic regions downwind of continental aerosol source regions, Quinn and Bates (2005) reported MSEs of sub-10-micron aerosols varying from 0.78 to 3 m2 g−1. Thus, accurate and experimental estimation of MSE has global significance. Accurate estimates of MSE are also important in aerosol modules of global circulation and chemical transport models that compute the radiative forcing effects of aerosols and in chemical extinction budgets used for visibility regulatory purposes (Hand and Malm 2007). The collocated measurements of the mass concentration of composite aerosols and the total scattering coefficient during our campaign facilitated estimation of the mass scattering efficiency over BoB by performing a linear regression analysis, as shown by the dotted line in Fig. 9. In the figure, the daily mean values of σ550 obtained from the nephelometer are plotted against daily mean simultaneous estimates of total mass concentration MT using QCM. The daily mean values were considered here because of the difference in the sampling intervals of the two instruments. The linear regression analysis yields a slope of 2.66 ± 0.1 m2 g−1 with a correlation coefficient of 0.85 (R2 = 0.73) for the 27 pairs of points. Such an estimate is done for the first time over the BoB. Measurements over NIO, from KCO, and onboard Ronald H. Brown have yielded MSE values >3 m2 g−1 (Clarke et al. 2002) for accumulation-mode aerosols.

e. Latitudinal and longitudinal gradients

During ICARB, all the optical and physical properties of BoB aerosols revealed sharp latitudinal gradients, decreasing from north to south. Figure 10 shows the latitudinal variations of σ550 and β550, å, black carbon mass concentration (MB) and total mass concentration (MT), and BC mass fraction (FBC = MB/MT) and accumulation mode fraction (FA = MA/MT). A nearly threefold decrease is observed in σ550 and β550 from 20° to 09°N, which thereafter both increased (though weakly) toward 5°N. Similarly MT and MB also showed sharp gradients with a nearly eightfold decrease from north to south. Below 7°N, there is a definite, although weak, increase in the concentration of MT and MB. This increase, observed in almost all aerosol parameters, is associated with the increased particulate abundance in this region associated with the very high density of marine traffic (∼30% of the global traffic) prevailing over this narrow channel (Nair et al. 2008a). In contrast, the latitudinal variation of å showed highest values at 16°N (not at the northernmost BoB where MB, MT, σ, and β were highest). However, FBC and FA (generally used as the tracers of anthropogenic influence) did not show any remarkable latitudinal trends; rather, they fluctuated about mean values of 3.1 ± 0.2 and 0.6 ± 0.04, respectively, showing that BC forms a homogeneous component of the composite, accumulation-mode aerosols, such that its contribution to the accumulation mode mass remains nearly invariant with latitude.

The latitudinal variations of aerosol parameters (MT, MB, σ, β, and å) were parameterized using an exponential growth function of the form
i1520-0469-66-9-2640-e3
where C is the measured parameter, C0 the latitude-independent component of C, Λ the latitude in degrees, and ΛD the scaling distance for an e-fold increase in the value of C. Such parameterization is important and useful in modeling the transport of anthropogenic aerosols from the continental source regions to the ocean and will provide a quantitative estimate of the spatial extent over the oceans to which the anthropogenic impacts are felt significantly. Performing nonlinear regression analyses of the measurements using Eq. (3), ΛD and C0 are estimated for MB, MT, σ550, β550, and å. The results are summarized in Table 2 along with the squared correlation coefficient (R2). While the mass concentrations MB and MT showed sharp gradients, with ΛD of 8.4 and 9°, gradients in σ550 and β550 were shallower with ΛD ∼ 13°.

Longitudinal variations of MB and σ550 were not as conspicuous as the latitudinal variations, even though over the northern BoB an increase in scattering coefficient with longitude was clearly indicated. In Fig. 11, we examine the longitudinal variations of σ550 for the northern (above 13°N) and southern BoB, indicated respectively by solid lines connecting filled points and dotted lines of connecting unfilled points. Whereas in the northern BoB σ550 shows a positive gradient toward the east (indicating probable source regions in East Asia), over the southern BoB σ550 remains nearly steady in the longitude range of 81°–89°E and decreases to lower values further eastward. BC also exhibited a similar longitudinal variation (Nair et al. 2008a) over the entire BoB north of 12°N. All of these factors point to a significant contribution from the East Asian regions to BC and submicrometer aerosol concentration in the BoB during this season.

f. Columnar optical properties

Having examined the aerosol characteristics in the MABL and their distinctiveness between the northern and southern BoB, we now examine the columnar characteristics as revealed by the AOD, vis-à-vis the surface optical properties. For this, we used the collocated spectral AOD measurements made at 30-min intervals onboard the ORV and spectral AOD (daily mean) values from the measurements made using the multiwavelength radiometer (MWR) at the network observatories over the island stations, Port Blair (PBR) in BoB and Minicoy in the Arabian Sea. The MWR provided mean AODs (separately for the forenoon and afternoon parts of the day) at 10 wavelengths centered at 380, 400, 450, 500, 600, 650, 750, 850, 935, and 1025 nm having full-width half-maximum bandwidths of 5 nm (Moorthy et al. 2008). During ICARB, continuous estimates of AODs from these islands were made using this instrument. The AODs estimated using these MWRs were compared with good agreement with the mean values estimated simultaneously using the microtops, which was taken onboard the cruise. The latitudinal distribution of AOD at 500 nm (τ500) over the BoB showed a minimum value of 0.09 at ∼9°N (in the southern BoB) and a maximum value of 1.2 at ∼16°N, as shown in Fig. 12 (top). For the entire BoB, the mean value was 0.36 ± 0.17 for τ500. Similar to MT, MB, and σ in Fig. 10, AOD also showed a gradient decreasing southward. Examining the values from PBR, we note that the average value of τ500 was 0.3 at Port Blair for the cruise period, with very little (<10%) temporal variation. This also confirmed the near-temporal stability of the aerosol environment during the cruise period, so that the variations seen in the cruise measurements corresponded mainly to spatial variations. Aerosol Robotic Network (AERONET) stations Mukdahan (16.6°N, 104.7°E) and Pimai (15.2°N, 102.6°E), lying further east, reported higher values (respectively 0.61 and 0.45) for the mean during the campaign period. Measurements from Minicoy Island (8.3°N, 73°E) in the Arabian Sea and Hanimaadhoo (6.7°N, 73.1°E) in the northern Indian Ocean also showed relatively higher optical depths of ∼0.4. A spatial synthesis of AODs during the ICARB period over the Indian mainland also showed higher values at most of the locations, except perhaps over the central Himalayas (Beegum et al. 2008).

From each measurement of the spectral AOD made on board, the Angstrom parameters α and β were estimated by performing regression analysis with the Angstrom relation τp(λ) = βλα, where α (an indicator of the steepness of the AOD spectrum) is a measure of the ratio of the accumulation-mode to the coarse-mode abundance of aerosols, β is a measure of the total aerosol loading, and λ is the wavelength expressed in μm. The latitudinal variations of α and β, shown in Fig. 12 (middle and bottom, respectively) also reveal gradients similar to that of AOD. During the campaign period α varied from 1.4 (in the northern BoB) to 0.4 (in the southern BoB) and β was in the range 0.75 to 0.04. Such a large spatial variation in α is suggestive of significant changes in the columnar size spectrum of aerosols as well, as we move from north to south. The frequency of occurrence of α (Fig. 13) peaks sharply at 1.13 (with a standard deviation of 0.13), with more than 90% of the values lying between 0.9 to 1.4. The sharp monomodal feature of α in Fig. 13 is distinctly different from the bimodal distribution of å (the wavelength exponent of scattering coefficient) for the marine atmospheric boundary layer (MABL) shown in Fig. 6 (bottom). This indicates that the size distributions of aerosols in the MABL differ significantly from the mean columnar size distribution.

With a view to delineating the particular oceanic region where this distinctiveness occurs, the latitudinal variations of the daily mean values of α and å are examined in Fig. 14. Quite interestingly, we observe that while over the southern BoB (particularly below 10°N) magnitudes of both α and å are quite close to each other, they start increasingly differing as we move northward. In the latitudinal region of 15° to 20°N, while α varies around the mean value of 1.2, å was as high as 1.4 to 1.5, suggesting a much steeper λ dependency for σ than for τ. This suggests two possibilities: (i) strong spatial variation of aerosol absorption and/or (ii) vertical heterogeneity of aerosol properties due to the prevalence of different type of aerosols at higher altitudes, probably brought in through long-range transport. The first aspect arises because we are comparing the spectral dependences of column-integrated extinction (scattering + absorption) coefficient (AOD) with the ambient scattering coefficient, σ. The implied assumption is a steady and low contribution of BC (absorption) to the total aerosol extinction. This could be the case during ICARB, when BC contributed only ∼3% to the total aerosol mass (Fig. 10); this was almost steady throughout the BoB, irrespective of latitude. In addition to BC, mineral dust also would contribute, although to a much less extent, to absorption at 880 nm. From the multiple wavelength measurements using the aethalometer (at seven wavelengths), the spectral absorption coefficients (σabs) were estimated (Nair et al. 2008b) and the index spectral dependence is evaluated by fitting the measurements to the Angstrom equation of the form σabs = p, where p is the wavelength exponent, a measure of spectral variation of absorption coefficient. For pure BC aerosols (or cases in which BC contributes to most of the aerosol absorption), p will be close to 1, whereas for biomass burning aerosols and/or mineral dust, p > 2 (Fialho et al. 2005). In the present study we found that the p value is ∼1 over the entire BoB, indicating the dominance of BC aerosols in the absorption. This almost eliminates the first possibility. As such, the change in the aerosol properties at higher altitudes assumes importance. The significantly lower values of α (compared to å) in the northern BoB imply the presence of higher abundance of coarser-mode aerosols above the MABL. This could arise if different types of aerosols, such as mineral dust, are brought in by the synoptic circulations through long-range transport. This aspect is examined with the aid of the aircraft measurements and back-trajectory analyses.

g. Altitude profiles from aircraft measurements

As pointed out in section 2, airborne measurements of extinction profiles (using MPL) and in situ measurements of the aerosol concentrations were carried out during ICARB from four coastal bases. Of this, the base at Bhubaneswar (BBR, Fig. 1) corresponded to northern BoB, and air sorties were carried out from here during 25–28 March 2008. During the same period the ship was sailing that part of the oceanic region making longitudinal transects. So the aircraft measurements provided the average aerosol characteristics aloft in the region where we had the shipborne measurements of the MABL aerosols. While the MPL was operated in the early nighttime of 25 March from an altitude of ∼8.2 km above mean sea level, in situ measurements were made during the daytime, including on 25 March. The average profile of these was used to understand the aerosol properties above the MABL and look for a possible elevated layer of aerosols. The results are presented in Fig. 15. The lidar data very clearly show the presence of a strong elevated layer of aerosol extinction in the altitude region 2 to 3 km, extending as far as 150 km off the coastline. The thickness of the layer varied from more than 1 km close to the coast to ∼500 m at the farthest point. Within this layer the extinction coefficient is 2 to 3 times more than the values just below the layer. Here it should be noted that occurrences of such elevated aerosol layers extending up to ∼400 off the east coast have been reported during ICARB by Babu et al. (2008) and Moorthy et al. (2008) based on aircraft measurements. The offshore extent at Bhubaneswar was limited to <200 km due to the presence of an off-shore “no fly zone” in the neighborhood. Figure 15 (left) shows the concentrations of total (M*T) and accumulation mode (M*A) aerosols derived from the OPC data. For the accumulation mode, we used 1 μm as the upper cutoff size. A well-defined peak in aerosol concentration is clearly seen between 2 and 3 km in both the profiles. This indicates the presence of strong elevated layer of aerosols above the boundary layer, probably by the advection of particles from the IGP and west Asia (WA). Here it is to be borne in mind that the in situ sampling of aerosols onboard aircraft may have uncertainties in the absolute magnitude mainly because the inlet system of aircraft may modify the mass concentration, especially in the coarse mode (Hermann et al. 2001). However, in the present study these uncertainties will be smaller because of the low sampling speed of the aircraft (∼80 m s−1). The scattering and extinction characteristics of this layer could be different from that of the aerosols in the MABL and might be leading to the difference in the spectral dependency of AOD. To ascertain this, we resorted to airmass back-trajectory analysis.

h. Trajectory analysis

Seven-day, isentropic airmass back trajectories arriving at the ship’s daily mean position were calculated using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Rolph 2003) at various heights between 2000 and 3000 m AGL altitude (the region where the elevated aerosol layer was seen in Fig. 15) for all the cruise days. The trajectories were then grouped into four groups; two each for the northern part and the southern part of BoB, based on the continental regions covered by these trajectories. The mean paths and their spatial spreads are shown in Fig. 16, where the end points of the trajectories correspond to the mean position of the ship during the one week considered. The figure shows a substantial decrease in the continental travel and increase in the over-ocean travel of the trajectories as we move southward from the head of the BoB (left to right and top to bottom). In the southern BoB (Fig. 16, bottom) the trajectories either are confined to the oceanic environment or have a considerable sea travel after a short travel of one or two days over the central Indian region. In either case, the temporal variations of α and å were quite similar and comparable, indicating the presence of particles having similar size distribution in the whole column, with very little vertical heterogeneity. However, in the northern BoB, where α and å differed significantly, the trajectories have significant travel across the hot and dry continental regions and comparatively much shorter travel over the ocean. In Table 3 we have given the altitude of the mean trajectory of each group as a function the days counting backward from the point of arrival. As can be seen there, the trajectories from the Indo-Gangetic plains had traveled through the boundary layer (which typically extend up to 1.5 km during this season), lifted upward and traveled eastward (similar to the case of a weak warm conveyor belt), and arrived at the ship’s location. However, the trajectories from west Asia originated from higher levels of the atmosphere and descend gradually. Nevertheless, fine dust could be transported through these trajectories because the intense resulting convection over the arid regions would lift the fine dust particles to higher levels than over the relatively cooler and vegetated IGP. Over the head of the BoB, where the differences (between α and å) were the highest, the trajectories have extensive long tracks over the arid regions of west Asia, Africa, Pakistan, and northwest India. As we move southward, the length of continental travel of the trajectories progressively decreases and so too does the vertical heterogeneity as revealed by the latitudinal variation of å and α. These provide strong support for the long-range transport of mineral dust aerosols from the arid regions over to northern BoB, causing the heterogeneity in the column. Generally, transported mineral dust has a mode radius of 0.5 μm (Hess et al. 1998). These particles occurring at higher levels produce the enhanced concentration in Fig. 15 and also lead to lowering of the values of α (deduced from the spectral AOD).

These observations are also in line with the earlier report of Niranjan et al. (2007), who, based on extensive profiling using a micropulse lidar at the east coast of Visakhapatnam, have shown the existence of elevated aerosol layer at a 3–6-km altitude region during March and April. They also found that for >66% of such cases, the airmass trajectories originated from the west Asian region. Based on extensive observation from Kanpur in the IGP, Chinnam et al. (2006) have shown a significant influence of mineral dust advection from west Asia and the Thar Desert over the IGP during the spring and summer season. During INDOEX, the spectral variations of surface and columnar aerosol properties were reported to vary significantly over the Indian Ocean when elevated aerosol layers were observed (Quinn et al. 2002a; Muller et al. 2001). Similar results were also reported over Sea of Japan during ACE-Asia, when the airmass back trajectories showed significant influence of dust in the upper troposphere (Quinn et al. 2004).

4. Conclusions

Collocated and spatially resolved measurements of the optical and physical properties of atmospheric aerosols, both in the MABL and vertical column, were measured using shipboard and airborne instruments for the first time over the Bay of Bengal region, over which no such data existed before. Our results have shown that long-range transport of aerosols from the arid/semiarid and urban landmass significantly modifies the aerosol properties above the MABL, particularly in the northern BoB. Other chief findings are as follows:

  1. Large spatial heterogeneity exists in almost all properties (physical and optical) of aerosols, both within the MABL and in the vertical column. The scattering coefficient σ550 varied from the extremely high values (∼200 Mm−1) over the northern BoB to near-pristine values (as low as ∼25 Mm−1) over the southeastern BoB, within a span of about 10°, revealing the large spatial heterogeneity that prevails over this small ocean. This information itself was not available prior to ICARB.
  2. Similarly, the wavelength exponent of σ varied rapidly across the ocean, from a high value in the north to a lower value (by a factor of ∼2) in the southern BoB, showing large difference in the size distribution of aerosols. Simultaneous data from the QCM revealed that the above change is associated with a rapid decrease in the concentration of accumulation-mode aerosols (in the radius range 0.05 to 1 μm), which contribute significantly to the scattering at the optical wavelengths.
  3. Although all the parameters (σ, β, MT, MB, α, and å) showed a definite latitudinal gradient, decreasing from north to south, the rate of decrease was much slower for the scattering properties than for mass concentrations. In comparison, the longitudinal gradients were weak and were conspicuous only in the northern BoB, where the properties tended to increase eastward from the Indian coast, indicating potential source regions in East Asia.
  4. The wavelength exponent å of the scattering coefficient differed significantly from that (α) of the AOD over the northern BoB, whereas they were quite comparable over the southern BoB, thereby indicating significant vertical heterogeneity in the aerosol properties over northern BoB. Simultaneous aircraft measurements showed elevated aerosol layers (several hundred meters thick) of enhanced extinction in the altitude regions of 2–3 km, extending hundreds of kilometers into the ocean from the mainland. Within the layer, the extinction coefficients where more than double the values below, while particle concentrations showed an increase in the abundance of coarse-mode aerosols. Back-trajectory analysis revealed that these layers are associated with potential advection pathways mainly from the arid regions of west Asia and northwest India. As we move increasingly to the south, the trajectories have increasingly lesser continental travel, and associated with this the vertical heterogeneity decreases.
  5. The vertical distribution of aerosols assumes significance while addressing radiative impact due to aerosols. It is interesting to note that the radiative forcing due to aerosols still remains as a significant source of uncertainty because most of the current estimates are largely based on model studies. Moreover, it is usual practice to use measured aerosol properties at the surface and translate them to column properties for use as input to radiation models by making assumptions about vertical profiles (Satheesh et al. 1999; Babu et al. 2002). Our study clearly shows that the surface aerosol properties could be significantly different from the mean property of the column in the presence of distinct aerosol layers aloft, which if not considered could lead to large errors in the estimation of radiative forcing. Our results are expected to lead to improvement in the assessment of the radiative impacts of BoB aerosols and the significance of the sharp spatial gradients and heterogeneities modifying it.
The role of such elevated layers of enhanced extinction over the BoB, associated with western advection of aerosols, in contributing to the “elevated heat pump” process (Lau et al. 2006) in the western Himalayan region needs to be investigated.

Acknowledgments

This study was carried out as a part of the ICARB project of ISRO-GBP. The authors are thankful to the Director, National Centre for Antarctic and Ocean Studies, and M. Sudhakar for providing shipboard facilities. The authors greatly acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and READY website (available online at http://www.arl.noaa/gov/ready.htm) used in this publication. We thank the principal investigators and their staff for establishing and maintaining the AERONET sites used in this investigation. We thank the anonymous reviewers whose critical evaluation and useful comments have largely improved the paper.

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Fig. 1.
Fig. 1.

Cruise track of SK223A over the BoB and NIO during ICARB. The points on the lines correspond to the ships position at 0530 UTC on the day identified below, with the number standing for the day of month and M, A, and My indicating March, April, and May, respectively. Major ports and urban conglomerates on the mainland close to the cruise track are marked in the figure.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 2.
Fig. 2.

Time series of AOD and BC concentration during the period of ICARB, from the island station Port Blair in the Bay of Bengal.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 3.
Fig. 3.

Spatial distribution of scattering coefficients at 550 nm.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 4.
Fig. 4.

Mean spectral variations of scattering coefficients over the northern (dotted line with filled circle) and southern (dotted line with open circle) BoB.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 5.
Fig. 5.

Spatial distribution of å at 550 nm over BoB.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 6.
Fig. 6.

Frequency of occurrence of (top) σ550 and (bottom) å550 over the northern (filled circles) and southern BoB (open circles).

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 7.
Fig. 7.

(top) A comparison of the number size distributions of aerosols over the head BoB (northern BoB) with that over northern Indian Ocean. (bottom) The percentage decrease in the concentration of latter from the former as a function of the particle radius.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 8.
Fig. 8.

Scatterplot of scattering coefficients measured using the nephelometer and scattering coefficients estimated from the corresponding number–size distributions from the QCM measurements.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 9.
Fig. 9.

Scatterplot of scattering coefficient and total mass concentration.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 10.
Fig. 10.

Latitudinal variation of (top left) σ550 (left ordinate) and β550 (right ordinate); (bottom left) MB (left ordinate) and MT (right ordinate); (top right) å550; and (bottom right) FBC (left ordinate) and FMA (right ordinate).

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 11.
Fig. 11.

Longitudinal variations of scattering coefficient over the northern (solid line, filled circles) and southern (dotted line, open circles) BoB.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 12.
Fig. 12.

Latitudinal variations of the (top) aerosol optical depth at 500 nm (τ500), (middle) Angstrom exponent (α), and (bottom) turbidity parameter (β).

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 13.
Fig. 13.

Frequency of occurrence of α (open bars) over the BoB. Dotted lines show the Gaussian distribution fit and regression parameters are given in the figure.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 14.
Fig. 14.

Latitudinal variations of Angstrom exponents estimated from the spectral measurements of ambient scattering coefficients (dotted line with open circles) and column-integrated extinction coefficients (solid line with filled circles).

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 15.
Fig. 15.

(left) Vertical profiles of total mass concentration (MT, solid filled circles) and accumulation mode mass concentration) (MA, dotted line with open circles) over the northern BoB; horizontal lines show the standard deviation of data. (right) Altitude distribution of normalized aerosol backscatter derived from airborne micropulse lidar measurements over the same region. Note the elevated layer of enhanced aerosol concentration and extinction between 2- and 3-km altitude.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Fig. 16.
Fig. 16.

Cluster plots of 7-day airmass back trajectories reaching 2250-, 2500-, and 2750-m altitude over the ship’s mean position over (a) head-BoB, (b) northern-central BoB, (c) southern-central BoB, and (d) southern BoB. The regions where the clusters originated are written in the respective panels.

Citation: Journal of the Atmospheric Sciences 66, 9; 10.1175/2009JAS3032.1

Table 1.

Comparison of the scattering coefficient at 550 nm obtained in this study with other measurements around the Indian region.

Table 1.
Table 2.

Parameters pertaining to the latitudinal gradients of various aerosol parameters, deduced by nonlinear least squares fit to Eq. (3).

Table 2.
Table 3.

Altitude (AGL) of the mean trajectory of each group (in Fig. 16) as a function of day, starting backward from the end point.

Table 3.
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