**1. Introduction**

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Particulates in the Upper Troposphere and Lower Stratosphere (UTLS) gained considerable interest due to their role in the dehydration of tropospheric air entering the stratosphere [1,2] as well as their potential to influence the radiation budget of the Earth-atmosphere system [3]. The upper troposphere region, which is also conducive for the formation of cirrus, plays a major role in the transport of water vapour and other chemical constituents into the stratosphere. The physical processes responsible for maintaining the observed aerosol distribution in the tropical UTLS, the process with which it interacts with cirrus clouds and the effect of these particulates on the radiation budget are not fully understood [4]. Studies have shown that the microphysical (such as particle shape, size, and size distribution) as well as the chemical properties of particles in the UTLS region [5-7] are mainly governed by the strength of tropospheric convection and the prevailing dynamics of the underlying troposphere. The formation and persistence of cirrus clouds in the upper troposphere is mainly governed by the concentration of available condensation nuclei in this region and their physical and chemical properties. These clouds are believed to be a significant contributor to atmospheric greenhouse effect [8,9] as well as hypothesized to play a major role in the dehydration of the lower stratosphere [1,2] and thus becomes an important factor governing global climate, through their positive feedback.

The last four decades of the 20th century have been marked by relatively intense volcanic activity [10] and hence long-term measurements of aerosols during this period mostly characterize the volcanically perturbed aerosols system rather than 'background' conditions [11-13]. During this period the increase in aerosol loading in the stratosphere could accelerate the heterogeneous chemistry of sulfate aerosols leading to a decrease in ozone amount [14-17], altering the NO2 concentration [18-20] and hence modifying the earth's

© 2012 Sunilkumar et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Sunilkumar et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

radiation budget [21]. However, long-term studies on stratospheric aerosols show that on a global scale the stratospheric aerosol loading has returned to the pre-eruption levels (prevailed in the late 1970s) after the eruption of Mt. Pinatubo in 1991 [13,22] for the first time in 1998 and continued to remain almost at the same level for a couple of years. Only very few studies are carried out on the characteristics of these background stratospheric aerosols using observational data [7,13,23]. These studies, however, have shown that the global distribution of stratospheric aerosols will be significantly influenced by the atmospheric dynamics which includes periodic variations, such as the Quasi-Biennial Oscillation (QBO), seasonal cycles and long-term secular changes in addition to small perturbations due to the feeble volcanic eruptions and also due to degassing from the Earth's crust. Identifying these secular trends in the background stratospheric aerosol system is crucial to predict future aerosol levels [24]. While *Deshler et al.* [13,25] observed no discernable long-term trends in the non-volcanic component of stratospheric aerosols over an extended period (1970-2004), *Hofmann et al.* [26] could observe a significant enhancement in the lower stratospheric aerosol load for the past several years (2000-2008) which they attributed to the increase in anthropogenic sulfur emission. Moreover, eruptions of a few minor volcanoes such as Manam, Ruang, Revantador and Soufriere also might have disturbed the background stratospheric aerosol level to a smaller extent. Although the volcanic degassing during the quiescent and the small eruptive volcanic periods contributes only 14% to total global SO2 emissions, its efficiency on total atmospheric SO2 burden is found to be much higher (factor of 5) than that of anthropogenic emissions. Model calculations [27] shows that even though the source strength of volcanic emissions is less than 20 % of the anthropogenic component, the flux of sulfur gases from volcanoes during this period leads to a sulfate burden in the free troposphere which is comparable to that from anthropogenic emissions. This is caused mainly by the altitude-latitude distribution of volcanic emissions, and is most pronounced in tropical latitudes [27].

Distribution of Particulates in the Tropical UTLS over

the Asian Summer Monsoon Region and Its Association with Atmospheric Dynamics 115

particulates in the UTLS region. This Lidar [7] is equipped with Nd:YAG laser (Model: PL8020, Continuum, USA) emitting linearly polarized pulses with 7 ns width and 20 Hz repetition rate at its second harmonic wavelength of 532 nm with a pulse energy of 550 mJ. The basic beam emerging from the laser source with a divergence of 0.45 m rad is expanded using a 10X beam expander to reduce the divergence to <0.1 m rad before transmitting vertically into the atmosphere. The time series of backscattered photons from different altitudes corresponding to each transmitter pulse are received using a 350 mm diameter Schmidt-Cassegrain telescope having a field of view of ~1 m rad. Both the transmitted beam and vertically looking receiving telescope are configured with a fixed horizontal separation of ~3 m. For this lidar configuration, as the lowest altitude at which the full overlap of the transmitter beam with the receiver field-of-view (beam-filled condition) is encountered around 7 km, the data from altitudes above 8 km only are used for retrieving the aerosol properties. A polarized beam splitter in the receiver beam path splits the beam into copolarized and cross-polarized components which are detected independently using two identical photomultiplier tubes operated in photon counting mode and acquired with a bin width of 2 μs corresponding to an altitude resolution of 300 m. These photon-number profiles corresponding to each transmitted pulse are summed over 250 s to achieve a good

The SAGE-II onboard the Earth Radiation Budget Satellite (ERBS) employs solar occultation technique to measure the attenuation of solar radiation at the Earth's limb between the satellite and the Sun due to scattering and absorption by different atmospheric species [32]. These measurements provide the altitude profile of the volume extinction coefficients of atmospheric particulates which includes particles of thin sub-visual cirrus clouds and aerosols at four different wavelengths in the visible and near-IR range (1020, 525, 453 and 385 nm) with a horizontal resolution of about 200 km and a vertical resolution of 0.5 km [33]. This sensor takes 30 occultation observations on a single day, which are equally spaced in longitude round the globe but vary in latitude by a few degrees giving a near global coverage over a period of 25−40 days. Details regarding the SAGE instrumentation and algorithms are discussed in earlier publications [32,34]. The upper limit of the particulate extinction measurable by SAGE sensor at 1020 nm is ∼2 x 10-2 km-1, which is much larger than that in the UTLS region (∼2 x 10-4 km-1) under volcanically quiescent period. Cirrus cloud with extinction greater than this value, generally referred to as 'opaque clouds' [35,36], are not measurable by the SAGE-II sensor. Presence of such clouds, limits the SAGE

The lidar data (backscattered signal) on different nights from Gadanki are used to derive the altitude profiles of particulate backscatter coefficient (βp) and volume depolarization ratio (δ) [37-39]. In the lidar system, received backscattered signal (at 532 nm wavelength) is separated into co-polarized and cross-polarized components (⊥ and ‖ channels, respectively) and recorded separately in two channels. The data in these two channels are analyzed separately employing the Fernald's algorithm [40] to estimate the total

signal to noise ratio up to altitude above ∼40 km.

measurements below tropopause.

**3. Estimation of particulate extinction from lidar data** 

The quasi-biennial oscillation in stratospheric zonal wind (QBOU) is found to influence significantly the distribution of volcanic stratospheric aerosols mainly over the tropics [28- 30]. Even though, recent observational studies [26,31] revealed significant seasonal and inter-annual variations in the stratospheric aerosol load during the volcanically quiescent period, studies on the influence of these types of periodic oscillations on the stratospheric aerosol distribution over the tropics during volcanically quiescent periods are very rare [11]. This study involves an attempt to study the features of particulates in the UTLS region during the relatively quiescent volcanic period (1998-2005) using global aerosol data from Stratospheric Aerosol and Gas Experiment (SAGE-II) archive and lidar data from Gadanki [13.5°N, 79.2°E]. Formation of semitransparent cirrus (STC) is very common in the upper troposphere. The characteristics of these STCs and their contribution to particulate scattering in UTLS region are also investigated.

## **2. Extinction/Backscatter data from LIDAR and SAGE-II**

The biaxial, monostatic dual polarization Lidar at the National Atmospheric Research Laboratory (NARL), Gadanki, is used to study the scattering properties of atmospheric particulates in the UTLS region. This Lidar [7] is equipped with Nd:YAG laser (Model: PL8020, Continuum, USA) emitting linearly polarized pulses with 7 ns width and 20 Hz repetition rate at its second harmonic wavelength of 532 nm with a pulse energy of 550 mJ. The basic beam emerging from the laser source with a divergence of 0.45 m rad is expanded using a 10X beam expander to reduce the divergence to <0.1 m rad before transmitting vertically into the atmosphere. The time series of backscattered photons from different altitudes corresponding to each transmitter pulse are received using a 350 mm diameter Schmidt-Cassegrain telescope having a field of view of ~1 m rad. Both the transmitted beam and vertically looking receiving telescope are configured with a fixed horizontal separation of ~3 m. For this lidar configuration, as the lowest altitude at which the full overlap of the transmitter beam with the receiver field-of-view (beam-filled condition) is encountered around 7 km, the data from altitudes above 8 km only are used for retrieving the aerosol properties. A polarized beam splitter in the receiver beam path splits the beam into copolarized and cross-polarized components which are detected independently using two identical photomultiplier tubes operated in photon counting mode and acquired with a bin width of 2 μs corresponding to an altitude resolution of 300 m. These photon-number profiles corresponding to each transmitted pulse are summed over 250 s to achieve a good signal to noise ratio up to altitude above ∼40 km.

114 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

volcanic emissions, and is most pronounced in tropical latitudes [27].

**2. Extinction/Backscatter data from LIDAR and SAGE-II** 

scattering in UTLS region are also investigated.

The quasi-biennial oscillation in stratospheric zonal wind (QBOU) is found to influence significantly the distribution of volcanic stratospheric aerosols mainly over the tropics [28- 30]. Even though, recent observational studies [26,31] revealed significant seasonal and inter-annual variations in the stratospheric aerosol load during the volcanically quiescent period, studies on the influence of these types of periodic oscillations on the stratospheric aerosol distribution over the tropics during volcanically quiescent periods are very rare [11]. This study involves an attempt to study the features of particulates in the UTLS region during the relatively quiescent volcanic period (1998-2005) using global aerosol data from Stratospheric Aerosol and Gas Experiment (SAGE-II) archive and lidar data from Gadanki [13.5°N, 79.2°E]. Formation of semitransparent cirrus (STC) is very common in the upper troposphere. The characteristics of these STCs and their contribution to particulate

The biaxial, monostatic dual polarization Lidar at the National Atmospheric Research Laboratory (NARL), Gadanki, is used to study the scattering properties of atmospheric

radiation budget [21]. However, long-term studies on stratospheric aerosols show that on a global scale the stratospheric aerosol loading has returned to the pre-eruption levels (prevailed in the late 1970s) after the eruption of Mt. Pinatubo in 1991 [13,22] for the first time in 1998 and continued to remain almost at the same level for a couple of years. Only very few studies are carried out on the characteristics of these background stratospheric aerosols using observational data [7,13,23]. These studies, however, have shown that the global distribution of stratospheric aerosols will be significantly influenced by the atmospheric dynamics which includes periodic variations, such as the Quasi-Biennial Oscillation (QBO), seasonal cycles and long-term secular changes in addition to small perturbations due to the feeble volcanic eruptions and also due to degassing from the Earth's crust. Identifying these secular trends in the background stratospheric aerosol system is crucial to predict future aerosol levels [24]. While *Deshler et al.* [13,25] observed no discernable long-term trends in the non-volcanic component of stratospheric aerosols over an extended period (1970-2004), *Hofmann et al.* [26] could observe a significant enhancement in the lower stratospheric aerosol load for the past several years (2000-2008) which they attributed to the increase in anthropogenic sulfur emission. Moreover, eruptions of a few minor volcanoes such as Manam, Ruang, Revantador and Soufriere also might have disturbed the background stratospheric aerosol level to a smaller extent. Although the volcanic degassing during the quiescent and the small eruptive volcanic periods contributes only 14% to total global SO2 emissions, its efficiency on total atmospheric SO2 burden is found to be much higher (factor of 5) than that of anthropogenic emissions. Model calculations [27] shows that even though the source strength of volcanic emissions is less than 20 % of the anthropogenic component, the flux of sulfur gases from volcanoes during this period leads to a sulfate burden in the free troposphere which is comparable to that from anthropogenic emissions. This is caused mainly by the altitude-latitude distribution of

The SAGE-II onboard the Earth Radiation Budget Satellite (ERBS) employs solar occultation technique to measure the attenuation of solar radiation at the Earth's limb between the satellite and the Sun due to scattering and absorption by different atmospheric species [32]. These measurements provide the altitude profile of the volume extinction coefficients of atmospheric particulates which includes particles of thin sub-visual cirrus clouds and aerosols at four different wavelengths in the visible and near-IR range (1020, 525, 453 and 385 nm) with a horizontal resolution of about 200 km and a vertical resolution of 0.5 km [33]. This sensor takes 30 occultation observations on a single day, which are equally spaced in longitude round the globe but vary in latitude by a few degrees giving a near global coverage over a period of 25−40 days. Details regarding the SAGE instrumentation and algorithms are discussed in earlier publications [32,34]. The upper limit of the particulate extinction measurable by SAGE sensor at 1020 nm is ∼2 x 10-2 km-1, which is much larger than that in the UTLS region (∼2 x 10-4 km-1) under volcanically quiescent period. Cirrus cloud with extinction greater than this value, generally referred to as 'opaque clouds' [35,36], are not measurable by the SAGE-II sensor. Presence of such clouds, limits the SAGE measurements below tropopause.

## **3. Estimation of particulate extinction from lidar data**

The lidar data (backscattered signal) on different nights from Gadanki are used to derive the altitude profiles of particulate backscatter coefficient (βp) and volume depolarization ratio (δ) [37-39]. In the lidar system, received backscattered signal (at 532 nm wavelength) is separated into co-polarized and cross-polarized components (⊥ and ‖ channels, respectively) and recorded separately in two channels. The data in these two channels are analyzed separately employing the Fernald's algorithm [40] to estimate the total

backscattering coefficients (β⊥ and β║, respectively) taking 30 km as the reference altitude where the aerosol contribution is assumed to be negligible. For this inversion a value of 40 sr-1 is assigned for the lidar ratio (SP) and its variation with altitude depending on δ is also accounted appropriately. With this correction the value of SP reduces to ∼26 sr-1 within the STC [39] in the upper troposphere. Further incorporating the correction for multiple scattering the value of SP within the STC reduces to 20 sr-1 (which is used to study the properties of STCs). The molecular backscatter coefficients for the two polarized components are estimated from the mean molecular number density profile taking a molecular depolarization factor (δm) of 0.028 [41]. The molecular backscatter coefficient of the co-polarized component (β<sup>m</sup>⊥) is related to that of the cross-polarized component (β<sup>m</sup>║) as β<sup>m</sup>⊥ =δmβ<sup>m</sup>║. Subtracting β<sup>m</sup>⊥ and β<sup>m</sup>║ from the altitude profiles of β⊥ and β║, respectively obtained from lidar data employing the Fernald's algorithm, the altitude profiles of particulate backscatter coefficient, β<sup>p</sup>⊥ and β<sup>p</sup>║, are estimated. The respective backscatter ratios for the co-polarized (R⊥) and cross-polarized (R║) components are estimated [37] as R⊥=β⊥/β<sup>m</sup>⊥ and R║=β║/β<sup>m</sup>║. As far the net atmospheric backscattering is concerned, the ''unbiased'' or effective backscatter ratio (R) is to be defined , to quantify the gross property of the medium, which on mathematical simplification can be written as R(h)=[R⊥(h)+δmR║(h)]/(1+δm). The volume depolarization ratio is obtained from the ratios of R⊥ and R║ as δ(h)= [δm R║(h)]/ R⊥(h). This ratio is a good indicator for distinguishing the cirrus based on 'particle habit'. While for small spherical particles, the values of δ will be relatively small, its value increases significantly as they become large and non- spherical. Using this property of cloud particles, structure and altitude extent of cirrus can be estimated from each lidar profile, which will be used to study the temporal variation of cirrus properties during the entire period of lidar observation. Based on a detailed scrutiny of a number of profiles at different cloud conditions a threshold value of δ ≥0.04 is assigned for discriminating the STC [42]. If the value of δ exceeds this threshold value it is classified as STC. The effective particulate backscatter coefficient, βP, is the sum of β<sup>p</sup>⊥ and β<sup>p</sup>║. The altitude profile of particulate extinction coefficient, αP, is estimated by multiplying the altitude profile of βp with the corresponding profile of SP.

Distribution of Particulates in the Tropical UTLS over

the Asian Summer Monsoon Region and Its Association with Atmospheric Dynamics 117

so transparent that the lidar beam could penetrate the cloud and provide measurable signal even from higher altitudes. The opacity of STCs are quantified using the cloud optical depth (τc) which is the height integrated particulate extinction coefficient (αp), obtained by multiplying βp with the Sp, from the cloud base (hcb) up to the cloud top (hct). In case if the cirrus is too dense (with τc exceeding 1.5) the lidar beam will not be able to penetrate the cirrus layer impeding useful lidar observations. In association with the enhancement in β<sup>p</sup> and R, a significant increase in δ also can be observed at these altitudes. This suggests that the scattering particles within the STCs are relatively large and significantly non-spherical in nature. Depending on τc, STCs are further classified [46] in three classes viz., sub-visual cirrus (SVC) with τc<0.03, thin cirrus (TC) with 0.03 < τc <0.3 and dense cirrus (DC) with τ<sup>c</sup> >0.3. General features of STCs from this tropical station [37] showed that while the occurrence of SVC is larger during winter, TC and DC occur more frequently during the monsoon period. The upper and lower boundaries of STCs are identified from altitude profiles of R and δ using a threshold condition [37,42] for R to exceed 2 in either of the two lidar channels (⊥ or ‖ channels) along with the value of δ exceeding 0.04 in the altitude

**Figure 1.** Altitude profiles of mean particulate backscatter coefficient (βp), effective backscatter ratio (R)

A detailed error analysis [42] showed that the estimated values of βp is less sensitive to the variability in SP. For a given uncertainty of 25% in Sp, the maximum uncertainty in βp is 10% in the absence of clouds, ~15% for thin cirrus and ~30% for thick cirrus. For the same uncertainty in SP, the maximum uncertainty in the retrieved backscatter coefficient and effective backscatter ratio are around 0.6%, 2% and 10%, respectively, for clear atmosphere, atmosphere with thin cirrus and atmosphere with thick cirrus. Including the possible errors in the lidar signal inversion associated with the uncertainty in the molecular backscatter coefficient, the resultant error in the derived optical depth would be ~20%. As the signal-tonoise ratio is >2 up to ∼45 km, for altitudes <30 km the system induced errors will be significantly small (<1%) compared to that from other sources. The error due to the influence

region where the STCs are usually observed ( 8 to 20 km).

and volume depolarization ratio (δ) for few nights during the year 1999
