On the maintenance of the Marine boundary layer-free tropospheric gradient of transient tracers

A simple model of the marine boundary layer is used to arrive at a steady-state budget for trace gases in this layer. Five processes maintain the tracer balance in the layer. The first is a large-scale horizontal convergence of the tracer; the second is the pumping by clouds of the tracer out of the boundary layer into the free troposphere; the third is penetration by the tracer from the free troposphere into the marine boundary from compensating subsidence in the cloud-free air surrounding cumulus clouds; the fourth is sea surface emission or deposition of the tracer; and the fifth is in situ photochemical production or loss of the tracer within the marine boundary layer. The budget equation is used in deriving an expression for the mixing ratio difference between the two regions. A scale analysis suggests that the mixing ratio difference between the marine boundary layer and the free troposphere can be an order of magnitude smaller in disturbed weather conditions than in suppressed conditions, showing that the difference in chemical concentrations between the free troposphere and the marine boundary layer cannot be rationalized solely on the basis of chemical measurements.