Four years of stratospheric aerosol measurements in the northern and southern hemispheres

Lidar data, collected from stations located in both the Northern and Southern hemispheres using 532nm wavelength, were employed for monitoring the evolution of stratospheric aerosol loading from January 1994 up to December 1997. Lidar data covering the pre-Pinatubo situation are available also for the 1989-1993 period only for the Antarctica station. These data are reported for comparison with the present situation. The analysis provided stratospheric aerosol loading measurements which show the return to a clean stratosphere as observed before the eruption of Mt. Pinatubo. During 1997, integrated backscattering values ranging between 8.2 +/- 3 10(-5) sr-1 for Lauder and Florence/ Brasimone and 6.0 +/- 1.5 10(-5) sr(-1) for the polar stations of Sodankyla and Dumont d'Urville were measured. These values are smaller then the ones registered before the Pinatubo eruption: for example, at Dumont d'Urville in 1989 an integrated backscattering of 0.75 +/- 0.12 10(-4) sr(-1) was measured. An evaluation of the decay time, i.e. the half time, for aerosols allowed a rough estimate of aerosol particle size. A comparison between the decay time for integrated backscattering in the 13 to 25-km region obtained for the four sites did not show any strong variability between the northern and southern hemispheres: only small differences are observed between the Arctic and Antarctic regions. A time constant of about 3 years and half was retrieved for the 1994-1997 period.