PSC AND VOLCANIC AEROSOL OBSERVATIONS DURING EASOE BY UV-VISIBLE GROUND-BASED SPECTROMETRY

Twilight sky colour measurements were made using ground-based UV-visib le SAOZ spectrometers at four stations along the Arctic circle during EASOE. The results show that volcanic aerosol from the Pinatubo erupti on of June 1991 first appeared above the Arctic in September 1991. The aerosol layer thickened progressively during the autumn and had sprea d to all four stations, outside in the very low stratosphere, as well as inside the polar vortex, by mid-January. For the rest of the campai gn, to mid-March, little further change was seen. The aerosol would ha ve masked any PSCs that may have occurred at altitudes less than 22 km . However, PSCs at higher altitudes should have been detected, had the ir optical thickness exceeded 0.01. Only one was recorded during EASOE : on 28-29 December 1991, in contrast to their frequent occurrence in January and early February 1990.