Assessment of the uncertainties in the NO₂ and O₃ measurements by visible spectrometers

The column amounts of nitrogen dioxide (NO₂) and ozone (O₃) were measured using a visible spectrometer based on the twilight zenith-sky technique at two observatories located at similar latitudes in the northern part of Japan separated by a distance of 150 km. The measurements began in April 1991 at the Moshiri Observatory (44.4°N, 142.3°E) and in April 1994 at the Rikubetsu Observatory (43.5°N, 143.8°E). Since weather conditions and the possible influence from tropospheric pollution were not always identical at these two observatories, the overall accuracy of the measurements was studied comparing these data sets. The first year data obtained at a solar zenith angle of 90 degrees indicated that the NO₂ slant column values at sunrise and sunset agreed within 0.36 and 0.54 x 10¹⁶ cm^-2, respectively, corresponding to 5% (June) and to 12% (December) of the columns. The O₃ values agreed within 0.76 x 10¹⁹ cm^-2, corresponding to 4% (March) ~ 6% (August) of the columns, although a part of the difference was systematic. The O₃ column amounts were also compared to those obtained by the Dobson spectrometer at Sapporo (43.5°N, 143.8°E), whose latitude is similar to these observatories. When an air mass factor of 17.5 was used, the two-year Moshiri vertical column values agreed with the Dobson direct sun values to within 15 Dobson Units, or 3 ~ 6% of the column. The difference between the two values was found to be due partly to the change in the air mass factor caused by seasonal and day-to-day changes in the shape of the O₃ vertical profiles. These results confirm the reliability of the NO₂ and O₃ measurements by visible spectrometers at these sites for the Network for the Detection of Stratospheric Change (NDSC).