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.
