Fluctuations in trace gas concentrations generated by entrainment in the boundary layer and free troposphere

By means of a simple analytical model, I show how entrainment across the top of the convective planetary boundary layer (PBL) can generate fluctuations in trace gas concentrations in the remote PBL and the free troposphere. The model assumes a well-mixed PBL overlain with a well-mixed free troposphere separated by a jump in species concentration. A jump in concentration is also assumed across the tropopause, and an exponential increase in diffusivity with height is assumed through the stratosphere. The model is applied to species with surface sources that have lifetimes of the order of weeks to months, and predicts that the standard deviations of the concentration fluctuations normalized by the mean concentration are proportional to the lifetimes of the trace gases to the -1/2 power. Species for which this approach is applicable include C2H6, C2Cl4, CHCl3, and CH3Cl. PBL Observations from the Pacific Exploratory Mission (PEM) Tropics A Mission, as well as other similar aircraft studies generally agree with this prediction. A relation of this type is useful for estimating lifetimes of trace gases in the atmosphere; or conversely, if the lifetimes are known, average entrainment rates in the measurement region.