Spatial heterogeneities in aerosol properties over Bay of Bengal inferred from ship-borne and MODIS observations during ICARB-W cruise campaign: Implications to radiative forcing

Atmospheric Environment 45 (2011) 404e412

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Spatial heterogeneities in aerosol properties over Bay of Bengal inferred from ship-borne and MODIS observations during ICARB-W cruise campaign: Implications to radiative forcing K. Raghavendra Kumar a, K. Narasimhulu b, G. Balakrishnaiah a, B. Suresh Kumar Reddy a, K. Rama Gopal a, R.R. Reddy a, *, L.S.S. Reddy a, K. Krishna Moorthy c, S. Suresh Babu c, C.B.S. Dutt d a

Department of Physics, Aerosol & Atmospheric Research Laboratory, Sri Krishnadevaraya University, Anantapur 515 055, Andhra Pradesh, India Department of Physics, Government First Grade College, Bellary 583 101, Karnataka, India Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, Kerala India d National Remote Sensing Centre, Balanagar, Hyderabad 500 037, Andhra Pradesh India b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 May 2010 Received in revised form 1 October 2010 Accepted 1 October 2010

Comprehensive investigations during the last decade have clearly established that aerosols have a significant impact on the climate. This paper reports the results of the spatial variations in aerosol optical depth (AOD) and fine mode fraction (FMF) characteristics as a function of latitude and longitude over the Bay of Bengal (BoB) and the Northern Indian Ocean (NIO) during ICARB-W cruise period of 27th December 2008e30th January 2009 from onboard Sunphotometer and MODIS (Terra, Aqua) satellite measurements. Very high AOD500 (0.7e0.8) occurred over the north head BoB adjacent to the northeastern Indian coast and the lowest AOD500 (0.1e0.2) occurred in central BoB far away from the coasts, and in a small area in the northeastern part close to Myanmar coast as well as over NIO. The highest values (as high as 1.2) of Ångström exponent, a occurring over northeast BoB (regions close to Bangladesh and Myanmar) indicate relative abundance of accumulation mode particles and very low values of a (below 0.7) over central part of BoB as well as southern BoB/NIO suggesting dominance of coarse-mode sea spray aerosols. Terra/Aqua MODIS AOD550 and cruise measured AOD500 using Sunphotometer showed good agreement (R2 ¼ 0.92) over the BoB. The total mass concentrations over BoB during cruise period were remarkably high, with a mean value of 28.4  5.7 mg m3. Aerosol FMF was higher than 0.7 over the BoB, while FMF over NIO was about 0.5. NCEP reanalysis data on winds at 850 hPa, along with 5-days airmass back trajectories via HYSPLIT model, suggested transport of continental aerosols from the central and northern India over the BoB by the strong westerly/northwesterly winds. Regionally averaged clear sky aerosol (net) forcing over BoB during the winter is 28.9 W m2 at the surface and 10.4 W m2 at the top of the atmosphere whereas, the ARF values estimated over NIO at TOA, surface and in the atmosphere are 6.4, 18.3 and þ11.9 W m2, respectively. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: ICARB-W Aerosol optical depth Fine mode fraction Ångström coefficients Direct aerosol radiative forcing

1. Introduction Aerosols affect global climate and earth’s radiation budget directly by scattering and absorbing solar and terrestrial radiation and are quantified through ‘aerosol radiative forcing’ (Pandithurai et al., 2008). Through their direct and indirect effects, aerosols have significant impact on solar radiation reaching the ground, the energy budget and precipitation formation and distribution (Kalapureddy et al., 2009). Aerosols found over the oceanic surface

* Corresponding author. Tel./fax: þ91 8554 255710. E-mail address: [email protected] (R.R. Reddy). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.10.004

are mainly sea salt particles produced by evaporation of water droplets sprayed into the atmosphere and windblown dust transported from the surrounding landmasses. These aerosols are produced by natural processes; induce both spatial and temporal perturbations in the radiation budget of the Eartheatmosphere system. The role of regional and/or synoptic scale airmass types in advecting aerosols from distinct source regions and causing changes in the optical depths/composition/physical characteristics over oceanic regions is quite recognized (Pillai and Moorthy, 2001; Moorthy et al., 2003). Over the Indian subcontinent and surrounding oceanic regions, a number of observational campaigns, viz., Arabian Sea Monsoon Experiment (ARMEX) (Moorthy et al., 2005), Indian Ocean

K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412

Experiment (INDOEX) (Ramanathan et al., 2001), Land Campaigns (LC) LC-I (Moorthy, 2005) and LC-II (Ramachandran et al., 2006), Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) (Moorthy et al., 2008) were conducted to investigate the role of aerosols in altering the atmospheric radiation budget and cloud properties. Despite the above campaigns, other few field experiments were limited to cruises along the Indian coasts and coastal northern BoB (e.g., Satheesh, 2002; Quinn et al., 2000; Vinoj et al., 2004; Satheesh et al., 2006) and focused mainly on columnar aerosol optical depth (AOD) and ambient total mass concentration measurements. The ICARB was conducted during the pre-monsoon season of MarcheMay 2006 over the Indian subcontinent and adjoining oceanic regions, e.g., Bay of Bengal (BoB), Northern Indian Ocean (NIO) and Arabian Sea (AS). The main results of this campaign have been highlighted in numerous studies by several researchers (e.g. Nair et al., 2008, 2009; Moorthy et al., 2009; Satheesh et al., 2009). However, dedicated ship-borne measurements to explore the aerosol characteristics in the entire BoB are almost nil to the best of our knowledge, since the eastern BoB had remained fairly unexplored during ICARB 2006. In continuation of the previous campaign, ICARB during winter (ICARB-W) was conducted from 27th December 2008 to 30th January 2009 for a period of 35 days over Indian mainland and BoB (longitude: 76e98 E and latitude: 3e21 N) as a successor to the ICARB 2006. This campaign, formulated on the same guiding principles as the ICARB 2006 (Moorthy et al., 2008) also consisted of three segments (viz., the land segment, air segment and the ocean segment) like its predecessor. This paper presents the spatial features of AOD (sp), mass concentrations of composite aerosols and direct aerosol radiative forcing estimated over BoB and NIO during the cruise and the results are compared with the concurrent values of the columnar aerosol extinction and fine mode fraction (FMF), derived from Moderate resolution Imaging Spectroradiometer (MODIS) in order to substantiate the inferences and their distinctiveness with respect to airmass back trajectories. 2. Cruise track, instrumentation and methodology The cruise track during the ICARB-W campaign is shown in Fig. 1. During this campaign, which focused on the winter season, the entire BoB, has been explored extensively and concurrent measurements of aerosol parameters using identical, inter

25N ICARB-W Cruise

Bangladesh

th th (Chennai, 27 December 2008 - Kochi, 30 January 2009)

J02

20N

INDIA

J07

D31

Myanmar

J08

Latitude (°N)

J01 D28

15N

J06

J03

J09

D30

Chennai

D27

10N

J21

J17

J25 J29

J26

J28

5N

J11 J14 J12

J20 J24

3. Prevailing meteorology

J15 J05 J16

J30

J22

J18

J13

J27 J23

J19 D27

0N 70E

75E

80E

calibrated instruments onboard the oceanographic research vessel (ORV) Sagar Kanya (SK) during its cruise # SK-254. The ship started from the southeast coast of India at Chennai port (13.1 N, 80.2 E) on 27th December 2008, and during its return journey, the ORV passed Sri Lanka on 28th January 2009 and returned back to Kochi (9.6 N, 76.1 E) at Arabian Sea (AS) on 30th January 2009. The arrow head shows direction of the ship along the cruise track while the open circles show the position of the ship at 05:30 UTC for each day. Aerosol observations were made using a handheld MICROTOPSII Sunphotometer (Solar Light Company, USA). The sunphotometer provides AOD at five channels centered at 380, 500, 870, 936 and 1020 nm with a full width half maximum bad width of 5e10 nm and columnar water vapor (WV) derived from instantaneous solar flux measurements using its internal calibration. The field-of-view of (FOV) of sunphotometer is 2.5 . A Global Positioning System (GPS) receiver attached to the sunphotometer provides information on time, location and altitude (including pressure and temperature). After eliminating data recorded around cloud passage or near the FOV of the instrument, the observations were possible on a total of about 32 clear-sky days with 1000 data sets during the entire cruise period. More details on the measurement protocols followed in the present cruise are similar to earlier campaign ICARB 2006 reported in several studies (Kalapureddy and Devara, 2008; Kalapureddy et al., 2009). The details of design, calibration, performance and error budget have been described elsewhere (Morys et al., 2001; Ichoku et al., 2002). Attempts have been made to compare the daily average sunphotometer AOD500 with respect to the corresponding geographical location observation of Moderate Resolution Imaging Spectroradiometer’s (MODIS) Aqua and Terra satellites AOD550 (Level 3-Daily Global 1 1 Degree Products). The overall differences in the measurements, errors and uncertainties associated in deriving AODs from both MODIS Terra and Aqua satellites over land and ocean are reported elsewhere (Remer et al., 2005; Ramachandran, 2007). Aerosol mass concentration measurements onboard the ORV were made (similar to the earlier set-up during ICARB, Nair et al., 2008) using a 10-stage Quartz Crystal Microbalance (QCM) cascade impactor (California measurements Inc., USA). A total of 596 samples for 35 days were collected during the cruise period formed the database for this investigation. The details of QCM, measurement protocols followed for sampling, data analysis, uncertainties and error budget are available in several earlier studies (Jayaraman et al., 1998; Pillai and Moorthy, 2001; Ramachandran and Jayaraman 2002; Nair et al., 2009) and these were strictly adhered to during data collection. Black carbon (BC) data was obtained from an Aethalometer continuously operated a sample flow rate of about 3 L min1 and BC mass concentration in mg m3 was detected at 5-min interval round the clock. More details on the instrument and the principle of operation are given elsewhere (Hansen et al., 1984; Wiengartner et al., 2003).

J10 J04

D29

Kochi

405

85E

90E

95E

100E

Longitude (°E) Fig. 1. Cruise track of the Integrated Campaign for Aerosols, gases and Radiation Budget during winter (ICARB-W) over the Bay of Bengal. The numeric stands for the day and the alphabet for the month, D for December and J for January.

The synoptic meteorological conditions prevailing over the BoB adjacent to the Indian subcontinent during the cruise period comprised predominantly with moderate winds (
406

K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412

day-to-day variation with a sharp latitudinal gradient consisted of larger values in the northern BoB in close agreement with the previous ICARB 2006 results (Kedia and Ramachandran, 2008; Nair et al., 2009). Very high AOD500 (0.7e0.8) occurred over the entire north head BoB adjacent to the northeastern Indian coast due to direct influence as well as the long-range transport of continental aerosols. The lowest AOD500 (0.1e0.2) occurred in central BoB, far away from the coasts, and in a small area in the northeastern part close to Myanmar coast as well as over NIO. AOD500 ranging from 0.4 to 0.6 and from 0.2 to 0.5 were observed over eastern and southern BoB, respectively. Earlier studies during ICARB 2006 (Kalapureddy and Devara, 2008; Moorthy et al., 2008) also suggested higher AODs over northern and western BoB in the premonsoon season. The high AOD500 close to the Indian coast arises mainly from the anthropogenic activities over the coastal regions. This was mainly due to higher population density and industrial activity in coastal regions of India and Bangladesh, directly influencing the aerosol load in coastal BoB (e.g., Niranjan et al., 2007). As the anthropogenic aerosols are generally in the submicron size having longer residence times and, for this reason, they get transported to longer distances over the ocean before they settle down under gravity. Such cases of aerosol and pollution transport from urbanized regions and mineral dust from the arid areas of northwestern India and Arabian countries during the pre-monsoon season have been reported from the back trajectory analysis (Kalapureddy and Devara, 2008; Kedia and Ramachandran, 2008). The AOD decreases in central BoB and increased again over east BoB. The daily mean AODs in the 0.5 mm wavelength band are in the range of 0.2e0.8 with an average AOD500 of 0.42  0.09 over BoB and 0.30  0.05 over NIO and the corresponding values for other wavelengths are given in Table 1. This mean value is comparable in magnitude with that of 0.36  0.12 and 0.26  0.10 over BoB and NIO during ICARB 2006 (Kalapureddy and Devara, 2008). The differences in AOD spatial distribution between this study and the results of those researchers may mainly be due to the differences in the period and the measurements obtained. The different aerosol properties obtained from our study are compared with the various earlier campaign results reported in the literature are shown in Table 2. The observed AOD500 values over northern BoB from sunphotometer measurements are seen corroborated with MODIS AOD550 observations during the cruise period (see right panel of Fig. 3). Thus, the mean AOD550 shows enhanced aerosol load over north head BoB , along the eastern Indian coast and in the far eastern BoB in close agreement with the ship-borne

Fig. 2. Synoptic wind vectors (from NCEP data) at 850 hPa levels during ICARB-W field experiment over the Indian region.

Calm easterly winds are encountered in the AS till the ship approaches coastal regions of the Indian peninsula. These changing wind patterns will provide corresponding signature in aerosol characteristics, as can be seen in later sections.

4. Results and discussion 4.1. Spatial variations of AOD and Ångström parameters The spatial variation of aerosol loading (AOD500) derived from sunphotometer over the oceanic regions around the Indian subcontinent is shown in Fig. 3 (left panel), which shows relatively higher aerosol loading over BoB than NIO during the winter season. In fact, similar variation is observed at all other wavelengths (figure not shown). This feature is consistent with the long-term satellite observations over the oceanic regions around India (e.g., Leon et al., 2001). During the campaign period the AOD500 exhibited large

AOD

AOD 500nm

23N

Kolkata

Kolkata

0.65

Bhubhaneswar

0.7

Bhubhaneswar

18N

0.6

Pyinmana

Pyinmana

Visakhapatnam

Visakhapatnam

0.55

18N

0.6

Rangoon

550nm

23N

Rangoon

Chennai Port Blair

13N

0.4

0.3

Kochi Trivandrum

8N

Latitude (N)

Latitude (N)

0.5 0.5

0.45 Chennai

0.4

Port Blair

13N

0.35 0.3

Kochi Trivandrum

0.25

8N 0.2

0.2 0.15

3N 75E

80E

85E

90E

Longitude (E)

95E

100E

3N 75E

80E

85E

90E

95E

100E

Longitude (E)

Fig. 3. Spatial variation of columnar AOD at (left panel) 500 nm measured from Sunphotometer and (right panel) AOD at 550 nm derived from MODIS over BoB during ICARB-W.

K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412

407

Table 1 Mean AODs and Angstrom parameters over BoB during ICARB-W campaign. Location

NW BoB N BoB NE BoB E BoB C BoB S BoB/NIO

Aerosol optical depth at l, nm

Ångström exponent

380

500

870

936

1020

a380e1020

b

0.58  0.04 0.62  0.04 0.27  0.03 0.52  0.04 0.38  0.04 0.35  0.05

0.44  0.03 0.47  0.03 0.21  0.03 0.41  0.04 0.29  0.03 0.29  0.05

0.20  0.03 0.24  0.02 0.09  0.02 0.19  0.02 0.15  0.03 0.17  0.04

0.20  0.03 0.23  0.02 0.09  0.01 0.19  0.02 0.17  0.03 0.18  0.04

0.20  0.03 0.23  0.03 0.09  0.01 0.18  0.02 0.18  0.03 0.18  0.04

1.17  0.06 1.03  0.05 1.22  0.05 1.16  0.03 0.85  0.05 0.71  0.07

0.19  0.03 0.22  0.02 0.09  0.01 0.17  0.02 0.17  0.03 0.17  0.04

NW e NorthWest, N e Northern, NE e NorthEast, E e Eastern, C e Central, S e Southern.

measurements. Note also, the great similarity with the ship-borne observations as far as it concerns the lower AOD550 in centralsouthern BoB and close to the Myanmar coast. The Ångström’s formula on spectral AOD, AODl ¼ bla, was used to estimate the Ångström’s turbidity coefficient ‘b’, which equals AOD at l ¼ 1 mm, and the Ångström wavelength exponent ‘a’ (Angstrom, 1964) in the wavelength band 380e1020 nm using the least squares method. a is a measure of the ratio of accumulation mode to coarse-mode concentrations of the columnar aerosols and is an indicator of the aerosol columnar size distribution (Schuster et al., 2006; Kaskaoutis et al., 2007). The turbidity coefficient is a measure of the column abundance of particles. The spatial distribution of the a380e1020 (left panel) and b (right panel) values over the entire BoB region is shown in Fig. 4. Over the entire oceanic regions having proximity to mainland/islands, a remained high (>1), with the highest values (as high as 1.2) occurring over northeast BoB (regions close to Bangladesh and Myanmar) where, interestingly, the AOD values were very low (8e9 January) indicate relative abundance of accumulation mode (0.1 mm < radius < 1.0 mm) particles. The a380e1020 values are high (above 1.2) over the northern BoB (2e7 January) with the moderate a values near the east (28e31 December) and west coast of Indian peninsular regions (28e29 January) and very low values of a (below 0.7) over central part of BoB (5 and 19 January) as well as southern BoB/NIO (21 January) suggesting dominance of coarsemode sea salt aerosols. These particles could be associated with additional production of sea spray aerosols (Ramachandran, 2004), due to higher winds over this area. It is worth noting that in the far eastern BoB (11e14 January), the a380e1020 values remained high, showing influence from southeast Asia. However, the majority of a380e1020 (>0.9), indicating a mixed aerosol field over the central BoB and the nearby Indian coast which was due to both influences of anthropogenic and natural sources. The mean a380e1020 is 1.14  0.24, slightly lower, but with larger variation from the value 1.21  0.11 found over BoB during ICARB 2006 (see Table 2). Both values are larger than that (0.90  0.18) reported for AS during ICARB 2006 (Kalapureddy et al., 2009). The Ångström turbidity coefficient b followed the pattern similar to that of AOD. Values of b observed to be higher near the

coasts and decreases monotonically as the ship moves away from coast and it is less when it enters into the open ocean environments. Thus, low (<0.15) values were observed near to Myanmar coast and in central BoB and higher in northern (<0.4) and western (0.2e0.3) BoB. The higher b values near the coast and northern BoB are associated with Chennai, Vishakhapatnam, Bhubhaneswar, Kolkata and Paradip port activities (Fig. 1). During the campaign b was found to be 0.17  0.09, larger than that (0.14  0.08) reported during ICARB-06 (Kedia and Ramachandran, 2008; Kalapureddy and Devara, 2008). 4.2. Comparison of AODs derived from MODIS and sunphotometer The first time comparison of MODIS derived AOD over the oceanic regions surrounding India by Vinoj et al. (2004) showed good agreement with sunphotometer measurements with a standard deviation 0.03 and a mean difference 0.01. Fig. 5 shows a scatter plot between AODs from the MODIS and the corresponding values from ship-borne measurements which displays a good agreement between the two with correlation coefficient of 0.92 (p < 0.0001). The standard deviation between the two measurements was 0.06 with a mean MODIS underestimation by 0.02. Recently, Vinoj et al. (2007) reported a standard deviation of 0.06 with a mean difference of 0.02 for validation of AODs derived from sunphotometer and MODIS Aqua and Terra over oceanic regions during a trans-continental cruise experiment conducted during JanuaryeApril 2006. 4.3. Back trajectories With a view to examining the effect of trajectories, which act as potential conduits for aerosol transport, we have computed fiveday (120 h) back trajectories, for all the days on which the measurements were available, using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model of the National Oceanic and Atmospheric Administration (NOAA) (www.arl.noaa. gov/ready/hysplit.html) (Draxler and Rolph, 2003). The most dominant pathways have been identified following a cluster analysis (Owen, 2003) and the results are shown in Fig. 6.

Table 2 Comparison of aerosol properties from present observations with previous cruise campaigns over BoB and NIO. Cruise number

Region

Campaign Period

Location

AOD500nm

a

b

Reference

ICARB-W (SK-254) ICARB (SK-223A) SK-188 SK-161A

Entire BoB including eastern BoB BoB

December 2008eJanuary 2009 MarcheApril 2006 February 2003 February 2001

SK-161B

BoB

FebruaryeMarch 2001

0.42  0.09 0.30  0.05 0.36  0.12 0.26  0.10 0.41  0.14 0.39  0.15 0.25  0.03 0.44  0.19 0.21  0.03

1.14  0.24 0.65  0.11 1.21  0.11 1.06  0.16 1.10 1.80  0.12 1.34  0.04 1.31 0.50

0.17  0.09 0.19  0.05 0.14  0.01 0.12  0.04 0.18 0.10 0.02 0.20 0.24

Present study

BoB BoB

BoB NIO BoB NIO BoB BoB NIO BoB NIO

Kalapureddy and Devara (2008) Vinoj et al. (2004) Ramachandran and Jayaraman (2003) Satheesh (2002)

408

K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412

alpha

23N Kolkata

beta

23N

Bhubhaneswar

0.36

Kolkata

1.3

Bhubhaneswar

1.2

0.32

Pyinmana Rangoon

0.9 0.8

Chennai

Port Blair

13N

0.7 0.6 0.5

Kochi Trivandrum

0.4

8N

Visakhapatnam

18N

1

Latitude (N)

18N

Latitude (N)

Pyinmana

1.1

Visakhapatnam

0.28

Rangoon

0.24 Chennai

0.2

Port Blair

13N

0.16 Kochi Trivandrum

0.12

8N

0.3

0.08

0.2

3N 75E

80E

85E

90E

95E

3N 75E

100E

80E

85E

90E

95E

100E

Longitude (E)

Longitude (E)

Fig. 4. (Left to right panels) Spatial distribution of Angstrom wavelength exponent (a) and turbidity coefficient (b) observed with Sunphotometer onboard cruise # SK-254.

The retrieved back trajectories could be grouped into five distinct regions, namely, east coast BoB (ECB), northern BoB (NB), central BoB (CB), Northern Indian Ocean (NIO) and west coast BoB (WCB), mainly depending on the regions bounded by the Indian subcontinent surrounding the BoB. The trajectory clusters arriving at ECB have very long continental overpass through the arid regions of west Asia and northwest India before arriving at ECB, across central India. These trajectories would thus be conducive for the significant advection of transported mineral dust aerosols to the measurement region (with significant anthropogenic components due to the local human activities). Beegum et al. (2008) have shown that the advected mineral dust aerosols from the arid west Asian regions lead to significant enhancement in the columnar AOD over IGP. The observed high values of AOD and moderately high a at ECB is the consequence of these. At NB, where the highest values of AOD are observed, the trajectories were found to have a wide spread

0.8 Mean Diff. = 0.02 RMS Diff. = 0.06 R = 0.92

0.7

35N

0.6

30N

0.5

25N

Latitude (oN)

MODIS AOD (550 nm)

over the Indian subcontinent covering the entire central India providing conduits for transport of continental aerosols, mostly mixed type (anthropogenic þ natural), to the measurement location and this would contribute significantly to the observed high values of AOD. Coming to the trajectories reaching CB 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 vast traverse across the oceanic regions where the wind speeds were moderately high (Fig. 2). This would result in mixed type of aerosols (coarse mode sea salt þ accumulation mode anthropogenic aerosols) and this explains moderate values of AOD and fairly low values of a. The trajectories arriving at NIO and WCB were mostly oceanic in nature. They would be mostly characterized by smaller amount of accumulation mode aerosols and larger amount of coarse mode sea salt as 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 a. As the southern BoB opens to the vast Indian Ocean, anthropogenic aerosol concentration would be insignificant.

0.4 0.3

20N NB

15N ECB

10N

0.2 0.1 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Sunphotometer AOD (500 nm)

NIO

0N -5N 65E

Fig. 5. Scatter plot of Sunphotometer measured AODs with those retrieved from MODIS Terra and Aqua satellites. Solid circles represent TERRA and open circles represent AQUA satellites. Vertical bars represent standard deviation of MODIS AODs and horizontal bars represent standard deviation of AODs measured from Sunphotometer over BoB during campaign period.

CB

WCB

5N

70E

75E

80E

85E

90E

95E

100E

Longitude (oE) Fig. 6. The mean trajectory clusters arriving at different regions during the cruise period.

K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412

4.4. Spatial distribution of total mass concentration of surface aerosols The spatial distribution of Mt is shown in Fig. 7. Total aerosol mass concentration (Mt) varied from the lowest value of 10.4  3.3 mg m3 to the highest value of 68.5  9.2 mg m3 over the Bay of Bengal, with a mean value of 28.4  5.7 mg m3 during the cruise period. In general, the mass concentrations reported over BoB were remarkably higher than those over AS during ICARB 2006 (Nair et al., 2008) and INDOEX (Ramachandran and Jayaraman, 2002). Interestingly, during ICARB-W, about 60% of the Mt values were between 11 and 30 mg m3. Significant deviations from the mean pattern were observed mainly in three regions (1) central BoB (10e15 N and 85e90 E), (2) northern BoB (19e21 N and 86e91 E) and (3) along the east coast of peninsular India. Extremely low aerosol mass loading was observed over the south and central BoB. Total mass concentration over this region was always <30 mg m3 (mean 23.6  4.3 mg m3). These low values of Mt appear to be associated with the strong winds that persisted over this region (Fig. 2). During our campaign, sea surface wind speed in this region was comparable to those outside this region of BoB, so that the local production of sea salt through bubble bursting is not significantly different. Even, the airmass back trajectory analysis estimated from HYSPLIT model were confined to BoB indicating absence of any significant long-range transport from the adjoining continents. Northern BoB (19e21 N and 86e91 E) showed the highest values of Mt (mean 45.4  10.2 mg m3), a region of enhanced aerosol concentration which has been reported similar variation in earlier studies (Ramachandran and Jayaraman, 2003). This higher aerosol loading is mainly attributed to the airmass transport from East Asia, north and west India and Arabian dust (Kedia and Ramachandran, 2008). Five-day airmass back trajectories at 500 m above ground level for the measurement days in which the ship was sailing in this region showed advection patterns along the Arabian arid areas, Oman coast and Saharan desert dust particles. Thus, advection of aerosols from these arid regions is a strong candidate for the observed high mass concentration over northern BoB. Moderate mass concentrations with a mean value of 32.3  7.1 mg m3, persisted along the east coast of India with higher values very near to the ports. Anthropogenic activities in the industrialized, urban areas and ports along the east

Mt

23N

70 Kolkata

65 Bhubhaneswar

60

Pyinmana

18N

Vishakhapatnam

55

Rangoon

409

coast of peninsular India and several other small ports and fishing harbors in between strongly influence Mt over the coastal regions. The advection of these continental aerosols, which are smaller in size, over the marine environment, increases the optical depth as well as mass loading (Moorthy et al., 2005, Kalapureddy and Devara, 2008).

4.5. Spatial variation of FMF obtained from QCM and MODIS Fig. 8 shows the spatial distribution of accumulation mode fraction (Af ¼ Ma/Mt) (left panel) over the BoB. Enhanced dominance of accumulation mode aerosols (with Af > 0.9) is seen only along the east coast of India, especially near the industrialized ports indicating the advection of continental aerosols from the peninsula. It is also very interesting to note that the higher percentage contribution of accumulation fraction at the northern BoB, which may be attributed to the anthropogenic aerosols advected along with fine mineral dust from the East Asian regions to BoB. Transported mineral dust is known to be mostly in accumulation size ranges with a typical mode radius of 0.5 mm (Hess et al., 1998). However, excluding these regions of high Af, vast regions of BoB were dominated by coarse mode aerosols so that on an average the Af for the BoB was only 0.63, and for 60% of the measurements, Af was within 0.4e0.75. Only 4% of the total observations, Af was >0.9. This shows that, in general, supermicron/coarse mode (>1.0 mm) aerosols (probably sea salt) dominate the entire BoB. FMF derived from MODIS can be used as a surrogate for separating anthropogenic from natural sources thereby increasing the accuracy of estimating the manmade impact on aerosol forcing. It is expected that AODs dominated by pollution will result in a high FMF and AOD, if the aerosols are mechanically generated (e.g., sea salt and desert dust) then that can result in a low FMF and a range of AODs (Kaufman et al., 2002; Remer et al., 2005). FMF variation as a function of latitude and longitude over the BoB is shown in Fig. 8. FMF is found to be >0.7 in the 10 N to 21 N latitudes. FMF is about 0.4e0.6 over NIO in the 3e10 N latitude regions. A higher FMF value indicates the dominance of fine mode particles which were produced mainly due to manmade sources in the surrounding regions and got transported across the oceanic region. The average FMF over the BoB is higher at 0.75 when compared to 0.55 over NIO, though the variations are large over the NIO. A higher FMF over the BoB indicates a relatively higher fine mode contribution to the AOD than over NIO. It is worth mentioning that the decrease in FMF over NIO in the 3e10 N latitude could be due to higher wind speeds that can produce larger amount of sea salt aerosols and/or due to the transport of larger size mineral dust particles from the nearby desert regions. Dominance of larger size aerosols in the aerosol distribution would result in lower FMFs.

Latitude (N)

50 45 Port Blair

13N

35 30 Kochi Trivandrum

25 20

8N

15 10

3N 75E

80E

4.6. Implications to radiative forcing

40

Chennai

85E

90E

95E

100E

Longitude (E) Fig. 7. Spatial variation of total aerosol mass concentration over Bay of Bengal.

The direct aerosols radiative forcing (ARF) is the change (DF) in the flux (F) either at the top of the atmosphere (TOA), or at the surface (S) due to the presence of the particles in the atmosphere. Thus

    ðDFÞTOA=S ¼ ðFY  F[ÞA TOA=S  ðFY  F[ÞNA TOA=S

(1)

where the subscripts NA and A represent the condition without and with aerosols, respectively. If DF is negative, the aerosols cause an increase in atmosphere flux leading to a cooling, while if it is positive, there will be a warming. The difference between the TOA forcing and surface forcing gives the atmospheric forcing which represents the energy absorbed in the atmosphere and is defined as,

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K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412 FMF

Ma /Mt

23N

23N

1

Kolkata

0.83

Kokata 0.9

Bhubhaneswar

Bhubhaneswar

Pyinmana

Visakhapatnam

0.8

Rangoon

0.7 0.6

Chennai

0.5

Port Blair

13N

18N

0.4 Kochi

0.3

Latitude (N)

Latitude (N)

18N

0.2

Pyinmana

0.78

Rangoon

0.73 0.68

Chennai

Port Blair

13N

0.63 0.58

Kochi Trivandrum

Trivandrum

8N

Visakhapatnam

8N

0.53 0.48

3N 75E

80E

85E

90E

95E

3N 75E

100E

80E

Longitude (E)

85E

90E

95E

100E

Longitude (E)

Fig. 8. Spatial distribution of the fraction of accumulation mode aerosols to the total mass concentration and fine mode fraction over BoB obtained from QCM and MODIS observations.

ðDFÞatm ¼ ðDFÞTOA ðDFÞS

(2)

The Santa Barbara Discrete Ordinate Atmospheric Radiative Transfer model (SBDART) (Ricchhiazzi et al., 1998) used in this study for the estimation of clear sky ARF at the surface and TOA, accounts for multiple scattering in a vertically inhomogeneous non-isothermal plane-parallel atmosphere. The input parameters used in this model were the sunphotometer measured optical properties such as AOD, WV, mass concentration of composite aerosols, soot and Optical properties of Aerosols and Clouds (OPAC) computed single scattering albedo (SSA) and asymmetry parameter (ASP) (Panicker et al., 2008, 2009). For the accurate estimation of ARF, the chemical composition of aerosols is to be known. As the chemical speciation of the aerosols was not done in the present study, we have used an aerosol model developed by Hess et al. (1998). Based on the observations, the maritime polluted aerosol model of Hess et al. (1998) was adapted as a zero order model. The chief components of the model were sea salt particles (coarse and accumulation), water-soluble particles (to represent continental accumulation mode), and soot. The mean aerosol characteristics, such as spectral AODs and Ångström exponent, observed over BoB during the campaign are used to constrain the aerosol model. The relative abundance of the components in the model were varied such that the spectral AODs and a estimated, using the composite aerosol model thus generated, agreed with the measured values within the measurement uncertainties (5%, in general). The corresponding values of the SSA at 500 nm estimated over BoB is found to be w0.88. This is comparable to the value observed over this region by several investigators in previous studies (Moorthy et al., 2009). Based on the measurements at KCO (Kashidhoo Climate Observatory) as well as the cruise onboard SK during January to March period of 1998 and 1999, Satheesh and Ramanathan (2000) reported SSA in the range of 0.87e0.90; Jayaraman et al. (2001) reported a value of 0.91 for coastal AS and Ramanathan et al. (2001a) reported a value of 0.86  0.06 for the west coast of India. From the extensive measurements of SSA during ICARB 2006, over BoB, Nair et al. (2009) reported a wide range of values varied from 0.84 to 0.98 with a mean value of 0.93  0.03. He further reported that the latitudinal variation of SSA over BoB is insignificant, indicating that the absorbing aerosols are well mixed in the marine atmospheric boundary layer (MABL) over BoB. The resulting diurnally averaged aerosol forcing over entire BoB at the surface is 28.9 W m2 and at the TOA is 10.4 W m2. The difference between the TOA and the surface forcing is þ18.5 W m2

which is absorbed in the atmosphere. Similarly, the ARF values estimated over NIO at TOA, surface and in the atmosphere are 6.4, 18.3 and þ11.9 W m2, respectively. Earlier experiments over BoB and AS did not consider this large heterogeneity in ARF; rather they tended to use a mean value (e.g., Ramanathan et al., 2001; Moorthy et al., 2005). The regional (0e20 N) average ARF values estimated during the INDOEX (JanuaryeMarch 1999) were 7 W m2 at TOA and 20 W m2 at the surface are comparable only to those observed over the BoB during ICARB, over the AS the values are only half of the INDOEX reported values (Moorthy et al., 2009). While the ARF values reported in earlier campaigns are lower than those observed over the BoB during ICARB-W. The difference arises mainly because of the seasonal difference in these campaign measurements. 5. Conclusions A cruise expedition was conducted as part of ICARB-W during winter season of December 2008eJanuary 2009 over the BoB to map the aerosol and trace gases environment. In the present study, variations in AOD, FMF and Angstrom wavelength exponent, a were analyzed and discussed. The main results are summarized below: 1. The spatial distribution of columnar measurements of aerosol characteristics showed large variability. High values of AOD500 were observed over northern/northwestern parts of the BoB in which AODs were as high as w0.7; while the low values of AOD (<0.2) were observed in the northeastern regions of BoB (Myanmar/Bangladesh coast) and southern BoB which opens to the vast Indian Ocean. The Ångström wavelength exponent, a, showed highest values of w1.5, even though generally high values prevailed over the eastern as well as northern coastal regions of India. The Ångström turbidity coefficient b followed the pattern similar to that of AOD with a mean value of 0.17  0.09 over BoB. 2. Total mass concentration showed low values over the central BoB and equatorial Indian Ocean with a mean value of 23.6  4.3 mg m3, high values over northern BoB (mean 45.4  10.2 mg m3) and moderate mass concentrations with a mean value of 32.3  7.1 mg m3, persisted along the east coast of India with higher values very near to the ports. This high in aerosol loading is mainly attributed to the transport of airmasses from East Asia, north and west India and Arabian mineral dust.

K. Raghavendra Kumar et al. / Atmospheric Environment 45 (2011) 404e412

3. The aerosol fine mode fraction (FMF) which can be utilized as a proxy to separate the urban and natural aerosol sources is found to be higher than 0.7 over BoB. A higher FMF value indicates the dominance of fine mode particles which were produced mainly due to manmade sources in the surrounding regions and got transported across the oceanic region. 4. The diurnally averaged aerosol forcing over entire BoB at the surface is 28.9 W m2 and at the TOA is 10.4 W m2. The difference between the TOA and the surface forcing is þ18.5 W m2 which is absorbed in the atmosphere. Similarly, the ARF values estimated over NIO at TOA, surface and in the atmosphere are 6.4, 18.3 and þ11.9 W m2, respectively. Acknowledgements The present study was carried out as part of the ICARB-W project of ISRO-GBP. The authors are thankful to NCAOR/DOD for providing onboard facilities and also to the Sagar Kanya cruise members for their excellent support and cooperation. S. Naseema Beegum and Vajja Dinakar Prasad are acknowledged for their help in measurements onboard the cruise. The authors gratefully acknowledge the NOAA ARL for the provision of the HYSPLIT data and GES-DISC MOVAS for the retrieval of the MODIS data from their websites. The authors wish to thank the Journal Editor Prof. H.B. Singh and the two anonymous reviewers for their constructive and useful comments which improve the scientific content of the original paper. References Angstrom, A., 1964. Techniques of determining the turbidity of the atmosphere. Tellus XIII, 214e223. Asnani, G.C., 1993. Tropical Meteorology, vols. 1 and 2. Indian Institute of Tropical Meteorology, Pashan, Pune, 1012 pp. Beegum, S.N., Moorthy, K.K., Nair, V.S., Babu, S.S., et al., 2008. Characteristics of spectral aerosol optical depths over India during ICARB. Journal of Earth System Science 117 (S1), 303e313. Draxler, R.R., Rolph, G.D., 2003. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model Access Via NOAA ARL READY Website. http://www. arl.noaa.gov/ready/hysplit4.html. NOAA Air Resources Laboratory, Silver Spring, MD. Hansen, A.D.A., Rosen, H., Novakov, T., 1984. The aethalometer: an instrument for the real-time measurements of optical absorption by aerosol particles. Science of the Total Environment 36, 191e196. Hess, M., Keopke, P., Schult, I., 1998. Optical properties of aerosols and clouds: the software package OPAC. Bulletin of American Meteorological Society 79, 831e844. Ichoku, C., Levy, R., Kaufmann, Y.J., Remer, L.A., Li, R.R., Marins, V.J., Holben, B.N., Abuhassan, N., Slutsker, I., Eck, T.F., Pietras, C., 2002. Analysis of the performance characteristics of the five channel Microtops-II Sunphotometer for measuring aerosol optical thickness and precipitable water vapor. Journal of Geophysical Research 107 (D13), 4179. doi:10.1029/2001JD001302. Jayaraman, A., Lubin, D., Ramachandran, S., Ramanathan, V., Woodbridge, E., CollinsZalpuri, K.S., 1998. Direct observations of aerosol radiative forcing over the tropical Indian Ocean during the JanuaryeFebruary 1996 pre-INDOEX cruise. Journal of Geophysical Research 103, 13827e13836. Jayaraman, A., Satheesh, S.K., Mitra, A.P., Ramanathan, V., 2001. Latitudinal gradient in aerosol properties across the Inter Tropical Convergence Zone: results from the joint Indo-US study on board Sagar Kanya. Current Science 80, 128e137. Kalapureddy, M.C.R., Devara, P.C.S., 2008. Characterization of aerosols over oceanic regions around India during pre-monsoon 2006. Atmospheric Environment 42, 6816e6827. doi:10.1016/j.atmosenv.2008.05.022. Kalapureddy, M.C.R., Kaskaoutis, D.G., Raj, P.E., Devara, P.C.S., Kambezidis, H.D., Kosmopoulos, P.G., Nastos, P.T., 2009. Identification of aerosol type over the Arabian Sea in the premonsoon season during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB). Journal of Geophysical Research 114, D17203. doi:10.1029/2009JD011826. Kaskaoutis, D.G., Kambezidis, H.D., Hatzianastassiou, N., Kosmopoulos, P.G., Badarinath, K.V.S., 2007. Aerosol climatology: dependence of the Angstrom exponent on wavelength over four AERONET site. Atmospheric Chemistry and Physics Discussion 7, 7347e7397. Kaufman, Y.J., Tanre, D., Boucher, O., 2002. A satellite view of aerosols in the climate system. Nature 419, 215e223. Kedia, S., Ramachandran, S., 2008. Features of aerosol optical depths over the Bay of Bengal and the Arabian Sea during premonsoon season: variabilities and

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