Indian Journal of Radio & Space Physics Vol 42, October 2013, pp 332-339

Detection of long range transport of aerosols with elevated layers over high altitude station in the central Himalayas: A case study on 22 and 24 March 2012 at

ARIES, Nainital

Raman Solanki1, Narendra Singh1,$,*, P Pant1, U C Dumka1, Y Bhavani Kumar 2, A K Srivastava3, Sanjay Bist4

& H C Chandola5 1Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263 002, Uttarakhand, India

2National Atmospheric Research Laboratory, Gadanki 517 502, Andhra Pradesh, India 3Indian Institute of Tropical Meteorology (Branch), Prof Ramnath Vij Marg, New Delhi 110 012, India

4India Meteorological Department, Lodhi Road 110 003, New Delhi, India 5Department of Physics, Kumaun University, Nainital 263 002, Uttarakhand, India

$E-mail: narendra@aries.res.in

Received 5 August 2012; revised 1 April 2013; accepted 5 June 2013

An advanced version of Boundary Layer LiDAR system, termed as LiDAR for atmospheric measurement and probing (LAMP) has been operational, at Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, a high altitude station (29.4°N, 79.5°E, ~1960 m above mean sea level), in central Himalayas, since October 2011. The site is at an altitude, which is well above the planetary boundary layer particularly during the night when observations are taken and thus, lies in the free troposphere. Also, there are no anthropogenic sources of aerosols nearby. However, from March to June, due to strong convection, the aerosols get transported to higher altitudes, up to 2 km or more, from the nearby urban and distant regions as well. Here, a case study of each long range transport and convectively driven elevated aerosol layers, observed with LAMP on 22 and 24 March 2012, has been presented. A normal profile observed on 28 March 2012 without any signature of elevated layer of aerosol is also discussed. The seven days back air mass trajectories over three altitude levels, viz. at 4.5, 3 and 1 km on 22 March; at 4.5, 2 km and 700 m on 24 March, and at 4.5, 2 km and 500 m on 28 March have been derived. The upper levels delineate that the possible origin of the multiple elevated aerosol layers on 22 and 24 March may be transported from far-off regions, such as the dry arid regions of North Africa and Saudi Arabia. To confirm the same, the observations were further substantiated with the TERRA satellite yielded aerosol optical depth (555 nm) obtained from the on board instrument Multi-angle Imaging Spectro Radiometer (MISR), which explicitly shows the high value of time averaged columnar aerosol optical depth (AOD) over Saudi Arabia and Red Sea during 18-23 March 2012 and an appreciable decrease during the period 25-29 March 2012, confirming the origin of long range transport. For the first time, such a high altitude aerosol layers (~4.5 km) are observed over this region. The lowest aerosol layer observed on 24 and 28 March 2012 in vertical aerosol backscatter profile is attributed to the transport from adjoining regions via boundary layer evolution and associated mixing.

Keywords: Aerosol optical depth, Aerosol backscatter profile, Air mass back trajectory

PACS No: 92.60.Mt

1 Introduction

Studies on vertical distribution of tropospheric aerosols and optical properties are essential for radiative transfer calculation and are of great importance for understanding the aerosol effect on climate. Dust storms have been recognized as having a wide range of impacts on climate and environment1. Desert dust constitutes a large fraction (50%) of the naturally occurring tropospheric aerosols2. These particles are responsible for an additional cooling effect in the lower troposphere, while in the middle and upper troposphere, these particles lead to an additional warming2. Dust particles, which are

generally of size greater than 1 mm, mainly consist of quartz, mica and clay minerals, are usually produced by the natural weathering that breaks the rocks3. Desertification is another potential source of dust production. However, natural weathering has greater influence on Asian dust emissions and associated dust storm occurrences when compared to the desertification processes4. Further, dust from East Asia is typically of mineral type5, while limited studies suggest it of mix type over South Asia6. Globally, there have been different studies on dust storms using satellite and ground based observations7,8. Such studies in Asian region are

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largely focused on dust storms originating from East Asia during spring mainly in April9 and forming elevated layers over higher altitudes.

Atmospheric aerosols from both natural and anthropogenic sources play an important role in the Earth atmosphere radiation budget directly by scattering and absorbing the incoming solar radiation; and indirectly by modifying the cloud radiative properties through altering the microphysical properties of clouds10-12. The magnitude and sign of aerosol radiative impact depend on physical, chemical as well as radiative properties of these aerosols. Due to the short residence time as well as the diverse aerosol types, the direct and indirect affect of aerosols exhibit large spatio-temporal variations. The direct and indirect effect of aerosol radiative forcing still remains a significant uncertainty for climate studies11 due to the large heterogeneity in aerosol properties and inadequate data information particularly in the vertical distribution. In order to assess the radiative forcing of aerosols, the knowledge of vertical distribution of aerosols in the Earth atmosphere is very important. Also, it is well known that the vertical distribution of aerosols in the atmosphere critically modifies the thermal structure of the atmosphere and hence, its stability with serious implications in cloud formation processes13. The formation of elevated aerosol layers in the atmosphere has great significance in the radiative forcing applications14,15. In this context, LiDAR observations are very useful as they potentially provide the vertical distribution on the aerosol optical properties. Such elevated aerosol layers are found in northern India, particularly during the winter season due to dry convective lifting of pollutants at distant sources and subsequent horizontal upper air long range transport16. However, at Manora Peak, Nainital, by and large these layers are observed in all months during the spring season of the year, but here, only two cases, of multiple aerosol layers as observed with LAMP system, are presented.

Ignoring vertical layering of aerosol can lead to several uncertainties in aerosol remote sensing. A micro pulse LiDAR based on the elastic backscattering of emitted low power laser is a useful tool to provide the vertical profiling of aerosols with both high range and temporal resolution. Aerosol vertical distribution and integral optical properties, such as aerosol optical depth (AOD), can be obtained with this technique simultaneously. More importantly, the surface aerosol extinction coefficient is one of a

few quantities of interest to determine aerosol effect on visibility. Here, the emphasis has been only on coarse particles because during the long range transport, the aerosols tend to grow in size, forming the elevated aerosol layers.

2 LAMP system

The advanced version of micro pulse LiDAR System18, popularly known as LAMP, designed and developed by National Atmospheric Research Laboratory (NARL), installed in a temperature and humidity controlled environment at ARIES, Nainital under a joint scientific collaborative program, has been utilized for the vertical profiling of atmospheric aerosols in the lower troposphere. This portable micro pulse LiDAR system is basically operated during night in the presence of stable boundary layer when day time well mixed aerosols settle and form layers of suspended particles at various levels in the atmosphere.

LAMP system employs a diode pumped Nd-YAG laser system, a coaxial transceiver for transmitting the laser pulses and detecting the collected photons, a dedicated data acquisition system, and a computer control with an interface system. The pulses of light (532 nm) energy are transmitted from telescope into the atmosphere. As the pulse propagates, part of it is scattered by molecules, water droplets, ice crystals (cirrus clouds), dust and haze aerosols in the atmosphere, and small portion of back scattered light is collected by the telescope and then detected by an in-built photo multiplier tube. The distance to the particle layer is derived from the time delay between each outgoing laser pulse and the backscattered signal. Schematic of the system and details are provided by Bhavani Kumar18. Major specifications and system parameters are given in Table 1.

3 Data and Methodology

The LAMP actually sends a laser pulse of width 10 ns, and receives backscattered signal at the vertical resolution of 15 m. The range corrected, normalized return signal for one transmitted laser pulse is a combination of backscatter energy from the Rayleigh and aerosol components. LAMP is coaxial LiDAR and field of view (FOV) is blocked by the steering mirror at the centre of the telescope and the complete overlap of the transmitter and receiver FOV occurs at an altitude of about ~90 m. Thus, the system does not require overlap correction and the data is acquired only after 90 m. The micro pulse LiDAR used in the

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present study transmits pulse energy of 4 µJ per pulse. Each profile is derived with an averaging for 120 sec. The standard methodology for deriving backscattered signals is opted19,20. The backscattered power (photon counts) received at acquisition unit are further subjected to retrieve the aerosol backscatter coefficients from backward solution as suggested by Klett21.

Aerosol layers were distinguished from the low level clouds by applying a criterion22, according to which if the backscatter ratio (BSR) profile at any height bin is exceeding 2, then it is identified as the case of a cloud. The BSR, which is the relative magnitude between Rayleigh (βm) and Mie (βa) backscattering coefficients, is defined as:

a

a

( ) ( )( )

( )m

z zBSR z

z

β β

β

+=

LAMP is operated during clear sky nights continuously for 4-6 hours once a week. Out of 51 available observations, two cases of elevated aerosol layers observed on 22 and 24 March 2012 are presented here. The aerosol optical depth (AOD) of dust particles (optical thickness) can be calculated by integrating the aerosol extinction coefficient σa(z) profile over the vertical distance between any two heights. The σa(z) can be obtained by multiplying the aerosol backscatter coefficient βa(z) profile with the LiDAR ratio for aerosols (LRa) for that particular day. To demonstrate the long range transport, back trajectories analysis has been carried out.

4 Results and Discussion

LiDAR profiles show multiple elevated layers present over Nainital as a result of local meteorology and mostly upper level long range transport of aerosols originating from arid and semi-arid regions of the western part of the Indian subcontinent and other parts of the countries to the west of India. Aerosol layers found above the boundary layer could have been transported several thousands of kilometers without significant removal and as a consequence can contribute significantly to the columnar aerosol optical depth at times, more than what local boundary layer does. Elevated aerosol layer has also been observed during the month of June 2006 (Ref. 17) at an altitude of 1300 m. On 22 March 2012, elevated aerosol layers were observed at altitude of about 3.5 and 4.5 m for the first time over this region, indicating that a strong surface forcing have lifted dry air masses to the higher altitudes, followed by horizontal transport to the site under observation.

A total number of 51 data sets were analysed for this study, nine data sets were found to have the signature of elevated aerosol layers particularly in spring. Figure 1 shows the typical vertical profile of range-time-intensity (RTI) measured during 1900 hrs IST to about mid night on 22 March 2012. A shallow intensity layer at about 4.5 km, very strong backscattering signals at about 3.5 km and around 1 km are observed. The aerosol backscatter coefficient derived from the signal intensity is further averaged to get the vertical profile up to 6 km. Signature of a shallow layer of aerosols is evident in RTI plot, this shallow layer gradually settles down at around 2230 hrs IST and mingles to the denser lower aerosol layer at about 3 km. The lowest aerosol layer is detected at about 1 km, which also settles down around mid night and becomes a part of the stable

Table 1 — LAMP relevant parameters and specifications

Laser source

Type Pulsed diode pumped solid state laser

Operating wavelength 532 nm

Pulse width <10 ns

Repetition rate 2500 Hz

Beam size 0.4 mm

Polarization Linear

Beam divergence < 1.5 mrad

Beam expander magnification

8X

Divergence of the expanded laser beam

< 200 µrad

Telescope

Type Cassegrain

Size Dia 150mm, f# 9

Field of view < 400 µrad

Telescope mount Gimbal

Filter

Type Interference

Central wavelength 532 nm

Full width at half maximum 0.5 nm

Recommended operating temperature

30 ± 1°C

Detector

Type Photomultiplier, Hamamatsu R3234

Gain 2.5 × 107(typical)

Quantum efficiency <10%

Data Acquisition System

Model EG&G Ortec, MCS-pci

Maximum counting rate 150 MHz

Dwell time selectable 100 to 1300 ns

Number of channels 4 – 65536

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layer or localized nocturnal boundary layer. Average vertical profile of backscatter coefficient is given in Fig. 2, where three peaks at the corresponding RTI heights are clearly seen, however, the peak at 3 km is prominent and an indicative of denser elevated aerosol layer.

The back air mass trajectory analysis using the HYbrid Single Particle Lagrangian Integrated Trajectory23 (HYSPLIT) model reveals the source of aerosols transported from distant regions. The HYSPLIT23 model of Air Resources Laboratory (ARL), National Oceanic and Atmospheric Administration (NOAA), is a complete system for computing sample air parcel trajectories to complex dispersion and deposition simulations. The unregistered version available on the ARL site can be downloaded and run as per the requirement. The model requires meteorological Global Data Assimilation System (GDAS) data files which are also available online. The starting location, the

specified time for which the air mass is to be traced backwards in time, and the starting height of the air parcel above the ground level were specified while running the model.

Figure 3 depicts the seven days back trajectories of air mass, at three altitudes 4.5, 3 and 1 km, for 21 (one day prior), 22 (day of observation) and 23 (one day later) March 2012. The trajectory analysis reveals

Fig. 1 — Altitude vs time contour plots of range corrected photon counts on 22 March 2012

Fig. 2 — Aerosol backscatter profile on 22 March 2012 [Standard deviation shown in horizontal error bars]

Fig. 3 — HYSPLIT seven days back trajectories of air mass analyzed for 21, 22 and 23 March 2012 at three different altitudes: (top panel) 4.5 km, (middle panel) 3 km, and (bottom panel) 1 km

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that the air masses at 4.5 km are coming near westerly and dust seems to be originating from Europe and Saudi Arabian regions; whereas at 3 km, air masses are originating from Europe and extreme North African regions with northwesterly winds that turn westerly as it passes over Iran, Afghanistan and northern Pakistan regions. At 1 km, the trajectory shows that the air masses are mostly from north-west and western part of India and neighboring country like Afghanistan. This explicitly depicts that the air masses reaching at high altitudes over the site are basically transported from very far distances, whereas the lower altitude layers are mostly contributed by the dry air masses from western part of the country.

LiDAR observation on 24 March 2012, presented in Fig. 4, shows the aerosol layers observed at about 700 m, ~2 km and a very shallow layer at about 4 km. The patches of cirrus clouds are also observed above 6 km. The mean vertical profile of the aerosol backscatter coefficient in Fig. 5 shows the peaks at

corresponding elevated aerosol layers with maxima at ~700 m. Figure 6 depicts the back air mass trajectories derived at 4.5 and 2 km, for 23, 24 and 25 March, which indicate that the winds are coming mostly from western and north-western part of the world. The 4.5 km elevated layer gradually weakens and this could be justified from the 4.5 km trajectory

Fig. 6 — HYSPLIT seven days back trajectories of air mass analyzed for 23, 24 and 25 March 2012 at three different altitudes: (top panel) 4.5 km, (middle panel) 2 km, and (bottom panel) 700 m

Fig. 4 — Altitude vs time contour plots of range corrected photon counts on 24 March 2012

Fig. 5 — Aerosol backscatter profile on 24 March 2012 [Standard deviation shown in horizontal error bars]

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which shows far separated blue and red circles indicating that the air mass now spends less time over the aerosol loaded region. However, strong aerosol layer at 700 m is due to air masses that are transported from adjoining regions via boundary layer evolution and associated mixing.

RTI profile for a normal day (28 March 2012) is shown in Fig. 7, demonstrating a very shallow air mass above 1.5 km, and a significant aerosol layer only up to about 1 km. Corresponding backscatter coefficient given in Fig. 8 shows the prominent peak at about 1 km and no signature of multiple layers are seen. The back air mass trajectory at 4.5 km agl reveals that the prevailing wind during the previous day have the same origin as the other days; and on the subsequent day, the air masses are quite westerly but without aerosol loading as shown in Fig. 9 (top

panel). Trajectory at 2 km shows that air masses are following the same path during all three days; whereas, at 500 m the air masses are originating from the nearby regions within few 100 km.

Figure 10 (top panel) shows contour plot of time averaged AOD for 18-23 March 2012 derived from MISR (555 nm) instrument on TERRA satellite,

Fig. 7 — Altitude vs time contour plots of range corrected photon counts on 28 March 2012

Fig. 8 — Aerosol backscatter profile on 28 March 2012 [Standard deviation shown in horizontal error bars]

Fig. 9 — HYSPLIT seven days back trajectories of air mass analyzed for 27, 28 and 29 March 2012 at three different altitudes: (top panel) 4.5 km, (middle panel) 2 km, and (bottom panel) 500 m

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indicating a high AOD (>1.0) over Saudi Arabia, Red Sea, Punjab and North East Pakistan regions. Satellite derived AOD values compliment the origin of aerosols, which are transported to long distances, as explained by the trajectory analysis particularly to the higher altitude aerosol layers above 4 km. The 4.5 km elevated layer is due to the long range transport from Saudi Arabia as the time spent by the air mass is more in this region (shown by the clustering of red circles over the region). An elevated layer at about 3.5 km may be attributed to have been originated from the nearby regions of Punjab and North East Pakistan as clusters of air masses over this region are evident in the trajectory. Bottom panel of Fig. 10 depicts the MISR derived contour plot of AOD (time averaged from 25 to 29 March 2012), representing appreciable decrease in AOD from being greater than 1 during 18-23 March to less than 0.6 for 25-29 March 2012; and thus, vindicating the absence of elevated layer on 28 March. The LiDAR derived AOD values for 22, 24 and 28 March 2012 are 0.023, 0.025 and 0.018,

respectively. The cause for the formation of these elevated aerosol layers may be the convective lifting of the dry air to higher levels, at a distant source containing mostly aerosols and dust that are long range transported to the pristine site Nainital. Such elevated layers of aerosol which primarily consists of coarse mode particle primarily dust, as suggested by the angstrom exponent being less than one (0.817) for 22 March, can play crucial role in heating the atmosphere24. Also, the long wave absorption caused due to these dust particles can significantly contribute to tropospheric warming over the central Himalayas. 5 Conclusions

A micro pulse Lidar (532 nm) is operational at ARIES, Nainital and intermittent observations on the vertical profiling of aerosol are made over the year. As a result of such observations, elevated layers (basically dust) were detected on 22 and 24 March 2012. For the first time, formation of aerosol layer at an altitude of about 4.5 km was observed over the site on 22 March 2012, which is quite unusual. The origin of such layers is found to have associated with convectively lifted dust over dry arid North Africa and Saudi Arabian region. The source and transport of the aerosols is explained by Satellite observations (TERRA) and back air mass trajectory analysis. Acknowledgement

This work has been carried out as a part of project, ‘Atmospheric Boundary Layer Network & Characterization (ABLN&C)’, under the Indian Space Research Organization-Geosphere Biosphere Programme (ISRO-GBP). The authors are thankful to the Director, ARIES, Nainital for his constant support in carrying out this work. The authors are also grateful to NARL for development and installation of LAMP at ARIES Nainital. They also thank Atmospheric Sciences Data Center at NASA Langley Research Center for providing MISR data online.

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