A STUDY OF AEROSOLS

Abstract

The thesis entitled 'A Study of Aerosol3' deals with a few properties of the aerosols, especially the optical properties which have been measured continuously in the night time in ambient conditions in order to study the effect of relative humidity variations and the seasonal patterns. Chapter I which is an introduction, deals with the processes of aerosols origin in the atmosphere and their removal from there. The properties like shape, size, number density and size distribution functions, chemical composition etc. have been discussed. The theories of elastic scattering of electromagnetic radiation by particles, Rayleigh and Arlie theory, are reviewed in detail as they are used in computing the number density and Angular scattering functions. The various methods of sampling the aerosols and measuring the scattering properties are also discussed. Chapter II deals with the experimental studies on light scattering by aerosols in the ambient atmosphere. A simple and sturdy experimental arrangement is used to measure the scattered light energy in a fixed scattering angle zone (90° < 9 < 120°) over long periods in the night time using an artificial source of light , here being Halogen bulb search light. The_ recorded variations in the signal are function of the aerosols' properties as for the visible electromagnetic radiations contribution of the molecules of atmospheric gases is very less and is almost constant with time. In the present case to confine the study for a single wavelength of the halogen bulb spectrum, a narrow band pass filter (Xmax = 0.5764mi Half Band Width = 0.01 tim) is placed in front of the photo-detector (Photomultiplier tube RCA 931A). The background diffused light is taken into account by using another similar photodetector, operating at the same sensitivity, orientation and height as the previous one. The difference of two signals gives the scattered light energy from the volume of scatterers in the atmosphere formed by the intersection of fields of view of the search light and the photomultiplier tube. Such observa-tions taken over the months of April, May, June, Sept., Oct., Nov., Dec. , 1982 and May, 1983 are reported and the effect of the relative humidity and the seasonal variation pattern of the angular scattering properties of the aerosols are studied. Using observations of the two nights, Sept.20, 1982 and May 12,1983, the time variation in aerosols' number density is calculated assuming a bimodal logm)rmal size distribution function, given by Shettle and Fenn (85), for the local aerosols, incorporating the changes in scattered light energy due to changes in the complex refractive index and the parameters (mode radii) of the size distribution function caused by the variations in a atmospheric relative humidity. The computational work is based on the Fast Vector Speed Computer Codes of Wiscombe (94). In calculations, contribution of the air mole-cules has been duly taken into account. In Chapter III behaviour of the Angular Scattering Function, i(G) (function giving the intensity of scattered light for incident radiation having unit irradiance) for three particles differing in size and representing urban aerosols is computed theoretically using Mie theory for different values of atmospheric relative humidity. The calculations are based on the experimental data of Hanel (34) and are performed on DEC-20 computer system. Besides, the computations are done for the volume angular scattering function, ri(G), for power law model of aerosols for different values of imaginary part of the complex refractive index keep-ing the real part fixed and the vice-versa, to see the effect of individual components on the angular scattering. The work is extended for real atmospheric models by computingoi(G), for aerosols of rural, urban and marine environments at different values of relative humidity. The bimodal lognormal size distribution f unction is used for the calculations and changes in the complex refractive index as well as mode radii are incorporated while studying the effect of relative humidity on the angular function. For measuring angular distribution of scattered light in the ambient atmosphere in the night time, a simple experimental set-up employing He-Ne Laser as the source of light is used. To take care of the varying intensity of background diffused light at different orientations of the photomultiplier tube (v) a black plate is used in the field of view of the photomulti-plier tube. It provides a constant, background. Besides it, as a caution measurement at each angle of scattering is taken with and without switching on the lie—Ne Laser. The difference of two gives the actual light scattered by the scatterers in the volume of interest. In the arrangement source of light is kept fixed and the photomultiplier tube is revolved along a fixed axis to measure the scattered light energy at different angles. At different angles, velure of scattering zone changes as the laser beam makes a cylindrical column and the path intercepted by the photomultiplier tube is different at different angles of scattering. For sake of comparison the measured signal is divided by the volume at each angle of scattering, the former being minimum at 90' and maximum at 0' and 180' . In Chapter IV is presented the size distribution function for the local aerosols lying in the size range, 0.2 < r < 5.0 pm. The sampling is done using a four stage cascade impactor, the fourth stage followed by filter paper on a porous polyethy-lene disc. The particles deposited on the circular glass slides of the different stages of the impactor are scanned and photographed on Scanning Electron Microscope. On the basis of photographs, particle s are m easured and classified into different size ranges thus enabling us to give a size distribution function. Proper correction factor is applied on the data as for smaller particles collection efficiency of the last stage is pot hundred percent.