Background Information

Atmospheric Dispersion Index

Summary

The Atmospheric Dispersion Index (ADI) is a numeric rating of the atmosphere's capability of transporting pollutants away from their sources. The ADI is based on the Gaussian distribution statistical models described by Lavdas (1986), in which a normal distribution of pollutant concentration is expected within the plume. The dispersion index scale was developed for the Florida Division of Forestry, which regulates controlled burns depending on dispersion conditions and the burner’s certification class.

The ADI uses a 50-km rectangular control volume, with the pollutant source at its rear edge. An equal weighting of an atmospheric ventilation factor and a surface-based climatological dispersion model described by Busse and Zimmerman (1973) provide the index value. This model assumes that the concentration of the pollutant downwind of the volume changes linearly with emission rate within the control volume. With a doubling of the emission, a doubling of the concentration at the downwind edge of the control volume is expected.

The ADI ranges from 1 to over 100. Low values indicate poor dispersion, where smoke will be trapped near the ground, and high values indicate good dispersion, where smoke will be quickly carried away from the source and high into the atmosphere. Normal daytime values in good prescribed burning conditions vary from 60 to 100. Values much larger than 100 indicate blow-up conditions, under which crown fires are likely. Overnight values of 10 or less are common, although forcing events such as frontal passages may produce high values.

Surface and rawinsonde (weather balloon) observations provide information about atmospheric stability, the mixing height, and the mean wind within the mixing layer (transport wind vector). Upper air conditions are normally observed every 12 hours at stations much farther apart than surface stations. This requires estimates of upper air conditions between stations at the time the ADI is calculated.

The following components are used to calculate the ADI.

Ceiling
The ceiling is the height at which cloud layers obscure at least 50% of the sky. The ceiling helps determine the stability index and may be used as guidance for pilots engaged in fire suppression.

The ceiling is estimated from surface-based observations of cloud from human observers or automatic devices. Cloud type, base height, and a description of the fractional coverage appear in hourly meteorological aviation routine reports. The coverage descriptions, which indicate few, scattered, broken, or overcast, are translated into numeric coverage classes. The ceiling is taken from the last cloud layer used when the sum of coverage values for successive layers equals 0.5 (50%).

Pasquill–Gifford–Turner Stability
Static stability of the atmosphere determines the amount of mixing that occurs between layers. An atmospheric layer is generally unstable, producing mixing and greater dispersion, if cooler air lies above it. An atmospheric layer is generally stable, preventing mixing and trapping smoke, if warmer air lies above it.

This stability calculation uses the Pasquill (1961, 1974) method, modified by Gifford (1962) and applied to computers by Turner (1961, 1964). Stability calculation using the Richardson number or a combination of the two methods is optional.

The Pasquill–Gifford–Turner method estimates stability on the basis of recent surface-based observations, including wind speed, sun incidence angle, and ceiling. The Richardson number, which uses surface and rawinsonde observations, provides a ratio between buoyant and mechanical forces in an atmospheric layer.

The Pasquill–Gifford–Turner stability values range from 1 to 7. Low values indicate that the atmosphere is unstable and that smoke will be dispersed easily, forming high plumes resembling cumulonimbus clouds. Midrange values indicate that the atmosphere is neutral, with possible weak, sporadic buoyancy providing some dispersion. High values indicate that the atmosphere is stable and buoyant forces are weak, trapping smoke close the ground.

Mixing Height
The mixing height is the distance above ground level to which smoke will rise. Mixing heights are determined by surface and rawinsonde observations. The mixing height calculation assumes that inversions are not deformed by heat sources such as urban areas or large fires.

The mixing height is usually low in a stable atmosphere, a common event at night. A low mixing height traps smoke near the ground, so that it duffuses through a layer only about 150 meters deep by the time it reaches the downwind edge of the control volume. The mixing height may extend several kilometers above ground in unstable air, common during daytime convection.

Transport Wind Vector
The transport wind vector is the mean wind speed and direction in the layer between the ground and the mixing height. The transport wind vector provides the expected direction and speed of smoke movement.

Transport wind calculations depend on surface and rawinsonde observations. At locations where no rawindsonde observations exist, observed surface winds and upper air winds interpolated from the closest few rawinsonde stations are used.

References

Lavdas, L.G. 1986. An atmospheric dispersion index for prescribed burning. U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station, Asheville, NC. Research Paper SE-256.

Busse, A.R.; Zimmerman, J.R. 1973. User’s guide for the climatological dispersion model. US Environmental Protection Agency, Research Triangle Park, NC. EPA-R4-73-024.

Gifford, F.A. 1962. Uses of routine meteorological observations for estimating atmospheric dispersion. Nuclear Safety 2(4):47-51.

Pasquill, F. 1961. The estimation of dispersion of wind-borne material. Meteorological Magazine 90:33-49.

Pasquill, F. 1974. Atmospheric diffusion. 2nd ed. John Wiley and Sons, New York NY.

Turner, D.B. 1961. Relationships between 24-hour mean air quality measurements and meteorological factors in Nashville, TN. Journal of Air Pollution Control Association 11:483-489.

Turner, D.B. 1964. A diffusion model for an urban area. Journal of Applied Meteorology 3:83-91.