Observation of several chlorine nitrate (ClONO2) bands in stratospheric infrared spectra

Four of the most prominent and sharpest infrared absorption features of chlorine nitrate at 780.2, 807.7, 809.4 and 1292.6 cm−1 have been observed in a series of infrared solar spectra obtained at an unapodized spectral resolution of 0.01 cm−1, using the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument from on-board Spacelab 3. A quantitative analysis of the ν4 Q branch at 780.2 cm−1 has provided insight into the concentration of ClONO2 between 19 and 40 km altitude. While the mean profile deduced from 3 sunset occultations near 30° N latitude exhibits a shape close to that predicted by model calculations, its conconcentrations in the 20 to 32 km altitude range are, however, about 30% larger, reaching a peak concentration of 9 × 108 molecules/cm³ at 25 km. The concentrations above 32 km, deduced from one sunrise occultation at 47° S, are even larger than the corresponding sunset values at 30° N latitude. Some of these discrepancies may be caused by the rather large uncertainty in the assumed Q branch strength. The results reported here constitute a significant input towards understanding the chemistry prevailing in the stratosphere as well as for model calculations predicting the secular evolution of our atmosphere.