Atmospheric infrared emission has been recorded with a scanning variable-filter radiometer on eight stratospheric balloon flights at Alert, inside the arctic vortex, from December 1991 to March 1992. The radiometer and the data analysis for the determination of the altitude distribution of nitric acid vapor are described. The time development of the nitric acid altitude profile at Alert is presented. A sudden change in the profile and vertical column density appears to be associated with the mid-January stratospheric warming. There is good overall agreement between our results and other arctic winter measurements. ¿ American Geophysical Union 1993