Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Studies of Aerosol optical depth using multiwavelength sun photometer measurements at the semi-arid region : Rajkot

Auteur : Ritweej, Rajeev R.

Université de soutenance : Saurasthra University

Grade : Doctor of Philosophy (PhD) 2009

Description
Atmospheric aerosols are defined as tiny particles either in the solid phase or the liquid phase or as a mixture of both (a solid core covered with a liquid envelope) suspended in air. The importance of atmospheric aerosols relies mainly on their interaction with solar radiation and consequent effects on Earth’s climate system. Aerosols are generated by a variety of natural and anthropogenic mechanism, both directly and indirectly ; while they are removed by sedimentation (dry deposition) and washout (wet removal). During their lifetime in the atmosphere, aerosols also undergo transformation in their physical and chemical properties with consequent impact on their effects. Thus, the aerosol properties and effects show large variation in space and time. Moreover, the combined effects of generation, sustenance and removal mechanism make the atmospheric aerosols generally polydispersive. Particles of different sizes co exist in the atmosphere. The size spectrum runs well over four decades, from 10–3 µm to 102 µm. This wide size spectrum makes the aerosols a complex species and several techniques are required to characterize them. Different instrumental setups used for these studies include both active and passive remote sensing techniques. Multi wavelength sun photometer has been one of the instruments used as an effective passive remote sensing means to determine the spectral transmission characteristics of the atmosphere. After subtracting the effects due to molecular scattering and absorption features, one can estimate the spectral attenuation characteristics of aerosols. By applying the Mie theory to the viii spectral features of optical effects of aerosols, it is possible to derive the size distribution of aerosols. In India, under the IMAP programme, multi wavelength radiometers were developed and deployed at Trivandrum, Mysore and Vishakapatnam, which is monitoring AOD for the past one decade (Krishna Moorthy et al, 1999) as a part of Indian Middle Atmosphere Programme (IMAP). They are all at low latitudes in southern India. Recently, AOD monitoring has been started at other places also like Jodhpur, Indore and Udaipur in central India (Pandey and Vyas, 2001 ; Gupta et al, 2003). Aerosol studies using the sophisticated LIDAR system has been undertaken at Mt. Abu by Jayaraman et al (1998). Recent studies suggest that tropospheric aerosols contribute substantially to radiative forcing and that anthropogenic sulphate aerosols and soot particles, in particular, have imposed major perturbations to this forcing (Satheesh et al, 2000). In view of the high variability due to orographic, marine, desert etc influence on aerosol parameters, it is desirable to monitor from different locations in the same region. In Saurashtra region there has not been any measurements of aerosols and water vapour although IMD measurements of relative humidity may exist. Therefore a systematic investigation of these quantities from Rajkot is highly desirable, which is attempted in this thesis.
This thesis deals with the results of investigation of aerosol characteristics obtained using a multi wavelength sun photometer. Rajkot (22o18′ N, 70o44′ E, 142 m above sea level) is a semi-arid tropical Indian station situated near the Arabian Sea in the Saurashtra region of Gujarat. It is a major residential and ix Industrial area. AOD at six different wavelengths (330, 440, 500, 675. 870 and 1020 nm) are measured using two hand held microprocessor based sun photometers [MICROTOPS-II (version 2.43 & 5.5)] in the premises of Physics Department (Saurashtra University) located on the outskirts of the city. However there is no major industry nearby. The AOD at 1020 nm for the period July 2004 - February 2008 and at five other wavelengths ranging from 380 nm to 870 nm for March 2005 – February 2008 is regularly measured, analysed and the results are presented in this thesis which will comprise of six chapters.

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