This paper describes some recent work involving the enhancement, application, and evaluation of the REgional Modeling System for Aerosols and Deposition (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 modeling system that uses streamlined algorithms that reflect the state-of-the science and current knowledge of the important physical and chemical processes to simulate the chemistry, transport, and deposition of airborne pollutants (with emphasis on particulate matter (PM), ozone, and mercury) using. The REMSAD modeling system was developed and is maintained and distributed by Systems Applications International, Inc. (SAI), a wholly owned subsidiary of ICF Consulting. The U.S. Environmental Protection Agency (EPA) funded major portions of the development of REMSAD.

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. Simplification of the photochemical and other processes enables one to use REMSAD to obtain regional and annual estimates of PM, visibility, and deposition using readily available (standard) computer technology in a reasonable amount of time (a typical continental-scale (for the U.S.), annual simulation takes about two weeks of computer run time on a high-end , by today’s standards, work station). Despite the simplifications, many of the key features and components of the REMSAD model are consistent with the state-of-the-science knowledge and techniques as related to particulates, mercury, and other toxic species.

Recent updates to REMSAD have focused on improved use of information from meteorological models (including cloud cover and liquid water content), improved representation of photochemistry (using the micro-CB mechanism) and mercury chemistry, and a mercury tagging scheme. The model has been applied to the simulation of PM air quality and particulate and mercury deposition for the continental U.S. for two different years (1996 and 1998) using output from two different meteorological models. The results have been evaluated using available air quality data (on a monthly, seasonal, and annual basis). These recent applications have indicated that the modeling results are sensitive to the distribution and amount of cloud cover, rainfall rate, effective mixing height, and biogenic emissions. Results from the mercury tagging scheme indicate that both local emissions and background concentrations of mercury contribute to deposition at the Mercury Data Network (MDN) monitoring sites.