Rethinking the Ozone Problem in Urban and Regional Air Pollution (1991) / Chapter Skim
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10 Ozone Air-Quality Models
10 Ozone Air-Quality Models
Pages 303-350
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From page 303... ...
Photochemical air-quality models can be used to demonstrate NAAQS attainment or to educate planners about the emissions controls needed to head toward attainment. Whether or not they are actually used in determining abatement 303
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EPA guidelines (EPA, 1986b) identify two kinds of photochemical model: The urban airshed model (UAM)
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There is a need for a set of directives about how to specify the size of the modeling domain, the horizontal grid spacing to be used, the vertical extent of the modeling region, and the number and resolution of vertical layers. These directives must be based on the exercise of models having a wide range of spatial resolutions and on the comparison of model performance against a wide variety of high-quality field data.
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306 .~ en - ~ of At Go Ci\r ~°° ttS At 4)
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307 oo on ox _ oo on _ C
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For this reason, and because such regional models are currently being used to assess ozone abatement strategies in areas like the northeastern United States, we will review regional grid models in this chapter. The particular model on which we focus is EPA's Regional Oxidant Model (ROM)
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The simplest example is a single-layer model in which the height and quantities of potential temperature and moisture, for example, are predicted in the boundary layer (Lavoie, 1972~. This hybrid of numerical prediction and layer-averaged approaches is the basis of the EPA Regional Oxidant Model (ROM)
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These concentration fields are called the initial conditions. Throughout the simulation it is necessary to specify the species concentrations called the boundary conditions in the air entering the three-dimensional geographic domain.
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Because few measurements of air-quality data are made aloft, it is generally assumed that species concentrations are initially uniform in the mixed layer and above it. To ensure that the initial conditions do not dominate the performance statistics, model performance should not be assessed until the effects of the initial conditions have been swept out of the grid.
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An aerometric data base is a critical component of a modeling application. Such a data base is needed to provide input to the model and to serve as a tool for assessing model performance.
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pecles Number of profiles per day 23 per site 4-8 100 meters None Ozone, NO,, None NO, speciated VOCs None or limited to routine National Weather Service or military installation observations None
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Another major issue is what should be done when sufficient air-quality data do not exist to perform a definitive model performance evaluation, but a SIP is still required. For some areas in this category, a grid-based, three-dimensional model simulation is inappropriate.
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REGIONAL GRID MODELS Grid-based models have been developed to simulate oxidant production and acid deposition over the eastern United States. The grid size of these models ranges from 18.5 to 127 kilometers (km)
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is a grid-based photochemical air-quality model with a grid size of ~ 18.5 km, designed to simulate ozone formation and transport over the eastern United States (Figures 10-1 and 10-2~. The transport is governed by wind fields interpolated on an hour-by-hour basis from observations at surface and upper air stations.
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· . · N 38 00 § § ,5, FIGURE 10-2a Regional oxidant model (ROM)
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Numerical prediction models with 15 to 30 levels in the vertical direction and 15 km resolution laterally are now being run operationally in the United Kingdom and elsewhere.
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ROM takes boundary conditions for ozone to be the average of values (6-hour mean) at three "relatively rural sites in Ohio, West Virginia, and Virginia; the model is then re-equilibrated for 1.5 hours, starting with the equilibrated initial conditions and the new ozone value, to obtain daytime and nighttime boundary conditions.
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with model results are shown in Figure 10~. The figure illustrates the tendency of the model to overpredict ozone at lower concentrations ~ < 50 ppb)
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~ 204= it. ~ ~.20 1 ~ ~< ~ · ~K'~ FIGURE 10-3 ROM grid cell locations (darkened)
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to 7:00 p.m., LST) hourly ozone concentrations at each of six groups of receptor locations over the period from July 14 to August 31, 1980.
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326 RETHINKING THE OZONE PROBLEM 200 Q Q Q AL _ 150 llJ LL fir lL _ 0 50 O 100 80196 - 80244 (08 - 19 furs.
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OZONE AIR QUALllY NEEDS 80196-80244 (08-19hrs.only)
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examined the spatial pattern of ozone in the Northeast corridor for five episodes in the six-week period. They compared model results to the pattern of maximum concentrations (the maximum for each station during the episode)
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EVALUATION OF MODEL PERFORMANCE Air-quality models are evaluated by comparing their predictions with ambient
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The term "verifications might be reserved to describe a successful, or positive, outcome of the model evaluation process; to determine whether a model is in fact valid. Traditional photochemical model performance evaluations do not provide
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This is not the case in the example presented here; the model underestimates the peak ozone concentration, and the variability in afternoon ozone predictions that results from mixing-height uncertainties appears to be comparable to the estimated uncertainty in the ozone measurements. Model performance evaluation procedures and tests must be designed to reveal flaws in assumed input information and model components in order to ensure that a model is producing the right answer for the right reason.
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From page 336... ...
Current grid-based photochemical models reproduce hourly averaged ozone concen trations to within 30-35 % of measured values, and the peak 1-fur concentration is often reproduced to within 15-20%. Tesche (1988)
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Model Performance Evaluation Procedures Before a photochemical model is applied to simulate the effects of emissions controls, the model must be shown to reproduce the chemical and physical processes that govern ozone formation. This kind of assessment, referred to as model performance evaluation, involves the compilation of emissions, meteorological, air quality, and chemical data drawn from an actual episode.
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Diagnostic Evaluation Procedures of Model Performance The diagnostic evaluation procedures for models discussed in this section are used to determine the causes of failure of a flawed model; to stress a model to ensure failure if the model is flawed; and to provide additional insight into model performance beyond that supplied through the operational evaluation procedures previously discussed. Frequently, operational model evaluation does not convey enough information about the model and data base to allow their use in emissions control strategy development and testing.
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From page 339... ...
For stagnation episodes, some residual effects of initial conditions could be seen even on the third day of a multiday simulation (Tesche and McNally, 1989~. Zero Boundary Conditions The zero-boundary-condition simulation examines the influence of boundary
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This simulation helps identify situations in which the base-case results are greatly affected by the boundary conditions. The simulation is performed by setting all inflow and outflow boundary values to zero, including those for the top surface of the modeling region.
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From page 341... ...
Boundary conditions primarily influence the regions near the boundary, with a small influence in the central modeling region. Reducing both boundary and initial conditions to background concentrations led to less than a 4% reduction in peals ozone and exposure predictions from the base-case simulation that used more representative values.
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For model results outside the ranges given for any one of these areas, it should be incumbent on the modeler to explain why the performance is poorer than that commonly achieved in similar applications and whether the causes of poorer performance will adversely affect the use of the model in control strategy evaluations. This method provides reviewing agencies with a general model performance target, but still guards against the inappropriate rejection of less accurate model simulations when appropriate explanations can be provided.
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From page 343... ...
, also offer the potential for evaluations of alternative chemical kinetic mechanisms. Evaluation of model performance for precursor and intermediate species as well as for product species other than ozone is recommended when ambient concentration data for these species are available.
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Four mass balance and flu* calculation procedures are suggested to accompany detailed performance evaluations.
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Sensitivity analysis of air-quality models meets two objectives: to determine qualitatively whether a model responds to changes in a manner consistent with what is understood about the basic physics and chemistry of the system, and to estimate quantitatively the uncertainty in model predictions that arise from uncertainties in the inputs and parameters. Various methods applicable to sensitivity-uncertainty analysis of photochemical air-quality models are available (Dunker, 1980, 1984; Seigneur et al., 1981; Tesche et al., 1981; Tilden et al., 1981; McRae et al., 1982; Brost, 1988; and Derwent and Hov, 1988~.
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From page 346... ...
would trace a path within the upper and lower uncertainty bounds of the hourly ozone measurements. TESTING THE ADEQUACY OF MODEL RESPONSE TO CHANGES IN EMISSIONS It is important to assess the ability of models to correctly simulate the effects of emissions changes because of the direct connection between changes in emissions and the intended regulatory application of photochemical models.
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The problems raised in this kind of evaluation are serious. To allow a meaningful model performance evaluation, detailed emissions inventories of comparable accuracy are required for base years long enough apart (at least 10 years)
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found that NOx reductions led to decreased ozone concentrations in both locations. The model inputs needed to simulate historical ozone episodes boundary and initial conditions, both on the ground and aloft, and emissions inventories have associated uncertainties, often of a magnitude difficult to estimate.
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Computationally demanding. Sensitive to boundary conditions when long-range transport is important.
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Model performance evaluation procedures must be designed to reveal flaws in a basecase simulation to ensure that a model gives the right answer for the right rea son. Computational constraints historically have limited the use of advanced threedimensional, photoche~cal air-quality models.
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