Stratospheric and total NO2 column measurements with a modified Canterbury filter photometer

A programme of ground-based stratospheric and total NO₂ column measurements was instituted at the Laboratory of Atmospheric Physics (40.5° N, 22.9° E) in August 1985. We present here the results of the first two years of measurements with a modified Canterbury filter photometer, details of which are given in the text. The stratospheric NO₂ column, obtained at twilight during low local NO₂ levels, shows the seasonal variation with monthly mean values of about 6×10^-15 molec. cm^-2 in the summertime to about 2.2×10^-15 molec. cm^-2 in the wintertime. These measurements compare well with measurements obtained with different instruments by other groups at similar latitudes (about 40° N) but in different places. Also, the asymmetry of the evening-to-morning stratospheric NO₂ over Thessaloniki was found to be on the average equal to 1.58. Total NO₂ column over Thessaloniki has a pronounced seasonal variation with amplitude of 0.68 matm. cm which can be explained partly from measured local NO₂ sources which discharge in the mixing layer and partly from photolysis of the NO₂ reservoir species.