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*MOP., •..✓ r� 4.4.3.4 Air Emissions from Soil Volatilization Chemicals may be emitted from soil into the air due to volatilization and wind erosion �-. of soil particles. These emissions are transported to receptors by wind advection and turbulence-driven dispersion. This risk analysis only evaluated volatilization emission rates and not particles due to the paving of the site. Exposure to human receptors may then occur by inhalation of airborne contaminants. The air dispersion models described in this section simulate the airborne transport of vapor emissions from the source to the receptor. Volatilization is the release mechanism by which a compound evaporates in the vapor phase to the atmosphere from the soil. Contaminants can move upward through the soil primarily by three mechanisms. The first is vaporization followed by vapor-phase diffusion of the chemical through air-filled soil pores. When the upper soil layers dry out due to evaporation of water, capillary forces create an upward soil moisture flow, which then carries soluble contaminants toward the soil surface. Liquid-phase diffusion of contaminants through the soil moisture can also contribute to the flux. The rates of each of these processes are determined by the chemical properties of the chemicals of concern (vapor pressure, diffusivity, solubility, partition coefficient, half-life), physical `-' properties of the soil matrix (bulk density, porosity, fraction of organic carbon, moisture content), and environmental factors (temperature, wind speed, humidity, rainfall). At ground level, the wind speed and ground surface roughness control the rate at which chemicals leave the soil matrix and diffuse into the atmosphere. M For the Yellow Freight site, the Jury Behavior Assessment model (1983) was used to �.. estimate the volatilization flux emission rates from soil to the atmosphere. The emissions from the Jury model were subsequently used with a box model to calculate exposure concentrations in air at the site. Because site-specific characteristics for input y parameters were not available,conservative assumptions were made for input parameters in the modeling to give a worst-case concentration. e• The box model was used to estimate the steady-state average concentration of the chemicals at the site within a well-mixed cubic volume of air. The model assumes that the volume of air is bounded at the top of the mixing zone and is ventilated by a steady S:\LDC\YELLO.RPT May 4,1995 4-12