The total column abundance of NO3in the stratosphere at night has been measured for 5 years at 40¿N, with additional observations at 19¿, 31¿, 51¿, and 64¿, and 64¿N. In this set of measurements the NO3 abundance varied from slightly greater than 1014 cm-2 to below the detection limit of 1013 cm-2. At low latitude the mean abundance appears to be in general agreement with a model based upon simple NOx chemistry, except that the summer values tend to be smaller than spring values. At mid-latitude the variation in abundance exceeds a factor of 5,which is greater than predicted. Large day-to-day variations in are observed, especially in summer. The winter values mostly fall below the detection limit, which can be as low as 1013 cm-2. The abundance appears to exhibit a close relation to the highest latitude experienced by stratospheric air prior to its arrival at mid-latitude; the higher the latitude the lower is the NO3 abundance. At 64¿N the NO3 abundance was below the detection limit of 1.5¿1013 cm-2, both in April and November. This is far below what is predicted, especially in April. These observations suggest that a scavenger of NO3 is continually produced in the stratosphere, particularly at high latitude, and when there is a strong equatorward flow, the scavenger can at least reach mid-latitude. The identity of the scavenger remains unknown, as does the importance of its role as a sink for NOx in the stratosphere.