AER Inc., Report December 1999

The effect of the boundary layer processes on atmospheric CH3Br mixing ratio

M. Ko and C. Andronache

AER Inc., Cambridge, Massachusetts, USA


Abstract. Atmospheric methyl bromide, CH3Br, exhibits a free troposphere (FT) mixing ratio of about 10 pptv, with seasonal and regional variations of ~ 2 pptv. In contrast, the boundary layer (BL) mixing ratio of CH3Br can have large variations, connected to inhomogeneities in surface flux, sinks in soils and ocean, and to entrainment between BL and FT. Given the observed variability of CH3Br in the BL, the lifetime of this compound can be significantly shorter in the lower part of the atmosphere in the vicinity of sources and sinks. The goal of this work is to determine the flux of CH3Br above the BL as a function of parameters describing the BL processes. We use a conceptual one-dimensional model to illustrate the role of deposition velocity, surface flux of CH3Br and vertical eddy diffusion intensity on a long-term average vertical distribution of CH3Br and on the CH3Br flux between BL and FT. The role of eddy diffusion and vertical velocity are investigated using the GFDL SKYHI model, which has a BL physics parameterization similar to CCM3. Based on our preliminary results, the flux of CH3Br above the BL is mainly sensitive to the surface flux, the deposition velocity, and to a less extent to the eddy diffusion coefficient. Typically, the flux of CH3Br at the top of the BL is about 10-40% from the surface flux.