Lidar observations of the stratospheric aerosol layer over southern Italy in the period 1991-1995

Lidar measurements of the stratospheric aerosol layer have been carried out in Napoli (40 degrees 50' N-14 degrees 10' E) and Potenza (40 degrees 36' N-15 degrees 44' E) during the period 1991-1995, covering the history of the aerosol cloud produced by the eruption of Mount Pinatubo (June 1991, Philippines), Measurements are expressed in terms of aerosol backscattering coefficient beta(A),(z), aerosol integrated backscattering IB and aerosol optical thickness tau(A), at lambda = 351 nm and 355 nm; beta(A)(z) and tau(A) are determined from a single-wavelength elastic return. IB, tau(A), and beta(max), the peak aerosol backscattering, reached their maximum value in December 1991, displaying a subsequent decay with e-folding times of 237 +/- 25, 250 +/- 111, and 257 +/- 33 days, respectively. R(max), the peak scattering ratio, is characterized by a decay time of 235 +/- 13 days. Measurements of the extinction-to-backscatteriny; ratio, alpha(A)(z)/beta(A)(z), and of the column parameter, tau(A)/IB, allowed us to retrieve aerosol dimensional characteristics. The time evolution of the height, z(c), of the aerosol cloud center of mass was also determined. Downward gravitational settling of stratospheric aerosols with time suggests aerosol particles fall within the size range 0.1-0.3 mu m. An abrupt change in IB and beta(max) is observed approximately 1000 days after the eruption as a result of the winter-summer transition and the tropospheric removal of the lower portion of the stratospheric aerosol layer, Changes in the values of IB, tau(A)/IB, and z(c) suggest that this transition is characterized by a change in the aerosol mean radius from 0.3 to 0.1 mu m.