Spectral extinction coefficients of rural aerosol in southern italy-a case study of cause and effect of variability of atmospheric aerosol

The spectral extinction coefficient of the atomospheric aerosol at a rural location in Southern Italy was determined by means of a telephotometer: the radiance of a target at a horizontal distance of 12 km was measured at nine different wavelengths in the visible. Therefore the extinction coefficient of the aerosol contained within a conical volume of about 450,000 m3 and a length of 12 km was measured. All measurements were performed in summer 1993 during a period of stagnant air, presumably always with the same air mass and thus a similar type of aerosol. The daily variation usually followed a similar pattern: decrease in extinction coefficients in the morning and early afternoon and an increase towards the evening. This variation correlated well with the change in humidity. The Ångstrom exponent of the spectral extinction coefficient, was lowest at high humidity and highest at low humidity. Comparing the horizontal attenuation measurements and vertical transmission measurements done with a solar photometer, a considerable vertical extent of the aerosol was found. This is important for climate considerations, since additional light absorption and scattering by the aerosol can lead to an increase or a decrease of the temperature. Inversion of the spectral extinction coefficient data to obtain particle size distributions shows that the dry (< 50% r.h.) particles have a peak of the volume size distribution at 0.38 μm, which increases to 0.81 μm at 80% r.h. Light absorption by the aerosol varied little on a day to day basis, amounting to 20 to 40% of the light extiniction coefficient. Values this high are common in Europe, also outside densely populated areas. The average single scattering albedo of the dry aerosol was 0.76, thus the aerosol will have a heating effect due to its light absorption.