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Atmospheric Environment 40 (2006) 6494–6503

www.elsevier.com/locate/atmosenv

A study of aerosol optical depth in the central Indian region(17.3–8.61N) during ISRO-GBP field campaign

Sachchidanand Singh�, Bhupender Singh, B.S. Gera, Manoj K. Srivastava,H.N. Dutta, S.C. Garg, Risal Singh

National Physical Laboratory, New Delhi, India

Received 30 July 2005; received in revised form 24 January 2006; accepted 24 January 2006

Abstract

We present the ground-based, clear-sky daytime measurements of column aerosol optical depths (AODs) at different

wavelengths and the mixing height of atmospheric boundary layers at various locations, conducted during ISRO-GBP

aerosol-radiation-trace gases measurements campaign in the central Indian region (17.3–28.61N and 77.21–78.21E) from

Delhi to Hyderabad and back, in the month of February 2004. The measurements show AOD variation at 500 nm in the

range of 0.2–0.5 and Angstrom exponent in the range of 1.0–1.4 throughout the region. The entire experimental region has

been classified into four categories depending upon their locations and surroundings, viz., forest, rural, semi-urban, and

rural-dusty. The volume size distribution of the aerosol particles at all these categories showed a bi-modal distribution with

fine mode dominating around 0.23mm effective radius and the coarse mode dominating around 1mm. The forest site

showed minimum AOD and a values with an equal contribution of aerosol particles in the fine and coarse mode. As

the fine-mode particle concentration relatively increased at other regions, the a value also went on increasing, along with

the increase in AOD. The AOD also showed a latitudinal variation with a minimum occurring at about 23.51N. During the

whole campaign, the average mixing height of planetary boundary layer (PBL) during the daytime was found to be

between 650 and 950m.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: AOD; Central India; Size distribution; MICROTOPS; SODAR

0. Introduction

Atmospheric aerosols and trace gases playimportant roles in the regional as well as globalradiation budget and forcing (IPCC, 2001 and thereferences therein). The basic parameter thatdetermines the aerosol loading in the atmosphereis the column aerosol optical depth (AOD). Apartfrom its varied physico-chemical characteristics, the

e front matter r 2006 Elsevier Ltd. All rights reserved

mosenv.2006.01.033

ing author.

atmospheric aerosols also have a large spatial aswell as temporal variability. Although satellitemeasurements provide large spatial coverage andlong-term data on aerosol and trace gas measure-ments, their accuracies are limited due to theretrieval and other processes (Vachon et al., 2004).On the other hand, the ground-based fixed measure-ments provide more accurate measurements butlimit the spatial coverage. The ground-based mea-surements in a mobile experimental mode, particu-larly in remote areas where no fixed measurement

.

ARTICLE IN PRESSS. Singh et al. / Atmospheric Environment 40 (2006) 6494–6503 6495

stations are available, can give valuable data withlarge spatial coverage. National Physical Labora-tory (NPL), New Delhi participated in one suchexperimental campaign under the ISRO-GBP aero-sol-radiation-trace gases measurements during 31January–1 March 2004 along the Delhi–Hydera-bad–Delhi corridor route covering nearly 3200 kmin the remote areas of central India. It covered thelatitudinal range of 17.3–28.61N and the longitude77.21–78.21E. Fig. 1 shows the stations en-route themeasurements were taken.

The month of February was particularly suitablefor such measurements due to availability of clearsky conditions, the stable boundary layer duringdaytime, and more or less similar meteorologicalconditions throughout the central India (region ofexperiment). The average meteorological conditionsreveal a more or less calm and quite atmosphere atthe surface throughout the campaign. The surfacewind speed was observed to be below 4m s�1 andrelative humidity less than 50% at almost all themeasurement sites. The visual observations of cloudconditions were reconfirmed during the presentanalysis using the MODIS satellite pictures, whichalso show more or less clear sky conditionsthroughout the observation month along the regionof experiment.

In this paper, we present the study of columnAOD and its temporal and spatial variationsthroughout the experimental campaign. The ob-servations show appreciable variation of AODalong the latitude (17.3–28.61N). It shows a gradualdecrease from Delhi at latitude 28.61N to reach aminimum at around 23.51N near Jabalpur and thenagain an increase in AOD with latitude. Further, thebehavior of AOD with respect to the mixing heightof the atmospheric boundary layer (ABL) at various

Onward JourneyReturn Journey

888072

20

28

72 80 88 96

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DELHI

Fig. 1. Site map of the ISRO-GBP campaign route between

Delhi and Hyderabad.

study sites is also studied. The ABL information isobtained using a mono-static SODAR system. Theobservation shows a moderate average height ofboundary layer during the day (10:00–16:00 h) atalmost all the observation sites and varied between650 and 950m. At few sites, however, the maximumheight of ABL reached more than a kilometer.Generally the AOD variation is found to beindependent of the mixing height of ABL. Thevolume size distributions of aerosols were generallybi-modal with a minimum Angstrom exponent aoccurring at the forest location of Tikaria nearJabalpur and the maximum at the Shaadnagar nearHyderabad. In addition, the contours of windspeeds at different altitudes were also studied indetails along with the trajectory analysis (backwardand forward) at several stations to understand theaerosol behavior at different locations during thecampaign experiment.

1. Experimental details

The campaign experiment started on 1 February2004 from Delhi in a mobile mode. The list ofstations at which experiments were conducted isgiven in Table 1. It may be noticed that theobservation covered the latitude range of about111 going through various ecosystems like urban,semi-urban, rural, and forest within a time span ofabout a month. For the present study, the AODmeasurements were taken using the Solar Lightportable spectrometer MICROTOPS II instrumentsat 340, 500, 675, 870, and 1020 nm. AnotherMICROTOPS II instrument with 305, 312, 320,936, and 1020 nm channel was used for column-integrated ozone measurements in DU and theintegrated column water vapor measurements incm. The instruments were calibrated at Solar LightCompany, USA, and the measurements were takenwithin a year of the calibration. The accuracy ofwavelength channels of this instrument is 70.3 nmin the UV range and 71.3 nm in the visible andnear-infrared range. The full-width at half-max-imum (FWHM) bandwidth for UV channels is2.470.4 nm and for other channels it is 1071.5 nm.More details about these MICROTOPS instrumentsare described elsewhere (Morys et al., 2001).

The determination of mixing height of the ABLwas done at each site along the study route using amono-static SODAR system developed at NPL,New Delhi. The system is capable of real-timemonitoring of the ABL up to a height range of

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Table 1

Measurement sites during ISRO-GBP field campaign

Sl. no. Date Place Latitude (1N) Longitude (1E) Category

1 1 February 2004 CRRI, Delhi 28.583 77.020 —

2 2 February 2004 Kosi Kalan 27.824 77.414 R

3 3 February 2004 Garrhi Nohbar 27.027 77.969 R

4 4 February 2004 Orcha 25.297 78.624 F

5 5 February 2004 Orcha 25.297 78.624 F

6 6 February 2004 Satravasa 24.575 78.471 R

7 7 February 2004 Sanuda 23.88 78.905 F

8 8 February 2004 Murgakheda 23.103 79.25 F

9 9 February 2004 Tikaria 23.367 80.072 F

10 11 February 2004 Kurari 21.771 79.519 R

11 13 February 2004 Bhivapur 20.402 78.742 RD

12 14 February 2004 Bussapur 18.95 78.377 RD

13 16 February 2004 Shaadnagar 17.028 78.189 SU

14 17 February 2004 Shaadnagar 17.028 78.189 SU

15 18 February 2004 Shaadnagar 17.028 78.189 SU

16 19 February 2004 Shaadnagar 17.028 78.189 SU

17 20 February 2004 Shaadnagar 17.028 78.189 SU

18 21 February 2004 Shaadnagar 17.028 78.189 SU

19 22 February 2004 Ramampeth 18.11 78.448 R

20 23 February 2004 NEERI 20.091 78.562 SU

21 24 February 2004 NEERI 21.121 79.071 SU

22 25 February 2004 Jhiria Tola 21.901 79.528 R

23 26 February 2004 Mehar 24.293 80.769 F

24 27 February 2004 Khajuraho 24.878 79.934 SU

25 29 February 2004 Dholpur 26.672 77.897 RD

R–rural, F–forest, RD–rural-dusty, SU–semi-urban.

S. Singh et al. / Atmospheric Environment 40 (2006) 6494–65036496

about 1 km with a resolution of 1m. High-power(10 acoustic watts) acoustic bursts of 100ms istransmitted vertically at a pulse repetition frequencyof 15 pulse/min using a SODAR antenna consistingof a parabolic disc with a transducer placed at itsfocus. The backscattered acoustic waves from theturbulent atmospheric region along the path ofacoustic wave are received using the same antenna.The signals are processed to produce onlinefacsimile display of the dynamics of ABL thermalstructures. The echogram structural details arefurther used to derive the mixing height of ABL(Singal et al., 1994).

2. Observations and results

2.1. Spatial variation of AOD along campaign route

Fig. 2 shows the average latitudinal variations ofmeasured quantities—column AOD at 500 nm,ozone and water vapor along the campaign pathat all the observation sites. The vertical bars indicatethe standard deviations at the daily daytime meanvalues. The onward (Delhi–Hyderabad) journey

during the campaign is shown by filled rectangleswhereas open rectangles denote the backward(Hyderabad–Delhi) trip. A dip in AOD values near23.51N latitude can be noticed during both, theonward as well as backward trips. The AOD valuesare further compared with the MODIS satellitederived values, when the satellite passes are directlyoverhead the observational sites. The values aregenerally comparable except at a few occasionswhen satellite AOD values are slightly less than themeasured values. Similarly, the column ozone andthe water vapor also show a dip around the samelocation (23.51N). However, it is more pronouncedin the forward journey. As the value of AODdepends on several factors like, production of localaerosols, transport of aerosols by winds, andecological factors including surface properties, thereason for this decrease should be ascertainedcarefully. In order to check these effects, we dividedthe entire experimental region into four ecologicalsub-categories depending upon their locations andsurroundings. These categories are semi-urban,rural-dusty, rural, and forest. The AOD variationsfor these categories of observation sites are shown in

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Fig. 2. Average AOD at 500nm (bottom), ozone (center), and water vapor (top) at different sites during the ISRO-GBP campaign during

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Fig. 3. Average AOD categorized under four different cate-

gories—semi-urban (SU), rural-dusty (RD), rural (R), and forest

(F).

S. Singh et al. / Atmospheric Environment 40 (2006) 6494–6503 6497

Fig. 3. The highest value of AOD (at 500 nm)averaging about 0.49 is observed at rural-dustylocations due to their proximity to desert and semi-arid areas. This is followed by the semi-urbanlocations where the average AOD value is of theorder of 0.40. The value of AOD is reasonably lowin the rural locations with average value of 0.26.The lowest value of AOD is however observed inand around the forest locations, where averagevalue is only 0.19.

Apart from the ecological and local effects, thecolumn aerosols are also supposed to be affected bythe prevailing wind patterns, particularly in thelower troposphere. As the surface winds at almostall the experimental sites were more or less same andbelow 4m s�1, we tried to see if there are appreciablevariations at higher altitudes. In order to see this,

ARTICLE IN PRESSS. Singh et al. / Atmospheric Environment 40 (2006) 6494–65036498

the variation of wind components (U and V winds)as a function of latitude in the central Indian regionfor each day was examined during the month of

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Fig. 4. Latitudinal variation of zonal wind U (left) and meridional wind

Dots denote the observational locations on different days. (a) MODIS

grid for February 2004. Dots denote the observation points during on

February 2004 (observation period). The latitudinalvariations of daily zonal (U) and meridional (V)winds are shown in Fig. 4 for the 77.51 longitude

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Terra monthly average aerosol optical depth obtained at 11� 11

ward (black) and backward (gray) campaign trips.

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Tikaria (F) Kurari (R)Bussapur (RD) Shaadnagar (SU)

Time, hrs

Fig. 5. Daytime temporal variation of AOD at 500 nm at the four representative sites under different categories described in Fig. 4.

0

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Tikaria (1.02) FKurari (1.30) RBussapur (1.39) RDShaadnagar (1.27) SU

Wavelength, nm

Fig. 6. Spectral variation of the aerosol optical depth at the four

representative sites (same sites as in Fig. 5).

S. Singh et al. / Atmospheric Environment 40 (2006) 6494–6503 6499

and at 850-mbar level. The contours were obtainedfrom NOAA-CIRES Climate Diagnostics Centerfrom their website http://www.cdc.noaa.gov/docs/cite.html. Figures show that in the central Indianregion, although there are variable wind patternsduring the month of February at higher altitudes nodefinite conclusion on the removal of aerosols dueto wind can be derived at. Hence the AODminimum indicated around 23.51N latitude cannotbe explained on this basis alone. Moreover, thisminimum AOD seems to be a normal feature and isalso reflected in the monthly average AOD pictureobtained from the MODIS satellite observationshown in Fig. 4a.

2.2. Temporal variation of AOD and ABL

Apart from spatial variations along latitudes,AOD measurements also show temporal variationsat different locations. The daytime variations ofAODs at four locations representing the four typesdescribed above are plotted in Fig. 5. The maximumaerosol is observed at rural-dusty site and minimumat the forest location. Further, both, the forest andrural sites, represented by Tikaria and Kurari,respectively show minimum temporal variation inthe AOD throughout the day. On the other hand,the rural-dusty site Bussapur and semi-urban siteShaadnagar both show an increase in AOD in theafternoon. This increase may be attributed to thelocal aerosol generation during the day.

The spectral variations of average aerosols forthese locations are plotted in Fig. 6. The variationsare more or less similar except at the forest locationof Tikaria, where the AODs are minimum. Thenumbers in the brackets (Fig. 6) denote theAngstrom wavelength exponent a. It is estimated

by a least-squares fit on a log–log plot scale of theexperimental AOD versus wavelength measure-ments (as the measured AOD spectra follow theusual Angstrom power law, t(l) ¼ bl�a) andindicates the aerosol size distribution. In the presentcase, a are estimated using AOD measurements at340, 500, 675, 870, and 1020 nm. The value of a isminimum at the forest location and maximum at therural-dusty site. The representative values for thefour sites, i.e., forest, rural, rural-dusty, and semi-urban are 1.02, 1.30, 1.39, and 1.27, respectively. Ahigh value of a generally indicates an abundance ofsmaller-sized particles in the atmosphere.

In order to see the columnar aerosol sizedistribution of the particles at the four category ofsites, the spectral distribution of columnar AODs ateach site were put to constrained linear inversionfollowing the method of King et al. (1978) and King(1982). The radiation inversion program developedby Michael King was obtained from the websitehttp://ltpwww.gsfc.nasa.gov/crg and converted togive volume size distribution. A moderate refractiveindex value of 1.54–0.002i was used for theinversion calculation. This value lies in the range

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Kurari (R)Bussapur (RD)Shadnagar (SU)

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m3 /

µm)

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Fig. 7. Column aerosol volume size distribution obtained by inversion of AOD spectrum (Fig. 6).

S. Singh et al. / Atmospheric Environment 40 (2006) 6494–65036500

of values obtained for the urban and mixed aerosoltypes (Dubovik et al., 2002). Fig. 7 shows thecolumn volume size distribution of the aerosols forthe four representative sites described above. Gen-erally at all these sites, the distribution is bi-modalwith fine mode dominating around 0.24 mm radiusand the coarse mode dominating around 1 mmradius. A clear dominance of fine-mode particlesover the coarse mode can be noticed at all the sitesexcept at the forest site of Tikaria, where both, thefine and coarse mode are more or less of similarmagnitude. It is because of this reason that theAngstrom exponent a is minimum (�1) at Tikaria.As the relative contribution of fine-mode particlesincrease at other locations, the Angstrom parametera also increases along with the increased AODvalues.

As the height profile of aerosol concentration,particularly black carbon (BC) shows rapid decreasewithin ABL (Moorthy et al., 2004), a comparison ofAOD measurements and the ABL height has alsobeen done at different locations during this cam-paign experiment. Fig. 8 shows such a comparisonof column AOD at 500 nm and ABL mixing heightfor four representative locations during the daytime.At forest site, the mixing height varies between 646and 943m during the day time when AOD variesfrom 0.11 to 0.17. At the rural site, a small variationin mixing height from 731 to 816m corresponds to asmall variation in AOD from 0.28 to 0.35. Similarly,at the rural-dusty site also, a very small change inAOD (0.46–0.52) is observed when planetaryboundary layer (PBL) changes from 731 to 858m.At the semi-urban site, however, a small change in

mixing height (731–858m) is coupled with compara-tively large change in AOD from 0.28 to 0.44.

3. Discussions and conclusions

The month long road campaign experiment fromDelhi to Hyderabad and back in the central Indianregion gives important insight into the aerosolproperties and behavior of the region. The stablesurface wind (o 4m s�1), small relative humidity(o50%), and more or less clear sky conditions(o 10%) throughout the month of observation(February 2004) provided excellent conditions forAOD and boundary layer measurements. It appearsthat the ozone variation somehow correlates withthe AOD variation showing a minimum at 23.51N.However, from our limited observations, it isdifficult to ascertain whether it is due to aerosolsor because of latitudinal character of ozone varia-tion that coincides with the forest location ofJabalpur where AOD is minimum. The water vaporincreases as we go from north to south during thecampaign. This is because we are approachingequator and hence an increase in the ambienttemperature and humidity is expected.

The average AOD at 500 nm for the whole regionof experiment (17.3–28.61N and 77.2–78.21E)spread over the whole month of February 2004had a value of 0.32 with a standard deviation of0.12. Large standard deviation value is because ofthe fact that it covers different types of environmentand the ecosystem. Four such types categorized hereare semi-urban, rural-dusty, rural, and forest. TheAOD at 500 nm in these categories averaged

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Feb 17 2004, Semi-Urban siteFeb 14 2004, Rural-Dusty site

ABL

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AOD

Fig. 8. Variation of AOD at 500nm with the mixing height of atmospheric boundary layer during daytime at four representative sites.

S. Singh et al. / Atmospheric Environment 40 (2006) 6494–6503 6501

0.4070.06, 0.4970.04, 0.2670.03, and 0.1970.05,respectively. The values are well within the range ofmeasurements observed in the similar surroundingselsewhere (Dubovik et al., 2002). However, thesevalues are less compared to those in the Indo-Gangetic plain region (Singh et al., 2004; Dey et al.,2004) and higher than the coastal and deep-seaAOD values in Indian Ocean (Jayaraman et al.,1998; Moorthy et al., 2001; Ramachandran andJayaraman, 2003).

The general characteristics of AOD representedby the Angstrom exponent a in all the categoriesshows an abundance of smaller-sized particles, exceptat the forest location, where an equal contributionfrom fine and coarse particles is indicated by a beingclose to unity. The minimum a value (1.02) at forestlocation is also associated with the minimum ofAOD values (0.1970.05). The moderate-to-low avalue coupled with the low AOD may be associatedwith forest and other biogenic aerosols (Holbenet al., 2001). The a value gradually increases at otherlocations and reaches a maximum at the rural-dustysite. As we know that an increase in a value is causedeither by increase in number of smaller particlesor by a decrease in the number of bigger-sizedparticles. The increase in AOD associated with an

increase in a value at these locations indicates anincrease in the concentration of fine particles.This behavior is further confirmed by the columnvolume aerosol size distribution obtained fromthe inversion of AOD spectral variation (Fig. 7).It clearly shows a strong bi-modal aerosol distribu-tion at all the regions. The forest site clearlyshows equal contributions from the fine- andcoarse-mode aerosol particles. As the contributionof fine-mode particles gradually increases at otherlocations, the Angstrom exponent a also increases(Fig. 6). The increase in AOD and the Angstromexponent are consistent with the retrieved aerosolvolume size distribution.

In order to access the influence of transport onthe aerosol concentration, back-trajectory analysiswere done at some of these representative sites atdifferent altitudes and for varying time durationsusing Hybrid Single-Particle Lagrangian IntegratedTrajectory (HYSPLIT) model (Version 4) devel-oped by Draxler and Rolph (2003) at Air ResourcesLaboratory, NOAA. Although the aerosols aresupposed to have a residence time of 5–7 days, wehave considered only 48 h back-trajectory due totwo main reasons—one that in the present mobilecampaign we have only 1 day of observation at a

ARTICLE IN PRESSS. Singh et al. / Atmospheric Environment 40 (2006) 6494–65036502

particular location (except at Shaadnagar), andsecondly the error in the trajectory increases forlonger time periods and will become too large foranalysis of more than 72 h. Further, even the short-term (2-day) transport can influence the ambientaerosols, especially when no source region (majorindustry etc.) is present nearby. Figs. 9a and brepresent such 48-h back-trajectories ending atTikaria (a forest location site with low AOD and

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February 2004.

low a value) and Shaadnagar (a high AOD and higha site) at 06UTC at three altitudes at 0.5, 1.0 and3.0 km above ground level on 9 and 19 February,respectively.

The backward trajectories ending at Tikaria on 9February 2004 shows that for the last 48 h, theincoming winds are generally above the mixinglayer, showing that the particles are traveling fromcleaner and vertically higher altitudes. This gave riseto the condition of low AOD over Tikaria. On theother hand, for the case of Shadnagar, where theAOD is comparatively high and the Angstromexponent is also high, the wind is reaching throughthe coastal area and inside the mixing layer travelingthrough the land mass. Travel at lower altitude inthe land mass causes the inclusion of boundary layeraerosols in the air mass that increases the AOD. Theincrease in polluted fine particles, due to this, causesan increase in the Angstrom exponent. Similarbehavior is also manifested in the size distributionof the aerosol at this site.

Finally, the impact of ABL on the overalldistribution of AOD for the whole region is foundto be more or less negligible. The average mixingheight is nearly 750–950m during the daytimethroughout the region, while the AOD variation isquite appreciable. This indicated that the localcharacteristics of aerosols and the aerosol concen-tration above boundary layer both have a role indetermining the value of column AOD. This isfurther strengthened from the fact that, at indivi-dual locations, the AOD variation throughout theday is generally found to be independent of theboundary layer height (Fig. 8). Due to the verynature of this campaign and lack of data, it is notpossible to quantify the contribution of boundarylayer aerosol in the total AOD measured at differentlocations, but still the contribution may be sig-nificant. Further, there may be some effect ofvariation in the ground surface altitude on AODmeasurements and the PBL. Although the variationin surface altitude during the whole campaign(difference of maximum and minimum) is about450m, the effect has not been taken into considera-tion in the present study.

From the results and discussions presented above,we may conclude that during the month ofFebruary 2004, the average AOD in the centralIndian region (17.3–28.61N) is generally below 0.5at 500 nm. The Angstrom exponent a that deter-mines the AOD is moderate and lies in the range1.0–1.4. A close-to-unity a value associated with low

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AOD (o0.2 at 500 nm) near Jabalpur suggests theimpact of forest surroundings on aerosols. The sizedistribution at this location shows an equal con-tribution of fine- and coarse-mode particles. Atother more polluted locations, both the fine- andcoarse-mode concentration of aerosol particlesincreases but the relative contribution of fine-modeparticles increases substantially, causing an increasein the Angstrom exponent. Some of these fine-modeparticles might have been brought by transport ofair within the boundary layer. The regional (latitu-dinal) variation in the column AOD may be acombined influence of the type of ecological regionand the transport within the ABL.

Acknowledgments

We acknowledge the GES-DISC InteractiveOnline Visualization and Analysis Infrastructure(Giovanni), the NASA’s Goddard Earth Sciences(GES) Data and Information Services Center(DISC) for MODIS data and images, and NOAAAir Resources Laboratory (ARL) for the provisionof the HYSPLIT transport and dispersion model.One of the authors (MKS) is thankful to CSIR forsenior research associateship.

References

Dey, S., Tripathi, S.N., Singh, R.P., Holben, B.N., 2004.

Influence of dust storms on the aerosol optical properties

over the Indo-Gangetic basin. Journal of Geophysical

Research 109, D20211.

Draxler, R.R., Rolph, G.D., 2003. HYSPLIT (HYbrid Single-

Particle Lagrangian Integrated Trajectory) Model access via

NOAA ARL READY Website (http://www.arl.noaa.gov/

ready/hysplit4.html). NOAA Air Resources Laboratory,

Silver Spring, MD.

Dubovik, O., Holben, B., Eck, T.F., Smirnov, A., Kaufman, Y.J.,

King, M.D., Tanre, D., Slutsker, I., 2002. Variability of

absorption and optical properties of key aerosol types

observed in worldwide locations. Journal of Atmospheric

Sciences 59, 590–608.

Holben, B.N., Tanre, D., Smirnov, A., Eck, T.F., Slutsker, I.,

Abuhassan, N., Newcomb, W.W., Schafer, J., Chatenet, B.,

Lavenue, F., Kaufman, Y.J., Vande Castle, J., Setzer, A.,

Markham, B., Clark, D., Frouin, R., Halthore, R., Karnieli,

A., O’Neill, N.T., Pietras, C., Pinker, R.T., Voss, K., Zibordi,

G., 2001. An emerging ground-based aerosol climatology:

aerosol optical depth from AERONET. Journal of Geophy-

sical Research 106, 12,067–12,097.

International Panel on Climate Change, 2001. The Scientific

Basis. Cambridge University Press, New York.

Jayaraman, A., Lubin, D., Ramachandran, S., Ramanathan, V.,

Woodbridge, E., Collins, W.D., Zalpuri, K.S., 1998. Direct

observations of aerosol forcing over the tropical Indian Ocean

during the January–February 1996 pre-INDOEX cruise.

Journal of Geophysical Research 103, 13,827–13,836.

King, M.D., 1982. Sensitivity of constrained linear inversions to

the selection of the Lagrange multiplier. Journal of Atmo-

spheric Sciences 39, 1356–1369.

King, M.D., Byrne, D.M., Herman, B.M., Reagan, J.A., 1978.

Aerosol size distributions obtained by inversion of spectral

optical depth measurements. Journal of Atmospheric Sciences

35, 2153–2167.

Moorthy, K.K., Saha, A., Prasad, B.S.N., Niranjan, K., Jhurry,

D., Pillai, P.S., 2001. Aerosol optical depths over peninsular

India and adjoining oceans during INDOEX campaign:

spatial, temporal, and spectral characteristics. Journal of

Geophysical Research 106, 28,539–28,554.

Moorthy, K.K., Babu, S.S., Sunilkumar, S.V., Gupta, P.K.,

Gera, B.S., 2004. Altitude profiles of aerosol BC, derived

from aircraft measurements over an inland urban location in

India. Geophysical Research Letters 31, L22103.

Morys, M., Mims III, F.M., Hagerup, S., Anderson, S.E., Baker,

A., Kia, J., Walkup, T., 2001. Design, calibration, and

performance of Microtops II handheld ozone monitor and

sun photometer. Journal of Geophysical Research 106,

14,573–14,582.

Ramachandran, S., Jayaraman, A., 2003. Spectral aerosol optical

depths over Bay of Bengal and Chennai: I—measurements.

Atmospheric Environment 37, 1941–1949.

Singal, S.P., Gera, B.S., Pahwa, D.R., 1994. Application of sodar

to air pollution meteorology. International Journal of Remote

Sensing 15, 427–441.

Singh, R.P., Dey, S., Tripathi, S.N., Tare, V., Holben, B., 2004.

Variability of aerosol parameters over Kanpur, northern

India. Journal of Geophysical Research 109, D23206.

Vachon, F., Royer, A., Aube, M., Toubbe, B., O’Neill, N.T.,

Teillet, P.M., 2004. Remote sensing of aerosols over North

American land surfaces from POLDER and MODIS

measurements. Atmospheric Environment 38, 3501–3515.