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Model Report

REGIONAL MODELING SYSTEM FOR AEROSOLS AND DEPOSITION

Last Revision Date: 08/25/2009 View as PDF
General Information Back to Top
Model Abbreviated Name:

REMSAD
Model Extended Name:

REGIONAL MODELING SYSTEM FOR AEROSOLS AND DEPOSITION
Model Overview/Abstract:
REMSAD is a three-dimensional grid model designed to calculate the concentrations of both inert and chemically reactive pollutants by simulating the physical and chemical processes in the atmosphere that affect pollutant concentrations. REMSAD provides estimates of the concentrations and deposition of the simulated pollutants at each grid location in the modeling domain. REMSAD includes the Particle and Precursor Tagging Methodology (PPTM) for sulfur, nitrogen, and mercury to allow estimates of source contributions to air concentrations and deposition. Post-processing can provide concentration averages and deposition totals over any subset of the time span of the simulation.

Keywords: Particulate, deposition, aerosol, grid-model, PM, sulfate, nitrate, mercury, regional
Model Technical Contact Information:
Agency Contact
Dwight Atkinson
EPA Office of Water
Atkinson.Dwight@epamail.epa.gov
202 566-1226

Developer Contact
Sharon Douglas
Systems Applications International
ICF Consulting Inc.
sdouglas@icfconsulting.com
(415) 507-7108

Model Homepage: http://www.remsad.com Exiting the EPA Site
Substantive Changes from Prior Version: Version 8 includes the Carbon Bond V chemical mechanism

User Information Back to Top
Technical Requirements
Computer Hardware
PC or Workstation, min 500Mb RAM, 100 Gb disk storage
Compatible Operating Systems
Unix, Linux
Download Information
See www.remsad.com exit EPA for registration and download instructions.
Using the Model
Basic Model Inputs
REMSAD requires a number of input files containing spatially resolved (3-dimensionally gridded) and temporally resolved (time-varying) data on meteorological parameters, emissions and physical characteristics. The major inputs are summarized below, grouped by input type:
Emissions:

  • Low-level emissions for area, mobile, low-level point, non-road, and biogenic sources
  • Elevated point-source emissions Initial and Boundary Conditions:
  • Initial species concentrations for each grid cell within the modeling domain
  • Species concentrations along the lateral boundaries of the modeling domain
  • Surface chlorine concentrations (for simulations of mercury)

Meteorological Fields:

  • u- and v- wind components
  • Temperature
  • Surface pressure
  • Water vapor concentration
  • Vertical diffusivities or exchange coefficients
  • Cloud-water mixing ratio
  • Rain-water mixing ratio
  • Rainfall rate

Surface Characteristics:

  • Land-use characteristics
  • Terrain heights

Chemistry Parameters:

  • Chemical reaction rates and other micro-CB parmeters
  • Photolysis rates

Simulation Control:

  • Simulation control parameters and option specifications
Basic Model Outputs
REMSAD provides gridded, averaged surface and multi-layer instantaneous concentrations, and surface deposition output for all species and grids simulated. The averaged surface concentrations and deposition are intended for comparison with measurements and ambient standards. The instantaneous concentration output is primarily used to restart the model, and to examine model results in the upper levels. Concentrations of particulates are passed as input to a postprocessor module that estimates atmospheric visibility. Wet and dry deposition fluxes are calculated hourly and may be accumulated for any desired interval. Post-processors can prepare graphical displays of concentration and deposition patterns and comparisons of simulated values with data observed at individual monitors. Species simulated can include NO, NO2, formaldehyde, acetaldehyde, xylene, toluene, olefins, parafins, and other hydrocarbons, CO, O3, PNA, PAN, HNO2, H2O2, HNO3, organic nitrate, SO2, sulfate, isoprene, mono-terpenes, NH3 and ammonium nitrate and sulfate, polycyclic organic matter, mercury, dioxin, atrazene, cadmium, particulate nitrate, primary and secondary organic aerosols, primary carbon, PM2.5, and PM10. When PPTM is utilized, REMSAD outputs include estimates of contributions of tagged sources to sulfate, nitrate, ammonium, and mercury concentrations.
User Support
User's Guide Available?
To download the User's Guide to the Regional Modeling System for Aerosols and Deposition (REMSAD) Version 7, go to: www.remsad.com/info.htm. exit EPA

User Qualifications
Some prior experience in modeling and atmospheric science is helpful. User should be comfortable running applications in a Unix/Linux environment. User must be capable of organizing and handling large data sets.

Model Science Back to Top
Problem Identification
REMSAD is a three-dimensional grid model designed to calculate the concentrations of both inert and chemically reactive pollutants by simulating the physical and chemical processes in the atmosphere that affect pollutant concentrations. The basis for the model is the atmospheric diffusion or species continuity equation. This equation represents a mass balance in which all of the relevant emissions, transport, diffusion, chemical reactions, and removal processes are expressed in mathematical terms. The REMSAD system consists of a series of preprocessor programs, the core model, and several postprocessing programs. The model solves the species continuity equation using the method of fractional steps, in which the individual terms in the equation are solved separately in the following order: emissions are injected; horizontal advection/diffusion is solved; vertical advection/diffusion and deposition is solved; and chemical transformations are performed for reactive pollutants. The model performs this four-step solution procedure during one half of each advective (driving) time step, and then reverses the order for the following half time step. The maximum advective time step for stability is a function of the grid size and the maximum wind velocity or horizontal diffusion coefficient. Vertical diffusion is solved on fractions of the advective time step to keep their individual numerical schemes stable. A typical advective time step for coarse (50–80 km) grid spacing is 10–15 minutes, whereas time steps for fine grid spacing (10–30 km) are on the order of a few minutes.

Model inputs are prepared for meteorological and emissions data for the simulation days. Once the model results have been evaluated and determined to perform within prescribed levels, a projected emission inventory can be used to simulate possible policy-driven emission scenarios.

The representation of the various processes in the model is based on current scientific understanding.

Summary of Model Structure and Methods
REMSAD includes:
  • A detailed gas phase chemical mechanism (the Carbon Bond V mechanism) consisting of 30 gas phase species and over 100 reactions.
  • An algorithm treating the formation of secondary organic aerosols.
  • The MARS-A algorithm to calculate particulate partitioning of nitrates
  • Particulate partitioning algorithms for toxics and organic aerosols.
  • Algorithms for transport and diffusion of all pollutants.
  • Convective transport effects.
  • Detailed treatments of dry and wet deposition processes.
  • Aqueous and gas phase chemistry of mercury.
  • Chemical transformations of toxics such as POM and dioxins.
  • Particle and Precursor Tagging Methodology (PPTM) for sulfur, nitrogen, and mercury to allow estimation of source contributions
Model Evaluation
REMSAD has undergone multiple peer reviews sponsored by the EPA. E.g., see Seigneur, C., G. Hidy, I. Tombach, J. Vimont, P. Amar. 1999. “Scientific Peer-Review Of The Regulatory Modeling System For Aerosols And Deposition (REMSAD).” The KEVRIC Company, Inc., Durham, NC; and see Myers, T., Y. Wei, B. Hudischewskyj, and S. Douglas. 2006. “Application of the REMSAD Modeling System to Estimate the Deposition of Mercury in Support of the Wisconsin TMDL Pilot (Revised).” Systems Applications International/ICF International, San Rafael, California (06-017)]. For each application, REMSAD is normally evaluated against available observed data such as from the IMPROVE, CASTNET, NADP, and MDN monitoring networks. Reports such as the one cited above include examples of sensitivity analyses conducted with the model and discussions of uncertainty.
Key Limitations to Model Scope
Highest resolution is about 1 – 2 km.
Long simulation periods and high resolution require considerable computer resources.
Preparation of input data for large applications can be time and computer resource intensive.

Case Studies
Myers, T., Y. Wei, B. Hudischewskyj, and S. Douglas. 2006. “Application of the REMSAD Modeling System to Estimate the Deposition of Mercury in Support of the Wisconsin TMDL Pilot (Revised).” Systems Applications International/ICF International, San Rafael, California (06-017)].

Myers, T. and Y. Wei. 2004. “REMSAD Air Deposition Modeling in Support of TMDL Development for Southern Louisiana.” ICF International, San Rafael, California (04-038).

Myers, T., Y. Wei, B. Hudischewskyj, J. Haney, and S. Douglas. 2008. “Model-Based Analysis and Tracking of Airborne Mercury Emissions to Assist in Watershed Planning: Revised Final Report.” ICF International, San Rafael, California (08-034).


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