The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model

American Journal of Remote Sensing 2014; 2(4): 20-29 Published online October 20, 2014 (http://www.sciencepublishinggroup.com/j/ajrs) doi: 10.11648/j.ajrs.20140204.11 ISSN: 2328-5788 (Print); ISSN: 2328-580X (Online)

The seasonal variability of aerosol optical depth over Bangladesh based on satellite data and HYSPLIT model

Mainul Islam Mamun*, Monirul Islam, Pallab Kumar Mondol

Department of Applied Physics and Electronic Engineering, University of Rajshahi, Rajshahi 6205, Bangladesh

Email address: [email protected] (M. I. Mamun), [email protected] (M. Islam), [email protected] (P. K. Mondol)

To cite this article: Mainul Islam Mamun, Monirul Islam, Pallab Kumar Mondol. The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model. American Journal of Remote Sensing. Vol. 2, No. 4, 2014, pp. 20-29. doi: 10.11648/j.ajrs.20140204.11

Abstract: Atmospheric aerosols have constituted a crucial environmental and climate issue. There is lack of studies dealing with monitoring of aerosol patterns over Bangladesh. This review attempts to analyze the seasonal variations in AOD over Bangladesh during the period 2002-2011, using MODerate resolution Imaging Spectroradiometer (MODIS) Level 3 remote sensing data. A Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to generate a backward trajectory in order to identify the origins of air masses, with the aim of understanding the spatio-temporal variability in aerosol concentration. Seasonal variations during the last decade show maximum AOD values during pre-monsoon season; while minimum AOD during post-monsoon. An evidence of decadal decreasing trend in AOD is found during monsoon season while all other seasons show increasing trends. High spatio-temporal variations of AOD are observed during different seasons in 2010. Back trajectory analysis indicates that Bangladesh is mainly affected by the pollutants and desert dust of India combining with sea salt particles blown from the Arabian Sea. The air masses are arriving at lower altitudes (500m, 1500m) mainly from western India and Indian subcontinent but at higher altitude (2500m) especially in winter season it comes from far western regions, such as Europe and various sub-Saharan regions of Africa. Different flow patterns of air masses during post-monsoon are observed that the air masses are arriving from southeast in the direction; in case of Sylhet division the sources of air masses are in the coastal regions of Thailand, border regions of Myanmar and China. These studies become helpful to understand the nature and influence of aerosols on seasonal dynamics over Bangladesh.

Keywords: Aerosol Optical Depth, MODIS, Seasonal Variations, HYSPLIT, Bangladesh

1. IntroductionAtmospheric aerosols have many impacts on global

climatic system. Aerosols attenuate the solar radiation to reach in earth surface by scattering and absorbing the radiation, altering the solar spectrum, affecting the earth’s energy budget, influencing the cloud properties, hydrological cycle and also human health [9]. Earth’s atmosphere is changing due to the variations of atmospheric aerosol load, greenhouse gases, solar radiation, and land surface properties [18]. Aerosols are suspended liquid or solid particles in air. They exhibit a wide range of compositions and shapes depending on the origins and associated atmospheric processes. Aerosol processes are very complex and highly inhomogeneous in their distributions. As a result accurately modeling of anthropogenic aerosols and their effects remains a challenge till to present. Mass concentration quantification by an optical measure known aerosol optical depth (AOD) is usually used to estimate the aerosol loading or amount in the atmosphere [27]. Various scientific researchers have been

recognized that atmospheric aerosol is a key parameter in climate change studies especially over South Asia. Atmospheric temperature fluctuations [15], [8] radiative forcing (e.g. [22]), rainfall pattern changes (e.g. [19], [25]), intensive tropical cyclone (e.g. [4]) and desertification (e.g. [20]) are examined by aerosol studies. Aerosols exhibit high spatio-temporal variations which influence the cloud properties and precipitation processes. These indirect effects are recognized by several literatures on regional and global scale [16], [17]. Satellite data is important and useful for the study about air quality research [6], [12]. Satellite retrievals of column aerosol optical depth (AOD) are a cost effective way to monitor and study aerosols distribution and effects on climate [3]. [15] reported that for mapping the distribution and properties of aerosols satellite data has tremendous potential. Due to sparse of ground measurements in many regions of the world satellite data is the only way to monitoring aerosol loads and aerosol properties in a regional

21 Mainul Islam et al.: The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and

and global scale. The USA’s NationalAtmospheric Administration (NOAA) HYSPLITindirectly describe and map the spatio-temporalaerosols. Aerosol concentration in Asia is increasingindustrial and urban both areas due to growingincreasing urbanization and industrialization,energy, changes in land use, increasing increasing temperature and increasing pollution. Different studies in India show increasingaerosol optical depth which reflects its pollutedas increasing anthropogenic aerosols. [14]spatial and temporal variations of aerosolproperties and their climate implicationsdiversity in population density, regional emissions,and seasonally-changed air masses. [23]seasonal AOD variability over Rajkot, in India,values in winter and higher values in summer.found the maximum AOD in the summer winter season over Indo-Gangetic plain. Similarly,higher AOD values in summer and lower valuesvarious cities in Pakistan. But a detaileddistribution of AOD over South Asia basicallyand seasonally basis are rather sparer. Toproperties and impacts of aerosols monthlystudies are needed.

The objective of this study is to investigatevariability of AOD over Bangladesh during2011 using MODIS level-3 data. A detailedeach season is done in the year of 2010. HYSPLITalso used to understand the spatial and temporalin aerosol load for seven divisions in Bangladeshseasons.

2. Methodology 2.1. Study Area

Bangladesh is a low-lying, riverine countrySouth Asia with 710 km coastline largely the northern littoral of the Bay of Bengal.Bangladesh is formed by the Gangesconstructed by the confluence of the Meghna and Brahmaputra (Jamuna) tributaries. The west, north, and east ofbordered with India by a 4,095-kilometerthe southeast is bordered with Myanmar bywater frontier (193 km) (Fig. 1). Four distinctconsidered in the study area from climaticFirstly the dry winter season from Decembersecondly the pre monsoon hot summer fromthirdly the monsoon or rainy season from Juneand fourthly the post monsoon season October to November [24]. Wind velocitysummer than in winter. The average temperatureis about 25°C. Mean monthly temperatures18°C in January and 30°C in April-May. Maximumtemperature ranges between 38 °C and 41hottest month in most parts of the country coolest month. Significant differencestemperatures occur across the country.

The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model

National Oceanic and HYSPLIT model can temporal variations of

increasing over the growing populations,

industrialization, demands for motorized traffic, other kinds of increasing trend of

polluted environment [14] reported that

aerosol load, optical implications due to large

emissions, land use [23] analyzed the India, found lower summer. [19] also season than in the

Similarly, [1] found values in winter for

detailed analysis of basically on monthly

To understand the monthly and diurnal

investigate the seasonal during the period 2002-detailed observation in

HYSPLIT model is temporal variability

Bangladesh of different

country located in marshy jungle on

Bengal. Geographically Ganges delta which is

Ganges (Padma), rivers and their of Bangladesh are land frontier and

by a short land and distinct seasons are

climatic point of view. December to February,

from March to April, June to September

which lasts from velocity is higher in

temperature everywhere temperatures range between

Maximum summer 41 °C. April is the and January is the

differences in seasonal Heavy rainfall is

characteristic of Bangladesh asyear. The rainy season is characterizedwesterly winds, very high humidity,long consecutive days of rainfall.rainfall occurs during the monsoon

Fig 1. Geographical location

2.2. Dataset and Analyses

Aerosol optical depth (AOD)situ and remote sensing measurementsatellite remote sensing measurementlarge spatial and temporal distributions satellite remoteMODerate resolution ImagingAOD datasets are used. The NASA’s Terra and Aqua satellitesDecember 1999 and in May spectral channels to provide informationland and oceanic conditions. MODISdata collection which is importantlong- and short-term change Aerosol retrieval is different over[26]. The MODIS provides observations(250m to1 km) and temporal (1different portions of the electromagneticinvestigation MODIS Terra Level

A 7-day air-mass back trajectorySingle Particle Lagrangian Intregratedmodel to reveal the sources ofseven different divisions withThe FNL dataset which is reprocessedOceanic and Atmospheric Administrationthe meteorological input for thecan calculate up to 40 forwarddifferent altitudes with a horizontalresolution of 500 × 500m usingstudy, the backward trajectories1500m, and 2500m heights abovethe origin of air masses bringingdesert dust, sea salt) into the variousduring different seasons.

The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and

as a result flood occurred every characterized by southerly or south-

humidity, and heavy rainfall and rainfall. About 80 % of total

monsoon season.

location of Bangladesh.

(AOD) can be measured by both in measurement systems. Here the

measurement is used. To study the heterogeneities of aerosol

remote sensing is essential. The Imaging Spectroradiometer (MODIS)

MODIS instruments onboard satellites which launched in 2002 respectively. It has 36 information about atmosphere, MODIS provides a continuous

important for understanding both in the global environment.

over land [11] from over oceans observations at moderate spatial

(1 to 2 days) resolutions using electromagnetic spectrum. For this

Level-3 data is used. trajectory is made using the Hybrid

Intregrated Trajectory (HYSPLIT) of aerosols over Bangladesh of

with four representative seasons. reprocessed from the National

Administration (NOAA) is used as the trajectory model. The user

forward or backward trajectories at horizontal grid of 1.5° × 1.5° and a using the HYSPLIT model. In this

trajectories are calculated at 500m, above ground level to investigate

bringing particular matters (such as various divisions of Bangladesh

American Journal of Remote Sensing 2014; 2(4): 20-29 22

3. Result and Discussions 3.1. Seasonal variations in AOD

Fig. 2 shows the spatial distribution of seasonal mean AOD over Bangladesh in 2010, for four representative seasons namely pre-monsoon (March-May), monsoon (June-September), post-monsoon (October-November) and winter (December-February) which reveals high spatial variability. The seasonality of AOD exhibits the patterns of higher values during pre-monsoon & monsoon seasons whereas lower values during post-monsoon & winter seasons.

Fig. 3 shows the temporal variations of seasonal AOD by time series plots over Bangladesh during 2010. The results show that at the beginning of pre-monsoon season the AOD is increasing then from July AOD is rapidly decreasing and this decreasing trend remains constant throughout the whole monsoon season. Again from starting of post-monsoon season the AOD is increasing rapidly and this increasing trend follows the winter season.

3.1.1. Variations in AOD During Pre–Monsoon Season In pre-monsoon (March-May) the spatial distribution of

AOD is presenting deviations from region to region as shown in Fig. 2a. The northern, southern and eastern parts show higher AOD distributions during pre-monsoon season. It is found that the AOD starts to increase rapidly from the beginning of pre-monsoon (March), reaching its maximum in June as shown in Fig. 3a. The summer increase could be due to high temperature and high wind velocities producing larger quantities of wind-driven dust particles [1]. During pre-monsoon season also the water soluble aerosols grow hygroscopically in the presence of water vapor causes higher AOD values [1]. A higher concentration of water vapor in pre-monsoon season leads to a higher AOD because water vapor and AOD are directly related to each other’s [1], [23]. The increasing AOD also indicate to increased emissions of aerosols from biomass burning in Bangladesh, as the other source of aerosols which must be examined [10]. Dust activity is maximum during May-July. Dust frequency and intensity during pre-monsoon depend on various meteorological phenomenons such as rainfall amount, soil moisture, wind speed and wind direction etc. [8].

3.1.2. Variations in AOD During Monsoon Season The strong gradient in AOD distribution is shown during

monsoon season over Bangladesh in 2010 (Fig. 2b). The AOD concentration is increasing from very low to very high from northeastern part to southwestern part of the country. Khulna, Barisal and the coastal areas are shown highest AOD distribution in monsoon period; this may be due to a high concentration of water vapors and sea salt spray. From monsoonal time series plot it is shown that the high AOD value is decreased rapidly from July and reached its minimum value (~0.1) in October. This decrease just before the winter season could be due to cloud scavenging and rain washout process [23]. Bangladesh and northeast India is affected by monsoon at the end of June, and as a consequence, the MODIS retrievals in July and August are limited due to extensive cloudiness. So, there are possibilities of lack of data over some times over specific

pixels in this area leading to the observed complicated pattern as regards the AOD variations [10].

3.1.3. Variations in AOD During Post- Monsoon Season In post-monsoon, spatial distribution shows comparatively

lower values of AOD distribution than other seasons over Bangladesh while the south-western region (Khulna and Barisal) shows higher AOD distribution than other regions. As this area are affected by sea salt and marine aerosols. The eastern regions especially Chittagong and Sylhet divisions show very lower AOD distribution. From the AOD time series graph (Fig. 3c) it is shown that AOD is rapidly increasing from September, reaching a maximum AOD value of ~1.00 in November. This increasing manner may be due to local activities which are started from October [10].

3.1.4. Variations of AOD During Winter Season Winter season shows high aerosol loading over

Bangladesh in 2010 except the Chittagong and Sylhet divisions, as shown in Fig. 2d. The spatial distribution shows a strong gradient in AOD from very high to low from west to east. From Fig. 3d the winter period clearly reveals the lowest variability of AOD in 2010 over Bangladesh among all seasons. Note that some area in northwestern Rajshahi division shows extremely high AOD distributions in winter. Dense fog is a common activity in Bangladesh during winter which may be the reason for this peak AOD. Dense fog causes enormous economic loss and also the daily life become stand still. [10] also found a pronounced eastwards increasing AOD over Eastern IGP during winter season. [5] reported that “the Bihar pollution pool” strongly influencing the aerosol load & properties over the northern Bay of Bengal. Again dryer weather and lower humidity causes a lower AOD values in winter season than summer season [1].

3.1.5. Seasonal Mean Variations of AOD Seasonal mean variations and trends of AOD are also

analyzed over Bangladesh for a period of ten years (2002-2011) as shown in Fig. 4. The highest AOD level is observed in pre-monsoon and the lowest AOD level in post-monsoon. The annual mean AOD in summer is 0.61 and in post monsoon it is 0.33. Where winter season and monsoon season show medium to high AOD levels. It is also evident that pre-monsoon AODs are higher than those for all other seasons in the year, followed in order by winter, monsoon and post-monsoon. Our analysis shows that the AOD trends since 2002 are increasing in pre-monsoon, post-monsoon and winter seasons but decreasing in monsoon season. It is also shows that the AOD trend is rapidly increasing in winter than all other seasons, ordered by post-monsoon and pre-monsoon. The rapid increase of AOD in winter may be due to increase in fog and local activities with conjunction of anthropogenic aerosols. The decreasing trend of AOD in monsoon during the last decade indicates that the rainfall amounts and patterns may be changing as the rainy season shifts backward. It is also found that AOD values varied between low values in post-monsoon and high values in pre-monsoon, where the pattern values in winter and monsoon are quite similar to each other. During pre-monsoon high temperature plays a vital role in heating lifting loose material from soil and also due to higher wind speed consequently higher AOD

23 Mainul Islam et al.: The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model

values observed. Rain washout processes reduce the AOD values in post-monsoon season [1]. Anthropogenic aerosols are the major type over south Asia as they contribute about 70 – 80% to the total AOD [7]. They also the dominant type both over urban areas and downwind Oceanic areas [21]. In rural and agricultural areas the fine mode aerosols mainly originate from burning of bio-fuels, while in urban areas they are originating from fossil-fuel combustions [10].

(a) Pre-monsoon

(b) Monsoon

(c) Post-monsoon

(d) Winter

Fig 2. AOD distribution images of Terra-MODIS over Bangladesh during (a) pre-monsoon, (b) monsoon (c) post-monsoon and (d) winter seasons in 2010.

American Journal of Remote Sensing 2014;

Fig 3. Seasonal time series plots of AOD during monsoon (c) post-monsoon and (d) winter seasons overyear 2010.

Fig 4. Annual-mean seasonal variations and trendsBangladesh for the period 2002-2011.

American Journal of Remote Sensing 2014; 2(4): 20-29

(a) pre-monsoon, (b) over Bangladesh for the

trends of AOD over

4. Influence of Air MassesUsing Back Trajectory

Seven day back trajectory analysesHYSPLIT model are studiedsources of the air masses in regions are the seven divisionspre-monsoon, monsoon and posttrajectories are computed at severaland 500m). The trajectories for2010, 22 July 2010 and 15 OctoberThese trajectories are only includedbecause similar trajectories found5 reveals that most of the backmasses flow originating either fromBay of Bengal, the great TharDesert (Cholistan), the SaharaLibya, various western regionsAfrica & Europe also from Atlanticfrom south-eastern regions suchSea and that consequently a significantthis areas could be observed. Inthe western land mass and responsible for the increase in AOD

Barisal

24

Masses on AOD, Trajectory Analysis

analyses based on the NOAA studied in order to understand the

the study regions. The study divisions of Bangladesh, for winter,

post-monsoon seasons. These several altitudes (2500m, 1500m, for 08 December 2009, 29 May

October 2010 are shown in Fig. 5. included here as a representative found for other time period. Fig. back trajectories identified air from Arabian Sea (extremely),

Thar Desert of India, Pakistani Sahara and sub-Saharan regions,

regions of India towards north-south, Atlantic Ocean and sometimes

such as China, Myanmar, Luzon significant increase in AOD over

In general the air masses from from the Arabian Sea are AOD values over Bangladesh.

25 Mainul Islam et al.: The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and

During winter (08 December 2009) backseveral areas reveal that the trajectories areand north-west in the direction. For 500maltitudes the sources of air mass loadingIndian and east Pakistani deserts and the middleregions of India, all over the seven divisions.and Sylhet divisions at 1500m altitude a loading are observed. The sources are Redand Bay of Bengal for Sylhet. Where in 2500mair masses are loading from far westernEurope and various sub-Saharan region ofPoland, Estonia, Libya, Sahara desert etc.source is Saudi Arab and in Rangpur thisGulf.

Fig. 5 shows that, in pre-monsoon and the highest AOD values in Bangladesh occurredsalt bringing from the Arabian Sea viathrough southwest direction. This highestobserved in Bangladesh in most of the divisions.northern divisions Rajshahi and Rangpur in summer other type of air masses areconjunction of Arabian Sea salt. Such Ocean salt and air masses from Nepal (closeare loading in Rajshahi and Rangpur respectively.Chittagong division at 1500m altitude theloading from Goa of India (coastal region).

During post-monsoon (15 October 2010)observed on air masses loading. Fig. 5 revealsloading are observed from both southeastdirection in Bangladesh. Such as Arabiananthropogenic air masses of different regionsMyanmar, Thailand, Luzon Sea etc. Dhakadivisions are shown highest differences of

The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model

back trajectories at are from the west

500m and 1500m loading mainly are west

middle & western divisions. But in Khulna

different air mass Red Sea for Khulna

2500m altitude the western regions, such as

of Africa such as etc. In Sylhet this

this source is Persian

monsoon seasons occurred due to Sea via Bay of Bengal

highest AOD value is divisions. Only in at 2500m altitude are loading with

as north Atlantic (close to Indian border) respectively. Again in

the air masses are region).

2010) verities are reveals that air masses

southeast and southwest Arabian Sea salt,

regions in India, China, Dhaka and Sylhet of air mass loading

among all of the divisions. Myanmar, coastal regions of Thailandthe sources in Sylhet are southwesternChina and Myanmar, costal Bengal and Luzon Sea.

Therefore the air mass backpre-monsoon, monsoon, postsources of air masses loadingwinters air masses are reachinglong distances from northwest,masses have traveled shorter likely that the air masses spentthe summer than during the months it would explain becauselevels of AOD are observed. Thisdistribution of solar flax reachinghigher latitudes air is warmertropical air which rises verticallyhigh altitudes while the coolerequator at lower levels. Theincreases at equatorial latitudesmass and most equatorial air movesthis pressure gradient. Since overthe air moves away from the equatorhigh altitudes and the coriolis south and south-westerly windsvariations of sources, directionsloading may be due to variousas wind speed, wind direction, clear from the back trajectoriesinfluenced by the combine effectOcean and that this has resultedover Bangladesh.

Chittagong

The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and

The sources in Dhaka are Thailand and Arabian Sea and

southwestern border region of region of Thailand, Bay of

back trajectories indicate that in post-monsoon and winter the

loading are different. For example, reaching in Bangladesh have traveled

northwest, while the other seasonal air distances. Therefore it seems

spent more time over land during winter. During the summer

because in this time the higher This is also due to the uneven

reaching the earth’s surface. The warmer than at lower latitudes. The vertically moves towards the poles at

cooler polar air moves towards the The air presser consequently

latitudes due to the outflow of air moves towards the poles under over the region of Bangladesh equator in northerly direction at force has a significant effect, winds dominate [1]. These

directions and distances of air mass metrological parameters such temperature, pressure etc. It is

trajectories that the air masses have been effects of land, industries and the

resulted in increased AOD values

American Journal of Remote Sensing 2014; American Journal of Remote Sensing 2014; 2(4): 20-29

Dhaka

Khulna

26

27 Mainul Islam et al.: The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model

Rajshahi

Rangpur

The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and

American Journal of Remote Sensing 2014;

Fig 5. Seven-day back trajectories for

5. Conclusion The aim of this study is to analysis the

variations over Bangladesh based on Terrasensing data during the last decade (2002develop an understanding of the impactclimate system. Aerosol air masses loadinga backward trajectory with the help of Particle Lagrangian Integrated Trajectory (HYSPLIT)The seasonal mean AOD variations and trendsfor a period of 10 years (2002-2011), also variations and trends for one specific yearexamined over Bangladesh. Seasonal analyseshighest AODs are found in pre-monsoonminimum AODs are found in post-monsoon.AODs in pre-monsoon are interpreted as higher temperature, higher humidity andparticles blown from Arabian Sea and Baylow AODs in post-monsoon due to a resultenvironment after rainy season by rain washoutSeasonal variations in 2010 present high spatialeach season over Bangladesh. Examinationvariations using back trajectory analysismasses from Indian deserts and sub-Saharaparticles from Arabian Sea along with (industrial emissions, locally derived dustburning) are the main sources of aerosolBangladesh. These air masses cause differentin various divisions, and overall influence

American Journal of Remote Sensing 2014; 2(4): 20-29

Sylhet

for 7 divisions in Bangladesh for 8 Dec 2009, 29 May 2010, 22 Jul 2010

the seasonal AOD Terra-MODIS remote (2002-2011) and to

impact of aerosols on loading are identified by

a Hybrid Single (HYSPLIT) model.

trends are estimated the seasonal AOD year of 2010 are

analyses report that the monsoon season and monsoon. These high

being a result of and lots of sea salt Bay of Bengal, where

result of clean and cool washout processes. spatial variations in

Examination of these analysis reveal that air

Saharan regions, sea Indian pollutants dust and biomass

aerosol loading in different AOD loading

influence in Bangladesh.

This study has some limitationsdata and it is a newer study in Bangladesh.this research over Bangladeshsatellite AOD retrievals don’tdata also with other researchrecommends that further muchabout aerosols to achieve a bettertemporal variations in aerosolsimpacts. On the whole, this studyuseful in the assessments of aerosols on regional and globalaerosols.

Acknowledgements MODIS data used in this

maintained by the GES-DISCacknowledged with thanks. NOAA(ARL) is also gratefully acknowledgedHYSPLIT model to analyze the Back

Research Highlights � Seasonal variations of

MODIS data and the HYSPLIT� Highest AOD level is reported

while lowest in post-monsoon.� The assessment of AOD

variations in seasonal pattern.

28

2010 and 15 Oct 2010.

limitations as there is no ground-based Bangladesh. The comments of

Bangladesh may be less accurate as this don’t compare with ground based

research projects. It is therefore much more researches are needed

better understanding of spatio- and their various atmospheric

study in Bangladesh will prove spatio-temporal variations in

global climate impact due to

this study were developed and DISC of NASA is gratefully NOAA Air Resources Laboratory acknowledged for providing the

Back Trajectories.

AOD are examined using

HYSPLIT model. reported in pre-monsoon season

monsoon. AOD shows high spatio-temporal

pattern.

29 Mainul Islam et al.: The Seasonal Variability of Aerosol Optical Depth over Bangladesh Based on Satellite Data and HYSPLIT Model

� Back trajectory analysis uses to identify the origin of air masses.

� The aerosol sources of high AOD over Bangladesh are mainly external.

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