6.2 USE OF AREA SOURCE ALGORITHM IN AERMOD (09/27/05)

Because of issues related to excessive run times and technical issues with model formulation, the

approach that AERMOD uses to address plume meander has not been implemented for area

sources. As a result, concentration predictions for area sources may be overestimated under very

light wind conditions (i.e., u << 1.0 m/s). In general, this is not expected to be a problem for

meteorological data collected using standard wind instruments since instrument thresholds are

generally too high. However, the problem could arise with meteorological data derived from

very low threshold instruments, such as sonic anemometers. While not currently accepted for

regulatory applications of AERMOD, this problem has also arisen when data from a gridded

meteorological model was used to drive AERMOD. Meteorological grid models can at times

produce extremely light winds. During such conditions time-averaged plumes tend to spread

primarily as a result of low frequency eddy translation rather than eddy diffusion. AERMOD

treats this meander effect by estimating the concentration from two limiting states: 1) a coherent

plume state that considers lateral diffusive turbulence only (the mean wind direction is well

defined) and 2) a random plume state (mean wind direction is poorly defined) that allows the

plume to spread uniformly, about the source, in the x-y plane. The final concentration predicted

by AERMOD is a weighted sum of these two bounding concentrations. Interpolation between

the coherent and random plume concentrations is accomplished by assuming that the total

horizontal “energy” is distributed between the wind’s mean and turbulent components.

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