A probability density function (PDF) dispersion model is presented for buoyant plumes in the convective boundary layer (CBL), where the mean concentration field C is obtained from the PDFs p_y and p_z of tracer particle position in the lateral y and vertical z directions. The p_y is assumed to be Gaussian, whereas the p_z is derived from the the vertical velocity PDF. which is skewed. Three primary sources contribute to the modeled C field: 1) the "direct" or real source at the stack. 2) an "indirect" source to account for the slow downward dispersion of lofting plumes from the CBL top, and 3) a "penetrated" source to treat material that initially penetrates the elevated inversion but later fumigates into the CBL. Image sources are included to satisfy the zero-flux conditions at the ground and the CBL top. Comparisons between the modeled crosswind-integrated concentration fields C and convection tank data show fair to good agreement in the lower half of the CBL. In particular, the C profiles at thesurface agree with the data over a wide range of the dimensionless buoyancy flux F_* and show a systematic decrease in C with F_*. Comparisons between the modeled and observed ground-level concentrations around several power plants exhibit good agreement on average and are considerably better than those obtained with a standard Gaussian plume model. A residual analysis suggests some areas for future model development.
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