Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 1251–1259

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Journal of Atmospheric and Solar-Terrestrial Physics

1364-68

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jastp

Micro pulse lidar observations of mineral dust layer in the lowertroposphere over the southwest coast of Peninsular India duringthe Asian summer monsoon season

Manoj Kumar Mishra, K. Rajeev n, Bijoy V. Thampi, K. Parameswaran, Anish Kumar M. Nair

Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695022, India

a r t i c l e i n f o

Article history:

Received 12 April 2010

Received in revised form

14 August 2010

Accepted 16 August 2010Available online 22 August 2010

Keywords:

Aerosol transport

Mineral dust

Lidar

Asian summer monsoon

Indian region

26/$ - see front matter & 2010 Elsevier Ltd. A

016/j.jastp.2010.08.012

esponding author. Tel.: +91 471 2563886.

ail address: k_rajeev@vssc.gov.in (K. Rajeev).

a b s t r a c t

A large aerosol plume with optical depth exceeding 0.7 engulfs most parts of the Arabian Sea during the

Asian summer monsoon season. Based on Micro Pulse Lidar observations during the June–September

period of 2008 and 2009, the present study depicts, for the first time, the existence of an elevated dust

layer occurring very frequently in the altitude band of 1–3.5 km over the west coast of peninsular

India with relatively large values of linear depolarization ratio (dL). Large values of dL indicate the

dominance of significantly non-spherical aerosols. The aerosol optical depth of this layer (0.2) is

comparable to that of the entire atmospheric column during dust-free days. Back-trajectory analysis

clearly shows the advection of airmass from the arid regions of Arabia and the west Arabian Sea,

through the altitude region centered around 3 km. This is in contrast to the airmass below 1 km

originating from the pristine Indian Ocean region which contains relatively spherical aerosols of marine

origin with dL generally o0.05.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Atmospheric aerosols play a significant role in the radiationbudget and climate of the earth-atmosphere system (Forster et al.,2007). During the Asian summer monsoon season (June–September),the strong westerly winds (5–20 m/s) in the lower and middletroposphere causes significant long-range transport of Arabian dustover to the north Arabian Sea while the large sea surface windsproduce considerable amounts of sea salt aerosols. Aerosols fromthese two sources lead to the manifestation of an intense aerosolplume over the entire north Arabian Sea with visible band aerosoloptical depth (AOD) often exceeding 0.7 (Zhu et al., 2007; Remeret al., 2008). However, southerly wind in the lower troposphere overthe south Arabian Sea brings dust-free airmass from the pristineIndian Ocean into these regions leading to a strong latitude gradientin AOD around �8–101N (Nair et al., 2005). Strong precipitation overthe eastern Arabian Sea, which implies wet-removal of aerosols,further increases the spatial gradient in AOD over this region.

Regional distribution of aerosols over the Arabian Sea duringthe Asian summer monsoon season has been extensively studiedusing satellite observations (e.g., Husar et al., 2001; Li andRamanathan, 2002; Rajeev et al., 2004; Nair et al., 2005;Zhu et al., 2007; Remer et al., 2008); these studies show thepredominance of dust aerosol plume over the Arabian Sea during

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this season, with peak values of AOD occurring in July. Dominanceof the dust plume in this region is also evident from the groundbased observations of spectral AOD at Minicoy (8.5N1 and 731E),an island location in the equatorial Indian Ocean (Moorthy andSatheesh, 2000). This aerosol plume is dominated by coarse modeparticles (Remer et al., 2008) and the estimated contribution ofnon-sea salt aerosols is at least three times larger than sea saltaerosols produced by the prevailing high wind speed over theseoceanic regions (Parameswaran et al., 2007).

The amount of dust over to the east Arabian Sea region could besignificantly reduced by the large-scale precipitation prevailing inthis season. However, the observational data on aerosol opticaldepth over the east Arabian Sea is rather limited mainly becauseof the large frequency of occurrence of clouds over this region(Nair et al., 2005; Meenu et al., 2010). The regional mean aerosolradiative forcing at the surface during June–August period over theArabian Sea is estimated to be about �17.8 W/m2 and thecorresponding value of the aerosol radiative heating of the atmo-sphere is approximately 10.8 W/m2 (Zhu et al., 2007). Altitudedistribution of aerosols using CALIOP (Cloud-Aerosol Lidar withOrthogonal Polarization) onboard the Cloud-Aerosol Lidar andInfrared Pathfinder Satellite Observation (CALIPSO) showed thepresence of dust layers over the Arabian region extending up to�5 km in altitude, during the June–August period. The frequency ofoccurrence of such dust events are reported to be 478% (Liu et al.,2008). Altitude distribution of aerosols over Maldives (4.11N and73.31E) observed using a ground based Lidar also showed thepresence of an elevated aerosol layer (Muller et al., 2001). Except for

M.K. Mishra et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 1251–12591252

these, observations of the altitude distribution of aerosols over theArabian Sea and over the west coast of peninsular India duringthe Asian summer monsoon season are sparse. Such observationsare essential for determining the pathways of aerosol transport,radiative heating of the atmosphere, and the potential role ofaerosols in modulating cloud properties. Unlike satellite or groundbased sun-photometer observations, ground based Lidars have thepotential to provide altitude profile of aerosols even in the presenceof dense high altitude clouds, a scenario often observed over the eastArabian Sea region. In this paper, the altitude distribution of aerosolsduring the Asian summer monsoon season (June–September) of2008 and 2009 are investigated using the Micro Pulse Lidar atTrivandrum (8.51N and 771E), a station located in the Arabian Seacoast of south peninsular India. Lidar observations of the lineardepolarization ratio are used to characterize the dust layers. The roleof long range transport and percentage contribution of the dust layerto the columnar AOD is also investigated.

2. Data and method of analysis

Altitude profiles of aerosol backscatter coefficient (ba) andlinear depolarization ratio (dL) obtained using the Micro PulseLidar (Model: MPL-4B-532/POL-SW of Sigma Space Corporation,USA) during June–September period of 2008 and 2009 are usedfor the present study. This Lidar employs a frequency doubleddiode pumped solid state Nd:YAG laser delivering optical pulsesat 532 nm wavelength at a rate of 2.5 kHz and pulse width of 7 ns.The maximum pulse energy is 8 mJ. A Maksutov–Cassegrain typetelescope (diameter of 178 mm) is used for transmitting the laserbeam and receiving the backscattered signal. Two identicalinterference filters (each having a bandwidth of 0.14 nm andout-of-band rejection of 105) are used in cascade to limit thebackground. This makes the MPL suitable for daytime operationalso. The detector is a Silicon Avalanche Photodiode (Si-APD)operated in photon counting mode.

In MPL, the polarization sensitive detection of backscatteredlight is achieved by activating a liquid crystal retarder (LCR) suchthat it alternately transmits linearly (when no retarding potentialis applied) and circularly (when a quarter-wave retardingpotential is applied) polarized light. The backscattered radiationis received in both cases through a polarizing beam splitter, whichdirects the co-polarized and depolarized signals (in the two cases,respectively) to the detector. The details of the polarizationsensitive Lidar measurements and determination of the lineardepolarization ratio are given by Flynn et al. (2007). Using the co-polarized and depolarized lidar signals, the depolarization ratio,dMPL, and hence the linear depolarization ratio (dL) are estimated(Flynn et al., 2007) as

dMPL ¼9Pð0Þ9

9Pðp=2Þ9ð1Þ

and

dL ¼dMPL

dMPLþ1ð2Þ

where P(0) is the received power when the LCR is operated withzero retardation and P(p/2) that when the LCR is operated with aquarter wave retardation. The value of dL is an indicator of theextent of non-sphericity associated with the scatterers (Sakaiet al., 2000). The larger the value of dL, the higher is the non-sphericity of aerosols. The value of dL is used as a potential tool fordifferentiating different aerosol types (Rajeev et al., 2010).

The lidar system alternates between the two states of polar-ization at an interval of 1 min. The co-polarized and depolarizedlidar backscattered signals are collected alternately with a range

resolution of 30 m and time integration for 1 min. The total lidarbackscattered signal power (Flynn et al., 2007) is estimated as

P¼ 2Pð0ÞþPðp=2Þ ð3Þ

The range-dependent lidar geometrical correction factor g(r)(‘r’ being the range) is obtained by operating the lidar horizontallyfrom an elevated location, following the method described byWelton et al. (2002) assuming a horizontally stratified atmo-sphere. The dead-time correction of the APD detector is carriedout based on the calibration curve provided by the manufacturer.For each profile, the average photon count observed between thealtitudes of 50 and 60 km is considered as the background, whichis subtracted from the measured signal to estimate the truebackscattered signal. Extreme care is taken to avoid cloud-contamination. The Lidar data when clear sky condition prevailsat least up to an altitude of 6 km, for a minimum period of 30 mincontinuously, only are used to derive the altitude profile of ba byinverting the lidar backscattered signal using Fernald’s algorithm(Fernald, 1984), assuming an aerosol extinction-to-backscatterratio (S) of 30 Steradian (Sr). The molecular backscattering andextinction coefficients are derived from the monthly meanstandard atmospheric model of pressure (P) and temperature (T)for Trivandrum.

For inverting the lidar data, the reference altitude ZR isassumed to be 20 km (up to which signal-to-noise ratio is fairlygood during night), where the aerosol backscatter coefficient isassumed to be zero. However, during the daytime (as the signal-to-noise ratio is good only up to 6–10 km) or when optically thickhigh altitude clouds are present above 6 km, the referencealtitude also is correspondingly reduced. The aerosol backscatterat this reference level is taken from the closest nighttime invertedaltitude profile of ba, for which good signal-to-noise ratio isachievable up to 20 km.

Sensitivity analysis (for the variations in the assumed value of S,and the boundary condition of ba at ZR) is carried out to ascertain theaccuracy of the lidar-derived ba, which shows that the uncertaintyassociated with the estimated value of ba is approximately 30%.Utilization of the same telescope and detector system for the mea-surement of co- and cross-polarized signal reduces the uncertaintyin dL, which is typically about 40%. However, above �5 km, thestrength of the cross-polarized lidar signal due to scattering byaerosols is very weak, and the uncertainty in dL will be 450%. Thecolumnar aerosol optical depth (t) is measured during cloud-freeconditions using a sun-photometer (Microtops-II, Solar Light Co.)which provides AOD at 5 wavelengths: 440, 500, 675, 936, and1020 nm. In order to minimize the uncertainty in the lidar invertedprofile, the aerosol backscatter and extinction profiles derived fromMPL data are weighted by the AOD obtained using the sun-photometer on the same day. Regional distribution of AOD (t at550 nm) over the Arabian Sea is extracted from the data obtainedfrom MODIS (Moderate Resolution Imaging Spectroradiometer:MOD08_D3.005 product) onboard the Aqua satellite. The standarderror associated with this AOD is 0.0370.05t over the ocean.

3. Results and discussion

3.1. Altitude structure of ba and dL

Fig. 1 shows the altitude–time cross section of the attenuatedlidar backscatter signal (PN¼z2P(z), where z is the altitude andP(z) is the backscattered power from z) in the co-polarizedchannel along with dL obtained from MPL data during 11–12 Juneand 02–03 September 2008 as two typical examples. For acomparison of aerosol loading over the Arabian Sea on these days,

Fig. 1. Time–altitude variations of (a) range-compensated attenuated lidar backscatter signal (PN, co-polarized component) and (b) linear depolarization ratio (dL) during

11–12 June 2008, (c) Spatial distribution of average AOD during 11–12 June 2008 derived from MODIS data. (d, e, f) are same as (a, b, c), respectively that for 02–03

September 2008. The location of Trivandrum is indicated by a ‘+’ sign in (c) and (f).

Fig. 2. Mean profile of (a) aerosol backscatter coefficient (ßa, shown in log-scale),

(b) linear depolarization ratio (dL), and (c) normalized columnar contribution to

AOD for the cases shown in Fig. 1.

M.K. Mishra et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 1251–1259 1253

the regional distribution of AOD (at 550 nm) obtained fromMODIS at 13:30 local time (averaged for the respective twoconsecutive days) is also shown in this figure. During the night of11 June 2008 (Fig.1a) the attenuated backscatter signal ismaximum near the surface and decreases with increase inaltitude. A relatively large decrease in the values of PN above3 km is worth noting. The corresponding time–altitude variationof dL (Fig. 1b) shows a prominent aerosol layer with significantlylarge value of dL (in the range 0.1–0.25) between 1 and 3.5 km,while its value below 1 km is considerably small (o0.05). Thisclearly indicates that the layer at 1–3.5 km is dominated by highlynon-spherical aerosols, while those below 1 km are relativelymore spherical. Remarkably, the upper boundary of the elevatedlayer (observed in PN and dL) descends from 3.5 km to less than3 km during the course of this night. The spatial distribution ofAOD in Fig.1c shows high values over the Arabian Sea, extendingfrom the northwest Arabian Sea up to the west coast of peninsularIndia with a pronounced latitude gradient across �81N.

In contrast, the time–altitude cross-sections of PN and dL

during 2–3 September 2008 (Fig. 1d and e) do not show anyprominent elevated layer. The values of dL are generally o0.03showing the dominance of nearly spherical aerosols throughoutthe altitude. The regional distribution of AOD during 2–3September (Fig. 1f) shows substantially low values of AOD overthe east Arabian Sea compared to that during 11–12 June 2008.The high values of AOD and the dust plume during these days arerather confined to the west Arabian Sea. This is in agreement withthe MPL observations (Fig. 1d and e) showing the absence of anydust layer over Trivandrum during this day.

Isolated low level clouds are observed on 2–3 September2008 as seen in Fig. 1d, manifested by the intermittent spikesbelow the altitude of �3 km, which are followed by largereduction in the value of PN above the altitude of their occurrence.The absence of Lidar signal above the clouds is because of theirlarge optical depth. The values of dL at the altitude of cloudoccurrence (in the lower troposphere) are generally small(o0.01). Similar observations of small value of dL associatedwith low-level clouds are found throughout the year (not shownin figure). This clearly shows that particles associated with theseclouds are mostly small water droplets which are fairly homo-geneous and spherical in nature and the enhancement in dL

observed around 3 km is not caused by the presence of such

low-level clouds. As explained earlier, the cloudy data areeliminated from the subsequent analysis to derive altitudeprofiles of ba and dL.

Mean altitude profiles of ba and dL during 11–12 June and 2–3September 2008 are shown in Fig. 2. The altitude gradient of ba

during 11–12 June is considerably small up to �3 km, followed bya rapid decrease above. The altitude profile of dL also shows adistinct layer between 1 and 3.3 km, with mean value of dL in therange 0.1–0.2. In contrast, the altitude profile of ba on 2–3September shows a steady decrease with increase in altitude rightfrom the surface, even though the rate of decrease varies withaltitude. Note that the values of dL for the entire altitude regionare less than 0.02 during these days. The mean value of ba

between 700 m and 3 km on 02–03 September is smaller thanthat on 11–12 June by a factor of 2, while the values of ba above�4 km are comparable.

The normalized cumulative contribution of aerosols (NCC) atdifferent altitudes to the total atmospheric aerosol loading isestimated by integrating ba from the surface up to a particularaltitude, z, and normalizing it with the column integrated aerosolbackscatter coefficient. This parameter provides a quantitativeassessment of the percentage contribution of aerosols from

M.K. Mishra et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 1251–12591254

different altitudes to the total columnar aerosol loading and AOD.The altitude variations of NCC during the above periods arepresented in Fig. 2c. While the mean AOD during 11–12 June is0.38, the layer integrated AOD of the dust layer on this daybetween 1 and 3.5 km is 0.21. This figure also shows that, whilethe contribution of aerosols in the altitude region above 1 km tothe AOD is �55% on 11–12 June, the corresponding value is only38% during 2–3 September.

3.2. Month-to-month variation in the mean altitude structure

of ba, dL, and NCC: influence of dust layer

It is observed that the dust layer persists for long time (evenfor few days) when the value of dL is very high (typically 40.12),while it is intermittent when dL is relatively less. The valuesof dL are o0.05 when the dust layer is not discernible in thetime–altitude cross sections of dL. Based on the altitude profilesof dL, the lidar data are grouped into three different categories:(1) days with no dust layer (dLo0.05 throughout the profile or

Fig. 3. Histograms of the frequency of occurrence of different values of dL in the

altitude range 1–3.5 km under different types of dust layer.

Table 1Frequency of occurrence of weak/moderate and strong dust layers and dust-free days

Months Number of days when ba,

dMPL could be estimated

Frequency of occurrence

strong elevated aerosol la

June 2008 9 45

July 2008 3 100

August 2008 10 50

September 2008 15 0

June 2009 12 0

July 2009 16 50

August 2009 14 57

September 2009 6 0

dLZ0.05 for an altitude extent of o0.5 km), (2) days with weak/moderate dust layer (0.05rdLo0.12 for an altitude extentofZ0.5 km), and (3) days with prominent dust layer (dLZ0.12for an altitude extent of Z0.5 km). However, this classification israther arbitrary. Histograms of the frequency of occurrence ofdifferent values of dL in the altitude range 1–3.5 km (the typicalaltitude of occurrence of the dust layer) under different conditionsare shown in Fig.3. The general characteristics of the histogramsare similar during 2008 and 2009. Whenever the dust layer isstrong, the values of dL in the above altitude band vary over a widerange 0.01–0.24 with a broad peak in the range 0.08–0.15. Thefrequency distribution of dL during the occurrence of weaklayers is narrower and less skewed compared to that duringthe occurrence of strong layers, and is centered around 0.06. Thevalue of dL never exceeds 0.05 when the dust layer is absent(note that, based on the criteria for classification of dust layers,the values of dLZ0.05 which is limited to an altitude extent ofo0.5 km are classified as profiles with no dust layer). Fig. 3substantiates the arbitrary criteria followed in the classification ofthe dust layers.

Table 1 shows the frequency of occurrence of profiles in eachof the above categories for June–September in years 2008 and2009, obtained by dividing the number of daily mean profiles ineach category with the total number for the respective month.Note that altitude profiles of ba are obtained only for cloud-freeconditions. Large frequency of occurrence of low and middle levelclouds and precipitation over the site during the Asian summermonsoon season significantly limits the total number of days ofuseful observations. (Usual date of onset of the summer monsoonover Trivandrum is 1 June.) Table 1 shows that the frequency ofoccurrence of dust layers is maximum during July (whenmoderate or strong dust layers occurred on all the days ofobservation during 2008 and 2009), which is followed by August(when the moderate or strong dust layers occurred during 70%and 64% of the days in 2008 and 2009). The frequency ofoccurrence of dust layer is minimum in June and September witha large variability from 2008 to 2009. Earlier satellite observationsshowed that, in general, the mineral dust plume over the ArabianSea maximizes during June–July period (e.g., Nair et al., 2005).However, the large frequency of occurrence of clouds over theeast Arabian Sea significantly impedes the number of cloud-freepixels observed by the satellites from which the AOD can beretrieved (e.g., Meenu et al., 2010). Notwithstanding this, thelargest values of MODIS-derived AOD (as shown latter in Fig. 8)over the southeastern Arabian Sea (near the coast of Trivandrum)during the present study period occur in June–July 2008 and July2009. In contrast, the AOD values over the Arabian Sea as a wholeand southeast Arabian Sea in particular are very small in June2009. This is in agreement with the MPL observations of thefrequency of occurrence of the dust layer, which shows thatthe dust layer is absent in June 2009 while strong dust layersare present in June 2008.

during June–September period of 2008 and 2009.

of

yer (%)

Frequency of occurrence of

weak/moderate aerosol layer (%)

Frequency of occurrence

of dust-free days (%)

22 33

0 0

20 30

60 40

16 84

50 0

7 36

17 83

Fig. 4. Monthly mean altitude profiles of aerosol backscatter coefficient (ßa, shown

in the left panel), linear depolarization ratio (dL, shown in the middle panel) and

normalized columnar contribution to AOD (NCC, shown in the right panel) during

June–September of 2008.

Fig. 5. Same as Fig. 3 but for 2009.

M.K. Mishra et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 1251–1259 1255

Altitude profiles of mean ba, dL, and NCC for the above threeclasses in June–September are presented in Figs. 4 and 5,respectively, for years 2008 and 2009. It is interesting to notethat the mean values of ba are significantly large in the altituderegion of 1–3.5 km when the dust layer is strong. Rapid decreasein ba above �3.5 km is particularly notable during days in whichthe dust layer is strong. The average value of ba in the altituderegion of 1–3.5 km is maximum during July and August. In thealtitude band of 0–3.5 km, the mean aerosol scale height (altitudeextent over which ba decreases to e-1 of its value at the basealtitude) is 1.470.1 km during the strong-dust days while itsvalue is �1 km during the other days.

The mean value of dL in the altitude region of 1–3.5 km duringthe occurrence of a weak/moderate or strong dust layer is foundto be significantly larger than that during the dust-free days. Thepeak value of dL often exceeds 0.15 during strong dust days. In allthe three classes, the value of dL is generally less than 0.05 below�1 km and above �4 km. This, together with the rapid decreaseof ba above 3.5 km clearly shows that the dust layer is fairlyconfined to an altitude region of 1–3.5 km. High value of ba andlow value of dL below 1 km suggests significant contribution ofnearly spherical aerosols in this region. The hygroscopic growth ofsea salt aerosols (with RH490% below 1 km) leads to theformation of nearly spherical aerosols, for which the value of dL

is relatively small. Note that, strong wet-removal of aerosolsprevailing during this period leads to a rapid decrease in aerosolconcentration (including dust) within the atmospheric boundarylayer (ABL). This process also might significantly contribute to theobserved decrease in ba in this region (o�1 km).

Altitude variation of NCC shows that the elevated aerosol layercontributes more than 50% of the AOD during the strong dustdays. The mean AOD on such days is significantly larger than that

during the other days. Weighting the altitude profiles of NCC withthe value of AOD on that day provide the mean layer integratedoptical depth of the dust layer between 1 and 3.5 km, which is 0.2.Note that, this value is comparable to the total column integratedAOD on the non-dusty days during this period. This showsthe importance of the elevated dust layer in modulating theAOD during the Asian summer monsoon season. Altitude profileof NCC shows that the contribution of aerosols above 1 km toAOD is rather small during September (�35%) compared to theJune–August period.

3.3. Sources of aerosols at different altitudes

As seen from the above analysis, the optical properties ofaerosols in the elevated layer can be different from those below.In order to identify the sources of these particles, we haveexamined the 7-day air back-trajectories at different altitudes inthe lower troposphere ending at this location for all the daysduring the study period using the NOAA ARL HYSPLIT modelobtained through their website (http://www.arl.noaa.gov/ready).

The air trajectories ending at 1 km over Trivandrum ondifferent days in June–September during 2008 and 2009 arepresented in Fig. 6 and those ending at 3 km in Fig. 7. In all thesemonths, most of the air trajectories ending at 1 km above thesurface originate from the Indian Ocean and the South ArabianSea and take a curve near the Horn of Africa (the Somalijet region) to reach Trivandrum. None of them originate fromthe northern side. However, in August and September, thesetrajectories mostly traverse the central Arabian Sea. It is quiteinteresting to note that significant number of trajectories endingat 3 km originate from Arabian landmass and traverse the westArabian Sea to reach Trivandrum. During this period the Arabian

Fig. 6. Air back-trajectories ending at 1 km over Trivandrum on all the days of the month during June–September 2008 and 2009.

Fig. 7. Same as Fig. 5, but for the air back-trajectories ending at 3 km over Trivandrum.

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landmass is heated to its maximum (boreal summer) and is also alocation of strong surface winds. Dust storm is very common overArabia during this period. Increase in convective activity is alsofavourable for the uplift of mineral dust originating from the aridsurface. On examining the air trajectories ending at 3 km (almostmiddle of the observed dust layer over Trivandrum) it can be seenthat in July, August and September significant fraction of thetrajectories have passed through the north of 101N including thewestern parts of Arabian Sea, the Horn of Africa or the Arabianregion. The back-trajectories between 2 and 4 km (not shownhere) are almost similar to that at 3 km. The back-trajectoriesending at o1 km (not shown here) are similar to those at 1 km(Fig. 6) but shows larger contribution of airmass from the southArabian Sea and Indian Ocean. This clearly shows that the airmass

below 1 km is distinctly different and originates from the pristineIndian Ocean region, while the airmass above contains significantcontribution from the west Arabian Sea or Arabia. Pronouncedincrease in dL around 1–3.5 km and the significant decrease in thevalue of dL (0.02–0.05) below 1 km are in agreement with theproposed transport derived from airmass trajectories. Althoughthe trajectories ending at 1 km traverse the central Arabian Seaduring August and September, the AOD over this region is lowercompared to that in July (as shown in Fig. 8).

The Potential of long-range transport in bringing mineral dustover Trivandrum is further examined using the regional distribu-tion of AOD (at 550 nm) observed by MODIS (Remer et al., 2008)during June–September period of 2008 and 2009. The spatialdistribution of AOD thus obtained over the Arabian Sea region is

Fig. 8. Monthly mean regional distribution of AOD over the Arabian Sea region during June–September months of 2008 and 2009. The location of Trivandrum is indicated

by a ‘+’ sign.

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depicted in Fig. 8. High values of AOD over the Arabian Sea, with itspeak in the northwestern parts, can be seen in all these months.This peak value and the southward extension of the region of highAOD increases from June to July and subsequently decreases. InSeptember, the region of high AOD shrinks significantly to remainfairly confined to the northwestern parts. Trivandrum is located inthe southern boundary of the region of this enhanced AOD. Dailymaps of AOD (e.g., Fig. 1) clearly depict the spatial extent the dustplume, which often engulf a large region up to Trivandrum.However, the strong influx of airmass from the Tropical IndianOcean over the Arabian Sea limits the southward extension of thedust plume to a great extent which leads to a large latitudegradient around 101N. Note that, the air back-trajectories around2–3 km (Fig. 7) are mostly oriented along 8–121N, which indicatethat the southward advection of the dust plume might be ratherweak. The frequency of occurrence of dust layers over Trivandrumis maximum in July (Table 1), when the southern boundary of thedust plume often reaches up to �81N (Fig. 8). Fig. 6 also shows thatthe back-trajectories ending at 1 km over Trivandrum duringAugust and September mostly traverse the central and northeastArabian Sea, where the value of AOD is relatively small in Augustcompared to July. Hence, the airmass from the central andnortheast Arabian Sea during August may be less efficient intransporting dust over to Trivandrum. This is further confirmed bythe low values of dL observed in the ABL (Figs. 4 and 5).

As can be seen from Fig. 7, the number of air trajectoriesoriginating from Arabia and ending at 3 km over Trivandrum is

significantly less in June and July 2009 compared to that in year2008. The relatively lower values of dL during June and July 2009indicate that the elevated dust layer is weaker compared to thatduring the corresponding periods of 2008. It is also interesting inthis context to note that the values of AOD over the east Arabian Seaduring June–July 2009 are also less compared to that in 2008. Thesefeatures lead to the inference that the observed difference in thealtitude structure of aerosols (and the dust layers) in these twoconsecutive years is mainly due to corresponding changes insynoptic circulation. The absence of strong dust days overTrivandrum in June 2009 also is consistent with the observed weaksouthward extension of the region of high AOD over the Arabian Sea.

Though the long-term satellite observations have revealed thetransport of dust over the Arabian Sea during the Asian summermonsoon season (e.g., Remer et al., 2008; Nair et al., 2005), theseinferences were based on a limited amount of data in each year asthe occurrence of clouds are very frequent over the easternArabian Sea (Nair et al., 2005; Meenu et al., 2010) during thisperiod. Strong sea surface winds (often exceeding 410 m/s)during this season generate abundant sea salt aerosols from thesea surface over the Arabian Sea, which also is added to thetransported Arabian dust. A thin layer of widespread cirrus overthe east Arabian Sea during this season (Rajeev et al., 2008)further complicates the discrimination of dust from satellite orin situ measured AOD. Note that both the dust and the seaspray aerosols are relatively large in size and they cannotbe discriminated based on AOD data alone. Observations using

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CALIPSO, however, have revealed the significant depolarizationproperty of aerosols over the Arabian Sea (Liu et al., 2008), whichcould mainly be attributed to the dust aerosols transported fromArabia, as the locally generated sea-salt aerosols tend to be nearlyspherical with low values of dL. However, due to the lowfrequency of observation by CALIPSO (orbit repeatability of 16days) and high frequency of occurrence of optically thick high-altitude clouds, the amount of aerosol data collected fromCALIPSO over the Arabian Sea during the summer monsoon isconsiderably small. The present dual polarization lidar observa-tions of aerosols over Trivandrum, during most of the days inwhich clear sky conditions prevailed at least up to 6 km, showthat the transport of dust over the east Arabian Sea and west coastof India occur mostly during the July–August period through thealtitude region of 1–3.5 km. This, however, is consistent with thesatellite observations which show that the southern boundary ofthe dust plume reaches only up to �81N during this period.Though the integrated extinction coefficient of the dust layer isrelatively small (�0.2 even when the dust layer is strong)compared to the AOD observed at the northwestern parts of theArabian Sea during July–August (mean AOD at 550 nm is �0.8over this region; see Fig. 8), it is comparable to the total AODobserved during the dust-free days over Trivandrum. Persistenttransport of pristine airmass from the Tropical Indian Ocean(with column AODo0.15 observed during the dust-free days)significantly impedes large scale spreading of the dust plume overto the southern Arabian Sea and Indian Ocean.

4. Conclusion

Earlier studies based on satellite data revealed that thenorthern part of the Arabian Sea is significantly influenced bythe large-scale transport of dust from the Arabian Desert duringthe Asian summer monsoon season (June–September). However,the high frequency of occurrence of dense clouds over the easternparts of the Arabian Sea during this season had considerablyreduced the frequency of observations of AOD using satellite andground-based radiometers. A first-time observation on thealtitude distribution of aerosol backscatter coefficient and lineardepolarization ratio (an indicator of the non-sphericity ofaerosols) over the west coast of Peninsular India (Trivandrum)during the Asian summer monsoon season (June–September) inthe two consecutive years, 2008 and 2009, is carried out using aMicro Pulse Lidar. These observations are further supported byregional distribution of MODIS AOD and HYSPLIT air back-trajectory analyses. The lidar observations revealed the existenceof an elevated layer of highly non-spherical aerosols in thealtitude region of 1–3.5 km (just above the ABL), which is verycommon during the July–August period. The appearance of thislayer starts in June, with the arrival of monsoon over the westcoast of India, and subsequently intensifies with the advancementof monsoon further northwards. This layer, which is mostprominent in July–August, starts weakening by September. Theobserved increase in dL within the elevated layer suggests that theparticles constituting this layer are relatively non-sphericalcompared to those above and below. This elevated layercontributes 455% of the column integrated backscatter coeffi-cient and AOD during the strong dust days. The optical depth ofthis layer is �0.20 which is similar to the total columnar AODobserved during dust-free days. Low values of dL below this layer(within ABL) indicate that the particles in this altitude region arenearly spherical in nature.

The day-to-day and monthly variations observed in thesatellite derived AOD over the southeast Arabian Sea are inagreement with those of the altitude structure of lidar-derived

aerosol backscatter coefficient at Trivandrum. Satellite observa-tions of the regional distribution of AOD show a pronouncedlatitude gradient across 8–121N, which encompasses the presentobservation site. This leads to large day-to-day variability in thestrength of the dust layer at this location, depending strongly onthe prevailing lower-tropospheric circulation. The back trajectoryanalysis shows that, over Trivandrum, while the advection ofairmass from the pristine Indian Ocean dominates below 1 km,the airmass around 3 km (the peak altitude of the dust layer) has asignificant contribution from the dust-laden air from the Arabianlandmass and the western Arabian Sea. These observations alongwith the values of dL at different altitudes unambiguously showthat the elevated layer composed of highly non-sphericalparticles, formed as a consequence of a long-range transport ofmineral dust from the arid regions of Arabia, while those below1 km are mostly produced at the sea surface.

Acknowledgement

The authors gratefully acknowledge the NOAA Air ResourcesLaboratory (ARL) for the provision of the HYSPLIT transport anddispersion model and READY website (http://www.arl.noaa.gov/ready.html) used in this publication. The authors would like toacknowledge the MODIS Science Team for the Science Algorithms,the Processing Team for producing MODIS data, and the GESDAAC tools for accessing Aqua MODIS data. One of the authors,Dr. K. Parameswaran would like to acknowledge CSIR forproviding grant through the Emeritus Scientist scheme.

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