Annual variations of the altitude distribution of aerosols and effect of long-range transport over the southwest Indian Peninsula

Atmospheric Environment 81 (2013) 51e59

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Annual variations of the altitude distribution of aerosols and effect of long-range transport over the southwest Indian Peninsula Manoj Kumar Mishra*, K. Rajeev, Bijoy V. Thampi 1, Anish Kumar M. Nair Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Long-term observations of aerosol altitude distribution in tropical Indian coast.
Elevated layers of highly nonspherical aerosols in widespread aerosol plumes.
Long-range transport enhances aerosol loading at 2e4 km altitude by 5e10 times.
Highly systematic and prominent annual variation of aerosols at 2 e4 km altitude.
Variation of aerosol loading at <1 km altitude during different seasons are <20%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 March 2013 Received in revised form 28 August 2013 Accepted 31 August 2013

Annual variations of the altitude distribution of aerosols and the effect of long-range transport in modulating the aerosol loading over Thiruvananthapuram (8.5 N, 77 E), a relatively clean tropical station located in the southwest coast of Peninsular India, are investigated using dual polarization Micro Pulse Lidar observations carried out during March 2008eMay 2011. Combined analysis of these lidar observations with the spatial distribution of aerosols derived from satellite data shows the occurrence of elevated layers of highly non-spherical aerosols in the 1.5e4 km altitude region, which are associated with the wide-spread aerosol plumes over the Arabian Sea during the pre-monsoon and summermonsoon seasons. In contrast, w90% of the column integrated aerosol backscatter coefficient (ba) (below 5 km altitude) occurs below w1.5 km during winter. Seasonal variation of mean ba below w1 km altitude is <20%. Altitude profiles of ba above w1 km during January e characterised by the smallest values of ba, absence of elevated aerosol layers, and weak atmospheric winds e may be considered as the upper limit of the contribution by locally produced aerosols for quantifying the effect of long-range transport during the other months. Compared to January, a 3e10 fold increase in ba occurs in the 2 e4 km altitude region during AprileMay and JulyeAugust. The elevated layers contribute w20e30% of the total aerosol loading during the above months. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Aerosols Lidar Indian region Elevated aerosol layer Depolarization ratio

1. Introduction * Corresponding author. Tel.: þ91 471 2563224; fax: þ91 471 2706535. E-mail addresses: [email protected], [email protected] (M.K. Mishra). 1 Presently at Science Systems and Applications, Inc., 1 Enterprise Parkway, Hampton, VA 23666, USA. 1352-2310/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2013.08.066

Atmospheric aerosols significantly modulate the radiation budget, cloud properties, atmospheric thermodynamics, and overall climate of the Earth-atmosphere system (e.g., IPCC, 2007).

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Knowledge of the spatial and vertical distribution of aerosols is essential for quantifying the effect of long-range transport in modulating the aerosol abundance at regions far from their sources (e.g., Rajeev et al., 2010). Altitude distribution of aerosols has an important bearing on the radiative heating and hence the thermal stability of the atmosphere (Thampi et al., 2009) as well as the semi-direct effect of aerosols (Ackerman et al., 2000). Aerosol-cloud interaction and the indirect effect of aerosols (Albrecht, 1989) also depend on their vertical distribution. These effects of aerosols on clouds, radiation budget and hydrological cycle are among the major uncertainty factors in climate prediction (IPCC, 2007). As the atmospheric residence time of aerosols in the lower and middle troposphere is rather small (typically less than a week), the aerosol distribution undergoes considerable spatial variations, and should be studied on a regional scale. Indian subcontinent and the oceanic regions surrounding it are considerably influenced by the long-range transport of aerosols (e.g., Moorthy et al., 1997, 2010; Satheesh and Ramanathan, 2000; Ramanathan et al., 2001; Ramachandran, 2004; Jayaraman et al., 2006; Niranjan et al., 2007; Lawrence and Lelieveld, 2010). Satellite observations showed that the transport of aerosols from the Asian continent to the adjoining oceans increases from winter (DecembereFebruary) to pre-monsoon (MareMay) season and maximizes during the summer monsoon season (JuneeSeptember) (Nair et al., 2005). Detailed investigations on the aerosol properties over the Arabian Sea were conducted during the Indian Ocean Experiment (INDOEX) (Ramanathan et al., 2001) and the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) (Moorthy et al., 2010), as well as the earlier ship-borne measurements conducted as part of various scientific expeditions (e.g., Moorthy et al., 1997). During the pre-monsoon season, oceanic regions around the Indian subcontinent are significantly influenced by the long-range transport of continental aerosols from Arabia, Indian subcontinent and Southeast Asia, leading to a pronounced aerosol plume off the southwest coast of Peninsular India, which reduce the diurnal mean surface-reaching solar flux by about 15e 35 Wm2 (Satheesh and Ramanathan, 2000; Ramanathan et al., 2001). During the summer monsoon season, intense plumes of mineral dust originating from the West Asian Deserts engulf almost the entire north and central Arabian Sea and reach at least up to the west coast of Peninsular India (e.g., Nair et al., 2005; Mishra et al., 2010). On the contrary, long-range transport of aerosols and total atmospheric aerosol loading over the Arabian Sea are considerably small during the winter season (e.g., Nair et al., 2005). Lidar observations of the altitude profiles of aerosols over Maldives (4.1 N, 73.3 E) revealed the presence of an elevated aerosol layer in the altitude band of 1.5e3.5 km during the premonsoon season (Ansmann et al., 2000; Müller et al., 2003). Airborne measurements and ship-borne lidar observations showed that this elevated aerosol layer covers a wide region in the Arabian Sea during the above period (Léon et al., 2002; Welton et al., 2002). Long-term (w15 years) bi-static lidar observations of the vertical distribution of aerosols in the altitude band of 501000 m over Thiruvananthapuram (8.5 N, 77 E) have brought out the aerosol distribution in the nocturnal atmospheric boundary layer and their annual and inter-annual variations in this coastal environment (Parameswaran, 2001). Micro pulse lidar observations of the vertical distribution of aerosols over this location showed layers of highly non-spherical aerosols during the premonsoon and summer monsoon seasons (Mishra et al., 2010; Rajeev et al., 2010). The above observations clearly showed that the Arabian Sea and southwest Peninsular India witnesses large, but distinctly different aerosol plumes during the pre-monsoon and summer monsoon seasons. However, studies on the altitude distribution of tropospheric aerosols during the post-monsoon

(OctobereNovember) and winter seasons as well as their annual and interannual variations over this region are highly limited. We aim to fill this gap using the dual polarization Micro Pulse Lidar (MPL) observations carried out at Thiruvananthapuram (8.5 N, 77 E), located in the southwest coast of Peninsular India adjoining the Arabian Sea. These lidar observations, integrated with satellite-based observations of regional aerosol distribution, provide improved understanding of the vertical distribution of aerosols in the highly prominent aerosol plumes which persist during different seasons. Thiruvananthapuram represents a relatively clean coastal environment devoid of major pollution sources. Hence, these observations can be used for investigating the effect of long-range transport in regulating the abundance and properties of aerosols in an otherwise clean environment. Main objectives of the present study are: (1) to present a systematic analysis of the mean altitude distribution of aerosols in the troposphere below w5 km and its monthly, seasonal, annual and interannual variations over this region, and (2) to estimate the potential effect of long-range transport in modulating the vertical distribution of aerosols and aerosol type during different seasons. 2. Lidar site, data and method of analysis 2.1. Geographical location of Thiruvananthapuram and its meteorological conditions Regular observations on the vertical distribution of aerosols using the dual polarization MPL were carried out at Thumba, Thiruvananthapuram (geographical location of Thumba is marked in Fig. 2). The lidar site is located w500 m inland from the southeast Arabian Sea. Monthly mean climatology of the daily minimum and maximum air temperatures near the surface, number of days with rain and thunder (thunder being a proxy for atmospheric convection) and mean rainfall at Thiruvananthapuram obtained from the long-term (>30 years) observations carried out by the India Meteorological Department are shown in Fig. 1. The monthly mean wind speed and direction at 1 km above the mean sea level derived from radiosonde data are also shown in Fig. 1. Based on the prevailing meteorological conditions, analysis of the MPL data are grouped into 4 seasons, viz. winter (DecembereFebruary), premonsoon (MarcheMay), summer monsoon (JuneeSeptember) and post-monsoon (OctobereNovember). Of these, winter and summer monsoon are the two contrasting seasons while premonsoon and post-monsoon are transition periods. On average, dry conditions with scanty rainfall prevail at Thiruvananthapuram during winter. The daily minimum and maximum temperatures, number of days with rain and thunder, and rainfall increases with the advancement of the pre-monsoon season. Thiruvananthapuram is the gateway for the Asian summer monsoon to the Indian subcontinent. Climatological onset date of summer monsoon rain over Thiruvananthapuram is 1 June. Rainfall is largest in June and decreases with the advancement of the summer monsoon season. The convective activity as well as rainfall shows a secondary peak during the post-monsoon season. The synoptic wind speed is low during the period of November to April and increases significantly from May reaching its peak value in July and decreases subsequently. The wind direction is northeasterly during the November to March period, and changes to northerly/ northwesterly during AprileMay and to westerly during June to October. 2.2. Lidar data and method of analysis Details of the dual polarization MPL system, method of data processing, incorporation of correction for the detector dead-time,

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Fig. 1. Monthly mean climatology of the meteorological parameters at Thiruvananthapuram: (a) minimum and maximum near-surface temperatures, (b) number of days with rain and thunder, (c) rainfall and (d) wind speed and direction at 1 km altitude.

after-pulse effects and geometrical factors, inversion of the data to retrieve aerosol backscatter coefficient (ba) and linear depolarization ratio (LDR), potential sources of errors and uncertainty limits of the retrieved parameters are described in detail elsewhere (Flynn

Fig. 2. Seasonal mean spatial distribution of MODIS-derived AOD over the oceanic regions around the Indian subcontinent during winter (DJF), pre-monsoon (MAM), summer monsoon (JJAS), and post-monsoon (ON) seasons (averaged during 2008e 2011). The black filled-circle in the southwest Peninsular India marks the location of Thiruvananthapuram.

et al., 2007; Mishra et al., 2010, 2012; Rajeev et al., 2010). The MPL consists of a diode-pumped Nd:YAG laser as transmitter which emit laser pulses of 7 ns width (selectable energy of 2e8 mJ pulse1) at the wavelength of 532 nm with a pulse repetition rate of 2500 Hz. The system was operated with a range resolution of 30 or 60 m and time integration of 60 s. As the MPL system uses same telescope (diameter of 178 mm) for both transmission and reception, the lidar signals are obtained from a reasonably short range of 90 m onwards. The lidar system alternates between two states of polarization (co-polarized and cross-polarized) at an interval of 60 s. The raw data are subsequently corrected for detector noise, dead-time, and range-dependent geometrical correction factor (Welton et al., 2002; Rajeev et al., 2010; Mishra et al., 2012). The system is operated regularly during rain-free days. Data recorded during the period of 25 March 2008e30 May 2011 are used for the present study. Since the focus is on the vertical distribution of aerosols, those data which are free of clouds at least up to an altitude of 6 km for a minimum duration of 30 min only are used. Extreme care is taken to avoid the cloud contamination by examining the temporal and altitude variations of the lidar backscatter signals as well as through visual observations of the prevailing cloud conditions recorded during the lidar operation. The raw lidar data are further integrated for a period of 30e60 min (depending on the availability of cloud-free data) before inverting to retrieve ba and LDR. This is essential to improve the signal-tonoise ratio at higher altitudes. Fernald’s method is used for inverting the lidar data (Fernald, 1984). The LDR is estimated following the method described by Flynn et al. (2007). The LDR is an indicator of sphericity of aerosols: larger the value of the LDR, larger is the non-sphericity. These profiles on individual days are averaged for each month to derive the monthly mean profiles. For inverting the lidar signal, it is necessary to assume an appropriate value of the lidar ratio (S1). Earlier observations over the southeast Arabian Sea (Welton et al., 2002; Franke et al., 2003) showed that the values of S1 during winter and pre-monsoon seasons generally vary in the range 20e70 Sr with a clustering

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around 30-50 Sr. In the present study, a mean value of 40 Sr is adopted for S1 for all seasons. The molecular backscattering and extinction coefficients are estimated using standard atmospheric model of pressure and temperature for the respective month applicable for Thiruvananthapuram. During night, the reference altitude (h0) is fixed at 20 km. During the daytime, due to the relatively low signal-to-noise ratio (SNR), the maximum altitude up to which useful lidar signal could be available (and hence h0) decreases to w6e8 km. Due to these reasons, the altitude profiles of ba and LDR up to 5 km altitude only are presented here. Sensitivity analysis (for variations in the assumed value of S1, and the boundary condition of ba at h0) shows a typical uncertainty of w20% in the derived ba. As the same receiving system is used to measure the lidar signal for both polarizations, the system-induced uncertainty in LDR is negligible at all altitudes. The overall uncertainty of LDR below 5 km is <30%. Aerosol optical depth (AOD) during cloud-free days is measured using a sunphotometer (Microtop-II, Solar Light Co.) at five wavelengths: 440, 500, 675, 936 and 1020 nm. This instrument is periodically calibrated using LangleyeBouger technique and compared with similar instruments. Maximum uncertainty in the measured AOD is 0.04. The AOD at 532 nm (the MPL wavelength) is estimated through interpolation by fitting an inverse power law for the wavelength dependence to the observed spectral variation of AOD. In order to further minimize the uncertainty in the lidar-derived altitude profiles of aerosol extinction and backscatter coefficients, they are further weighed by this AOD at 532 nm. 3. Results and discussion 3.1. Seasonal mean spatial distribution of AOD over the surrounding oceanic regions The seasonal mean spatial distribution of AOD over the Arabian Sea and the Bay of Bengal adjoining the coastal regions of peninsular India derived from Terra-MODIS satellite data (Remer et al.,

2005) during winter, pre-monsoon, summer monsoon, and postmonsoon seasons (averaged during 2008e2011) are depicted in Fig. 2. Daily mean AOD values gridded at a spatial resolution of 1  1, obtained from the Level-3 Terra-MODIS data (MOD08_D3, Collection 5) are used for estimating the seasonal mean AOD. Overall, the uncertainty of the MODIS-derived AOD is 0.03  0.05  AOD over the oceanic regions (Remer et al., 2005). As the regional distribution and transport of AOD over the Arabian Sea are reported in the literature (e.g., Rajeev et al., 2000; Nair et al., 2005), the description of regional aerosol distribution in this paper is limited to the essential information required to understand the distribution of aerosols and the effect of long-range transport over the observation site. The seasonal mean AOD and its spatial variations over the Arabian Sea are minimum during winter. On average, AOD over the west coast of the peninsular India increases from winter to pre-monsoon season. Associated with the lower tropospheric northerly winds from the northern continents, a well developed aerosol plume with AOD in the range of 0.4e0.5 spreads over the southeast Arabian Sea, covering the coastal region of Thiruvananthapuram, during the pre-monsoon season. It is most prominent at north of w7 N between the west coast of India and w68 E longitude, with a steady decrease in AOD with increasing distance from the coast. The northerly winds from the continental regions during the pre-monsoon season give way to the massive summer monsoon flow from the otherwise pristine Indian Ocean (Nair et al., 2005; Mishra et al., 2010). However, the wind flow over the north and central Arabian Sea during the summer monsoon are predominantly westerly/northwesterly, especially above w2 km altitude (Nair et al., 2005; Mishra et al., 2010). Associated with the above changes in atmospheric circulation, the aerosol plume in the Arabian Sea off the southwest Indian peninsular coast during the pre-monsoon season gets replaced by an intense mineral dust plume from the west Asian deserts, which engulfs the entire north and central Arabian Sea, during the summer monsoon season. The seasonal mean AOD in this plume is largest over the northwest Arabian Sea (AOD > 0.6), and decreases towards the east, with AOD

Fig. 3. Typical examples of the altitudeetime plots of the range-corrected lidar backscattered signal (RCS in the co-polarized channel, in log-scale) (left panels) and LDR (right panels) during typical days in winter (19 January 2011), pre-monsoon (03 March 2009), summer monsoon (05 August 2009) and post-monsoon (03 October 2008) seasons. The vertical white patches indicate data gaps.

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w0.4e0.5 over the west coast of India. The southeastern boundary of this mineral dust plume reaches up to the coastal regions of Thiruvananthapuram. The aerosol plume disappears during the post-monsoon season, when AOD over most parts of the Arabian Sea is <0.4. The spatial distribution of AOD over the Arabian Sea during the post-monsoon season is almost similar to that in winter. 3.2. Time-altitude cross sections of lidar signal during different seasons Typical examples of the time-altitude cross sections of rangecorrected lidar backscattered signal (RCS in the co-polarized channel, which is an indicator of aerosol abundance) and LDR during typical days in winter (19 January 2011), pre-monsoon (03 March 2009), summer monsoon (05 August 2009) and postmonsoon (03 October 2008) seasons are depicted in Fig. 3. Though the absolute magnitudes of RCS and LDR show significant day-to-day variations during any given season, general tendencies in their altitude variations are somewhat similar during the season (e.g., Rajeev et al., 2010). For example, as seen in Fig. 3, the aerosol abundance and their temporal variations during the course of the day are largest in the lower troposphere (below w2 km) during all seasons. During almost all days in winter, the aerosol abundance rapidly decreases with altitude above 2 km, and the values of LDR are <0.05, indicating the prevalence of highly spherical aerosols at all altitudes. Compared to winter, most of the days in the premonsoon season experience significant aerosol loading in the 2e 3.5 km altitude band and the decrease in RCS is remarkably rapid above w3.5 km. In general, the values of LDR are also larger (w0.05e0.15) at all altitudes during this season; however, the occurrence of higher values of LDR is more prominent in the 2e 3.5 km altitude band. During the summer monsoon season, altitude variation of RCS is rather weak up to w4 km during most of the days, which is also associated with larger values of LDR (typically 0.08e0.25) above w1 km. This indicates the existence of distinctly different aerosol types below and above w1 km altitude over the study region during the summer monsoon season. Vertical variations of RCS and LDR during most of the days in the post-monsoon season resemble those in winter. Altitude distribution of aerosols in the atmospheric boundary layer (ABL) undergoes significant

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diurnal variation, which is well discernible during winter, premonsoon and post-monsoon seasons. Typically, the mixing height varies from <500 m in the early morning to >1.5 km in the afternoon during the above seasons (e.g., Mishra et al., 2012). During the summer monsoon season, the afternoon peak value of mixing height is generally less compared to the other seasons (e.g., Parameswaran, 2001), typically <800 m. This study focuses on the monthly mean altitude profiles of aerosol distribution. 3.3. Monthly mean altitude variations of ba and LDR and their interannual variations Altitude-time cross sections of the monthly mean ba and LDR during March 2008eMay 2011 are depicted in Fig. 4. Fig. 4(a) shows a pronounced annual variation in the monthly mean profiles of ba, especially above w1 km altitude. In general, ba is least in January and September and largest during AprileMay. A secondary increase in ba is observed during JulyeAugust. During most of the years, this secondary peak is separated from the pre-monsoon peak by the relatively smaller values of ba observed in June, which witnesses a transition of airmass type associated with the onset of summer monsoon. In comparison with the pre-monsoon and summer monsoon seasons, aerosol abundance is generally small during the post-monsoon months of OctobereNovember. The magnitude of the annual variations of ba is largest above 2 km, where the aerosol loading during DecembereJanuary is the least, while it is markedly larger during AprileMay and JulyeAugust. The observed pattern of the annual variation is repeated every year. However, the year-toyear variation of ba is quite significant during June and the postmonsoon season. Fig. 4(b) shows a predominant annual variation of LDR, especially above w1 km altitude. During the post-monsoon and winter seasons, the aerosols are highly spherical at all altitudes with LDR<0.05. Interannual variations of LDR during these seasons are negligible. An elevated layer of highly non-spherical aerosols is observed in the altitude band of w2e4 km during the pre-monsoon season, especially in April and May. As shown in Rajeev et al. (2010), the elevated layer of non-spherical aerosols during the premonsoon season arises from the long-range transport of aerosols from the northern/northwestern parts of the Indian subcontinent

Fig. 4. Time-altitude cross sections of the monthly mean values of (a) log(ba) and (b) LDR during March 2008 to May 2011.

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by the prevailing northerly/northeasterly winds in the lower and middle troposphere. Considerable interannual variations occur in the monthly mean value of the LDR in this elevated layer during the pre-monsoon season. Mean values of LDR in the 2e4 km altitude band during AprileMay of 2010 are generally <0.05 while those during the corresponding months of 2008 are w0.1. In all years, the largest non-sphericity of aerosols is observed in July when a distinct elevated layer of high LDR values (monthly mean LDR values in the range of 0.1e0.2) is observed in the altitude band of w1.5e4 km. This layer persists during August as well, though the mean value of LDR is slightly reduced. On the contrary, aerosols below w1 km during this season are highly spherical (mean LDR < 0.05). During the summer monsoon season, the airmass reaching Thiruvananthapuram at <2 km altitude is predominantly from the pristine Indian Ocean carried by the strong southerly winds, while that at 2e4 km altitude is a mixture of the airmass from the west Arabian Sea (having considerable amount of mineral dust) and the pristine Indian Ocean (Mishra et al., 2010). The low value of LDR at <1 km altitude indicates highly spherical nature of these aerosols and might be due to the hygroscopic growth of marine aerosols (significant fraction of which might be sea salt) in the highly humid lower troposphere during the summer monsoon. On the contrary, the presence of highly non-spherical mineral dust results in higher values of LDR observed in the altitude band of 2e4 km during the summer monsoon season. The altitude profiles of LDR observed during JulyeAugust have the following major differences compared to those in AprileMay: (i) in general, the mean values of LDR are relatively smaller (<0.05) in the lower troposphere, and (ii) the non-sphericity of aerosols in the elevated layer is larger but its inter-annual variations are smaller. The month of June marks the transition of airmass types arriving over the Indian Peninsula. Satellite observations show that the transport of dust from the Arabian Desert during June is mostly limited to the northern Arabian Sea and the contribution of dust to the AOD over the southeast Arabian Sea and southwest coast of Peninsular India are usually small (e.g., Nair et al., 2005). In general, this results in a weak aerosol loading with relatively spherical aerosols over the southwest coast of Indian Peninsula in June, as observed in Fig. 4 (a,b). However, small changes in the prevailing wind direction can cause large changes in dust concentration over the southeast Arabian Sea. This leads to large interannual variations of the elevated aerosol layer of highly non-spherical aerosols in June. For example, the values of ba as well as LDR during June 2008 and 2010 are distinctly larger than those in June 2009. 3.4. Multi-year averaged monthly and seasonal variations of ba and LDR In order to clearly bring out the annual variation of ba and LDR, multi-year average (2008e2011) monthly mean altitude variations of these parameters are depicted in Fig. 5(a,b). The percentage contribution of aerosols at each altitude (z) to the total aerosol content in the atmosphere below 5 km is estimated by integrating ba between the surface and ‘z’ and normalizing it with the total column integrated backscatter coefficient (IBC) up to 5 km altitude. This parameter is defined as the normalized cumulative contribution (NCC) and is shown in Fig. 5(c). Throughout the year, altitude variation of mean ba is least below the altitude of w800 m. This is primarily because of the well-mixed aerosol distribution in the diurnally averaged mixing region. Variation in the absolute magnitude of mean ba in this altitude band during different months is <30%. Above w1 km altitude, the mean value of ba shows a systematic annual variation with an increase from January to Aprile May and JulyeAugust, followed by a general decreasing trend till December. The LDR profiles show highly spherical nature of

Fig. 5. Annual variation of the multi-year average (2008e2011) monthly mean altitude profiles of (a) log(ba), (b) LDR, and (c) NCC during JanuaryeDecember.

aerosols at all altitudes during October to February period, with the monthly mean values of LDR<0.05. Non-sphericity of aerosols consistently increases from March to May throughout the lowerand middle-troposphere. Above w1 km altitude, the LDR values continues to increase in the subsequent months to reach a peak value in July (mean LDR > 0.1). However, the LDR values below 1 km altitude substantially decreases from its peak value in May to JuneeAugust (mean LDR < 0.05), indicating a transition of aerosol type from relatively non-spherical to highly spherical type. Significance of the elevated aerosol loading from winter to summer monsoon season is seen in NCC, which shows that w90% contribution to the integrated ba occurs below w1.5 km during winter, while the corresponding altitude is w2.5 km during JulyeAugust. Annual variations of the monthly mean values of layerintegrated ba and layer-averaged LDR for altitude slabs of thickness 1 km are shown in Fig. 6. The strong annual cycle in ba with

Fig. 6. Annual variations of the multi-year (2008e2011) monthly mean (a) layerintegrated backscatter coefficient, and (b) layer-averaged LDR, in the altitude band of <1 km, 1e2, 2e3, 3e4, and 4e5 km.

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during the four seasons are shown in Fig. 7. Altitude profiles of ba and LDR are comparable during winter and post-monsoon seasons, when the aerosol loading is the least and the aerosols are highly spherical. The vertical distribution of aerosols undergoes largest seasonal variation above 2 km, while their values are comparable below w1 km. During the summer monsoon season, LDR shows a broad peak in the range of 1.5e3.5 km which is dominated by highly non-spherical aerosols. 3.5. Quantification of the effect of long-range transport of aerosols

Fig. 7. Seasonal mean altitude profiles of (a) ba, (b) LDR and (c) NCC during winter, premonsoon, summer monsoon and post-monsoon seasons, averaged during 2008e2011.

peak-to-trough ratio varying in the range of 5e10 is discernible in the 2e5 km altitude region. Phase of the annual variation of ba are different above and below 2 km altitude. For example, the highest values of ba at <2 km occurs in March while that at 3e5 km occurs in May and JulyeAugust. Similarly, the lowest values of ba at >2 km occurs in DecembereJanuary while that at <2 km is observed in September. The lower value of layer averaged ba observed during June to September might be associated with the efficient aerosol removal mechanisms by the summer monsoon rainfall. Note that the corresponding values of ba at <2 km are generally larger during winter and pre-monsoon seasons, when the rainfall is scanty. The annual cycle of LDR is prominent at all levels, with the largest variation occurring in the altitude band of 2e4 km. As seen from the above analyses, the vertical distribution of ba and LDR are generally consistent during different months of the same season. The multi-year seasonal mean altitude profiles of ba, LDR and NCC

The dominant sources of natural and anthropogenic aerosols might be different during different seasons. Notwithstanding this, monthly variations in the mean values of ba below w1 km altitude is <30%, while the corresponding variations above w2.5 km altitude is substantially larger. Aerosol loading is the least during winter, especially above w1 km in January. Altitude profiles of ba and LDR during this period show the absence of any prominent elevated aerosol layers and w90% of column integrated ba is contributed by aerosols occurring below w1.5 km altitude. At the study region, January marks the driest month with low wind speed (typically <2 m s1 at 1 km altitude). This shows that aerosols over the study region during this period are mainly produced locally or regionally rather than being brought by long-range transport. The wet removal processes are highly inefficient at the study region during January, resulting in generally larger residence time of aerosols. Based on the above, it is reasonable to assume that the altitude profiles of ba during January represent the upper limit of the locally/regionally produced aerosols and can be used as a bench-mark to quantify the effect of long-range transport in increasing aerosol abundance over the region during the other periods. The difference between the mean altitude profiles of ba during any given month and that in January (dba) is an index of the effect of long-range transport of aerosols during the month, especially above w1 km altitude. Fig. 8 shows the multi-year monthly mean altitude variations of dba during January to December. Corresponding altitude variations

Fig. 8. Multi-year (2008e2011) monthly mean altitude profiles of (a) dba, and (b) normalized dba [ ¼ dba/ba(January)]. (c) Altitude profiles of the seasonal mean normalized dba during pre-monsoon, summer monsoon and post-monsoon seasons (averaged during 2008e2011). See text for details.

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of the normalized values of dba obtained by dividing dba with the monthly mean values of ba in January at the respective altitudes are also shown in Fig. 8. This analysis reveals that the maximum increase in ba occurs at altitudes below w2.5 km during MarcheMay. The negative values of dba during the summer monsoon and postmonsoon seasons below w2 km altitude might be due to the reduced aerosol residence time (caused by the increase in precipitation) during these seasons compared to January. In terms of the normalized change at each altitude, Fig. 8 reveals that the effect of long-range transport is largest in the altitude band of 2e4 km where the values of ba increases by a factor of 3e10 during the premonsoon and summer monsoon seasons. The seasonally averaged normalized difference in ba (Fig. 8 (c)) shows that the seasonal mean variations in ba below w1.5 km are generally less than 20% while this contribution rapidly increases above w2 km altitude. The mean increase during the pre-monsoon season is >400% at above 2.5 km altitude. Though the largest percentage increase in aerosol loading is observed during the summer monsoon season, this increase mainly occurs above w3 km altitude.

4. Conclusions Monthly, seasonal and annual variations of the altitude profiles of aerosol backscatter coefficient and LDR derived from the MPL observations carried out at Thiruvananthapuram, a relatively clean coastal station located in the southwest Peninsular India near the Arabian Sea, during the period of March 2008eMay 2011 are presented here. In-situ production of anthropogenic aerosols is rather small at this station, which is located far from major pollution sources; hence the present observations have the potential to quantify the effect of long-range transport in influencing the abundance and properties of aerosols in an otherwise clean coastal environment. Satellite-derived spatial distribution of AOD shows that Thiruvananthapuram is located within the wide-spread aerosol plumes that occur over the Arabian Sea and adjoining regions during the pre-monsoon and summer monsoon seasons. Altitude variations of the monthly mean ba is least below w800 m altitude during all seasons. Seasonal variation in the mean values of ba in this altitude band is <20%. The winter season is characterized by a rapid exponential decrease of ba with altitude above this well-mixed region, with w90% of the aerosol loading confined to <1.5 km altitude. Spherical aerosols (LDR < 0.05) dominate at all altitudes during this season. An elevated layer of significantly non-spherical aerosols, which contributes w25% of the column integrated aerosol content, is observed in the altitude region of 2e4 km during AprileMay. This elevated layer is associated with a well-defined aerosol plume observed over the southeast Arabian Sea region. The JulyeAugust period is manifested by contrasting aerosol types below and above w1 km altitude. A prominent elevated layer of highly non-spherical aerosols (mean LDR of w0.1e0.2) occur above w1 km altitude, which is associated with the large-scale mineral dust plume that engulf the entire north and central Arabian Sea during this season. In contrast, highly spherical marine aerosols (LDR < 0.05) dominate below w1 km altitude. The aerosol distribution and sphericity during postmonsoon season are comparable to those in winter; both these seasons are marked by the absence of any distinct elevated aerosol layers. The annual cycle of both aerosol loading and non-sphericity are most prominent in the 2e4 km altitude region. As a result of the elevated aerosol layer formed by the long-range transport, the values of ba increases by a factor of 3e10 in the altitude band of 2e 4 km during AprileMay and JulyeAugust months, compared to the corresponding values in January which is marked by the lowest aerosol loading and absence of elevated aerosol layers.

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