Knowledge of air mass is vital for the interpretation of twilight measurements of trace gases, as well as the conversion of measured slant column amounts to vertical abundances for comparison with model predictions. Radiative transfer computations were used to determine NO2 air mass values for clear skies at 450 and 650 nm using a discrete ordinate (two different formulations), Monte Carlo, and an integral equation method. All four methods yielded agreement to within 6% at a solar zenith angle of 90¿ when the absorber was located in the stratosphere. For a tropospheric absorber, differences as large as 21% occurred at 90¿. Since only the Monte Carlo method treats the scattered radiation in spherical geometry, it is more accurate for computing tropospheric air masses where multiple scattering is significant. The other three models use a conceptual approximation by treating the scattered radiation in plane parallel geometry. However, for absorbers in the stratosphere, major saving of computing time without any loss of accuracy is obtained using the discrete ordinate or integral equation method as compared to the Monte Carlo method. ¿ American Geophysical Union 1996