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) insrument from on-board Spacelab 3.

A quantitative analysis of the &ngr;4 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 concentrations 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 calculation predicting the secular evolution of our atmosphere.