Evidence of substantial ozone depletion in winter 1995/96 over northern Norway

In winter 1995/96, the stratospheric ozone layer over most of the European Arctic was exposed to extremely low temperatures and strong PSCs. At the same time, total ozone monitoring instruments at Tromsø and Andøya, Northern Norway, observed values up to 50% below the long-term average values of the Dobson instrument at Tromsø. The deviation increased systematically with time of the year, reaching a maximum in early March. Measurements with the ozone DIAL system at the ALOMAR facility at Andøya showed significant reduction of ozone mixing ratios in the 430 to 580 K potential temperature region. A maximum reduction of almost 50% occurred at about 500 K between early January and early March. Comparison with 3D model calculations revealed large reductions with respect to passive tracer ozone profiles in the 400 to 600 K region, reaching a maximum of about 60% by the end of March at the 475 K level. The 3D chemical model showed ozone depletion in the same altitude region as the measurements but seemed to underestimate the depletion systematically. The comparison also indicated ozone depletion of up to 30% in the 475 to 675 K region in extra-vortex air which remained close to the vortex edge during a period in mid-March.