Processes regulating short-lived species in the tropical tropopause layer

The annual cycle of species with lifetimes longer than 25 days is governed primarily by the variations of vertical velocity, not convection. This is particularly true for carbon monoxide, where a seasonal cycle in the lower stratosphere of the right phase is produced without variations in tropospheric emissions. An analysis of critical short-lived bromine species (CH₂Br₂ and CHBr₃) indicates that substantial amounts of these tracers may get advected into the lower stratosphere as source gases at 18 km, and are estimated to contribute 2.8 pptv (1.1–4.1) to stratospheric bromine.