IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 6, NO. 4, OCTOBER 2009 807

LIDAR Observations of Aerosol Properties OverTropical Urban Region—A Case Study During a

Low-Pressure System Over Bay of BengalAnu Rani Sharma, Shailesh Kumar Kharol, and K. V. S. Badarinath

Abstract—Variations in aerosol properties and ground-reachingsolar irradiance over a tropical urban environment in Hyderabad,India, associated with a low-pressure system during December 3–10, 2008, over Bay of Bengal (BoB) were analyzed. Considerablevariations in aerosol properties and ground-reaching solar irra-diance due to changes in wind velocity and direction associatedwith the low-pressure system formed over southeast BoB wereobserved. Terra/Aqua Moderate Resolution Imaging Spectrora-diometer AOD550 variations showed trends matching with groundobservations. Nighttime Light Detection and Ranging (LIDAR)observations suggested considerable reduction in atmosphericparticulate matter (PM) loading under the influence of low-pressure system. Results of the study have implications for mon-itoring urban air quality as synoptic weather systems are capableof modifying the atmospheric PM loading.

Index Terms—Aerosol optical depth (AOD), light detection andranging (lidar), PM2.5, solar irradiance, UVery.

I. INTRODUCTION

U RBAN areas were considered to be a major source ofatmospheric pollution due to population growth, migra-

tion, increasing industrialization, and energy use, particularlyin developing countries [8]. Physical and chemical propertiesof aerosols in urban environments are different from those inremote rural areas [18]. The short lifetime of aerosols underthe influence of meteorological parameters, particularly relativehumidity [9], makes the aerosol characterization and modelinga real challenge [24]. Air pollutants in urban areas containa complex mixture of small and large particles of varyingorigin and chemical composition and secondary pollutants fromatmospheric chemical processes. The air quality in urban areasis governed by temporal distribution of emissions from variousactivities in the city, the topography, and the weather, includingatmospheric circulation patterns in the region [10].

The extensive coastal belt of India is very vulnerable to low-pressure systems in the Bay of Bengal (BoB) or the ArabianSea. Most importantly, the formation of a low-pressure systemin the ocean is one of the most prominent weather systemscharacterized by high atmospheric pressure gradients and wind

Manuscript received April 1, 2009; revised April 30, 2009 and May 19, 2009.First published August 18, 2009; current version published October 14, 2009.This work was supported by the Geosphere Biosphere Programme, IndianSpace Research Organisation (ISRO-BGP).

The authors are with the Atmospheric Science Section, National RemoteSensing Centre, Department of Space, Government of India, Hyderabad500 625, India (e-mail: bhardwaj_anu2003@yahoo.co.in; shaileshan2000@yahoo.co.in; badrinath_kvs@nrsc.gov.in).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LGRS.2009.2025781

speeds. Physical characteristics of these atmospheric phenom-ena are usually obtained using remote-sensing techniques [3],[13]. In this letter, we report on ground- and satellite-based ob-servations of aerosol loading over an urban region in Hyderabadand its linkages with synoptic weather systems over BoB duringDecember 3–10, 2008.

II. STUDY AREA

The study area in Hyderabad is located between 17◦10′ and17◦50′ N latitude and 78◦10′ and 78◦50′ E longitude, in thesoutheastern part of the Indian region, 300 km from the BoB.Hyderabad is the fifth largest city in India; its population is5 751 780 according to the census of 2001, a purely urbanizedarea. The climate of the region is semiarid with a total rainfallamount of ∼700 mm occurring mostly during the monsoonseason in the period June–October. The climatology of thearea experiences four dominant seasons each year: winter(December–February), premonsoon (March–May), monsoon(June–August), and postmonsoon (September–November).Measurements under clear-sky conditions for the case studywere carried out in the premises of the National RemoteSensing Centre (NRSC) campus at Balanagar (17◦.28′ N and78◦.26′ E) located well within the urban center.

III. DATA SETS AND METHODOLOGY

A. Ground Measurements

Synchronous measurements of the aerosol optical depth(AOD) were carried out using a handheld multichannelmicroprocessor-controlled total-ozone portable sun photometer(MICROTOPS II) instrument in the premises of the NRSC cam-pus located at Balanagar. It performs measurements at six spec-tral bands centered on 380, 440, 500, 675, 870, and 1020 nm.Great care was taken to avoid any error in MICROTOPS IIsun targeting by mounting the instrument on a tripod stand.The details about the design, calibration, and performanceof MICROTOPS II have been described elsewhere [2], [21].Along with the daytime measurements of AOD500, continuousmeasurements of the vertical profile of aerosols and planetaryboundary layer were carried out using a portable micropulselidar (MPL) system at 532 nm fabricated by National At-mospheric Research Laboratory, Gadanki, India, which is basedon MPL technology [5], [6], [17], [25]. The system employsa diode-pumped Nd:YAG laser with second harmonic outputat 532 nm and operated at 2500-Hz repetition rate and 10-μJoutput pulse energy. The emitter beam is coaxial to the receiverfield of view (FOV) and operated in the zenith direction. The

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808 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 6, NO. 4, OCTOBER 2009

lidar receiver employs a 150-mm Cassegrain telescope and ahigh-gain photomultiplier tube operating in photon countingmode. The complete overlap between the laser beam and thetelescope FOV is expected at 150 m. This value representsthe lower limit of our vertical lidar profile. The backscatteredsignals are measured with a bin width of 200 ns correspondingto the altitude range of 30 m. A computer-based multichannelanalyzer was employed for recording the photon returns. Theraw data profile was integrated every 2-min interval, and thiscorresponds to three-lakh-shots integrated photon-count profile.The lidar measures the height profile of the backscattered signalfrom aerosols, during clear sky, which are converted into abackscattering ratio (R). The aerosol backscattering ratio R isdefined as

R =(βa + βm)

βm(1)

where βa and βm are the backscatter coefficients of aerosoland molecules, respectively. The aerosol backscattering ratiofrom the lidar return was estimated using the inversion methodsuggested in the literature [16]. The extinction coefficient “k”and the optical thickness τ532 were estimated from the lidar datausing

k = Sa ∗ βa (2)

τ532 =∫

k(z)dz (3)

where z is the range between the laser source and the target(∼7 km). Sa is the lidar ratio or the extinction-to-backscatteringratio taken as 50 sr.

An ultraviolet (UV)-B radiometer from Solar Light Company[11] was used to measure UVery in the range 280–315 nm.The cosine response of the instrument is 75% with a resolu-tion of 0.01 MEDh−1 [1]. Continuous measurements of theparticulate-matter (PM) size distribution were performed withGRIMM aerosol spectrometer model 1-108 [19]. The GRIMMinstrument works on the principle of counting the number ofparticles as it crosses a focused laser beam. Ambient air isdrawn into the instrument by a mass-flow-controlled pumpand passed through a light beam produced by a laser diode.Scattering induced by particles of various sizes is measured by aphotodiode detector, amplified, and, finally, binned to give thedistribution of PM in 15 different grain-size classes from 0.3to 20 μm. The instrument is capable of counting the particlesfrom one particle of air to two million particles l−1, and thelower detectable mass is 0.1 μg m−3. Ground-reaching solarradiation in 310- to 2800-nm broadband was carried out usingKipp & Zonen pyranometer model CMP 11 [22]. The col-located measurements provide better understanding of thechanges in aerosol properties and their influence on ground-reaching solar radiation associated with changes in synopticmeteorological conditions over the study site.

B. Satellite Observations

1) MODIS: The Moderate Resolution Imaging Spectrora-diometer (MODIS) has been acquiring daily global data in36 spectral bands from visible to thermal infrared [(TIR);29 spectral bands with 1-km, 5 spectral bands with 500-m,

Fig. 1. True color composite of MODIS showing the track of the low-pressuresystem during December 4–8, 2008. The arrow mark shows the direction of thelow-pressure system.

Fig. 2. TIR images from the Kalpana-1 VHRR geostationary satellite duringDecember 3–8, 2008, over India.

Fig. 3. Variation of the ground-level daily mean atmospheric pressure duringDecember 3–8, 2008, at Hyderabad, India.

and 2 spectral bands with 250-m nadir pixel dimensions].A MODIS sensor is onboard the polar-orbiting National Aero-nautics and Space Administration—Earth Observing SystemTerra and Aqua spacecrafts with equator crossing times of10:30 and 13:30 local time, respectively [20]. Aerosol re-trievals from the MODIS data are performed over land andocean surfaces by means of two separate algorithms describedin the literature [14], [15], [20], [23]. In this letter, theC005 Level 3 (spatial resolution 1◦ × 1◦) MODIS products areobtained from the Level 1 and Atmosphere Archive and Distri-bution System website (http://ladsweb.nascom.nasa.gov/).

2) Kalpana-1: Kalpana-1 (formerly MetSat-1), an exclu-sive Indian meteorological satellite weighing 1050 kg, waslaunched by Polar Satellite Launch Vehicle on September 12,2002. The satellite comprises one very high resolution ra-diometer (VHRR) and a data relay transponder (DRT) payload

SHARMA et al.: LIDAR OBSERVATIONS OF AEROSOL PROPERTIES 809

Fig. 4. (a)–(h) Mesoscale-model (MM5)-derived sea level pressure and winddirection at 850 hPa over the Indian region during December 3–10, 2008.

Fig. 5. Lidar-derived aerosol backscatter during December 3–8, 2008.

to meet meteorological requirements. The VHRR operates inthree bands: 1) the visible (0.55–0.75 μm); 2) the TIR (10.5–12.5 μm); and 3) the water-vapor infrared bands (5.70–7.10 μm), with a spatial resolution of 2 × 2 km for the visibleband and 8 × 8 km for the TIR and water-vapor ones, toobtain atmospheric cloud cover, water vapor, and temperature[4]. The DRT also provides data from fixed/mobile ground-level weather platforms. In addition, the winds at 700 hPaover the Indian peninsula were downloaded from the NationalCenters for Environmental Prediction (NCEP) website to studythe variations in wind speed over the region associated withlow-pressure system.

Fig. 6. Lidar nighttime aerosol backscatter profiles on December 3, 5, and 8,2008, at 2300 h.

IV. RESULTS AND DISCUSSION

Fig. 1 shows the MODIS true color composite showingthe track of a low-pressure system during December 4–7,2008. The low-pressure system formed over southeast BoB onDecember 4, 2008, moved westward and lay centered at 23:30Indian Standard Time (IST). On December 5, 2008, the low-pressure system was near 8.5◦ N latitude and 87.0◦ E longi-tude, about 900 km southeast of Chennai and 600 km east ofTrincomalee (Sri Lanka). The shear weakened the low-pressuresystem into a depression on December 7, 2008, and, a few hourslater, it weakened further into a well-marked low pressure areadue to land interaction with Sri Lanka. A time series of TIRimages derived from the Kalpana-1 VHRR was analyzed for abetter view of the spatiotemporal variations of the low-pressuresystem and is shown in Fig. 2. It is clear from Fig. 2 that thelow-pressure system formed over the southeast BoB region andmoved toward the southwest direction over a period of time.

Fig. 3 shows the daily mean surface pressure variationfrom an automatic weather station at Hyderabad, India, duringDecember 3–8, 2008. It is clear from the figure that the at-mospheric pressure showed a considerable increase and reachedup to 954 hPa on December 5, 2008, indicating that the studysite was under the influence of high pressure system. AfterDecember 5, 2008, as the low-pressure system over BoB wasmoving westwards, from the study region in Hyderabad, thebarometric pressure gradually decreased [7] and reached to itsminimum 952 hPa on December 7, 2008. Fig. 4(a)–(h) showsthe mesoscale-model (MM5)-derived wind directions overlaidon sea level pressure at 850 hPa during December 3–10, 2008,at 00:00 Coordinated Universal Time (UTC). The evolutionof the low-pressure system over southeast BoB and thecorresponding high pressure system over an urban region in Hy-derabad can clearly be seen in Fig. 4(a)–(h). The wind directionon December 3–10, 2008, was persistent northeasterly over theregion. The anticlockwise-wind-direction field clearly showsthe presence of the low-pressure system over southeast BoB.

The intense dynamic meteorological conditions caused bythe low-pressure system over BoB strongly affected theatmospheric structure, aerosol field, and properties overHyderabad, India. Fig. 5 shows the nighttime variations onthe backscatter coefficient profile measured from lidar duringDecember 3–8, 2008. The mixed layer characteristics are mostimportant in aerosol studies since the pollutants that are emittedinto the atmospheric boundary layer are subjected to gradual

810 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 6, NO. 4, OCTOBER 2009

Fig. 7. (a) BLL-derived total nighttime AOD variation during December 3–10, 2008, over an urban region in Hyderabad. (b) MICROTOPS-II sun-photometerAOD500 variation during December 3–10, 2008, over an urban region in Hyderabad. (c) Terra/Aqua MODIS AOD550 variation during December 3–10, 2008,over an urban region in Hyderabad.

Fig. 8. (a) Diurnal variation of the total solar irradiance measured using a pyranometer for during low-pressure system (December 4, 2008) and after low-pressure system (December 8, 2008). (b) Diurnal variation in UVery on December 4 and 8, 2008. (c) Scatter plot between the MICROTOPS II AOD500 and thepyranometer-derived total global solar irradiance.

dispersion and mixed through the action of turbulence in thisregion which ultimately facilitate in modeling the vertical ex-tent of the pollutant movement [12]. The boundaries betweenthe mixed layer and the free troposphere were distinctly char-acterized by the sharp gradient of the aerosol signals. It isclear from Fig. 5 that, on December 3, 2008, PM loading inthe mixed layer reached up to 1000 m and then showed agradual decreasing pattern from December 4 to 6, 2008. Thedecrease in PM loading in the mixing layer during December4–6, 2008, is associated with high pressure system over thestudy area due to low-pressure system over BoB. The gradualincrease in PM loading in the mixed layer was observed onDecember 7, 2008, and reached its maximum on December 8,2008. The high pressure system during December 4–6, 2008,suppresses the convective mixing over the region due to subsi-dence resulting in lower concentration of PM in the mixed layerof the atmosphere. Similar inferences were reported in lidarobservations during high pressure system over Taipei, Taiwan[7]. Fig. 6 shows the lidar-derived nighttime aerosol backscatterprofiles at 532 nm on December 3, 5, and 8, 2008, at 2300 h.It can be seen from the figure that the pollutant concentrationin the mixed layer decreased on December 5, 2008, due tosubsidence caused by high pressure system over the study areaassociated with low-pressure system over BoB.

Fig. 7(a) shows the boundary-layer-lidar (BLL)-derivednighttime AOD at 532 nm (AOD532) during the periodDecember 3–10, 2008, over an urban region in Hyderabad.The total AOD values showed a remarkable decrease onDecember 5, 2008, and are correlated well with the locationof the low-pressure system over BoB which was closer toHyderabad on this date. The nighttime AOD values showed adecrease of 60% on December 5, 2008 (when the low-pressuresystem was nearer to Hyderabad), compared to a normal day ofDecember 3, 2008. The marine air masses act as a ventilationmechanism for the whole troposphere despite the uplifting of

Fig. 9. Daily mean variation in the PM concentration during December 3–10,2008, over an urban region in Hyderabad, India.

soil particles near the surface due to the higher surface windswhich resulted in lower AODs. Fig. 7(b) shows the daily varia-tion of the MICROTOPS-II-derived AOD at 500 nm (AOD500)during the period of December 3–10, 2008, over an urbanregion in Hyderabad. The columnar values of AOD500 showeda remarkable decrease on December 4, 2008. Earlier stud-ies suggested similar atmospheric variations over Hyderabadduring the passage of the cyclone Mala in April 2006 [3].Terra/Aqua MODIS-derived AOD550 values also showed simi-lar reduction in the AODs during the study period as shown inFig. 7(c).

Fig. 8(a) shows the diurnal variation of the total solar ir-radiance measured using the Kipp & Zonen CMP 11 pyra-nometer during and after low-pressure system. The differentaerosol loading had a considerable influence on the intensityof ground-reaching solar radiation. The global solar irradianceshowed an increase of ∼6% on December 4, 2008, compared toDecember 8, 2008. Similar variations were also observed inUVery shown in Fig. 8(b). UVery showed an ∼6% increase

SHARMA et al.: LIDAR OBSERVATIONS OF AEROSOL PROPERTIES 811

on December 4, 2008, when compared to December 8, 2008.Fig. 8(c) shows the scatter plot between MICROTOPS AOD500

and pyranometer-derived total global solar irradiance. It is clearfrom the figure that a 0.1 increase in the AOD reduced the∼62-W/m2 ground-reaching solar irradiance. The variation ofthe daily mean PM at all size levels, viz., 1.0, 2.5, and 10 μm,is shown in Fig. 9. It can be observed from Fig. 9 that there was∼25% decrease in the ground-level PM concentrations duringDecember 5–7, 2008, under the influence of high pressuresystem over the study area. The PM concentrations startedincreasing after the synoptic meteorological conditions revertedback to normal conditions over the study region.

V. CONCLUSION

In this letter, we have analyzed the variations in aerosolproperties and ground-reaching solar irradiance for the periodof December 3–10, 2008, over a tropical urban environment inHyderabad, India, associated with a low-pressure system overBoB. The results of the study suggested the following.

1) LIDAR-measured mixed-layer aerosol concentrationsshowed low values during the period of low-pressuresystem over BoB.

2) The nighttime AOD values showed a 60% decrease onDecember 5, 2008, corresponding to the low-pressuresystem located nearer to the measurement site inHyderabad.

3) The global solar irradiance showed an ∼6% increase onDecember 4, 2008, during low pressure over BoB dueto reduction in columnar aerosol loading compared to anormal period.

Results of the study have implications for monitoring urbanair quality as synoptic weather system is capable of modifyingthe atmospheric PM loading. In the climate change scenario,increased occurrence of low-pressure systems over the regionwas anticipated, and this will have impact on the differentialloading of atmospheric pollutants over the region.

ACKNOWLEDGMENT

The authors would like to thank the Director of NRSCand the Deputy Director of Remote Sensing & GeographicalInformation System-Application Area (RS&GIS-AA), NRSC,for the necessary help at various stages.

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