In the stratosphere, NO₂ is in equilibrium with its reservoirs N₂O₅ and HNO₃. Both are photolysed during the day and re-form during the night. As a result, the amount of NO₂ in the stratosphere depends on the number of hours of sunshine. This factor leads to a strong seasonality with a maximum in summer and a minimum in winter in mid and high latitudes.

Because the amount of illumination depends only on season and latitude, the stratospheric NO₂ column is mainly a function of latitude and not of longitude. The transport of air masses, temperature variations and chemistry (within the polar vortex, for example) can lead to deviations from this simple picture, but as shown by the SCIAMACHY limb measurements in figure xxx, the assumption of a smoothly varying zonally invariant NO₂ field is a good approximation.