Atmospheric boundary layer ($H$)

The lowest part of the terrestrial atmosphere that responds directly to surface fluxes on relatively short timescales through turbulence. Its depth changes with heating, stability and weather; a sharp composition or temperature change can mark its top. A uniform-layer chemical budget converts a surface flux into a volume tendency by dividing by layer depth.

The atmospheric boundary layer over an ocean surface. A box-model ozone budget can include in-layer production, chemical loss, dry deposition, entrainment from the free troposphere and horizontal Advection. Fixed-depth local steady-state assumptions must be checked against transport and day–night variation.