UAM-V® Photochemical Modeling System
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The Urban Airshed Model® (UAM®) modeling system was developed and is maintained by Systems Applications International (SAI), a wholly owned subsidiary of ICF International. The UAM® system is the most widely used photochemical air quality model in the world today. Since SAI's pioneering attempts at photochemical air quality modeling in the early 1970s, the model has undergone nearly continuous cycles of application, performance evaluation, update, extension, and improvement. UAM-V®—with its variable grid feature—is a significant update to the UAM® system.

The UAM-V® has been applied to both regional and urban scale domains in the United States and abroad. There are more than 200 registered users of the UAM-V® worldwide. The UAM-V® model is supported by a comprehensive modeling system for developing the inputs required by such a three-dimensional photochemical model. Such extensive use and applications—across many computing platforms and by organizations of widely varying expertise—explains the system’s robustness.

The UAM-V® contains pioneering technical features and capabilities. Its two-way grid nesting allows multiple urban areas to be simulated within a larger region. In addition, UAM-V® allows variable vertical layer numbers and spacing, specification of three-dimensional meteorological variables, and explicit treatment of subgrid-scale photochemical plumes. Together these features advance the understanding of ozone formation within and ozone and ozone-precursor transport between urban areas. The model’s software has been completely rewritten in modular form and includes updated deposition, plume rise, solar flux, and chemical kinetics modules. Also available for the UAM-V® is a prognostic meteorological model with four-dimensional data assimilation and a complete gridded emission inventory preparation system; both are fully compatible with the formulation of UAM-V® and the latest CB-IV chemical mechanism.

The UAM-V® is often used to facilitate the identification of cost-effective regional- and urban-scale air quality management actions. Government agencies in the U. S. and Canada use, or have recently used, UAM-V® in this capacity to model:

  • The Eastern U. S. (as part of the Ozone Transport Assessment Group (OTAG))
  • Gulf of Mexico coastal region from Florida into Texas (as part of the Gulf Coast Ozone Study (GCOS))
  • Memphis, Knoxville, Nashville, Chattanooga, Little Rock, and Tupelo (as part of the Arkansas-Tennessee-Mississippi Ozone Study (ATMOS))
  • The Lake Michigan area (Chicago/Milwaukee)
  • Atlanta, Georgia
  • Los Angeles, California
  • The New England area
  • Vancouver, British Columbia.

The modeling system is also being applied to various European locales including Athens, Paris, Lyon, Milan, and the United Kingdom. Applications are underway in Latin America. To view an example of UAM-V® application results, click here.

Two versions of the model are currently available: Version 1.24, which has been in use for several years and was used in the OTAG studies, and Version 1.30 that includes an updated chemical mechanism and a generalized coordinate system.

Conceptual Overview of the Model

The Variable grid Urban Airshed Model (UAM-V)® system is a three-dimensional photochemical grid model that calculates concentrations of pollutants by simulating the physical and chemical processes in the atmosphere. The basis for the UAM-V® is the atmospheric diffusion or species continuity equation. This equation represents a mass balance that includes all of the relevant emissions, transport, diffusion, chemical reactions, and removal processes in mathematical terms. The model is usually applied to a 48- to 72-hour periods for urban applications and 360-hour periods for some regional-scale applications during which meteorological conditions result in high concentrations of pollutants.

The major factors that affect photochemical air quality include:

  • The pattern of emissions of NOx and volatile organic compounds (VOC), both natural and artificial,
  • The composition of the emitted VOC and NOx,
  • The spatial and temporal variations in the wind fields,
  • The dynamics of the boundary layer, including stability and the level of mixing,
  • The chemical reactions involving VOC, NOx, and other important species,
  • The diurnal and longitudinal variations of solar insolation and temperature,
  • The loss of ozone and ozone precursors by dry and wet deposition, and
  • The ambient background of VOC, NOx, and other species in, immediately upwind, and above the study region.

The UAM-V® simulates all of these processes when used to calculate ozone concentrations. When it is used to simulate carbon monoxide concentrations (or any other chemically inert pollutant), no chemical reactions are involved. The species continuity equation is solved using the following fractional steps: emissions are injected; horizontal advection/ diffusion is solved; vertical advection/diffusion and deposition is solved; and chemical transformations are performed for reactive pollutants. The UAM-V® performs these four steps during each time step. The maximum time step is a function of the grid size and the maximum wind velocity and diffusion coefficient. The typical time step is 10–30 minutes for coarse (10–40 km) grids and a few minutes for fine (1–2 km) grids.

Because it accounts for spatial and temporal variations as well as differences in the reactivity of emissions, the UAM-V® is ideal for evaluating the air-quality changes from emission control scenarios. This is done by first replicating a historical ozone episode to establish a base-case simulation. Model inputs are prepared from observed meteorological, emission, and air quality data for the episode days using prognostic meteorological modeling and/or diagnostic and interpolative modeling techniques. The model is then applied with these inputs, and the results are compared with available observations to determine its performance. Once the model results have been determined to perform within prescribed levels, the same base-case meteorological inputs are combined with projected emissions (with or without specified control measures) to project air quality.

Features of the UAM-V® Modeling System

In 1989–1991, SAI embarked on a massive research effort to rewrite the UAM® system. The UAM-IV, adopted by the U. S. EPA in 1987 as a preferred model, was refined and enhanced through the update of the computer code and inclusion of new capabilities, algorithms, modules, and features. Some significant features of the UAM-V® are:

  • Vertical grid structure: The structure of vertical layers can be arbitrarily defined and is no longer defined from the diffusion break (mixing height). This allows for higher resolution near the surface and better matching with output from prognostic meteorological models, which usually use a terrain-following coordinate system.
  • Three-dimensional inputs: Meteorological variables that are considered spatially constant in the UAM-IV (e.g., temperature, water vapor, pressure, and photolysis rates) vary temporally and spatially. Furthermore, horizontal diffusivities and vertical turbulent exchange coefficients are now required as input, usually calculated from a prognostic meteorological model.
  • Two-way nested grid: Finer grids can be imbedded in coarser grids for more detailed representation of advection/diffusion, chemistry, and emissions. Several levels of nesting can be accommodated.
  • Updated chemical mechanism: The Carbon Bond IV chemical mechanism has been updated to add the XO2–RO2 reaction, along with new temperature effects for PAN reactions, and a state-of-the science isoprene mechanism.
  • Dry deposition algorithm: The dry deposition algorithm has been updated to a detailed version similar to that used by the Regional Acid Deposition Model (RADM).
  • Advanced meteorological model: A prognostic meteorological model supports the application of the UAM-V® to regional or urban domains. It incorporates four-dimensional data assimilation, solution "nudging," and objective combination to obtain physically realistic three-dimensional meteorological fields for input into the UAM-V®.
  • Plume-in-grid: Emissions from point sources can be treated by a subgrid-scale Lagrangian photochemical plume model. Pollutant mass is released from the subgrid-scale model to the grid model when the plume size is commensurate with a grid cell size.
  • Plume rise algorithm: An updated plume rise algorithm has been implemented, based on the plume rise treatment for the Gaussian dispersion model TUPOS.
  • Process analysis extensions: Information can be saved on pollutant concentrations from each mechanism (chemistry, advection, etc) for each time step, allowing for an unprecedented level of detail in understanding simulated episodes.
  • Updated chemical mechanism (v 1.30): Acetaldehyde has been made an explicit species and ALDX has been added to represent higher aldehydes. These and other related changes make representation of aldehyde photolysis more accurate and facilitate the simulation of certain toxic species.
  • Generalized coordinate system (v 1.30): In addition to latitude/longitude and UTM, coordinates can be defined in terms of any rectangular system measured in km.

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