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especially the following:
· Wind speed and direction,
· Atmospheric stability,
· Mixing depth.
1 Source: EPA Office of Air Quality Planning and Standards, User’s Guide to CAL3QHC Version 2.0,
EPA-454/R-92-006, September 1995, p. 30.
I-11
A wide range of meteorological data is collected hourly at most airports. This information is
available from the National Climatic Data Center in a digital format ready for use by most
dispersion models. One year to several years’ data is required by regulatory agencies for a
modeling run to be considered valid.
I2.2.1 Wind Speed and Direction
Wind speed and direction are important parameters in modeling the dispersion of pollutants in
the atmosphere. The Gaussian approximation assumes that wind speed is constant from one
direction for a given time period being modeled, usually one hour. Wind speeds are usually
measured by an anemometer at a height of 20 feet or sometimes 10 m. These measurements may
or may not be corrected by the dispersion model to account for increasing wind speed with
height.
For those time periods in which the wind speed is given as zero or “calm”, a model will generally
assign a minimum wind speed. If a wind speed of zero were specified, the Gaussian equation
would compute an infinite concentration of the pollutant at the source, with no dispersion. In
reality, diffusion of pollutants into the surrounding atmosphere would take place in calm
conditions.
I2.2.2 Atmospheric Stability
Atmospheric stability and the presence of atmospheric turbulence are predominant factors that
determine the rate at which airborne pollutants are diffused. A region of the atmosphere with
strong vertical motion enhances dispersion by scattering pollutants through a larger volume of
air. Atmospheric stability determines the extent to which vertical mixing will occur and,
consequently, the degree to which airborne pollutants are mixed within a parcel of air. Stability is
influenced strongly by vertical temperature distribution. Horizontal mixing of the atmosphere
also influences the pathway of airborne pollutants through wind speed and related turbulence. In
general, atmospheric stability is a function of the temperature distribution with height, solar
radiation, cloud cover, and wind speed.
The stability is expressed in terms of the Pasquill-Gifford (P-G) stability classification system,
which identifies six classes ranging from A (very unstable) to F (very stable). In unstable
atmospheric conditions, the high turbulence and associated vertical mixing produce a peak
ground-level pollutant concentration near the emission source, with low concentrations at
distances far from the source. The most unstable conditions occur during daylight hours, with
low wind speeds and high solar radiation contributing to higher instability. In stable atmospheric
conditions, the low level of vertical mixing results in a low ground-level steady-state
concentration near the source, with comparatively higher concentrations at long distances from
the source. The most stable atmospheric conditions occur at night, during times of low wind
speeds and clear skies. Dispersion models convert wind speed data, cloud cover data, and solar
radiation to the corresponding stability category, according to Table I-2. Some models require
the direct input of stability categories along with other meteorological data.
Once the proper P-G stability category has been identified, the Gaussian standard deviation
factors sy and sz may be calculated. Dispersion models perform this calculation internally, with
no additional input required from the user. A methodology for calculating these values is given in
Turner’s “Workbook of Atmospheric Dispersion Estimates,” in EPA’s APTI Course 423:
Dispersion of Air Pollution — Theory and Model Application, Selected Readings Packet, p. 1-6
(Reference 40).
I-12
Surface Wind
Speed
Day Night
(at 10m) Incoming Solar Radiation Cloudy Clear2
(m/s) Strong Moderate Slight3
<2 A A-B B E F
2-3 A-B B C E F
3-5 B B-C C D E
5-6 C C-D D D D
>6 C D D D D
Table I-2: Key To P-G Stability Categories4
I2.2.3 Mixing Layer Height
In some models, the dispersion calculation is modified to consider the effects of the mixing layer
height on pollutant concentration. The mixing layer height (also known as the mixing depth or
inversion layer height) is the elevation of the boundary between the vertically mixed layer of air
closest to the earth’s surface and the relatively stable layer of air above. Vertical diffusion of
pollutants occurs readily within the mixing layer but does not occur to any significant degree
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