PREDICTION OF HIGHER MOMENTS OF NEAR-SOURCE CONCENTRATION BY SIMULATING THE MEANDERING OF POLLUTANT PUFFS

When modeling pollutant transport and dispersion using gridded meteorological flow fields on an hourly basis, significant parts of the turbulence spectra are not resolved in space and time. Parameterizations of dispersion (growth of puffs or plumes as a function of time, travel distance or puff size) commonly account for this by estimating one-hour averaged dispersion (so-called "absolute dispersion").

For risk assessments and odor impact analyses, the highest possibly occurring concentration during a time considerably shorter than one hour is more decisive than one-hour average values. For this, the probability density function of concentration for a given location and a specific averaging time is required.

In order to be able to estimate concentration probability density functions, the absolute dispersion has to be split up into its two components, i.e. the instantaneous puff/plume growth ("relative dispersion") and the dispersion caused by meandering of the puff during the time averaging period. The first of these two components is driven by the turbulent eddies being smaller than the size of the puff, which are thus able to increase the mean distance between particles within the puff. The second component accounts for the effect of eddies larger than the puff, which will displace the puff without enlarging it. For this, the concept of the Puff-Particle Model (de Haan and Rotach, "A Novel Approach To Atmospheric Dispersion Modeling: The Puff-Particle Model (PPM)", accepted for publication in Quart. J. Roy. Meteorol. Soc.) has been introduced as a new near-source dispersion model.

This contribution presents the estimation of the higher moments of near-source concentrations for different averaging times, using the combination of the CALPUFF model with the Puff-Particle Model (PPM) (de Haan, Rotach and Scire, "Introduction Of A Puff-Particle Approach For Near-Source Dispersion Into The CALPUFF Model", paper presented at the 23rd NATO/CCMS Int. Tech. Meeting, 1998). This combination improves the prediction of the highest occurring near-source concentration, as well as the distance down-wind from the source where it occurs, while retaining the advantages of puff models for the intermediate to far field range.

The PPM combines puff and particle dispersion models by moving the center of mass of each puff along a trajectory which mimics the quickly changing turbulent flow field. This trajectory is derived from the low-frequency part of trajectories as simulated by a Lagrangian stochastic particle model. CALPUFF is a Lagrangian puff dispersion model. Among its main fields of application are pollutant transport, chemical transformation, and deposition simulations for inhomogeneous and non-stationary conditions for periods of one year or more with a one-hour time step. Together with the flow fields of its meteorological model (CALMET), CALPUFF is applicable to complex terrain and coastal situations. The performance of the combined CALPUFF/PPM is validated using the data from tracer experiments with near-source measurements, and with measurements of concentration fluctuations.