Total nitrogen dioxide at the Arctic Polar Circle since 1990

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Abstract
Daily ground‐based NO2 column measurements have been conducted with UV‐visible SAOZ diode‐array spectrometers at Sodankyla in northern Finland since January 1990 and at 3 additional stations during EASOE. Inspection of the data indicates: a total NO2 reduction by 30% by the Pinatubo aerosol in spring and summer 1992 compared to the previous years; a poor correlation in winter with potential vorticity indicative of motion of the vortex and of advection of air from mid‐latitude, frequent during EASOE; an absence of significant global denitrification by sedimentation of nitric acid on PSC particles inside the vortex; and a high correlation between total NO2 and 30–50 hPa temperature. The latter is unexpected in winter 1991–92, since most of NOx (NO + NO2 + ClONO2 + 2N2O5) at the altitude of the aerosol layer should have been converted into liquid nitric acid. This is expected to revert only slowly to NOx at high latitude in winter. The correlation suggests a temperature dependent saturation of the aerosol water / sulfuric acid droplets and / or a temperature dependent mechanism of restitution of NOx to the gas phase.