Systems Applications International, Inc. (SAI)

15 November 2005

The Regional Modeling System for Aerosols and Deposition (REMSAD) is designed to support a better understanding of the distributions, sources, and removal processes relevant to fine particles and other airborne pollutants, including soluble acidic components and toxics. Consideration of the different processes that affect primary and secondary (i.e., formed by atmospheric processes) particulate matter at the regional scale is fundamental to advancing this understanding and to assessing the effects of proposed pollution control measures. These same control measures will, in most cases, affect ozone, particulate matter and deposition of pollutants to the surface.

The REMSAD modeling system was developed and is maintained and distributed by Systems Applications International, Inc. (SAI), an affiliate of ICF Consulting. The U.S. Environmental Protection Agency (EPA) funded portions of the development of REMSAD.

The REMSAD modeling system was originally intended as a screening tool – a model that could be run (quickly) for a continental-scale modeling domain (specifically the continental U.S.) and for a full-year simulation period – to provide information (although not very detailed) on the distribution and composition of particulate matter, the deposition of pollutant (including toxic) species onto the surfaces of inland and coastal bodies of water, and the expected change in air quality and deposition that results from changes in emissions. All of these parameters were intended to be primarily represented in terms of seasonal or annual averages or deposition totals. What began as a simple screening tool has evolved into a more complex “one atmosphere” modeling system that simulates the chemistry, transport, and deposition of airborne pollutants (with emphasis on particulate matter (PM), ozone, and mercury) using algorithms that reflect the state-of-the science and current knowledge of the important physical and chemical processes.

MODELING SYSTEM FEATURES

The REMSAD system is built on the foundation of the variable-grid Urban Airshed Model (UAM-V) – a regional-scale photochemical modeling system (SAI, 1999). Thus many of features of the UAM-V are also available in REMSAD. The REMSAD model is capable of "nesting" one or more finer-scale subgrids within a coarser overall grid. This two-way fully interactive nesting feature allows the user to apply higher resolution over selected source and/or receptor regions. The modeling system may be applied at scales ranging from a single metropolitan area to a continent containing multiple urban areas. To date, most applications have focused on the continental-scale.

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.

Fine particles (or aerosols) are currently thought to pose one of the greatest problems for human health impacts from air pollution. The major factors that affect the concentration and distribution of aerosols include:

The REMSAD model simulates these processes when it is used to simulate aerosol distribution and deposition. 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.

Gridded model inputs are prepared to represent meteorological conditions and emissions for each simulation day. Once the model has been evaluated and determined to perform within prescribed levels, a projected emission inventory can be used to simulate possible future policy-driven emission scenarios.

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 depositions 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 module that estimates atmospheric visibility. Wet and dry deposition fluxes are calculated hourly and may be accumulated for any desired interval.

The particulate matter species modeled by REMSAD include a primary coarse fraction (corresponding to particulates in the 2.5 to 10 micron size range), a primary fine fraction (corresponding to particulates less than 2.5 microns in diameter), and several secondary particulates (e.g., sulfates, nitrates, and organics). The sum of the primary fine fraction and all of the secondary species is assumed to be representative of PM_2.5.

REMSAD users have the option of choosing between two photochemical mechanisms. The full mechanism included in REMSAD, the Carbon-Bond Chemical Mechanism Version 5 (CB-V), is a state-of-the-science mechanism that includes updates to chemical reaction rates based on the most recent literature. The other option uses a photochemical mechanism module that is a reduced-form version of CB-V. This reduced-form version is termed “micro-CB” and is based on a reduction in the number of different organic compound species that are included. The inorganic and radical parts of the mechanism are identical to CB-V. The organic portion of the chemistry is based on three primary organic compound species (VOC, representing an average anthropogenic hydrocarbon species, and ISOP and TERP, representing biogenic hydrocarbon species) and one carbonyl species (CARB).

Secondary organic aerosol species (SOA) are known to result from the reactions of hydrocarbons in the atmosphere.  REMSAD Version 8 includes a calculation of the yield of SOA from both anthropogenic and biogenic hydrocarbon species.  Of the anthropogenic hydrocarbon emissions, the aromatic hydrocarbons are the principal contributors to SOA. Anthropogenic SOA is formed from reactions of TOL and XYL in CB-V. For microCB, a provision is included in REMSAD to establish the aromatic fraction of VOC as a function of space and time.  Biogenic emissions of the species TERP, representing monoterpenes, are the principal biogenic precursors of SOA.

REMSAD simulates both wet and dry deposition of gaseous and particulate species. Wet deposition occurs as a result of precipitation scavenging. Dry deposition is calculated for each species based on land-use characteristics and meteorological parameters.

The chemical transformations of mercury included in Version 8 of REMSAD are based on recent reviews of the current status of atmospheric chemistry of mercury. Species representing the oxidation state of mercury and the phase (gas or particulate) are tracked. These include HG0 (elemental mercury vapor), HG2 (divalent mercury compounds in gas phase), and HGP (divalent mercury compounds in particulate phase). A tagging scheme (PPTM, see below) for the mercury (and other) species is an optional feature of REMSAD.

The Particle and Precursor Tagging Methodology (PPTM) implemented in REMSAD allows one to tag and track the release, transport, chemical transformation, and deposition of precursor species and toxics (sulfur, nitrogen, mercury, cadmium, dioxin, and lead) from emissions sources, source categories, or source regions throughout the REMSAD modeling domain.

A number of issues are particularly important to a successful application of REMSAD for evaluating the atmospheric transport and deposition of pollutants. These include the accuracy and representativeness of the meteorological and emission inventory inputs; the resolution, structure and extent of the modeling grid; and the treatment of urban areas in both the source and receptor areas of the computational grid.

ATTRIBUTES AND LIMITATIONS

The REMSAD modeling system provides a relatively simple and cost effective means to begin to study and understand, through modeling, the factors that contribute to PM, mercury, and toxics concentrations and deposition totals, and the relative effectiveness of emission reductions measures in reducing the associated air quality related values. Attributes of the REMSAD modeling system, relevant to its use for current air quality modeling studies, include the following:

There is still much that is not known about the formation, composition, transport, and deposition of particulate and toxic species in the atmosphere. Although the availability, quality, and spatial and temporal representativeness of current measurements and experimental results do not support a comprehensive understanding of these processes, current PM modeling systems, including REMSAD, attempt to simulate these processes in a one-atmosphere modeling system. Further, the ambient air quality data are largely insufficient for a thorough evaluation of performance for REMSAD and other PM models.

Version 8 of the REMSAD modeling system has a few limitations that should be considered prior to its use in a regulatory or research application. These include: