This article was downloaded by: [Cornell University Library]On: 13 November 2014, At: 08:36Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

International Journal of RemoteSensingPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tres20

The influence of a south Asian duststorm on aerosol radiative forcingat a high-altitude station in centralHimalayasAtul K. Srivastava a b , P. Pant a , P. Hegde a c , SachchidanandSingh d , U. C. Dumka a , Manish Naja a , Narendra Singh a & Y.Bhavanikumar ea Aryabhatta Research Institute of Observational Sciences(ARIES) , Nainital, Indiab Indian Institute of Tropical Meteorology (Branch) (IITM) , NewRajendra Nagar, New Delhi, Indiac Space Physics Laboratory, Vikram Sarabhai Space Centre (VSSC) ,Trivandrumd Radio and Atmospheric Sciences Division, National PhysicalLaboratory , K.S. Krishnan Road, New Delhi, 110012, Indiae National Atmospheric Research Laboratory , Gadanki, IndiaPublished online: 27 Sep 2011.

To cite this article: Atul K. Srivastava , P. Pant , P. Hegde , Sachchidanand Singh , U. C. Dumka ,Manish Naja , Narendra Singh & Y. Bhavanikumar (2011) The influence of a south Asian dust stormon aerosol radiative forcing at a high-altitude station in central Himalayas, International Journal ofRemote Sensing, 32:22, 7827-7845, DOI: 10.1080/01431161.2010.531781

To link to this article: http://dx.doi.org/10.1080/01431161.2010.531781

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sources

of information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

International Journal of Remote SensingVol. 32, No. 22, 20 November 2011, 7827–7845

The influence of a south Asian dust storm on aerosol radiative forcingat a high-altitude station in central Himalayas

ATUL K. SRIVASTAVA†‡, P. PANT†, P. HEGDE†§, SACHCHIDANANDSINGH*¶, U. C. DUMKA†, MANISH NAJA†, NARENDRA SINGH† and

Y. BHAVANIKUMAR|†Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India‡Indian Institute of Tropical Meteorology (Branch) (IITM), New Rajendra Nagar,

New Delhi, India§Space Physics Laboratory, Vikram Sarabhai Space Centre (VSSC), Trivandrum

¶Radio and Atmospheric Sciences Division, National Physical Laboratory,K.S. Krishnan Road, New Delhi, 110012 India

|National Atmospheric Research Laboratory, Gadanki, India

(Received 12 November 2009; in final form 25 August 2010)

The impact of long-range transported dust aerosols, originating from the TharDesert region, to a high-altitude station in the central Himalayas was studiedwith the help of micro-pulse lidar (MPL) observations. A drastic change in lidarback-scatter profile was observed on a dust day as compared with that on a pre–dust day. The back-scatter coefficient on a dust day revealed that the dust layerpeaked at an altitude ∼1300 m above ground level (AGL) and extended up to∼3000 m AGL, with maximum value ∼3 × 10–5 m–1 sr–1. Aerosol Index (AI)and air mass back-trajectory analysis substantiate the transport of dust aerosolsfrom the far-off Thar Desert region to the experimental site. A significant effectof dust aerosols was also observed over the station on the spectral aerosol opticaldepths (AODs), measured using a Microtops-II Sunphotometer. It showed signif-icantly different spectral behaviour of AOD on a dust day as compared with thaton a pre-dust day. The Ångström exponent (α) showed a marked decrease from0.42 to 0.04 from the pre-dust day to the dust day. The aerosol radiative forcingestimated using the Santa Barbara DISORT (discrete ordinate radiative transfer)atmospheric radiative transfer (SBDART) model, in conjunction with the opti-cal properties of aerosol and cloud (OPAC) model, showed values of about –30,–45 and +15 W m–2, respectively, at top-of-atmosphere (TOA), surface and in theatmosphere on the dust day. The positive atmosphere forcing caused an estimatedheating of the lower atmosphere by ∼0.4 K day–1.

1. Introduction

Greenhouse gases and aerosols are the two most important elements that affect theradiation balance of the earth’s atmosphere. It is well established now that the anthro-pogenic emissions of greenhouse gases (most importantly carbon dioxide (CO2)) intothe atmosphere cause global warming of the troposphere (Kondratyev and Varotsos

*Corresponding author. Email: ssingh@nplindia.org

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online © 2011 Taylor & Francis

http://www.tandf.co.uk/journalshttp://dx.doi.org/10.1080/01431161.2010.531781

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7828 A. K. Srivastava et al.

1995a,b, IPCC 1996, 2001, 2007). The situation has come to such a level that,according to one estimate, even if the concentrations of all greenhouse gases (andaerosols) were kept constant at their levels of the year 2000, a further warming ofabout 0.1◦C per decade could not be ruled out (Cracknell and Varotsos 2007). As faras aerosols are concerned, the level of scientific understanding of their radiative effectsis still far below that of greenhouse gases (IPCC 2007). It has, however, been reportedthat the effect of aerosols on climate is estimated to be at least as much as 20% of thatdue to greenhouse gases (Forster et al. 2007). It is owing to such impacts of aerosols,particularly mineral dusts, on immediate environment and climate that their study hasrecently received a considerable importance (Liu et al. 2004, Varotsos 2005, IPCC 2007and references therein, Tzanis and Varotsos 2008).

By and large, the dust aerosols in the atmosphere have terrestrial sources and repre-sent an important process of land–atmosphere interaction (Tegen et al. 1996). In thenorthern hemisphere, dust particles play a key role in modulating climate through indi-rect aerosol effects that alter cloud properties (Sassen et al. 2003). As far as the duststorms in South Asia are concerned, they are common in north-west India, especiallyin the western Rajasthan region, which is mainly covered by the Thar and MargoDeserts (Dey et al. 2004, Prasad and Singh 2007, Pandithurai et al. 2008), and arethe single largest contributor to the mineral dust over the north-west Indian regions(Todd et al. 2007). Even though aerosols play a crucial role in the climate of the earth–atmosphere system by means of their direct and indirect impacts on climate (Twomey1977, Charlson et al. 1992, Ackerman et al. 2000), the estimation of aerosol radiativeforcing still remains one of the largest uncertain components, as far as global climatechange is concerned (IPCC 2007). Similarly, dust particles directly affect the climateby scattering and absorbing solar radiation, still their role in climate forcing is poorlyunderstood (Kaufman et al. 2002, Satheesh and Moorthy 2005). Hence, there is a needto study the radiative properties of dust aerosols in more detail to assess their impacton climate.

Satellite and ground-based observations have shown that the global sources ofatmospheric dusts are arid and semi-arid desert regions contributing to the long-rangetransport of dust particles during dust storms (Husar et al. 1997, Washington et al.2003). Comprehensive studies of dust transport events from the Sahara to Europeanregions have been carried out by means of a coordinated lidar network, under the aegisof the European Aerosol Research Lidar Network (EARLINET), using advancedmulti-wavelength lidar systems (Ansmann et al. 2003, Papayannis et al. 2008). Over theyears, several studies have been done on the characterization and impact assessmentof transported dust aerosols during dust events at different low-altitude stations (Deyet al. 2004, Park et al. 2005, Singh et al. 2005, Nee et al. 2007, Pandithurai et al. 2008).Most of these studies have focused mainly on urban and semi-urban low-altituderegions. However, not many studies have been reported on the impact of long-rangetransported dust aerosols into the pristine environment of high-altitude stations farfrom the dust origins (Hegde et al. 2007). Moreover, these investigations are veryimportant in the context of the recent theory of the ‘elevated heat pump’ as proposedby Lau et al. (2006), in which the rising hot air is forced by the increased heating in theupper troposphere, which draws in warm and moist air over the Indian subcontinent.

The present study is focused on a dust storm that primarily originated in the TharDesert area during the second week of June 2006, and reached the high-altitude stationof Manora Peak, Nainital, at an altitude of about 1958 m above mean sea level (m.s.l.).An elevated dust aerosol layer, associated with an enhanced back-scatter signal, wasobserved for the first time over the station using a ground-based micro-pulse lidar

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7829

(MPL) system. Apart from the lidar observation of the aerosol dust layer, the focus ofthe present study is also on the changes in aerosol radiative forcing due to enhanceddust concentration over this station in the central Himalayas.

2. Site description and measurement techniques

Observations were carried out from Manora Peak (29.4◦ N, 79.5◦ E, ∼1958 mabove m.s.l.), Nainital, located in the central Himalayas. The north and north-eastof Manora Peak has the sharply undulating topography of Himalayan mountainranges. However, the south-west side has low-elevated plain land merging into theGanga Basin. Owing to its high elevation associated with a very low atmosphericboundary-layer height, this site may be considered to be a free tropospheric site.As this high-altitude, remote, sparsely inhabited site is devoid of any major pollu-tion (Sagar et al. 2004, Pant et al. 2006), investigations there have the importanceof providing a background level against which the impact of aerosol loading duringdust episodes can be assessed. Since the long-range transport of dust makes a signif-icant contribution to atmospheric aerosols even at locations far from their sources,the impact of the enhanced aerosol concentration during the dust event over thisexperimental site can be distinctly seen in the vertical profiles of aerosol back-scatterintensity by the lidar measurements.

A portable lidar system was installed at Manora Peak, Nainital, in May 2006 forthe study of vertical profiles of aerosols and clouds. This system was designed anddeveloped by the National Atmospheric Research Laboratory (NARL), Gadanki, andis based on MPL technology (Spinhirne 1993). The lidar employs a diode-pumpedNd:YAG laser with second harmonic output at 532 nm and operated at a pulse rep-etition frequency of 2500 Hz. The emitter beam is coaxial to receiver field of view(FOV) and operated in the zenith direction. The lidar receiver employs a 150 mmCassegrain telescope. Further, an iris (pinhole) of diameter 0.5 mm was used to obtainthe receiver FOV of about 400 μrad. A computer-based multichannel analyser (modelMCA-pci, EG&G Ortec, Oak Ridge, TN, USA) was employed to record signalsback-scattered from atmospheric aerosols from 300 000 successively transmitted lasershots, which corresponds to a time resolution of 120 s for the applied repetition rate.The back-scattered signals were measured with a bin width of 200 ns, which corre-sponds to an altitude resolution of 30 m. The details of the system are given elsewhere(Bhavanikumar 2006).

The measured photon count profiles were corrected for noise and subjected to rangenormalization before inverting them to derive the back-scatter profiles using appro-priate inversion techniques as proposed by Fernald (1984) and Klett (1985). For theinversion of the lidar signal, a reference altitude of 6 km, where aerosol contributionis insignificant and the total volume back-scattering coefficient is equal to the molec-ular back-scattering coefficient, was considered, as most of the aerosols in the tropicsare located below 6 km (Gadhavi and Jayaraman 2006). The molecular (or Rayleigh)contribution to the signal was taken from the CIRA-1986 (Cospar InternationalReference Atmosphere) standard atmospheric model.

The received back-scatter signal can be expressed in terms of the back-scatteringratio (BSR), which is the relative magnitude between the molecular (or Rayleigh) andaerosol (or Mie) back-scattering coefficients, and given as:

BSR(z) = 1 + βa(z)βm(z)

, (1)

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7830 A. K. Srivastava et al.

where βm(z) and βa(z) are the Rayleigh and aerosol back-scattering coefficients at analtitude z, respectively. Further, the aerosol extinction coefficient (σ a) was estimatedby assuming an appropriate value of the lidar ratio (LR), which is the ratio of theextinction to the back-scattering coefficient. The LR value was considered to be 35 srfor an environment like Nainital, on the basis of extensively reviewed literature (Mülleret al. 2001, He et al. 2006 and references therein). Based on the above parameters,optical thickness of aerosol or dust particles (i.e. the aerosol optical depth (AOD)) canbe derived by integrating the aerosol extinction coefficient within the altitudes z1 andz2, expressed as:

Lidar(AOD) =z2�

z1

σa (z) dz =z2�

z1

βa (z) (LR) dz. (2)

Besides the lidar observations in the night-time, direct measurements of columnarAOD during the daytime, at different wavelengths from the ultraviolet (UV) to thenear-infrared (NIR) region, were also carried out at the station using a Microtops-IISunphotometer and Ozonometer (Solar Light Co., Glenside, PA, USA). The detailsof the instrument are available elsewhere (Morys et al. 2001). Measurements of sur-face Black Carbon (BC) mass concentrations were carried out using an aethalometer(model AE-42, Magee Scientific, Berkeley, CA, USA). More details about the instru-ment and the principle of measurements are available elsewhere (Hansen 2003). Someof the observations of the optical and physical properties of aerosols over the stationusing these instruments were reported by Hegde et al. (2007) during the dust episode.

3. Results and discussion

3.1 Identification of the dust episode

The moderate resolution imaging spectroradiometer (MODIS) aboard the Aqua satel-lite captured a large dust storm along the border between India and Pakistan on 12June 2006 (http://earthobservatory.nasa.gov/ NaturalHazards/Archive), which orig-inated over the Thar Desert and blew through the Indus valley, as shown in figure 1.The experimental site, Nainital, is indicated by a star in the figure. It shows the spatialdistribution of the dust storm, and reveals that plumes of dust aerosols engulfed theentire northern part of India. It is also apparent from the figure that the dust plumeswere heading towards the Himalayan Mountains, and reach as far as the high-altitudeexperimental station (top right corner of the image). In the lower left corner, sprays ofcloud also appear to blow in the same north-easterly direction as that of the dust. Oncethe dust reached the Himalayan Mountains, it changed its direction and blew alongtheir southern edge. Dust events such as this can also be discerned as an enhancementin the aerosol index (AI) as well as in spectral AODs.

3.2 Transport of dust at Nainital

The prevailing meteorology during the pre-monsoon season at Nainital is character-ized by north-westerly winds, which pass through the arid regions of western Indiaand mostly bring dry air mass from south-west Asia. Since the impact of ambient duston the radiation budget of the atmosphere changes with source regions and trans-port pathways, it is prerequisite to get these pieces of information for their impactassessment. In order to know the transport pathways of the dust aerosols due to dust

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7831

Pakistan

100 km

India

Nainital(29.4°N, 79.5°E)

Figure 1. Spatial distribution of dust storm observed by MODIS on 12 June 2006.

activities primarily originated over the Thar Desert region, analysis of backward airmass trajectories and the AI was carried out over the experimental station from 8to 15 June 2006. The ozone monitoring instrument (OMI) aboard the Aura satellitemeasures absorbing aerosols at UV wavelengths and their AI product. The AI can dif-ferentiate the nature of absorbing and non-absorbing aerosols, because it provides aqualitative measure of absorption of UV radiation by smoke and dust. Generally, theAI is positive for absorbing aerosols and negative for non-absorbing aerosols, that is,pure scattering (Varotsos et al. 2006, Li et al. 2009). Dust is the main contributor to theAI signal as far as the absorbing aerosols are concerned (Li et al. 2009). Therefore, sev-eral studies based on AI data have been made to identify the dust sources (Israelevichet al. 2002, Prospero et al. 2002, Washington et al. 2003, Dey et al. 2004).

The AI images obtained from the OMI during June 2006 show a regional distribu-tion of aerosols, which indicates the dispersion of dust aerosols over the Thar region(figure 2). It can be explicitly observed in the figure that the dust activity was at itspeak on 12 June 2006 with AI ∼4.0. However, the dust activity started on 11 June 2006and engulfed most parts of north-west India over the following days. The maximumvalue of AI ∼2.5 was observed over the experimental site on 12 June 2006. Earlier,Dey et al. (2004) had also observed positive AI values ranging from 1.7 to 3.2 overthe Indo-Gangetic Basin during dust events, which were largely attributed to the pres-ence of the dust along with absorbing aerosols. Moreover, recently Papayannis et al.

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7832 A. K. Srivastava et al.

408 June 2006

12 June 2006 13 June 2006 14 June 2006 15 June 2006

9 June 2006 10 June 2006 11 June 2006

35

30

25

20

15

10

5

40

La

titu

de

(°N

)

Longitude (°E)

AI

35

30

25

20

15

10

5

40

35

30

25

20

15

10

5

40

35

30

25

20

15

10

5

50 55 60 65 70 75 80 85 90 95 100

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 >

50 55 60 65 70 75 80 85 90 95 100

Figure 2. OMI AI images showing the source and the progressive movement of the dustaerosols before and after the dust storm activity in June 2006 (five-day air mass back-trajectoriesat 06:00 GMT (cyan line), 12:00 GMT (red line) and 18:00 GMT (green line) at an altitude of1500 m AGL are superimposed on respective days of the AI images).

(2007) found AI values over China ranged between 2.0 and 3.5 during dust events,which increased to 4.5 during intense dust events. The high values of AI attest to thepresence of absorbing aerosol particles over the region. Further, to substantiate thepathways for dust transport, five-day air mass back-trajectories employing the hybridsingle-particle Lagrangian integrated trajectory (HYSPLIT) model, were plotted andsuperimposed on the AI images (figure 2). The backward trajectories were analysed forthree different time intervals, at 06:00 Greenwich Mean Time (GMT) (cyan colouredline), 12:00 GMT (red coloured line) and 18:00 GMT (green coloured line), for analtitude of 1500 m above ground level (AGL). The back-trajectories show differentpathways for the transport of air masses from the source region to the experimentalsite over different time periods.

3.3 Lidar measurements

The lidar measurements were taken regularly at Manora Peak, Nainital, during thenight-time, to avoid the higher levels of background noise in the daytime. The impactof aerosol loading during the dust episode (which primarily originated over the TharDesert) over the experimental site was clearly observed in the lidar profiles, whichshowed a significant enhancement in the back-scatter coefficient on 12 June 2006as shown in figure 3 (open circles). The figure shows vertical profiles, derived fromthe lidar system, of the back-scatter coefficient, which depends on the variations inaerosol layer strength and height. To distinguish the existence of a dust layer from thedust free condition, a vertical profile of the back-scatter coefficient on 8 June 2006

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7833

5000

12 June 2006 (Dust)

8 June 2006 (Pre-dust)

4000

3000

2000

Altitude (

m)

1000

00.0 1.0 × 10–5 2.0 × 10–5

Back-scatter coefficient (m–1 sr

–1)

3.0 × 10–5 4.0 × 10–5

Figure 3. Average vertical profiles of the back-scatter coefficient on a dust day (12 June 2006)and a pre-dust day (8 June 2006).

(i.e. a pre-dust day, which is considered as a clean-sky day) has also been plotted inthe same figure (solid circles). The horizontal lines indicate the standard deviation ofthe back-scatter coefficient at each altitude level. It is quite obvious from the figurethat back-scatter coefficient values at each altitude (from ∼1000 up to 3000 m AGL)are significantly larger and with a broader peak on 12 June 2006 (dust day) than on8 June 2006 (pre-dust day). This is attributed mainly to the presence of an elevatedlayer of dust aerosols, transported from the far-off Thar Desert region to the experi-mental site. The peak value of back-scatter coefficient on the dust day was found tobe ∼3 × 10–5 m–1 sr–1 at the altitude of ∼1300 m AGL, extending up to ∼3000 mAGL, whereas the peak on the pre-dust day was found to be only ∼2 × 10–5 m–1 sr–1

at the slightly lower altitude of ∼1200 m AGL, which was due to the presence of back-ground aerosols within the boundary layer. The value of the back-scatter coefficientwas 1 × 10–9 m–1 sr–1 above and below the aerosol layer on both the dust and pre-dust days, which represents the back-scatter coefficient purely due to the molecularatmosphere.

Recently, Hegde et al. (2009) studied vertical profiles of aerosol over Nainital byanalysing aerosol back-scatter profiles derived from an MPL system, and found sig-nificantly large back-scatter ratio values, peaking at an altitude of around 1500 mAGL, during pre-monsoon months, which are highly associated with the prevailingstrong convection conditions. Similarly, Gautam et al. (2009), using space-borne lidarmeasurements (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations,CALIPSO), have also found an aerosol layer extending vertically over the foothillsof the Himalayas. These aerosols were found to have been transported from thenorth-western arid regions.

Temporal evolution of air masses from 120 hours before until the date of dust obser-vation over the station, at 1500 m AGL (approximate altitude of observed peak BSR),has been analysed with the HYSPLIT model and is shown in figure 4. Significantadvancements of air masses were observed near the altitude of peak BSR at differenttime intervals (as shown in figure 2) on 12 and 13 June 2006. This shows constancy in

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7834 A. K. Srivastava et al.

250012 June 2006 (06 GMT)

12 June 2006 (12 GMT)

12 June 2006 (18 GMT)

13 June 2006 (06 GMT)

13 June 2006 (12 GMT)

13 June 2006 (18 GMT)

2000

1500

1000Altitude (

m)

500

0–120 –96 –72 –48

Back days (h)

–24 0 24

Figure 4. Temporal evolution of air masses at 1500 m AGL for three different time intervalson 12 and 13 June 2006.

the vertically upward trend in the transport of the air masses, thereby facilitating theupward transport of dust from the source region to the experimental site.

Vertical profiles of aerosol in terms of back-scattered signals, having temporal reso-lution of 120 s, were obtained by the lidar system every day to study the height versustime variations in BSR. The temporal variation of BSR with height on 12 June 2006(figure 5) shows the vertical distribution of the dust aerosol layer with its strength andheight. The dust layer peaks at an altitude ∼1300 m AGL, having maximum BSRvalue ∼27, and extends up to an altitude ∼3000 m AGL. The corresponding aerosolextinction profiles were integrated within the altitude region, from ∼200 m AGL (low-est altitude detected by the lidar system) up to ∼4500 m AGL, to get the AOD (at532 nm) using equation (2). The temporal variation of lidar-derived AOD is shownover the same plot (continuous solid line, figure 5). By and large, the BSR and AODseem to be perfectly complementary to each other as far as their temporal variation isconcerned. The lidar-derived AOD (at 532 nm) values vary from ∼0.6 to 1.2 with anaverage value of ∼0.83 (± 0.12).

Recently, Nee et al. (2007) have investigated long-range transport of an Asian duststorm using a polarization lidar system at Chungli (24.6◦ N, 121.1◦ E). They haveobserved an average dust layer height ∼1.6 km, having an average BSR value ∼12and AOD ∼0.54 (± 0.31). However, Chen et al. (2007) have reported Raman anddepolarization lidar measurements of optical properties of Asian dust in the free tro-posphere during dust seasons over Taipei, Taiwan (25.14◦ N, 121.54◦ E). They haveobserved dust layers with altitude ranging from 1 to 6 km, having optical thicknessranging from 0.01 to 0.55. Studies based on coordinated lidar network data overEurope by Ansmann et al. (2003) and Papayannis et al. (2008) have found traces ofdust particles up to a height of about 7–8 km, after 2–5 days’ transport from theSahara Desert region. Ansmann et al. (2003) have reported the dust optical depthvarying between 0.1 and 0.8 at 532 nm, whereas Papayannis et al. (2008) found verylow desert dust optical depth (ranging from 0. 1 to 0.25) at the UV wavelength (351 or355 nm).

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7835

Altitude (

m)

AO

D

5000

4000

3000

2000

1000

0

Local time (h)

20:00 21:00 22:00 23:30

1.4

1.2

1.0

0.8

0.6

0.4 0

3

6

9

12

15

18

21

24

27BSR

Figure 5. Altitude (AGL) versus time variations in BSR on a dust day (12 June 2006) alongwith the variations in lidar-derived AOD at 532 nm (average AOD = 0.83 ± 0.12).

3.4 Effect of dust on surface aerosol properties

The effect of dust on surface aerosol properties at Nainital for the whole of June 2006has been described by Hegde et al. (2007). They have found a five to ten times increasein the coarse and giant mode particles during a dust day, as compared with theirrespective monthly means, using an optical particle counter, and a two to four timesincrease in the spectral AOD values compared with their normal day-mean. They havealso found a large increase in total suspended particulate matter concentration (∼280μg m–3) during the dust episode. Using sun/sky radiometer data, Dey et al. (2004),Singh et al. (2004) and Pandithurai et al. (2008) have studied the impact of trans-ported dust aerosols on various aerosol properties at different low-altitude stationsover the Ganga Basin. For the purpose of radiative forcing estimation in the presentanalysis, the spectral variations of AOD measured by Microtops-II on a dust day (13June 2006) and a pre-dust day (8 June 2006), as shown by vertical columns in figure6(a) and (b) respectively, have been used. A more than two fold increase in AOD val-ues was observed at all the wavelengths on the dust day as compared with those onthe pre-dust day. Mean AOD at 500 nm was observed to be as high as ∼0.63 duringthe dust day and ∼0.23 on the pre-dust day. A value of AOD of the order of 0.63 isvery high for a pristine site like Nainital, though it may be significantly low as com-pared with the values observed at other, low-altitude, stations in India during differentenvironmental conditions (Dey et al. 2004, Singh et al. 2005, Pandithurai et al. 2008,Srivastava et al. 2008).

An increasing trend of AOD with respect to wavelength observed on the dust dayclearly indicates the presence of coarser dust particles over the station. A similar trendin AOD has also been reported by Singh et al. (2005) during a dust event over NewDelhi. On the other hand, the AOD observed on the pre-dust day followed the normalbehaviour; that is, AOD decreases with increase in wavelength. The spectral depen-dence of extinction by particles, that is, AOD, may be approximated as a power lawrelationship, given by Ångström (1964):

τa(λ) = βλ−α, (3)

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7836 A. K. Srivastava et al.

Wavelength (nm) Wavelength (nm)

0.7213 June 2006 (Dust Day) 08 June 2006 (Pre-Dust Day)

0.64

(a)

0.56

0.48

0.40

AO

D

AO

D

0.32

0.24

0.16

0.08

0.00

0.72

0.64

0.56

0.48

0.40

0.32

0.24

0.16

0.08

0.00

300

400

500

600

700

800

900

1000

1100

(b)

Measured AOD

Model AOD

300

400

500

600

700

800

900

1000

1100

Figure 6. Spectral variations of Microtops-II-measured AOD values on (a) a dust day (13 June2006) and (b) a pre-dust day (8 June 2006) shown by vertical columns. Model-estimated AODvalues are shown by the dashed line with open circles.

where τ a(λ) is the AOD at wavelength λ (in μm), β is the turbidity coefficient, indi-cating aerosol loading, which is equal to τ a at λ = 1 μm, and α is widely knownas the Ångström exponent, and is a good indicator of aerosol particle size (Ecket al. 1999).

Ångström parameters were computed from the spectral variation of AOD measuredin the spectral range from 380 to 1020 nm using equation (3). During the dust day,the value of α was observed to be ∼0.04 with a corresponding value of β of ∼0.65.However, a significantly large value of α (∼0.42) was observed during the pre-dustday, whereas the value of β was observed to be significantly low (∼0.18). A highervalue of α is typically observed for accumulation of fine mode particles (Reid et al.1999, Eck et al. 1999) whereas a lower value is observed for coarse mode particlessuch as Saharan and Asian dusts (Eck et al. 1999, Sakai et al. 2002, Ansmann et al.2003). Moreover, the high β value during the dust day compared with that on the pre-dust day (more than 3.6 times larger) clearly indicates a heavy aerosol loading on thedust day. Unavailability of chemical data over Nainital makes it difficult to commenton the source of dust aerosols in terms of mineralogy. Nevertheless, the analysis ofback-trajectories and AI images during the dust storms strongly supports the conceptof the transport of dust aerosols from the western Thar Desert to the investigationsite at Nainital (figure 2). Moreover, the absence of enhancement in black carbon andaccumulation mode particles over the site suggests a negligible change in the influencesof anthropogenic activities on the aerosol properties during the study period (Hegdeet al. 2007).

3.5 Radiative effect of dust aerosols

Dust aerosols scatter as well as absorb short-wave solar radiation and also absorband emit outgoing long-wave radiation (Kaufman et al. 2001). Some of these dustaerosols are highly absorbing components of the atmosphere, which are significantcontributors to the radiative warming in the lower atmosphere due to short-waveabsorption (Alpert et al. 1998). The forcing at top-of-atmosphere (TOA) and at the

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7837

surface is defined as the difference in net fluxes (down minus up) with and withoutaerosols at their respective levels (Moorthy et al. 2005). In the present analysis, netflux has been estimated in the short-wave region (0.25–4.0 μm) with and withoutaerosols, separately at TOA and at the surface, using the Santa Barbara DISORT(discrete ordinate radiative transfer) atmospheric radiative transfer (SBDART) model(Ricchiazzi et al. 1998). SBDART is a plane-parallel radiative transfer model basedon a discrete ordinate approach to estimate aerosol radiative forcing in clear andcloudy conditions. The radiative transfer equations are numerically integrated withthe DISORT module (Stamnes et al. 1988) in the SBDART. This method uses anumerically stable algorithm to solve the equations of plane-parallel radiative trans-fer in a vertically inhomogeneous atmosphere (Ricchiazzi et al. 1998). All importantprocesses that affect the UV, visible and IR radiation fields are incorporated in theSBDART.

The main input parameters required in the SBDART for estimating aerosol forc-ing are AOD, single scattering albedo (SSA) and an asymmetry parameter. Amongthese input parameters, SSA plays a key role in forcing estimations, indicating therelative contribution of scattering and absorption to the total extinction by aerosolparticles. Besides the above optical parameters, the other important input parametersin the SBDART model are solar geometry, a model atmosphere and surface albedo.Since the surface albedo, or reflection, is one of the key parameters in forcing esti-mation, Hansen et al. (1997) have incorporated different kinds of surface reflectionin their detailed analysis of aerosol forcing, and suggested that owing to the highsurface albedo, aerosol forcing at TOA will change its sign from negative to posi-tive. Recently, Pandithurai et al. (2008) have reported positive TOA forcing at NewDelhi during the pre-monsoon month, which is mainly attributed to the lower SSAand higher surface albedo values. For estimating the radiative forcing over Nainital,surface albedo has been considered to be vegetation type among all the existing surfacetypes in SBDART.

As there are no direct measurements of SSA or the asymmetry parameter overthe station, measured spectral AOD (using Microtops-II) and BC mass concentra-tion (using the aethalometer) values were used to constrain the modelled SSA andthe asymmetry parameter using the optical properties of aerosol and cloud (OPAC)model given by Hess et al. (1998). To distinguish aerosol characteristics during dustand dust-free days, measured spectral AOD and BC mass concentration values werechosen on 13 June 2006 (dust day) and 8 June 2006 (dust-free, i.e. pre-dust day) and theapproach given by Moorthy et al. (2005) of anchoring points by defining new mixturesin the OPAC model was followed. The model was then fine-tuned to match the mea-sured AOD at different wavelengths. The components of the mixture used in the OPACmodel for the dust day included water-insoluble, water-soluble, transported mineraldust and soot. For the dust-free day the components used were water-insoluble, water-soluble, mineral (nuclei mode), mineral (accumulation mode) and soot. The value forsoot was taken from the actual measurements of BC during those two days. The num-ber density of each of these species was adjusted while maintaining the mass fractionof BC to the observed one, so that the modelled spectral AOD values agreed well, towithin 5%, with those of the observed mean AOD values. Because of this closure (withspectral AOD) and anchoring of the BC mass fraction, the initial assumption of themodel will not have any significant impact on the estimated forcing, as was pointedout by Satheesh and Srinivasan (2006).

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7838 A. K. Srivastava et al.

The AOD values estimated from the aerosol model were found to agree well withthe measured AOD values at all the wavelengths, as shown by dashed line with opencircles in figure 6(a) and (b) on the dust (13 June 2006) and pre-dust (8 June 2006)days, respectively. The model-estimated AOD values are well within the standarddeviations of the measured AOD values at each wavelength. The other correspond-ing optical parameters such as SSA and the asymmetry parameter, estimated usingthe OPAC model, were then used in the SBDART model for forcing calculations.Significantly low values of SSA were observed on both the pre-dust and dust days.The SSA (500 nm) was found to be ∼0.69 during the pre-dust day, which indicatessignificant aerosol absorption. The low SSA observed on the pre-dust day could bedue to the presence of large BC mass concentrations (∼1.95 μg m–3) over the sta-tion, and was found to be more than twice the value observed on the dust day (∼0.8μg m–3). On the contrary, a small increase in SSA (∼0.1) was observed during thedust day. This increase in SSA on the dust day could be explained by the combinedeffects of enhanced dust and low BC mass concentration over the experimental site.Although there was abundant dust loading on 13 June, the comparatively low value ofBC mass concentration on that day caused less absorption. Results are found to be ingood agreement with the values variously observed by Chinnam et al. (2006), Costaet al. (2006) and Pandithurai et al. (2008) during dust events at different locations.Ramana et al. (2004) have observed SSA values (500 nm) in the range of 0.70–0.90in the Himalayan region (Kathmandu, Nepal) by combining the actual measurementsof scattering and the absorption coefficient using a nephelometer and a particle sootabsorption photometer (PSAP). On the other hand, Pant et al. (2006) estimated theSSA value at Nainital in the range of 0.87–0.94, with a mean value of 0.90, during awinter month.

In order to obtain the diurnally averaged forcing at TOA and at the surface, radia-tive forcing was estimated for the solar zenith angle (SZA) at every 5◦ interval,following the procedure given by Dey and Tripathi (2007). The difference between theTOA and surface forcing is considered as atmospheric forcing (�F), which representsthe amount of energy trapped by aerosols within the atmosphere and transformed intoheat. The atmospheric heating rate due to aerosol absorption is given as:

∂T∂t

= gCp

�F�P

, (4)

where ∂T/∂t is the heating rate (K day–1), Cp is the specific heat capacity of air at con-stant pressure, g is the acceleration due to gravity and �P is the atmospheric pressuredifference between the top and bottom surfaces of each layer.

Based on the measured parameters and the prevailing atmospheric conditions overthe station, the mid-latitude summer model atmospheric profile was considered in theSBDART model. The measured values of columnar water vapour and total columnozone, retrieved by Microtops-II, were also used for model estimation, and were foundto be ∼0.68 cm and ∼358 Dobson Unit (DU) on the dust day (13 June 2006) and∼0.83 cm and ∼344 DU on the pre-dust day (8 June 2006). The estimated valuesof aerosol forcing at TOA, surface and in the atmosphere on the dust and pre-dustdays are shown in figure 7. The heating rates on the corresponding days are shownin parentheses. During the dust day, TOA and surface forcing values were found to

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7839

5013 June 2006 (Dust)

8 June 2006 (Pre-dust)40

30

20

10

–10

–20SW

forc

ing (

W m

–2)

–30

–40

–50

0TOA Surface

Atmosphere

(0.4

)

(0.1

)

Figure 7. Short-wave radiative forcing (in W m–2) at the TOA, surface and in the atmosphereon the dust (13 June 2006) and pre-dust (8 June 2006) days. Atmospheric heating rates (in Kday–1) on the corresponding days are given in parentheses.

be as high as about –30 and –45 W m–2 respectively, implying a net cooling effect.However, the resultant atmospheric forcing, of about +15 W m–2, was absorbed intothe atmosphere owing to the presence of mineral dust and soot aerosols. Consequently,the short-wave atmospheric absorption caused heating of the lower atmosphere by∼0.4 K day–1. Forcing due to mineral dust aerosols was observed to be higher thanon the pre-dust day by about two, three and five times, at the TOA, surface and in theatmosphere, respectively. The corresponding atmospheric heating rate was observedto be ∼0.1 K day–1 on the pre-dust day, which is one quarter of that on the dust day.Since the AOD is considered to be an index for the total columnar burden of aerosolsin the atmosphere, the results obtained on the two different days are highly associatedwith the spectral behaviour of the AODs.

In earlier studies, Ramana et al. (2004) observed a surface radiative forcing of about–25 W m–2 in the Himalayan region at Kathmandu, Nepal. However, Pant et al. (2006)estimated the aerosol forcing at Nainital during a winter month (December 2004)as +0.7, –4.2 and +4.9 W m–2 at TOA, surface and in the atmosphere, respectively.Recently, Prasad et al. (2007) found a change in average surface and TOA forcing ofabout –23 and –11 W m–2 respectively, during a dust event over Kanpur in the Indo-Gangetic (IG) plains. However, Pandithurai et al. (2008) estimated values of surfaceand TOA forcing ranging from –39 to –106 and –12 to +24 W m–2 respectively, at NewDelhi during the pre-monsoon season, which caused a high heating rate ranging from0.6 to 2.5 K day–1.

Such an enhanced dust layer over the high-altitude region of the central Himalayasduring pre-monsoon or late spring can play a crucial role in heating the atmo-sphere, as suggested by Lau et al. (2006). Recently, Gautam et al (2009) showedthe enhancement in early summer monsoon rainfall over India, and attributed thisto the possible pre-monsoon tropospheric warming observed over the Himalayas

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7840 A. K. Srivastava et al.

and subsequent strengthening of the land–sea thermal gradient. The effect could bemore pronounced due to the enhanced concentration of BC, which mixes with thedesert dust aerosols causing more heating at elevated heights. In the present analy-sis, the atmospheric heating, of the order of 0.4 K day–1, is only due to short-wave(0.25–4.0 μm) absorption by dust aerosol particles. Nonetheless, there may be a sig-nificant contribution to atmospheric heating by the long-wave absorption of these dustparticles as well. Such an elevated heating may lead to an advance of the rainy periodsand subsequently an intensification of the Indian summer monsoon as suggested byLau et al. (2006).

4. Conclusions

The impact of long-range transport of south Asian dust storms on optical andradiative properties of dust aerosols has been investigated, for the first time, over ahigh-altitude station in the central Himalayas. The salient features of the present studyare:

• The back-scatter coefficient values were observed to be significantly higher ateach altitude level on a dust day as compared to those on a pre-dust day, hav-ing a peak enhancement of ∼3 × 10–5 m–1 sr–1 at an altitude ∼1300 m AGLand extending up to the altitude ∼3000 m AGL, which is mainly attributedto the presence of a large amount of dust aerosols over the Himalayan region,transported from the associated intense dust episode in the Thar Desert.

• Both AI images and air mass back-trajectories strongly support the origin andtransport of dust aerosols from the western Thar Desert region towards theexperimental site.

• Microtops-II-measured mean AOD (500 nm) was observed to be as high as∼0.63 and 0.23 on dust and pre-dust days, respectively. The inferred Ångströmexponent (α), using the observed spectral AODs, was found to be ∼0.04 and0.42 on dust and pre-dust days respectively. These values of α clearly indicate thedominance of coarse mode particles on the dust day and accumulation of finemode particles on the pre-dust day.

• The presence of mineral dust over the station decreased the short-wave radiationreaching the earth’s surface. Thus, a net negative forcing of about –30 and –45W m–2 was observed at the TOA and surface respectively, which implies a netcooling effect. However, a net positive forcing of about +15 W m–2 was observedin the atmosphere, which implies a net warming effect and causes heating of thelower atmosphere by ∼0.4 K day–1.

AcknowledgementsSincere thanks are due to the directors of ARIES, Nainital, and IITM, Pune, for theirkind support and encouragement. This study is a part of ARFI and the ABLN&C pro-gram under the Indian Space Research Organization’s Geosphere Biosphere Program(ISRO-GBP). Authors P. Hegde and U.C. Dumka express their sincere gratitude toISRO-GBP for the financial support. The authors are grateful to the two anonymousreviewers for their constructive comments and suggestions, which helped to improvethe manuscript.

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7841

ReferencesACKERMAN, A.S., TOON, O.B., STEVENS, D.E., HEYMSFIELD, A.J., RAMANATHAN, V.

and WELTON, E.J., 2000, Reduction of tropical cloudiness by soot. Science, 288,pp. 1042–1047.

ALPERT, P., KAUFMAN, Y.J., SHAY-EL, Y., TANRE, D., DA SILVA, A., SCHUBERT, S. and JOSEPH,J.H., 1998, Quantification of dust-forced heating of the lower troposphere. Nature, 395,pp. 367–370.

ÅNGSTRÖM, A., 1964, The parameters of atmospheric turbidity. Tellus, 16, pp. 64–75.ANSMANN, A., BOSENBERG, J., CHAIKOVSKY, A., COMERO´N, A., ECKHARDT, S., EIXMANN,

R., FREUDENTHALER, V., GINOUX, P., KOMGUEM, L., LINNE, H., MA´RQUEZ, M.A.L.,MATTHIAS, V., MATTIS, I., MITEV, V., MULLER, D., MUSIC, S., NICKOVIC, S.,PELON, J., SAUVAGE, L., SOBOLEWSKY, P., SRIVASTAVA, M.K., STOHL, A., TORRES,O., VAUGHAN, G., WANDINGER, U. and WIEGNER, M., 2003, Long-range trans-port of Saharan dust to northern Europe: the 11–16 October 2001 outbreakobserved with EARLINET. Journal of Geophysical Research, 108, 4783, doi:10.1029/

2003JD003757.BHAVANIKUMAR, Y., 2006, Portable lidar system for atmospheric boundary layer (ABL)

measurements. Optical Engineering, 45, pp. 76201–76205.CHARLSON, R.J., SCHWARTZ, S.E., HALES, J.M., CESS, R.D., COAKLEY, J.A., HANSEN, J.E.

and HOFFMANN, D.J., 1992, Climate forcing by anthropogenic aerosols. Science, 255,pp. 423–430.

CHEN, W.N., TSAI, F.J., CHOU, C.C.K., CHANG, S.Y., CHEN, Y.W. and CHEN, J.P., 2007, Opticalproperties of Asian dusts in the free atmosphere measured by Raman lidar at Taipei,Taiwan. Atmospheric Environment, 41, pp. 7698–7714.

CHINNAM, N., DEY, S., TRIPATHI, S.N. and SHARMA, M., 2006, Dust events in Kanpur,northern India: chemical evidence for source and implications to radiative forcing.Geophysical Research Letters, 33, L08803, doi:10.1029/2005GL025278.

COSTA, M.J., SOHN, B.J., LEVIZZANI, V. and SILVA, A.M., 2006, Radiative forcing of Asian dustdetermined from the synergized GOME and GMS satellite data – a case study. Journalof the Meteorological Society of Japan, 84, pp. 85–95.

CRACKNELL, A.P. and VAROTSOS, C.A., 2007, The IPCC fourth assessment report and thefiftieth anniversary of Sputnik. Environmental Science and Pollution Research, 14,pp. 384–387.

DEY, S. and TRIPATHI, S.N., 2007, Estimation of aerosol optical properties and radiative effectsin the Ganga basin, northern India, during the wintertime. Journal of GeophysicalResearch, 112, D03203, doi:10.1029/2006JD007267.

DEY, S., TRIPATHI, S.N., SINGH, R.P. and HOLBEN, B.N., 2004, Influence of a dust stormon the aerosol optical properties over the Indo-Gangetic basin. Journal of GeophysicalResearch, 109, D20211, doi:10.1029/2004JD004924.

ECK, T.F., HOLBEN, B.N., REID, J.S., DUBOVIK, O., SMIRNOV, A., O’NEILL, N.T., SLUTSKER, I.and KINNE, S., 1999, Wavelength dependence of the optical depth of biomass burn-ing, urban, and desert dust aerosols. Journal of Geophysical Research, 104, pp. 31333–31 350.

FERNALD, F.G., 1984, Analysis of atmospheric lidar observations: some comments. AppliedOptics, 23, pp. 652–653.

FORSTER, P., RAMASWAMY, V., ARTAXO, P., BERNTSEN, T., BETTS, R., FAHEY, D.W., HAYWOOD,J., LEAN, J., LOWE, D.C., MYHRE, G., NGANGA, J., PRINN, R., RAGA, G., SCHULZ,M. and VAN DORLAND, R., 2007, Changes in atmospheric constituents and in radia-tive forcing. In Climate Change 2007: the Physical Science Basis. Contribution ofWorking Group I to the Fourth Assessment Report of the Intergovernmental Panel onClimate Change, S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt,M.Tignor and H.L. Miller (Eds.), pp. 129–234 (Cambridge: Cambridge UniversityPress).

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7842 A. K. Srivastava et al.

GADHAVI, H. and JAYARAMAN, A., 2006, Airborne lidar study of the vertical distribution ofaerosols over Hyderabad, an urban site in central India, and its implication for radiativeforcing calculations. Annales Geophysicae, 24, pp. 2461–2470.

GAUTAM, R., HSU, N.C., LAU, K.M., and KAFATOS, M., 2009, Aerosol and rainfall vari-ability over the Indian monsoon region: distributions, trends and coupling. AnnalesGeophysicae, 27, pp. 3691–3703.

HANSEN, A.D.A., 2003, The Aethalometer Manual (Berkeley, CA: Magee Scientific).HANSEN, J., SATO, M. and RUEDY, R., 1997, Radiative forcing and climate response. Journal of

Geophysical Research, 102, pp. 6831–6864.HE, Q.S., LI, C.C., MAO, J.T., LAU, A.K.H. and LI, P.R., 2006, A study on the aerosol

extinction-to-back-scatter ratio with a combination of micro-pulse LIDAR andMODIS over Hong Kong. Atmospheric Chemistry and Physics, 6, pp. 3243–3256.

HEGDE, P., PANT, P. and BHAVANIKUMAR, Y., 2009, An integrated analysis of lidar observationsin association with optical properties of aerosols from a high altitude location in centralHimalayas. Atmospheric Science Letters, 10, pp. 48–57.

HEGDE, P., PANT, P., NAJA, M., DUMKA, U.C. and SAGAR, R., 2007, South Asian dust episodein June 2006: aerosol observations in the central Himalayas, Geophysical ResearchLetters, 34, L23802, doi:10.1029/2007GL030692.

HESS, M., KOEPKE, P. and SCHULTZ, I., 1998, Optical properties of aerosols and clouds:the software package OPAC. Bulletin of the American Meteorological Society, 79,pp. 831–844.

HUSAR, R.B., PROSPERO, J.M. and STOWE, L.L., 1997, Characterization of troposphericaerosols over the oceans with the NOAA advanced very high resolution radiome-ter optical thickness operational product. Journal of Geophysical Research, 102, pp.16 889–16 909.

IPCC, 1996, Climate Change 1995: The Science of Climate Change. Contribution of WorkingGroup I to the Second Assessment of the Intergovernmental Panel on Climate Change,J.T. Houghton, L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg and K.Maskell (Eds.) (Cambridge: Cambridge University Press).

IPCC, 2001, Climate Change 2001: The Scientific Basis. Contribution of Working Group I to theThird Assessment of the Intergovernmental Panel on Climate Change, J.T. Houghton, Y.Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell and C.F. Johnson(Eds.) (Cambridge: Cambridge University Press).

IPCC, 2007, Climate Change 2007: The Physical Science Basis. Contribution of WorkingGroup I to the Fourth Assessment Report of the Intergovernmental Panel on ClimateChange, S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt,M.Tignor and H.L. Miller (Eds.), Chapter 2, p. 129 (Cambridge: Cambridge UniversityPress).

ISRAELEVICH, P.L., LEVIN, Z., JOSEPH, J.H. and GANOR, E., 2002, Desert aerosol transportin the Mediterranean region as inferred from the TOMS aerosol index. Journal ofGeophysical Research, 107, 4572, doi:10.1029/2001JD002011.

KAUFMAN, Y.J., TANRE, D. and BOUCHER, O., 2002, A satellite view of aerosols in the climatesystem. Nature, 419, pp. 215–223.

KAUFMAN, Y.J. , TANRE, D., DUBOVIK, O., KARNIELI, A. and REMER, L.A., 2001, Absorptionof sunlight by dust as inferred from satellite and ground-based remote sensing.Geophysical Research Letters, 28, pp. 1479–1483.

KLETT, J.D., 1985, Lidar inversion with variable back-scatter/extinction ratios. Applied Optics,24, pp. 1638–1643.

KONDRATYEV, K.Y. and VAROTSOS, C.A., 1995a, Atmospheric greenhouse effect in the contextof global climate change. Nuovo Cimento della Societa Italiana di fisica C, Geophysicsand Space Physics, 18, pp. 123–151.

KONDRATYEV, K.Y. and VAROTSOS, C.A., 1995b, Atmospheric ozone variability in the contextof global change. International Journal of Remote Sensing, 16, pp. 1851–1881.

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7843

LAU, K.-M., KIM, M.K. and KIM, K.M., 2006, Asian summer monsoon anomalies inducedby aerosol direct forcing: the role of the Tibetan Plateau. Climate Dynamics, 26, pp.855–864.

LI, J., CARLSON, B.E. and LACIS, A.A., 2009, A study on the temporal and spatial vari-ability of absorbing aerosols using total ozone mapping spectrometer and ozonemonitoring instrument aerosol index data. Journal of Geophysical Research, 114,D09213, doi:10.1029/2008JD011278.

LIU, X., YIN, Z.-Y., ZHANG, X. and YANG, X., 2004, Analyses of the spring dust storm fre-quency of northern China in relation to antecedent and concurrent wind, precipitation,vegetation, and soil moisture conditions. Journal of Geophysical Research, 109, D16210,doi:10.1029/2004JD004615.

MOORTHY, K.K., BABU, S.S. and SATHEESH, S.K., 2005, Aerosol characteristics and radiativeimpacts over the Arabian Sea during the inter-monsoon season: results from ARMEXfield campaign. Journal of Atmospheric Science, 62, pp. 192–206.

MORYS, M., MIMS, F.M., III, HAGERUP, S., ANDERSON, S.E., BAKER, A., KIA, J. andWALKUP, T., 2001, Design, calibration and performance of the Microtops-II hand-held ozone monitor and sun photometer. Journal of Geophysical Research, 106,pp. 14 573–14 582.

MÜLLER, D., FRANKE, K., WAGNER, F., ALTHAUSEN, D., ANSMANN, A., HEINTZENBERG, J.and VERVER, G., 2001, Vertical profiling of optical and physical particle propertiesover the tropical Indian Ocean with six-wavelength lidar 2: case studies. Journal ofGeophysical Research, 106, pp. 28 577–28 595.

NEE, J.B., CHIANG, C.W., HU, H., HU, S. and YU, J.Y., 2007, Lidar measurements of Asiandust storms and dust cloud interactions. Journal of Geophysical Research, 112, D15202,doi:10.1029/2007JD008476.

PANDITHURAI, G., DIPU, S., DANI, K.K., TIWARI, S., BISHT, D.S., DEVARA, P.C.S. and PINKER,R.T., 2008, Aerosol radiative forcing during dust events over New Delhi, India. Journalof Geophysical Research, 113, D13209, doi:10.1029/2008JD009804.

PANT, P., HEGDE, P., DUMKA, U.C., SAGAR, R., SATHEESH, S.K., MOORTHY, K.K., SAHA,A. and SRIVASTAVA, M.K., 2006, Aerosol characteristics at a high-altitude locationin central Himalayas: optical properties and radiative forcing. Journal of GeophysicalResearch, 111, D17206, doi:10.1029/2005JD006768.

PAPAYANNIS, A., AMIRIDIS, V., MONA, L., TSAKNAKIS, G., BALIS, D., BOSENBERG, J.,CHAIKOVSKI, A., DE TOMASI, F., GRIGOROV, I., MATTIS, I., MITEV, V., MÜLLER, D.,NICKOVIC, S., PE´REZ, C., PIETRUCZUK, A., PISANI, G., RAVETTA, F., RIZI, V., SICARD,M., TRICKL, T., WIEGNER, M., GERDING, M., MAMOURI, R.E., D’AMICO, G. andPAPPALARDO, G., 2008, Systematic lidar observations of Saharan dust over Europe inthe frame of EARLINET (2000–2002). Journal of Geophysical Research, 113, D10204,doi:10.1029/2007JD009028.

PAPAYANNIS, A., ZHANG, H.Q., AMIRIDIS, V., JU, H.B., CHOURDAKIS, G., GEORGOUSSIS, G.,PÉREZ, C., CHEN, H.B., GOLOUB, P., MAMOURI, R.E., KAZADZIS, S., PARONIS, D.,TSAKNAKIS, G. and BALDASANO, J.M., 2007, Extraordinary dust event over Beijing,China, during April 2006: lidar, sun photometric, satellite observations and modelvalidation. Geophysical Research Letters, 34, L07806, doi:10.1029/2006GL029125.

PARK, S.U., CHANG, L.S. and LEE, E.H., 2005, Direct radiative forcing due to aerosols in eastAsia during a Hwangsa (Asian dust) event observed on 19–23 March 2002 in Korea.Atmospheric Environment, 39, pp. 2593–2606.

PRASAD, A.K. and SINGH R.P., 2007, Changes in aerosol parameters during major dust stormevents (2001–2005) over the Indo-Gangetic Plains using AERONET and MODIS data.Journal of Geophysical Research, 112, D09208, doi:10.1029/2006JD007778.

PRASAD, A.K., SINGH, S., CHAUHAN, S.S., SRIVASTAVA, M.K., SINGH, R.P. and SINGH, R.,2007, Aerosol radiative forcing over the Indo-Gangetic plains during major dust storms.Atmospheric Environment, 41, pp. 6289–6301.

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

7844 A. K. Srivastava et al.

PROSPERO, J.M., GINOUX, P., TORRES, O., NICHOLSON, S.E. and GILL, T.E., 2002,Environmental characterization of global sources of atmospheric soil dust identi-fied with the nimbus7 total ozone mapping spectrometer (TOMS) absorbing aerosolproduct. Review of Geophysics, 40, 1002, doi:10.1029/2000RG000095.

RAMANA, M., RAMANATHAN, V., PODGORNY, I.A., PRADHAN, B.B. and SHRESTHA, B., 2004,The direct observations of large aerosol radiative forcing over the Himalayan region:validation. Geophysical Research Letters, 31, L05111, doi:10.1029/2003GL018824.

REID, J.S., ECK, T.F., CHRISTOPHER, S.A., HOBBS, P.V. and HOLBEN, B., 1999, Use ofthe Ångström exponent to estimate the variability of optical and physical prop-erties of aging smoke particles in Brazil. Journal of Geophysical Research, 104,pp. 27 473–27 490.

RICCHIAZZI, P., YANG, S., GAUTIER, C. and SOWLE, D., 1998, SBDART: a research and teachingsoftware tool for plane-parallel radiative transfer in the earth’s atmosphere. Bulletin ofthe American Meteorological Society, 79, pp. 2101–2114.

SAGAR, R., KUMAR, B., DUMKA, U.C., MOORTHY, K.K. and PANT, P., 2004, Characteristics ofaerosol spectral optical depths over Manora Peak: a high-altitude station in the centralHimalayas. Journal of Geophysical Research, 109, D06207, doi:10.1029/2003 JD003954.

SAKAI, T., SHIBATA, T., IWASAKA, Y., NAGAI, T., NAKAZATO, M., MATSUMURA, T., ICHIKI, A.,KIM, Y.-S., TAMURA, K., TROSHKIN, D. and HAMDI, S., 2002, Case study of Ramanlidar measurements of Asian dust events in 2000 and 2001 at Nagoya and Tsukuba,Japan. Atmospheric Environment, 36, pp. 5479–5489.

SASSEN, K., DEMOTT, P.J., PROSPERO, J.M. and POELLOT, M.R., 2003, Saharan dust stormsand indirect aerosol effects on clouds: CRYSTAL-FACE results. Geophysical ResearchLetters, 30, 1633, doi:10.1029/2003GL017371.

SATHEESH, S.K. and MOORTHY, K.K., 2005, Radiative effects of natural aerosols: a review.Atmospheric Environment, 39, pp. 2089–2110.

SATHEESH, S.K. and SRINIVASAN, J., 2006, A method to estimate aerosol radiative forcing fromspectral optical depths. Journal of Atmospheric Science, 63, pp. 1082–1092.

SINGH, R.P., DEY, S., TRIPATHI, S.N., TARE, V. and HOLBEN, B.N., 2004, Variability of aerosolparameters over Kanpur city, northern India. Journal of Geophysical Research, 109,D23206, doi:10.1029/2004JD004966.

SINGH, S., NATH, S., KOHLI, R. and SINGH, R., 2005. Aerosols over Delhi during pre-monsoonmonths: characteristics and effects on surface radiation forcing. Geophysical ResearchLetters, 32, L13808, doi:10.1029/2005GL023062.

SPINHIRNE, J.D., 1993, Micro-pulse lidar. IEEE Transactions on Geosciences and RemoteSensing, 31, pp. 48–54.

SRIVASTAVA, A.K., DEVARA, P.C.S., RAO, Y.J., BHAVANIKUMAR, Y. and RAO, D.N., 2008,Aerosol optical depth, ozone and water vapor measurements over Gadanki, a tropicalstation in peninsular India. Aerosol and Air Quality Research, 8, pp. 459–476.

STAMNES, K., TSAY, S., WISCOMBE, W. and JAYAWEERA, K., 1988, Numerically stable algo-rithm for discrete-ordinate-method radiative transfer in multiple scattering and emittinglayered media. Applied Optics, 27, pp. 2502–2509.

TEGEN, I., LACIS, A.A. and FUNG, I., 1996, The influence on climate forcing of mineral aerosolsfrom disturbed soils. Nature, 380, pp. 419–422.

TODD, M.C., WASHINGTON, R., MARTINS, J.V., DUBOVIK, O., LIZCANO, G., M’BAINAYEL, S.and ENGELSTAEDTER, S., 2007, Mineral dust emission from the Bodélé Depression,northern Chad, during BoDEx 2005. Journal of Geophysical Research, 112, D06207,doi:10.1029/2006JD007170.

TWOMEY, S.A., 1977, The influence of pollution on the shortwave albedo of clouds. Journal ofAtmospheric Science, 34, pp. 1149–1152.

TZANIS, C. and VAROTSOS, C.A., 2008, Tropospheric aerosol forcing of climate: a case study forthe greater area of Greece. International Journal of Remote Sensing, 29, pp. 2507–2517.

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14

Impact of a dust storm on aerosol in the central Himalayas 7845

VAROTSOS, C., 2005, Airborne measurements of aerosol, ozone, and solar ultraviolet irra-diance in the troposphere. Journal of Geophysical Research, 110, D09202, doi:10.1029/2004JD005397.

VAROTSOS, C.A., ONDOV, J.M., CRACKNELL, A.P., EFSTATHIOU, M.N. and ASSIMAKOPOULOS,M.N., 2006, Long-range persistence in global aerosol index dynamics. InternationalJournal of Remote Sensing, 27, pp. 3593–3603.

WASHINGTON, R., TODD, M., MIDDLETON, N.J. and GOUDIE, A.S., 2003, Dust storm sourceareas determined by the total ozone monitoring spectrometer and surface observations.Annals of the Association of American Geographers, 93, pp. 297–313.

Dow

nloa

ded

by [

Cor

nell

Uni

vers

ity L

ibra

ry]

at 0

8:36

13

Nov

embe

r 20

14