VERTICAL AND HORIZONTAL AEROSOL SPECTRAL EXTINCTION AT A RURAL LOCATION IN SOUTHERN ITALY

The vertical extinction of the aerosol has been measured with a multiw avelength sunphotometer at a rural location in southern Italy. All mea surements were performed during a period with relatively stable weathe r condition, thus it could be expected to have measured always the sam e air mass at least within each day. The direct solar radiation was re corded at wavelengths between 400 and 1100 nm with a resolution of 0.5 nm. Calibration of the instrument was performed by means of a Langley plot at an elevation of approximately 3000 m. From these measurements , vertical optical depths have been derived for a series of days in Ju ne 1993. The spectral extinction coefficient of the atmospheric aeroso l at the same location has been measured by means of a telephotometer. The radiance of a target at a distance of 12 km was measured at nine wavelengths in the visible. Thus the extinction coefficient of the aer osol contained within a conical volume of about 450,000 m(3) and a len gth of 12 km has been measured. The daily variation usually followed t he same pattern: a decrease in extinction coefficient in the morning a nd in the early afternoon and an increase toward the evening. This var iation correlated well with the change in humidity, so that it can be assumed that the aerosol particles have decreased their size due to ev aporation of water in the morning and uptake of water in the evening. The regular daily pattern of the horizontal aerosol extinction coeffic ient normally was not followed by the vertical aerosol optical density , but on the other hand, changes in vertical extinction have occurred on some days, which had no effect on the horizontal extinction. Vertic al extinction measurements were inverted to obtain aerosol size distri butions. Inversions were obtained by the Twomey-Phillips method and th ey confirm the presence of a hygroscopic aerosol as suggested directly by the variation of the extinction coefficients against the relative humidity. The aerosol undergoes very clear processes of growth which a re well correlated with the behavior of the humidity. Overall, the typ e of aerosol we have measured could be classified as rural aerosol. Th is kind of aerosol is typical of the boundary layer of clean continent al area and is composed prevalently by water-soluble substances and in minor abundance by dustlike aerosol.