Effects of stratospheric aerosols from the Mt. Pinatubo eruption on ozone measurements at Sodankylä, Finland in 1991/92

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Abstract
Total ozone amount is measured at Sodankylä, Finland using a Brewer spectrophotometer and a SAOZ instrument. The effect on these measurements of the stratospheric aerosols from the Pinatubo eruption is estimated, using a radiative transfer model. The aerosols is modelled as spherical particles of 75% sulphuric acid, with a size distribution inferred from lidar measurements. With the Brewer instrument looking directly at the sun or moon, measurements of ozone amount are not significantly affected by aerosols; the effect of aerosols on measurements looking at the zenith sky depends on the height of the aerosol layer and on the solar zenith angle. Ozone measurements from SAOZ observations of zenith skylight at sunrise and sunset are very sensitive to aerosol effects. Total ozone may be underestimated by up to 30% if airmass factors calculated for a clear sky are used. The SAOZ measurements can be brought into better agreement with the Brewer direct sun and moon measurements when airmass factors calculated taking account of aerosol scattering are used. A sensitivity test indicate that TOMS measurements are also affected by Pinatubo aerosols. For moderate and small solar zenith angles TOMS tends to overestimate the total ozone. For large solar zenith angle TOMS underestimates the total ozone significantly.