Aerosol absorption over Bay of Bengal during winter: Variability and sources

Atmospheric Environment 54 (2012) 738e745

Contents lists available at SciVerse ScienceDirect

Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Aerosol absorption over Bay of Bengal during winter: Variability and sources Sumita Kedia*, S. Ramachandran, T.A. Rajesh, Rohit Srivastava Space and Atmospheric Sciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad, Gujarat 380009, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2011 Received in revised form 15 December 2011 Accepted 19 December 2011

Measurements of black carbon (BC) mass concentration were made over the Bay of Bengal (BoB) during the period of 27 December 2008e29 January 2009. BC mass concentration is highest over the Coastal-BoB (5.1
3.0 mg m3) and is more than a factor of two higher than the South-BoB (2.5
1.4 mg m3). The source regions of BC over the study region is identified using the Total Potential Source Contribution Function (TPSCF) analysis. The probable source regions over the Coastal-BoB and North-BoB (India, IndoGangetic plain, Pakistan, Afghanistan) are found to be distinctly different than that over the East-BoB and South-BoB (mostly from southeast Asia). The spectral distribution of absorption coefficients suggested similar source types of BC present over the entire BoB, with significant contribution of absorbing aerosols from the sources other than fossil fuel burning. Our results suggest that the entire BoB remains dominantly influenced by aerosols emitted from biomass/biofuel burning during winter. Single scattering albedo (SSA) is found to vary in the range of 0.63e0.70 over different parts of BoB with the lowest value over Coastal-BoB and the highest value over South-BoB. SSA values observed in the present study are the lowest ever reported over the BoB in the last decade indicating highest concentration of absorbing aerosols over the BoB during winter. The present work and the results obtained will have strong implications while investigating the effect of anthropogenic aerosols over marine environment, and in estimating the spatiotemporal variation of aerosol radiative impact. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Black carbon Absorption Bay of Bengal PSCF Source region Variability

1. Introduction Atmospheric aerosols can scatter as well as absorb the solar and terrestrial radiation and exert a cooling effect on the Earth’s climate through direct and indirect effects which partially offset the warming caused due to greenhouse gases (Solomon et al., 2007). Once aerosols are produced, they can get transported from areas of high emissions to clean remote and marine environments under favorable wind conditions. The relative importance of scattering and absorption of radiation depends on the chemical composition and the size distribution of aerosols. Black carbon (BC) aerosols, produced from incomplete combustion, fossil fuel and biomass burning, can influence air quality and climate by absorbing the sunlight and thereby contribute to global warming. The radiative and climate impacts of BC are increasingly recognized as it is the second strongest contributor to global warming next to carbon dioxide (Ramanathan and Carmichael, 2008). BC emissions are reported to have varied in response to changes in the usage of fossil fuel and technology development, and the estimated BC emissions * Corresponding author. Present address: Laboratoire d'Aérologie, Observatoire Midi-Pyrénées TOULOUSE, France. Tel.: þ33 (0) 5 61 33 27 13. E-mail addresses: [email protected], [email protected] (S. Kedia), [email protected] (S. Ramachandran), [email protected] (T.A. Rajesh), [email protected] (R. Srivastava). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.12.047

are found to be the highest in developing countries, especially China and India (Novakov et al., 2003). Venkataraman et al. (2005) have shown that large amount of biofuel combustion (especially wood) is a potentially significant source of atmospheric BC in south Asia. Lifetime of BC in the lower atmosphere is of the order of a week (Ramanathan and Carmichael, 2008). Because of their fine size and relatively longer residence time, BC can easily get transported over longer distances and can pollute a pristine atmosphere. Bay of Bengal (BoB) occupies a special importance because of its proximity to the surrounding landmasses on its north, east and west which are densely populated and industrialized areas. In addition, BoB plays an important role in Indian summer monsoon system and precipitation pattern. A campaign for Aerosols, gases and Radiation Budget (ICARB) was conducted during the premonsoon season of MarcheMay 2006 with an aim to identify the major sources of aerosols (natural and anthropogenic) and to characterize their role on regional climate over the Bay of Bengal, Arabian Sea and India through intensive simultaneous measurements (Moorthy et al., 2008). The campaign revealed the existence of large heterogeneity in aerosol characteristics, sources and size distribution over the BoB. In addition, a higher aerosol loading was observed with higher anthropogenic contribution over the Bay of Bengal when compared to the Arabian Sea during ICARB (Moorthy et al., 2008; Nair et al., 2008; Kalapureddy et al., 2009; Kedia and Ramachandran, 2008, 2009).

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

In order to examine the seasonal variability in aerosol characteristics and to explore the BoB on a larger spatial extent, winter ICARB (W-ICARB) was conducted during December 2008eJanuary 2009 over the BoB (Fig. 1). The W-ICARB campaign also revealed the existence of large spatiotemporal as well as vertical heterogeneity in various aerosol properties over BoB during winter. The latitudinal variation of all the aerosol parameters (aerosol optical depth, total mass concentration, accumulation mode fraction, Ångström exponent) showed an increasing trend from the south to the north BoB. Similar trend in the vertical profile of aerosol concentration was also observed with the highest aerosol concentration over the North-BoB and the lowest over the South-BoB (Moorthy et al., 2010; Sreekanth et al., 2011; Sinha et al., 2011). In addition, remarkably high total mass concentrations were observed during W-ICARB indicating very high aerosol loading over BoB during winter (Raghavendra Kumar et al., 2011; Kaskaoutis et al., 2011; Kharol et al., 2011). In the present study, an attempt is made to delineate the heterogeneity in the measured aerosol absorption characteristics over different parts (Coastal, North, East and South) of BoB during winter. The possible causes for the differences in absorption properties of aerosols using measured BC mass concentrations over these regions of BoB are investigated. The potential source contribution function (PSCF) analysis is used to correlate the black carbon mass concentration observed over the BoB with the air mass back trajectories to identify probable source regions of BC aerosols over the study region.

739

Table 1 Surface level meteorological parameters during W-ICARB over different regions of BoB. Region

Temperature ( C)

Coastal-BoB North-BoB East-BoB South-BoB

26.7 26.1 26.8 27.6







2.0 1.0 0.7 0.6

Wind speed (m s1) 2.4 3.0 4.3 3.5







0.4 1.3 1.6 1.4

Relative humidity (%) 63 61 60 71







4 9 5 5

path shown in Fig. 1 before reaching Kochi on 29 January 2009 on completion of the campaign. During the study period, synoptic winds were calm and northeasterly favoring the outflow of airmass from distinct continental regions over the entire BoB (Moorthy et al., 2010). The meteorological parameters (relative humidity (RH), temperature and wind speed) were measured onboard at an hourly basis. Daily mean temperature is found to vary in the range of 24e30  C, wind speed varied between 1.2 and 6.3 m s1 and the RH varied between 49% and 79% during the study period. The daily mean meteorological parameters are further utilized to compute the regional mean values (Fig. 1) and are given in Table 1. Average wind speed is found to be highest over the East-BoB while RH is highest over the South-BoB (Table 1). The differences in temperature are not significant over the Bay of Bengal. 2.1. Black carbon mass concentration measurements Continuous measurements of BC mass concentration were made onboard the ship using a multiwavelength Aethalometer (AE-47, Magee Scientific, USA) during the study period. These measurements were made at seven different wavelengths viz., 370, 470, 520, 590, 660, 880 and 950 nm with a time interval of 5 min and a flow rate of 3.0 l min1 (Hansen et al.,1984). The aethalometer measures BC mass concentrations from the attenuation of a beam of light transmitted

2. Measurements and methodology The W-ICARB cruise campaign was conducted from 27 December 2008 to 29 January 2009 over the Bay of Bengal (Moorthy et al., 2010). The Oceanographic Research Vessel Sagar Kanya originated from the Chennai port on 27 December 2008 and sailed through the

25 Ahmedabad

Kolkata

India

Myanmar Bhubaneshwar

20

07/01

Mumbai

15

Goa

01/01

28/12

09/01

03/01

30/12

o

Latitude ( N)

Visakhapatnam

Chennai

Port Blair 15/01

26/12 05/01

Kochi

10

Sri Lanka

Trivandrum

11/01 25/01

21/01

17/01

29/01

5

0 70

Coastal-BoB North-BoB East-BoB South-BoB 75

13/01 27/01

19/01 23/01

80

85

90

95

100

o

Longitude ( E) Fig. 1. Cruise track over the Bay of Bengal during W-ICARB (27 December 2008e29 January 2009). Points represent the mean latitude and longitude corresponding to each day. Different parts of BoB (Coastal, North, East and South) are shown as a function of color. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

740

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

through the sample collected on a filter, which is proportional to the amount of BC mass loading in the filter deposit (Hansen et al., 1984). The amount of light transmitted through the filter is detected using a set of two photo diodes, one of which passes through the sample spot and the other passes through a blank portion of the filter. Absorption coefficients of aerosols as a function of wavelength (l) at a time interval of (Dt) are calculated following Bodhaine (1995) and Weingartner et al. (2003) as

potential source contribution function (TPSCF) which considers trajectories at different heights. As BC is measured near surface, we have restricted the maximum trajectory height to 1000 m. The TPSCF at ijth grid cell is given by,

  i Aln 1 1 i2 babs ðlÞ ¼ CR Q Dt

where nkij is the total number of trajectories passing through the ijth grid at height k, and mkij is the number of trajectories falling in ijth cell and at height k with BC concentration exceeding the median value. If the total number of endpoints in a particular cell is small, the uncertainty associated with the TPSCF for the cell will be higher. Therefore, to reduce the uncertainty caused by higher PSCF values estimated for grid cells with a small number of trajectory endpoints, TPSCF values were down weighted by multiplying with an arbitrary weight function (Hopke et al., 1995; Cherian et al., 2009; Sunder Raman et al., 2011).

(1)

where i1 and i2 are intensities of the sample and the reference beams respectively, Q is the volume of air sampled, and A is the area of the exposed spot on the filter where aerosols are collected. C is the correction factor applied to account for any change in the absorption occurring due to multiple light scattering effects on the filter. R is an empirical correction factor and describes the change in the aethalometer response with increased particle loading on the filter (Weingartner et al., 2003; Arnott et al., 2005; Collaud Coen et al., 2010). The corresponding BC mass can be calculated by dividing babs ðlÞ by specific absorption cross section (sabs ðlÞ) (Weingartner et al., 2003). Collaud Coen et al. (2010) have summarized different correction algorithms proposed by different authors for the correction of aethalometer absorption coefficients. They concluded that, if simultaneous measurements of scattering coefficients are not available (as in the present case) then Weingartner correction factors (Weingartner et al., 2003) should be used. Therefore, in the present work the correction factors (C and R) are calculated following (Weingartner et al., 2003) as a function of wavelength and utilized. The cumulative uncertainty in BC mass concentrations and absorption coefficients due to instrumental artifacts, changes in filter scattering caused due to aerosol loading, underestimation of aethalometer signal with increasing filter load, flow rate, filter spot area and detector response is found to lie in the range of 10e20% (Srivastava et al., 2011). 2.2. Potential source contribution function analysis Seven days air back trajectory analysis has been performed using a three dimensional (latitude, longitude and height) HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess, 1998) for individual days at a time interval of 1 h corresponding to four different altitudes (50 m, 100 m, 500 m and 1000 m). The identification of source locations for the measured BC mass concentrations is performed using Potential Source Contribution Function (PSCF) which is an appropriate technique to ascertain the sources and the primary pathways of aerosols present over the study region. PSCF has found wide application in identifying the source regions of aerosols on regional scales as well as in identifying the long range transported pollution (Hopke et al., 1995; Cherian et al., 2009; Sunder Raman et al., 2011). The measured BC mass concentrations are combined with air mass transport information for individual days. The median (50th percentile) of daily mean BC mass concentrations are used as the cut-off point and values above this are treated as high BC concentrations in PSCF analysis. For the PSCF analysis, the whole geographic region covered by the trajectories is divided into an array of 1  1 grid cell. A PSCF value is calculated for each 1  1 grid cell, indicating the extent of air transport over the cell, with higher values indicating greater air transport over a region coinciding with high pollutant concentrations at the receptor. As the air parcel can reach the receptor site from different heights, instead of PSCF we have calculated total

TPSCFij ¼

mkij nkij

(2)

3. Results and discussion Depending on the meteorological conditions and the backtrajectory analysis, the entire study region over BoB during W-ICARB has been divided into four parts (Fig. 1) as Coastal-BoB, North-BoB, East-BoB and South-BoB and the aerosol characteristics are analyzed. These regions are classified based on the distinct trajectory clusters reaching the study regions as shown in Fig. 2. The nature of the trajectories at all the four heights are found to be similar in a particular region over the BoB as seen in Fig. 2. Over Coastal-BoB and North-BoB, airmasses are found to arrive mostly from the Indian subcontinent and Indo-Gangetic plain (IGP) region. Therefore, it is expected that these regions of BoB are more influenced by outflow of pollutants from the densely populated and highly industrialized regions of India including IGP. East-BoB is found to be influenced by airmasses from east-Asia and the eastern part of India, whereas South-BoB is found to be mainly affected by the airmasses from southeast Asia at all the altitudes (Fig. 2). 3.1. Black carbon mass concentration and absorption coefficient Fig. 3aed portray the diurnal mean variation of BC mass concentration over Coastal-BoB, North-BoB, East-BoB and South-BoB respectively. At the outset, it is found that BC mass concentration as well as the variability is higher over Coastal-BoB and North-BoB when compared to other two regions. The average BC mass concentration is found to be highest over the Coastal-BoB (5.1
3.0 mg m3) while it is lowest over South-BoB (2.5
1.4 mg m3). BC mass over North-BoB and East-BoB are found to be 4.1
2.5 mg m3 and 3.2
1.0 mg m3 respectively. Fig. 3eeh show the frequency distribution of BC mass concentration over all the four regions. The probability distribution of BC mass is found to be widest over the Coastal-BoB which is due to the larger variability observed in BC mass over this region. The frequency of occurrence of BC mass >6 mg m3 is highest over Coastal-BoB (w30%) and is the lowest over South-BoB (w5%). This indicates that the occurrence of higher concentration of BC is most frequent over the North-BoB during winter. This could be because of the fact that the North-BoB is influenced by IGP where BC concentration remains highest during winter due to large anthropogenic emissions including biomass and crop residue burning (Badarinath et al., 2006; Dey and Di Girolamo, 2010). In order to quantify the relative change in the absorption characteristics of aerosols, the wavelength dependence of aerosol

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

741

Fig. 2. Average seven days trajectory pathways over (a) Coastal-BoB (b) North-BoB (c) East-BoB and (d) South-BoB (Fig. 1) at 50, 100, 500 and 1000 m. The vertical bars represent the latitudinal extent of trajectories.

absorption coefficient (babs calculated using Eq. (1)) is examined following a power law (Kirchstetter et al., 2004) which can be expressed as,

babs ¼ kla

(3)

where k and a are the absorption Ångström coefficients, and a is the measure of spectral dependence of aerosol absorption. The value of a gives an insight as to whether the absorbing aerosols are from fossil fuel or biomass/biofuel burning. Kirchstetter et al. (2004) found that light absorption by the motor vehicle aerosols exhibit a relatively weak spectral dependence, where absorption 1 varied approximately as l . On the other hand, biomass smoke aerosols exhibited a stronger wavelength dependence of 2 approximately l . Kirchstetter et al. (2004) concluded that the low temperature, incomplete combustion processes, including biomass burning, produce light absorbing aerosols that show much stronger spectral dependence than high temperature combustion processes such as diesel combustion. The knowledge on spectral dependence of aerosol absorption is important over Asia where the contribution of BC from biomass/biofuel burning has been reported to be equally important as the fossil fuel (Venkataraman et al., 2005). Fig. 4aed shows the spectral variation in absorption coefficient (babs) over different parts of BoB during W-ICARB. The mean absorption coefficient at 550 nm calculated using Eq. (1) is found to be highest over Coastal-BoB (0.67
0.39  104 m1) and is lowest

over South-BoB (0.31
0.18  104 m1) consistent with the variation in BC mass concentration. In order to get information on the nature of sources that contributed to the observed BC, regional average aerosol absorption spectra are fitted using Eq. (3) and a values have been estimated over all the regions of BoB. The absorption spectra of BC showed a good agreement with the power law (Eq. (3)) and no significant differences in the a is observed over all the four regions which varied between 1.81 and 1.98. Fig. 4eeh shows the frequency distribution of a over CoastalBoB, North-BoB, East-BoB and South-BoB calculated using individual measurements during W-ICARB. It is remarkable to see that a is >1.5 for >85% of the absorption spectra over all the four regions. This indicates that the contribution of fossil fuel burning to the measured BC is not very significant (<10%), rather contribution from biomass burning (biofuel, crop waste open burning and forest fires) dominates the entire BoB. These results support the fact that biofuel consumption is the dominating source of BC aerosols in south Asia during winter (Venkataraman et al., 2005; Habib et al., 2006). It should be noted that there exist no known sources of BC over oceans, therefore all the BC measured over the oceanic region (BoB) originated over the continents and got transported. In addition, it is well known that the continental outflow of pollutants such as biomass and biofuel burning from the IGP and southeast Asia dominates during winter over BoB. In this context, it is important to identify and quantify the source regions that contributed to the measured BC and its variation over BoB which is performed next by TPSCF analysis.

742

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

15

40

a

Coastal-BoB

e

Coastal-BoB

f

North-BoB

g

East-BoB

h

South-BoB

30

10

20 5

0 40

b

North-BoB

10 5 0 15

c

East-BoB

10 5 0 15

Frequency of occurence (%)

-3

BC mass concentration (μg m )

0 15

10

30 20 10 0 40 30 20 10 0 40

d

South-BoB 30

10

20 5

10

0

0 0

2

4

6

8 10 12 14 16 18 20 22 24

Time (Hours)

0

3

6

9

12

15 -3

BC mass concentration (μg m )

Fig. 3. Diurnal variation in black carbon (BC) mass concentration over (a) Coastal-BoB, (b) North-BoB, (c) East-BoB and (d) South-BoB. Frequency distribution of BC concentration over (e) Coastal-BoB, (f) North-BoB, (g) East-BoB and (h) South-BoB.

3.2. Identification of BC source region TPSCF analysis is performed to identify the probable source regions during the study period over Coastal-BoB, North-BoB, EastBoB and South-BoB. In the present study, regions with the TPSCF values ranging from 0.1 to 1 are assigned as the probable source regions for BC. A high TPSCF value over continental region represents the probable source region for BC, while a high TPSCF over oceanic region indicates preferred pathways (Cherian et al., 2009). The probable source regions for BC is found to be distinctly different over all the four regions indicating different origins of BC present over these regions. The probable source regions are largely from India, Indo-Gangetic plain (IGP), Pakistan and Afghanistan for the Coastal-BoB and North-BoB (Fig. 5). IGP is one of the largest river basins in the world, densely populated and highly polluted regions in India, where both natural and anthropogenic aerosols contribute to ambient aerosols throughout the year (Singh et al., 2004). During winter the entire IGP is highly influenced by the anthropogenic aerosols present especially near the surface as the boundary layer is shallow accompanied with colder temperatures. The outflow of pollutants from the IGP to the North-BoB during winter has been reported in many studies through ground based and satellite measurements (Singh et al., 2004; Badarinath et al., 2006; Dey and Di Girolamo, 2010). In contrast, the most probable sources over East-BoB and SouthBoB are predominantly from Port Blair, southeast-Asian region (Myanmar, Thailand and China) with some contribution from central India (over East-BoB) (Fig. 5). Streets et al. (2003) reported that BC emissions are mainly due to residential emissions such as

coal and biofuel consumption (64%) and biomass burning (18%) over Asia. During winter, large fluxes of anthropogenic absorbing aerosol emissions from various natural and anthropogenic sources (e.g., coal based power plant, industries, agricultural and biomass burning) have been reported in Indo-Gangetic plain, east and east-coast regions of India (Habib et al., 2006; Badarinath et al., 2006; Dey and Di Girolamo, 2010; Moorthy et al., 2010). Our analysis confirms that BC emission from biofuel/biomass burning dominates the Asian region and get transported over the otherwise clean marine environments during winter. 3.3. Aerosol single scattering albedo Aerosol single scattering albedo (SSA) is an important factor which can be used to determine the relative dominance of scattering versus absorbing aerosols, and can range from 0 (purely absorbing) to 1 (purely scattering). As simultaneous measurements of scattering coefficient are not available, we have utilized Optical Properties of Aerosols and Clouds (OPAC) model developed by Hess et al. (1998) to estimate aerosol scattering coefficients. OPAC calculates optical properties of aerosols in the atmosphere on the basis of microphysical data (size distribution and spectral refractive index) of individual aerosol species present in the atmosphere assuming aerosols as spherical particles which are externally mixed (Hess et al., 1998). In the ambient atmosphere aerosols exist as a mixture of different components. Thus, OPAC uses the optical and microphysical properties of basic aerosol components and calculates the optical properties of composite aerosol present in the ambient atmosphere. Maritime clean aerosol model

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

3e-04

a

100

Coastal-BoB y= 6.45 x

-1.82

80

2e-04

60

1e-04

40

5e-05

20

0e+00 3e-04

0 100

2e-04

b North-BoB

y= 5.75 x

-1.83

Frequency of occurence (%)

-1

Aerosol Absorption Coefficient (m )

2e-04

2e-04 1e-04 5e-05 0e+00 3e-04 2e-04

c

East-BoB

y= 11.86 x

-1.98

2e-04 1e-04

80

0 100 80

2e-04

40

80

h South-BoB

60

1e-04

40

5e-05

20

0e+00 300

g East-BoB

60

0 100 -1.81

North-BoB

20

20

y= 2.78 x

f

40

0e+00 3e-04

d South-BoB

e Coastal-BoB

60

5e-05

2e-04

743

0 400

500

600

700

800

900

Wavelength (nm)

1000

0-1

>1-1.5

>1.5-2

α

>2-2.5

>2.5-3

Fig. 4. Average aerosol absorption coefficient spectra, which are best fitted using power law over (a) Coastal-BoB, (b) North-BoB, (c) East-BoB and (d) South-BoB. Frequency distribution of a (Eq. (3)) over (e) Coastal-BoB, (f) North-BoB, (g) East-BoB and (h) South-BoB.

defined in OPAC represents undisturbed remote maritime conditions while maritime polluted model refers to a maritime environment which is influenced by anthropogenic aerosols. Maritime clean aerosol model contains water soluble and sea salt aerosols in accumulation and coarse modes. The number densities of water soluble, and sea salt aerosols in accumulation and coarse modes in Maritime clean model are 1500, 20 and 0.0032 particles cm3 respectively (Hess et al., 1998). Maritime polluted aerosol model consists of more than a factor of two higher number concentration of water soluble aerosols (3800 particles cm3) than maritime clean model. The number density of sea salt aerosols is the same in maritime clean and maritime polluted aerosol models. The maritime polluted model in addition has 5180 black carbon aerosol particles cm3. As the optical properties of aerosols can get modified by water uptake from the atmosphere, OPAC calculates the optical properties at eight relative humidity (RH) (0%, 50%, 70%, 80%, 90%, 95%, 98%, and 99%) conditions. In the present work, BoB is treated as maritime polluted environment as significant concentration of BC has been observed over BoB during the study period (Fig. 3). For a better approximation with the real atmosphere the number density of BC aerosols in the maritime polluted aerosol model is varied to obtain a match with BC mass concentrations measured using aethalometer for individual days. This number density of BC is then used in maritime polluted aerosol model of OPAC keeping other aerosol components (water soluble and sea salt) unaltered and corresponding scattering coefficients are calculated for the ambient RH which is similar to the work done by Ramachandran and Kedia (2010). The scattering coefficient for individual days is used along with the daily mean absorption coefficient calculated from aethalometer measurement

to get SSA at 550 nm. The average SSA at 550 nm is found to be about 0.63
0.14, 0.65
0.11, 0.68
0.06, 0.70
0.09 over the Coastal-BoB, North-BoB, East-BoB and South-BoB respectively using this approach. In order to estimate the uncertainty in SSA due to uncertainty in the scattering coefficient (bsca), the bsca values are altered by
10% for individual days and SSA is calculated. The maximum change in the SSA values with this approach is found to be <3% which confirms the robustness of the methodology used in the present study for SSA calculation. Difference in the SSA values over different parts of BoB confirms the presence of a large spatial heterogeneity in aerosol characteristics over the BoB which is expected as a large spatial heterogeneity is observed in BC mass concentration over the BoB. Table 2 summarizes the values of BC mass and SSA obtained over the BoB during the last decade. SSA values observed over the BoB in the present study is lower than the values reported earlier over BoB (Table 2). The reason for a lower SSA over the BoB in the present study could be due to the fact that the earlier studies were conducted during other seasons. It has been observed that large amount of pollutants from natural and anthropogenic activities such as biomass burning, crop residue burning, biofuel emissions get transported over the BoB from the surrounding continental locations during winter (Singh et al., 2004; Badarinath et al., 2006; Habib et al., 2006; Venkataraman et al., 2005; Dey and Di Girolamo, 2010) which is also reflected from a significantly higher BC mass observed over the BoB during winter in the present study (Table 2). BC mass concentrations measured during winter 2008 are higher than those measured during winter 1999 over BoB (Table 2), thus, indicating an increase in the amount of BC aerosol emissions during winter in the last decade. SSA over BoB during premonsoon varied

744

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

Fig. 5. Probable source regions determined from Total Potential Source Contribution Function (TPSCF) analysis for BC mass concentrations measured over (a) Coastal-BoB, (b) North-BoB, (c) East-BoB and (d) South-BoB.

between 0.84 and 0.98 with a mean of 0.93
0.03 (Nair et al., 2008). The lowest SSA value of 0.84 was observed in the central BoB (near Port Blair), whereas a higher SSA was observed over rest of the BoB during premonsoon. A higher SSA (w0.94) and a lower BC mass (varying from 0.1 to 1.3 mg m3) was also observed over the BoB indicating lower concentration of absorbing aerosols during October 2003 (postmonsoon) (Sumanth et al., 2004). Lower SSA values observed in the present study could be due to a higher concentration of BC mass over the entire BoB during winter as during this time frame the winds transport pollutants from the continents over BoB (Figs. 2 and 5). This indicates that the amount of absorbing aerosols remains higher during winter when compared to premonsoon and postmonsoon over the BoB. Our finding that the entire BoB remain dominantly influenced by the

Table 2 Summary of black carbon (BC) mass concentration and single scattering albedo (SSA) over the Bay of Bengal during different seasons. Duration 1 Februarye March 1999 2 October 2003 3 MarcheApril 2006 4 December 2008eJanuary 2009

Season

BC mass SSA (mg m3)

Reference

0.81
0.06 Clarke et al. (2002) Postmonsoon 1.8
1.6 0.94 Sumanth et al. (2004) Premonsoon 1.0e4.0 0.93
0.03 Nair et al. (2008) Winter 1.0e10.0 0.67
0.03 Present study Winter

1.5e1.9

aerosols emitted from biomass/biofuel burning will have strong implications while investigating the effect of anthropogenic aerosols over marine environment, and in estimating the spatiotemporal variation of aerosol radiative impact. 4. Conclusions Spatiotemporal variations in aerosol absorption properties have been investigated over the Coastal, North, East and South Bay of Bengal (BoB) during winter. Spectral measurements of black carbon (BC) mass concentrations and absorption coefficients were made during 27 December 2008e29 January 2009 which showed a large spatiotemporal heterogeneity in the BC mass over the BoB. The BC mass concentration is found to be the highest over the Coastal-BoB (5.1
3.0 mg m3) and lowest over South-BoB (2.5
1.4 mg m3). Spectral variation in absorption coefficients suggested that the contribution of fossil fuel to the measured BC over all the four regions of BoB is less than 15%, while >85% of BC mass originated from biomass/biofuel burning. The source regions of BC are identified using the TPSCF analysis which showed that the airmasses were mainly from India, IndoGangetic plain, Pakistan, Afghanistan region over the Coastal-BoB and North-BoB. Airmasses originated from the southeast Asia (Myanmar, Thailand and China) before reaching the East-BoB and South-BoB. Although the source regions are distinctly different over different regions of BoB, the nature of absorbing aerosols is found to be similar owing to similar spectral dependence.

S. Kedia et al. / Atmospheric Environment 54 (2012) 738e745

The SSA at 550 nm is found to vary in the range of 0.63 (CoastalBoB) to 0.70 (South-BoB). It is found that the amount of BC transported over the BoB is higher during winter when compared to premonsoon and postmonsoon. Thus, the results clearly indicate that biomass/biofuel burning aerosols dominate the entire BoB during winter which can have significant impact on regional climate. Acknowledgments We thank Dr. K.K. Moorthy, Project Director, ICARB, and Dr. C.B.S. Dutt, the chief scientist, for efficient planning and conduct of the campaign, and ISRO-GBP for funding support. We thank the Ministry of Earth Sciences for giving us the permission to sail onboard Sagar Kanya, the facilities onboard the ship and the ship staff. The air back trajectories are calculated using HYSPLIT model from http://www.arl.noaa.gov/ready/hysplit4.html. We thank Ribu Cherian, Indian Institute of Technology Bombay, Mumbai, for his help on PSCF analysis. References Arnott, W.P., Hamasha, K., Moosmüller, H., Sheridan, P.J., Ogren, J.A., 2005. Towards aerosol light-absorption measurements with a 7-wavelength aethalometer: evaluation with a photoacoustic instrument and 3-wavelength nephelometer. Aerosol Science and Technology 39, 17e29. Badarinath, K.V.S., Chand, T.R.K., Prasad, V.K., 2006. Agriculture crop residue burning in the Indo-Gangetic Plains- A study using IRS-P6 AWiFS satellite data. Current Science 91, 1065e1068. Bodhaine, B.A., 1995. Aerosol absorption measurements at Barrow, Mauna Loa and the South Pole. Journal of Geophysical Research 100 (D5), 8967e8975. Cherian, R., Venkataraman, C., Ramachandran, S., 2009. Temporal variability in emission category influence on organic matter aerosols in the Indian region. Geophysical Research Letters 36 (L06809). doi:10.1029/2008GL036311. Clarke, A.D., Howell, S., Quinn, P.K., Bates, T.S., Ogren, J.A., Andrews, E., Jefferson, A., Massling, A., Mayol-Bracero, O., Maring, H., Savoie, D., Cass, G., 2002. INDOEX aerosol: a comparison and summary of chemical, microphysical, and optical properties observed from land, ship, and aircraft. Journal of Geophysical Research 107 (D19). doi:10.1029/2001JD000572. Collaud Coen, M., Weingartner, E., Apituley, A., Ceburnis, D., Fierz-Schmidhauser, R., Flentje, H., Henzing, J.S., Jennings, S.G., Moerman, M., Petzold, A., Schmid, O., Baltensperger, U., 2010. Minimizing light absorption measurement artifacts of the Aethalometer: evaluation of five correction algorithms. Aerosol Measurement Techniques 3, 457e474. Dey, S., Di Girolamo, L., 2010. A climatology of aerosol optical and microphysical properties over the Indian subcontinent from 9 years (2000e2008) of multiangle imaging spectroradiometer (MISR) data. Journal of Geophysical Research 115 (D15204). doi:10.1029/2009JD013395. Draxler, R., Hess, G.D., 1998. An overview of the HYSPLIT-4 modeling system for trajectories, dispersion and deposition. Australian Meteorological Magazine 47, 295e308. Habib, G., Venkataraman, C., Chiapello, I., Ramachandran, S., Boucher, O., Reddy, M.S., 2006. Seasonal and interannual variability in absorbing aerosols over India derived from TOMS: relationship to regional meteorology and emissions. Atmospheric Environment, 1909e1921. Hansen, A.D.A., Rosen, H., Novakov, T., 1984. The aethalometer- an instrument for the real-time measurement of optical absorption by aerosol particles. Science of the Total Environment 36, 191e196. Hess, M., Koepke, P., Schult, I., 1998. Optical properties of aerosols and clouds: the software package OPAC. Bulletin of the American Meteorological Society 79 (5), 831e844. Hopke, P.K., Barrie, L.A., Li, S.-M., Cheng, M.-D., Li, C., Xie, Y., 1995. Possible sources and preferred pathways for biogenic and non-sea-salt sulfur for the high Arctic. Journal of Geophysical Research 100, 16595e16603. 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.

745

Kaskaoutis, D.G., Kharol, S.K., Sinha, P.R., Singh, R.P., Kambezidis, H.D., Sharma, A.R., Badarinath, K.V.S., 2011. Extremely large anthropogenic-aerosol contribution to total aerosol load over the Bay of Bengal during winter season. Atmospheric Chemistry and Physics 11, 7097e7117. Kedia, S., Ramachandran, S., 2008. Features of aerosol optical depths over the Bay of Bengal and the Arabian Sea during premonsoon season: variabilities and anthropogenic influence. Journal of Geophysical Research 113 (D11201). doi:10.1029/2007JD009070. Kedia, S., Ramachandran, S., 2009. Variability in aerosol optical and physical characteristics over the Bay of Bengal and the Arabian Sea deduced from Ångström exponents. Journal of Geophysical Research 114 (D14207). doi:10.1029/ 2009JD011950. Kharol, S.K., Badarinath, K.V.S., Kaskaoutis, D.G., Sharma, A.R., Gharai, B., 2011. Influence of continental advection on aerosol characteristics over Bay of Bengal (BoB) in winter: results from W-ICARB cruise experiment. Annales Geophysicae 29, 1423e1438. Kirchstetter, T.W., Novakov, T., Hobbs, P.V., 2004. Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon. Journal of Geophysical Research 109 (D21208). doi:10.1029/2004JD004999. Moorthy, K.K., Beegum, S.N., Babu, S.S., Smirnov, A., John, S.R., Raghavendra Kumar, K., Narasimhulu, K., Dutt, C.B.S., Nair, V.S., 2010. Optical and physical characteristics of Bay of Bengal aerosols during W-ICARB: spatial and vertical heterogeneities in the marine atmospheric boundary layer and in the vertical column. Journal of Geophysical Research 115 (D24213). doi:10.1029/2010JD014094. Moorthy, K.K., Satheesh, S.K., Babu, S.S., Dutt, C.B.S., 2008. Integrated campaign for aerosols, gases and radiation budget (ICARB): an overview. Journal of Earth System Science 117, 243e262. Nair, V.S., Babu, S.S., Moorthy, K.K., 2008. Spatial distribution and spectral characteristics of aerosol single scattering albedo over the Bay of Bengal inferred from shipborne measurements. Geophysical Research Letters 35 (L10806). doi:10.1029/2008GL033687. Novakov, T., Ramanathan, V., Hansen, J.E., Kirchstetter, T.W., Sato, M., Sinton, J.E., Sathaye, J.A., 2003. Large historical changes of fossil-fuel black carbon aerosols. Geophysical Research Letters 30. doi:10.1029/2002GL016345. Raghavendra Kumar, K., Narasimhulu, K., Balakrishnaiah, G., Reddy, B.S.K., Gopal, K.R., Reddy, R.R., Reddy, L.S.S., Moorthy, K.K., Babu, S.S., Dutt, C.B.S., 2011. 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, 404e412. Ramachandran, S., Kedia, S., 2010. Black carbon aerosols over an urban region: radiative forcing and climate impact. Journal of Geophysical Research 115. Ramanathan, V., Carmichael, G., 2008. Global and regional climate changes due to black carbon. Nature Geoscience 1, 221e227. Singh, R.P., Dey, S., Tripathi, S.N., Tare, V., 2004. Variability of aerosol parameters over Kanpur, northern India. Journal of Geophysical Research 109 (D23206). doi:10.1029/2004JD004966. Sinha, P.R., Manchanda, R.K., Subbarao, J.V., Dumka, U.C., Sreenivasan, S., Babu, S.S., Moorthy, K.K., 2011. Spatial distribution and vertical structure of the MABL aerosols over Bay of Bengal during winter: results from W-ICARB experiment. Journal of Atmospheric and Solar-Terrestrial Physics 73, 430e438. Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), 2007. Intergovernmental Panel on Climate Change. Cambridge Univ. Press, New York. Sreekanth, V., Moorthy, K.K., Satheesh, S.K., Babu, S.S., Nair, V.S., Niranjan, K., 2011. Airborne measurements of aerosol scattering properties above the MABL over Bay of Bengal during W_ICARB- characteristics and spatial gradients. Annales Geophysicae 29, 895e908. Srivastava, R., Ramachandran, S., Rajesh, T., Kedia, S., 2011. Aerosol radiative forcing deduced from observations and models over an urban location and sensitivity to single scattering albedo. Atmospheric Environment 45, 6163e6171. Streets, D.G., Bond, T.C., Carmichael, G.R., Fernandes, S.D., Fu, Q., He, D., Klimont, Z., Nelson, S.M., Tsai, N.Y., Wang, M.Q., Woo, J.-H., Yarber, K.F., 2003. Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon. Journal of Geophysical Research 108. doi:10.1029/2002JD003093. Sumanth, E., Mallikarjuna, K., Stephen, J., Moole, M., Vinoj, V., Satheesh, S.K., Moorthy, K.K., 2004. Measurements of aerosol optical depths and black carbon over Bay of Bengal during post-monsoon season. Geophysical Research Letters 31 (L16115). doi:10.1029/2004GL020681. Sunder Raman, R., Ramachandran, S., Kedia, S., 2011. A methodology to estimate source-specific aerosol radiative forcing. Journal of Aerosol Science 42, 305e320. Venkataraman, C., Habib, G., Eiguren-Fernandez, A., Miguel, A.H., Friedlander, S.K., 2005. Residential biofuels in South Asia: carbonaceous aerosol emissions and climate impacts. Science 307, 1454e1456. Weingartner, E., Saathoff, H., Schnaiter, M., Streit, N., Bitnar, B., Baltensperger, U., 2003. Absorption of light by soot particles: determination of the absorption coefficient by means of aethalometers. Journal of Aerosol Science 34, 1445e1463.