About the Empirical Kinetic Modeling Approach

Photochemical modeling, as captured in the Empirical Kinetic Modeling Approach (EKMA), is an effective tool for designing emission control strategies. Modeling can be used to estimate the effectiveness of a proposed control program for reducing peak ozone concentrations and exposure to ozone. Modeling can also be used to estimate the cost-effectiveness of proposed control measures, and can also be used to locate an optimal path toward attainment. In essence, the use of photochemical models provides the user with a way to ask many "what if" questions that assist in the design of an emission control program.

EKMA is just that - an approach or set of procedures that allow scientists and/or regulatory consultants to study the complex problem of calculating and predicting hourly ozone concentrations for a specific location, such as a city. The approach uses a variety of tools and techniques, including data gathering and modification, the use of a specific air quality model (the Ozone Isopleth Plotting Package, or OZIP), and guidelines for interpreting and repeating calcuations to make predictions and decisions. The key goal of EKMA is as follows:

to estimate the impact of controlling urban volatile organic compound (VOC) and/or NO_x emissions on peak hourly ozone concentrations

The name itself is instructive of the underlying implementation of the approach:

Empirical refers to the use of observed (i.e. data from the field) ozone (O₃) peaks to examine various strategies for reducing emissions, and thus reducing ozone levels
Kinetic modeling refers to the use of a well-established chemical model (built into the OZIP code) to simulate the transformation of the precursors of ozone (VOCs and NO_x) into tropospheric ozone
Approach, as mentioned above, conveys the mindset that a series of steps and guidelines are used to successfully understand the air pollution situation

The central focus of the EKMA is the graph generated as the product of the use of the OZIP modeling package. This graph is known as an isopleth, meaning "equal concentrations". The isopleth, shown below, is a graph with the concentrations of VOC emissions on the horizontal axis and the concentrations of the NO_x emissions on the vertical axisusually in units of parts per million (ppm) or parts per billion (ppb). VOC units are typically notated as ppmC, parts per million of carbon. The curved lines on the graph represent the concentration of ozone at values specified by the model user. For example, the model user can ask that a curve be drawn at every value of VOC and NO_x where the concentration of ozone is 0.12 ppm, which also happens to be the standard for air quality. Typically the user will ask the model to print a series of isopleths, such as 0.08, 0.12, 0.16, 0.20, etc. The user then can use these isopleths to make decisions about reducing one or both of the precursors to reach the desired maximum hourly concentration of ozone:

This modeling approach uses a Lagrangian modeling approach. In this approach (which differs from a fixed, or Eulerian model), the model considers a column of air which extends from the surface through a certain height above the surface. The column moves, and as a result, is subjected to fresh emissions, dilution, and chemical reactions. A graphic of a Lagrangian model is shown below:

EKMA can also be considered as a variant of a box model. In a box model, shown below, emissions enter the box and are transformed through a series of reactions (or a reaction mechanism). Transport in and out of the box is influenced by the surrounding meteorology. Dilution is usually taken into account:

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The Shodor Education Foundation, Inc.