Processes of long-term relaxation of stratospheric aerosol layer in Northern Hemisphere midlatitudes after a powerful volcanic eruption

Multiyear lidar measurements of characteristics of stratospheric aerosol la yer, made at midlatitude observatories in Tomsk (56.5 degreesN, 85.0 degree sE) and Minsk (53.9 degreesN, 27.5 degreesE), are analyzed and used to stud y the processes of long-term relaxation of the aerosol-perturbed stratosphe re after powerful volcanic eruptions to background state. The absence of si gnificant seasonal variations of vertical stratification of stratospheric a erosol and exponential altitudinal decrease of aerosol backscattering coeff icient are proposed as criteria of background state of stratospheric aeroso l layer for Northern Hemisphere midlatitudes. (C) 2001 Elsevier Science Ltd . All rights reserved.