such as sea breeze effects

aISC and AERMOD are available at: bSCIPUFF is available at: cCALPUFF is available at:

4 The temperature of air normally decreases the further above ground one goes and thus a temperature inversion occurs when a warmer layer of air exists above a cooler one. A temperature inversion influences dispersion because the layer of warmer air prevents dispersion of substances from the cooler air and vice versa. The net effect of a temperature inversion is to increase ground-level concentrations when the release occurs below the inversion and to decrease them when the release occurs above the inversion. The effect can be dramatic, increasing or decreasing concentrations by several fold.

figure 19.4 The more typical use of atmospheric dispersion models is in the simulation of a windborne outbreak.They compute downwind concentrations, then a separate model computes the effects of those concentrations on biosurveillance data.

There are far too many models to discuss in a chapter. Instead, we describe four models that are likely to be of value in biosurveillance, well known, and freely available: (1) the third version of the Industrial Source Complex model (ISC3), (2) AERMOD5 (Lakes Environmental, 2005), (3) the Second-order Closure Integrated Puff model (SCIPUFF), and (4) CALPUFF.6 Table 19.3 contrasts these models based on the additional aspects of weather, substance, and terrain that they take into account. We also note any unique features.

The ISC3 model (U.S. Environmental Protection Agency [EPA], 1995) is one of several models that the EPA recommends for regulatory purposes with respect to emissions of pollutants into the air by industry. It is based on the Gaussian plume model but makes adjustments for temperature inversions, dry and wet deposition of the substance, terrain, and building effects. The program, its source code (written in the FORTRAN programming language), and a user's guide are all freely available on the web (see website address in Table 19.3).

The AERMOD model is a proposed replacement for ISC3 for regulatory use. Its capabilities appear identical to ISC3 and it is also based on the Gaussian plume model, but it incorporates results from dispersion research that occurred subsequent to the development of ISC3. It includes improved modeling of atmospheric turbulence, dispersion in the layer of the atmosphere called the convective boundary layer, and the effects of terrain. Neither AERMOD nor ISC3 model weather conditions as changing with time, such as changes in wind speed and direction. As with ISC3, AERMOD, its source code (also in the FORTRAN programming language), and (a draft of) a user's guide are available for free on the web (see Table 19.3 for website address).

The SCIPUFF model is a model that accounts for changing weather conditions over time and location (Sykes et al., 1998). It is also the only model we mention here that is capable of modeling long-range dispersion up to the scale of a continent. The other models are limited to distances of 100 km or less. Despite its name, it can also estimate concentrations that result from continuous releases. SCIPUFF, its source code, and a user's guide are all available for free download (see Table 19.3 for website address).

Note that SCIPUFF is part of the Hazard Prediction and Assessment Capability (HPAC) software that the Defense Threat Reduction Agency makes available to first responders for a license fee. HPAC adds geographic information system features to SCIPUFF, enabling the user to visualize the dispersion of a substance over time on a map. HPAC also has the capability to predict the consequences of outdoor releases of various chemical and biological agents.

The CALPUFF model is a puff model. We include it here because it models over-water dispersion and other unique meteorological features that occur along coastlines such as sea breeze effects7 (Scire et al., 2000). Given the proximity of many population centers to coastal areas, its use may be preferred in these areas. Like SCIPUFF, it also handles weather conditions that vary by location. It is also freely available for download (see Table 19.3 for website address).

5 AERMOD stands for the American Meteorological Society/Environmental Protection Agency Regulatory Model.

6 CALPUFF presumably stands for the California Puff model, as its developers originally created it for the California Air Resources Board.

7 For the curious, "sea breeze" is the term for the phenomenon of the wind blowing from the ocean to shore during the day (and from the shore to the ocean at night) because of a temperature differential between the air over land and over water.

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