1.0 ATMOSPHERIC PATHWAY

        Pacific Northwest National Laboratory (PNNL) prepared this report for the U.S. Department of Energy (DOE). This report describes the atmospheric pathway component of the Multimedia Environmental Pollutant Assessment System (MEPAS), including the purpose and use of the component's modules and formulations. This document replaces earlier MEPAS formulation documents (Whelan et al. 1987; Droppo et al. 1989). The atmospheric pathway component modules, formulations, and rationale are discussed in the following sections:

• emission characterization, including particle suspension and volatilization
• contaminant transport, dispersion, and deposition, including dispersion coefficients
• initial source dispersion, including point and area sources, wind conditions, radioactive decay, chemical reactions, wet and dry deposition, source mass budget, and plume rise
• air-as-source, short-distance, disperse-regional-release, and complex terrain modules.

        The atmospheric pathway component of MEPAS estimates the pollutant exposures from atmospheric emissions to a regional human population. Long-term, average contaminant levels are computed using standard computation techniques for locations defined in terms of a direction and distance from the site. Using these methods, estimates of contaminant levels for exposure assessment can be made for both population centers and less populated rural areas. Short-term normalized air concentrations are provided for evaluation of acute exposures at nearby location



1.1 LONG-TERM AVERAGE EXPOSURE

        The long-term average exposure in the atmospheric pathway, based on a 70-year increment (i.e., approximately one human life span), represents the sum of exposures from individual atmospheric plumes. The travel time between release and exposure for these individual plumes is typically expressed in hours and minutes. Therefore, relatively short-term processes need to be incorporated into the computation of long-term, average concentrations.

        Modeling the long-term, average exposures to pollutants in the atmospheric pathway involves a weighted summation of exposures computed for a matrix of cases spanning ranges of ambient atmospheric conditions. The average conditions over the 70-year exposure period are represented by climatological data summaries (i.e., average frequency of occurrence of the various combinations of ambient atmospheric conditions) from a source such as the U.S. National Weather Service



1.2 SHORT-TERM AVERAGE EXPOSURE

        The short-term exposure to pollutants in the atmospheric pathway is based on maximum hourly concentrations. For a near-ground-level release, the 95% and 50% normalized air concentrations at 100 m from the release are provided. For all types of atmospheric releases, the maximum hourly normalized air concentration and its location are provided for each of the 16 direction sectors.



1.3 ATMOSPHERIC PROCESSES

        The fate of a contaminant released to the atmosphere depends on a number of complex processes including release mechanisms and characteristics, dilution and transport, chemical reactions, washout by cloud droplets and precipitation, and deposition onto the underlying surface cover. The MEPAS atmospheric pathway model accounts for each of these processes in computing long-term exposures. A schematic diagram illustrating the atmospheric pathway is presented in Figure 1.1.

        The atmospheric pathway for pollutants considers contaminant air concentrations and deposited surface concentrations. Air concentrations are needed for inhalation pathways. Surface concentrations resulting from dry and wet removal processes are needed for overland transport and for ingestion pathways for pollutants.

        The atmospheric pathway has several sequential components: suspension/emission, atmospheric transport and dispersion, and wet and dry deposition. The relationship of these atmospheric components in the MEPAS model is shown in Figure 1.2. If using a MEPAS emission module, site-specific data are
 
 



FIGURE 1.1. Schematic Diagram Illustrating the Atmospheric Environment




FIGURE 1.2 Atmospheric Pathway Computation Diagram

needed to estimate the gaseous and particulate release rates. An atmospheric transport and dispersion model is used to compute downwind air concentrations. As the plume travels away from a site, these airborne concentrations are reduced both by dispersion and deposition processes. Wet- and dry deposition models are used to compute the total deposition to the surface. These atmospheric pathway components maximize the validity of comparisons of environmental trends between sites by being based on empirical relationships that incorporate site characteristics (i.e., location, surface cover, climatology).

        The prediction of contaminant movement through the atmospheric pathway uses algorithms that address atmospheric suspension/emission of contaminants at a site and the subsequent transport, diffusion, and deposition of these airborne contaminants. Input to the model includes site-specific climatological information such as wind speed and direction, stability, and precipitation. Output from the model consists of average air and surface contaminant levels that are used in both the inhalation and ingestion components of the exposure assessment analysis. The computed surface contaminant levels also can be input to the overland transport components of MEPAS as a separate analysis run.