The code for tracers of mass transport in the atmosphere is initialised by setting #define TRACERS_ON in the rundeck. These tracers are mostly insoluble gases or ideal tracers such as air mass age or source. All changes in air mass (by advection, convection, the sea level pressure filter) and parameterised mixing (stratospheric diffusion, atmospheric turbulence) are followed by the tracers. There is one exception for the pure 'Air' tracer: this must follow the mass field identically, but during the operation of the sea level pressure filter, a conservation of mass implies a change in concentration. For most tracers, conservation of mass is more important, but for 'Air' concentration is conserved.