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EXECUTIVE SUMMARY<br />

ABSTRACT<br />

The <strong>CALPUFF</strong> <strong>Long</strong> Range Transport (LRT) air quality dispersion modeling system is evaluated<br />

against several atmospheric tracer field experiments. Meteorological inputs for <strong>CALPUFF</strong> were<br />

generated using MM5 prognostic meteorological model processed using <strong>the</strong> CALMET diagnostic<br />

wind model with <strong>and</strong> without meteorological observations. <strong>CALPUFF</strong> meteorological inputs<br />

were also generated using <strong>the</strong> Mesoscale Model Interface (MMIF) tool that performs a direct<br />

“pass through” <strong>of</strong> <strong>the</strong> MM5 meteorological variables to <strong>CALPUFF</strong> without any adjustments or<br />

re‐diagnosing <strong>of</strong> meteorological variables, as is done by CALMET. The effects <strong>of</strong> alternative<br />

options in CALMET on <strong>the</strong> CALMET meteorological model performance <strong>and</strong> <strong>the</strong> performance <strong>of</strong><br />

<strong>the</strong> <strong>CALPUFF</strong> LRT dispersion model for simulating observed atmospheric tracer concentrations<br />

was analyzed. The performance <strong>of</strong> <strong>CALPUFF</strong> was also compared against past <strong>CALPUFF</strong><br />

evaluation studies using an earlier version <strong>of</strong> <strong>CALPUFF</strong> <strong>and</strong> some <strong>of</strong> <strong>the</strong> same tracer test field<br />

experiments as used in this study. In addition, up to five o<strong>the</strong>r LRT dispersion models were also<br />

evaluated against some <strong>of</strong> <strong>the</strong> tracer field experiments. <strong>CALPUFF</strong> <strong>and</strong> <strong>the</strong> o<strong>the</strong>r LRT models<br />

represent three distinct types <strong>of</strong> LRT dispersion models: Gaussian puff, particle <strong>and</strong> Eulerian<br />

photochemical grid models. Numerous sensitivity tests were conducted using <strong>CALPUFF</strong> <strong>and</strong> <strong>the</strong><br />

o<strong>the</strong>r LRT models to elucidate <strong>the</strong> effects <strong>of</strong> alternative meteorological inputs on dispersion<br />

model performance for <strong>the</strong> tracer field studies, as well as to intercompare <strong>the</strong> performance <strong>of</strong><br />

<strong>the</strong> different dispersion models.<br />

INTRODUCTION<br />

Near‐Source <strong>and</strong> Far‐Field Dispersion Models<br />

Dispersion models, such as <strong>the</strong> Industrial Source Complex Short Term (ISCST) or American<br />

Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) typically<br />

assume steady‐state, horizontally homogeneous wind fields instantaneously over <strong>the</strong> entire<br />

modeling domain <strong>and</strong> are usually limited to distances <strong>of</strong> less than 50 kilometers from a source.<br />

However, dispersion model applications <strong>of</strong> distances <strong>of</strong> hundreds <strong>of</strong> kilometers from a source<br />

require o<strong>the</strong>r models or modeling systems. At <strong>the</strong>se distances, <strong>the</strong> transport times are<br />

sufficiently long that <strong>the</strong> mean wind fields can no longer be considered steady‐state or<br />

homogeneous. As part <strong>of</strong> <strong>the</strong> Prevention <strong>of</strong> Significant Deterioration (PSD) program, new<br />

sources or proposed modifications to existing sources may be required to assess <strong>the</strong> air quality<br />

<strong>and</strong> Air Quality Related Value (AQRV) impacts at Class I <strong>and</strong> sensitive Class II areas that may be<br />

far away from <strong>the</strong> source (e.g., > 50 km). AQRVs include visibility <strong>and</strong> acid (sulfur <strong>and</strong> nitrogen)<br />

deposition. At <strong>the</strong>se far downwind distances, <strong>the</strong> steady‐state Gaussian plume assumptions <strong>of</strong><br />

models like ISCST <strong>and</strong> AERMOD are likely not valid <strong>and</strong> <strong>Long</strong> Range Transport (LRT) dispersion<br />

models are required.<br />

The Interagency Workgroup on Air Quality Modeling (IWAQM) consists <strong>of</strong> <strong>the</strong> U.S. EPA <strong>and</strong><br />

Federal L<strong>and</strong> Managers (FLMs; i.e., NPS, USFS <strong>and</strong> FWS) <strong>and</strong> was formed to provide a focus for<br />

<strong>the</strong> development <strong>of</strong> technically sound recommendations regarding assessment <strong>of</strong> air pollutant<br />

source impacts on Federal Class I areas. One objective <strong>of</strong> <strong>the</strong> IWAQM is <strong>the</strong> recommendation<br />

<strong>of</strong> LRT dispersion models for assessing air quality <strong>and</strong> AQRVs at Class I areas. One such LRT<br />

dispersion model is <strong>the</strong> <strong>CALPUFF</strong> Gaussian puff modeling system, which includes <strong>the</strong> CALMET<br />

diagnostic wind model <strong>and</strong> <strong>the</strong> CALPOST post‐processor. In 1998, EPA published a report that<br />

evaluated <strong>CALPUFF</strong> against two short‐term tracer test field experiments (EPA, 1998a). Later in<br />

1998 IWAQM released <strong>the</strong>ir Phase II recommendations (EPA, 1998b) that included<br />

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