Atmospheric Eno~ronmmt Vol. 13, pp. 1311-1318. Pergamon Press zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ltd. 1979 Printed m Great Britam. SULPHUR DIOXIDE PLUM E STRUCTURE BY MASK CORRELATION SPECTROSCOPY G. GIOVANELLI, T. TIRAAASSI and S. SANDRONI* Istituto di Fisica dell’Atmosfera, Sezione Microfisica, C. N. R., Bologna, Italy zyxwvutsrqponmlkjihgfedc (First received 9 March 1978 and in,fina/form 15 March 1979) Abstract - A method for describing two dimensional plume structure using a ground based remote sensing technique, mask correlation spectroscopy, is presented. It consists essentially of a series of optical depth measurements at different zenith and azimuth angles. By geometrical projection and interpolation one can obtain the “isopleths”, which represent the contours of gas density inside the plume. This method has been tested in the field and some typical results for different meteorological conditions are given. The limitations of the method and possible future developments are discussed. INTRODUCTION The diffusion and transport of material from power station chimneys into the atmosphere is of primary importance in air pollution research. Remote sensing techniques are important new tools for the study of the structure and dynamics of the atmosphere and for the investigation of transport and diffusion phenomena. Photographic and radiometric techniques, amongst others, have been used in attempts to measure the shape, location, temperature and density distribution of plumes. The optical radar, Lidar, is effective in measuring plume rise and lateral spread from power plant stacks and can be used to follow the movement of dust clouds; Lidar has also been used to measure two dimensional particulate distribution in plumes (John- son, 1969). Recently the differential absorption Lidar (Dial) seems very promising in mapping atmospheric NO2 (Rothe et al., 1974) and SO2 (Hamilton et al., 1978). Vertical plume cross-sections are obtained by taking a series of Lidar shots through the plume at different angles of elevation. The backscattered signals give range resolved values for plume concentration, it being assumed that particle size distribution in the plume is homogeneous and that the plume itself produces negligible attenuation of a Lidar pulse. The Lidar is located a few hundred meters from the plume and is aimed in a direction approximately perpendicular to its axis. A series of cross-sections at increasing downwind distance reveal the rise, diffusion and change in the structure of the plume under the meteorological conditions existing at the time of the experiment. The dispersion from tall chimneys in the convective planetary boundary layer has been also studied by numerical (Lamb, 1978) or laboratory (Willis and Deardorff, 1978) simulation models in * Joint Research Centre, Ispra Establishment, Direction C, 21020 Ispra, Italy. terms of cross-wind integrated concentrations. The results could be verified by remote sensing techniques. In general these techniques can provide spatial and temporal integration which make observations more representative of the plume than those of a network of point sensors which may be affected by local or transient perturbations. In this paper a method similar to that described above for Lidar but applied to mask correlation spectroscopy is presented. It provides a two dimen- sional plume structure from measurements of SO, optical depths using diffused daylight as a light source. This method has been applied in field experiments and some preliminary results are presented. Such profiles obtained from ground based remote sensors can help in describing pollutant transport in the atmospheric boundary layer and in source monitoring. TWO-DIMENSIONAL PLUM E STRUCTURE Mask correlation spectrometry The mask correlation spectrometer is a dispersive type instrument based in the measurement of the degree of similarity between the molecular absorption spectrum of a gas and an optical correlation mask used as a fingerprint of the gas under investigation. The correlation mask consists of a series of photoetched slits reproducing the absorption spectrum of the gas. During the cyclic movement of the mask with respect to the dispersed incoming spectrum, a high degree of correlation is obtained for the target gas but not for other molecular species. A high specificity can be obtained from the technique only if the degree of correlation with other gases in the spectral region under investigation is low. Actually two mask correlation spectrometers are available: the Cospe? (Barringer Research) and the TeletecR (Tecneco). Both instruments can use an artificial light source (active mode) or diffused daylight 1311