Introduction to Air Quality Modeling: Ozone Modeling Procedure

1. Modeling Protocol and Historical Episode Selection

For any modeling used to support Attainment Demonstration SIP revisions, a Modeling Protocol is required by EPA (see Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze ). The Protocol includes plans for:

• Specification of policy and technical review groups
• Definition of modeling domains
• Periods to be modeled (episodes)
• Meteorological modeling
• Base-and future-case emissions modeling
• Specification of the photochemical modeling platform
• Base-case model performance evaluation
• Future-case modeling

2. Base-Case Modeling—Reproducing the Past

It is not possible to predict weather conditions in future years, so instead modeling for attainment demonstrations uses selected periods from recent years, sometimes called episodes. For ozone modeling, the selected periods typically last from one to a few weeks and include days where high concentrations of ozone were measured in the area of interest. For regional haze, the period modeled would normally include several weeks from each season or an entire year.

Once the period to be modeled and modeling domain are selected, meteorological modeling is conducted for the entire period to provide a three-dimensional characterization of important parameters, including wind, temperature, and solar radiation for each hour and in every location in the domain.

Simultaneously, emissions from all sources (vehicles, industry, construction, vegetation, ships, etc.) are modeled across the entire modeling domain for each hour. The emissions and meteorology are then fed into the photochemical model to produce base-case modeled pollutant concentrations.

To have confidence in the model's future predictions, it is necessary to evaluate the model's base-case performance by comparing its predictions with observations of the target pollutant (ozone, regional haze). Also, model predictions of important chemicals that are emitted, created, or destroyed in the model's chemical and physical processes are compared with available measurements - looking at how well the model handles these chemicals helps ensure that the model “gets the right answer for the right reason.”

3. Future-Year Modeling—Predicting the Future

Once satisfactory model performance has been achieved for the base case, econometric forecasts are used to estimate the growth of industry, traffic, population, etc. These estimates are in turn used to estimate future emissions from each source. These emissions are then adjusted to account for any federal, state, or local regulations.

Predicted future-year emissions are then substituted into the base-case model (the meteorology is left unchanged), and the model is run again. If future predicted levels are reduced sufficiently, then the model provides strong evidence that attainment will be reached in the future year modeled.

4. Modeling Emissions Controls for the SIP

If the existing regulations are not sufficient, the TCEQ works with cities and industry to find strategies that reduce emissions enough to achieve attainment. The model is used to evaluate the effectiveness of these strategies and to provide evidence that attainment will be reached once a strategy has been chosen.

In addition to the modeling, the attainment demonstration includes additional analyses designed to challenge the conclusion that attainment will be reached. These analyses may include trend analyses, third-party photochemical modeling, other types of modeling analyses, and anything else that provides evidence as to whether or not attainment will be reached. These analyses are weighed together with the conclusions of the modeling, and if the balance of evidence supports the conclusion that attainment will be reached, an attainment demonstration SIP amendment can be prepared for submittal to EPA.