This paper describes a method for obtaining the abundance and altitude of stratospheric NO2 by using ground-based spectroscopy at twilight. The method is then used to study the behavior of NO2 over Colorado at 40 ¿N; a companion paper discussed observations elsewhere. Large changes in both the abundance of NO2 and in its diurnal variation are commonly seen at mid-latitude and appear to reflect the role of quasi-horizontal transport in the stratosphere on a global scale.