Instructional Use of a Physical Model to Explain Quantitative Air Dispersion Modeling

Learning the concepts of atmospheric dispersion modeling for the first time can be quite a challenge. Even if the qualitative portions of the topic are not difficult to grasp, the quantitative, formulaic parts can be rather intimidating to students. Use of pedagogic tools, such as physical models, can reap terrific benefits in the form of accelerated student learning. Furthermore, instruction that combines physical models and computer models with basic theory can help students gain a deep and lasting understanding of complicated real world phenomena such as air pollution dispersion.

The construction of simple life-size and bench-scale artificial smokestacks has contributed to student learning of the fundamentals of atmospheric dispersion modeling. Specifically, two pieces of demonstration equipment were constructed. The first device, designed for outdoor use, is a 13-foot tall representative life-size stack with a 4-inch inner diameter. A small sampling port is positioned part way up the stack. A perforated cup at the base of the stack stores “contaminants” that travel up the stack via air supplied by a two-speed blower.

The second piece of equipment is a table-top unit that performs all the same simulation as its taller counterpart, except the miniature version has additional features for successful indoor use, including a stack-top fan to mimic the wind.

To simulate air contaminants, both models are stocked with “particulate matter” in the form of standard paper hole punches. Hundreds of these particles are loaded into the base of the stack where the blower projects them through to the ambient air. The wind disperses the particles that eventually settle in high concentrations not far from the stack base. A cubic-meter framework is then used to identify the area on the ground where the highest concentrations of particles are found. From this activity, the concept of mass per unit volume (e.g., grams per cubic meter) can be readily demonstrated.

Classroom use of the stacks has engaged students in active learning of the basic concepts of atmospheric dispersion modeling. This paper will examine the coupling of these physical models with the fundamental equations of air-dispersion modeling and computer models to assist student comprehension of dispersion theory.

Linkage will be made of the physical models and their simulation of dispersion to the basic relational formula, C = Q S / U, where C represents concentration (g/m3), Q is emission rate (g/s), S is stability (m-2), and U is wind speed (m/s). Discussion will then advance to the link of physical models to the Gaussian plume equation: X = (q / (pi u sigy sigz))(exp (-0.5 H2/sigz2)), where X represents concentration (g/m3), q is emission rate (g/s), pi equals 3.1416, u is wind speed (m/s), sigy is the lateral plume spread (m), sigz is the vertical plume spread (m), and H is the effective stack height (m).

For photographs and a more complete explanation of the instructional stacks see Pollution Engineering magazine “Emitting Education” (January 2009) (http://web.ebscohost.com/ehost/pdfviewer/pdfviewer?vid=3&sid=678969d1-ca07-415b-bb33-58324ba4a9a1%40sessionmgr4002&hid=4206) and “Mini-Modeling Measurements” (October 2010) (http://web.ebscohost.com/ehost/pdfviewer/pdfviewer?vid=4&sid=678969d1-ca07-415b-bb33-58324ba4a9a1%40sessionmgr4002&hid=4206).