Daily ground-based NO2 column measurements have been conducted with UV
-visible SAOZ diode-array spectrometers at Sodankyla in northern Finla
nd since January 1990 and at 3 additional stations during EASOE. Inspe
ction of the data indicates: a total NO2 reduction by 30% by the Pinat
ubo aerosol in spring and summer 1992 compared to the previous years;
a poor correlation in winter with potential vorticity indicative of mo
tion of the vortex and of advection of air from mid-latitude, frequent
during EASOE; an absence of significant global denitrification by sed
imentation of nitric acid on PSC particles inside the vortex; and a hi
gh correlation between total NO2 and 30 - 50 hPa temperature. The latt
er is unexpected in winter 1991-92, since most of NOx (NO + NO2 + ClON
O2 + 2N2O5) at the altitude of the aerosol layer should have been conv
erted into liquid nitric acid. This is expected to revert only slowly
to NOx at high latitude in winter. The correlation suggests a temperat
ure dependent saturation of the aerosol water / sulfuric acid droplets
and / or a temperature dependent mechanism of restitution of NOx to t
he gas phase.