IMPROVEMENTS TO THE ACCURACY OF MEASUREMENTS OF NO2 BY ZENITH-SKY VISIBLE SPECTROMETERS

Visible spectra of the zenith-sky were recorded at Faraday in Antarcti ca during 1990. Line-of-sight NO2 amounts were deduced by fitting labo ratory cross-sections to the spectra. A new procedure was used to dete rmine the offset to the zero of the measurement from the measurement s eries itself. This offset includes the effect of the reference spectru m-a single reference was used for the whole data set-plus any instrume ntal or analytic artefacts. It is important to determine this offset t o the zero because it affects the absolute calibration of the zero of the vertical amount of NO2. In the new procedure, offsets were determi ned from fits to plots of the measured line-of-sight NO2 vs calculated air-mass factors, whose curvature had been precomputed from model pre dictions rather than the straight-line fits of Langley plots. The resu lting vertical NO2 amounts were significantly lower than those deduced using a more subjective determination of offset, particularly in the early spring when denitrification might be expected.