# Trajectories versus Concentration Plumes

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HIGHLIGHTS
• Particle, Puff, Hybrid

A “puff” following a single trajectory cannot properly represent the growth of a pollutant cloud when the wind field varies in space and height.  In these situations, the single-puff must either split into multiple-puffs or the simulation must be conducted using many pollutant particles.  On the left, new trajectories are started every 4-h at 10, 100, and 200 m AGL to represent the boundary layer transport using the NAM 40 km forecast data, while on the right, 2500 particles were used to simulate the air concentration plume. Note how the upper level trajectories (and particles) are to the right of the lower-level trajectories. The CONTROL and SETUP.CFG files used in both of these calculations can be viewed by clicking on the links below each figure.

controlsetup.cfg        control  & setup.cfg

An hourly animation of the particles used to create the figure on the right can viewed by clicking HERE.

Particle:  The element (particle) is a point mass of contaminant. A fixed number of particles are released. They are moved by a wind having mean and random components. They never grow or split.

Puff:  The element is a fully 3-D cylindrical puff (see figure below left), having a defined concentration distribution in the vertical and horizontal.  Puffs grow horizontally and vertically according to the dispersion rules for puffs, and split if they become too large.

Hybrid:  The element is a circular 2-D object (planar mass, having zero vertical depth), in which the horizontal contaminant has a “puff” distribution (see figure below right). There are a fixed number of these in the vertical because they function as particles in that dimension. In the horizontal dimension, they grow according to the dispersion rules for puffs, and split if they get too large.
 Illustration of how a single particle (Q0) splits due to vertical diffusion into two particles Q2 and Q3. Illustration of how a single particle with radius R splits due to horizontal diffusion into four particles (Q1, Q2, Q3 and Q4) each with radius R/2.

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