1. Emission factors:

The first step in the use of the COMPDEP model is to determine "emission factors" for dioxin-like congeners. These factors are defined as the m g (or other mass unit) congener emitted per kg (or other mass unit) feed material combusted. Once assuming a rate of feed material combusted in appropriate units, kg/day, these emission factors can be translated to the units appropriate for atmospheric transport modeling, m g/sec. This assessment promotes the generation of specific congener emission factors, rather than TEQ or homologue group emission factors.

A TEQ concentration can be generated for exposure media concentrations once congener-specific concentrations are estimated using the Toxicity Equivalency Factor (TEF) scheme.

This recommendation is made because fate, transport, and transfer parameters, and TEFs, are different for specific congeners, leading to a TEQ exposure media concentration which would be different but more accurate than, say, assuming only a TEQ emission factor and one set of parameters for further modeling. Emission factors for the demonstration were generated from actual test data from an incinerator burning organic wastes (source otherwise unspecified).

Emission estimates for this example incinerator are similar to emissions that are known to be emitted from combustors employing sophisticated air pollution control devices (e.g., scrubbers combined with fabric filters). In order to place the demonstration scenario in context, the emissions from the hypothetical incinerator were ranked with other types of waste incinerators that are well controlled with some combination of a scrubber device and/or a fabric filter, as follows:

1. Medical waste incineration: 25 - 200 ng TEQ/kg waste combusted.
Hazardous waste incineration: 0.18 - 119 ng TEQ/kg waste combusted.
Hypothetical waste incinerator: 4.5 ng TEQ/kg waste combusted.
4. Municipal solid waste incineration: 0.05 - 3 ng TEQ/kg waste combusted.
. Sewage sludge incineration: 0.002 - 0.03 ng TEQ/kg sludge combusted.

2. Vapor/Particle Partitioning:

The second step in atmospheric transport modeling is to determine the percent of totally emitted dioxin-like congener which is in a vapor phase, and the percent which is in the particle phase.

The partitioning of stack emissions into these two phases was examined by reviewing stack testing data, ambient air sampling data, and a theoretical approach developed in Bidleman (1988). A summary of the vapor/particle (V/P) partitioning surmised from these three sources is given in Table III-1. From this review, it is generally concluded that:

table Table III-1. Percent distribution of CDDs and CDFs between vapor-phase (V) and particulate-phase (P) as interpreted by various stack sampling methods, ambient air monitoring, and ambient air theoretical partitioning.
a. Stack gas sampling:
The stack gas sampling methods in use today to monitor and measure the concentration of CDDs/CDFs emitted to the air from combustion sources do not provide a credible basis for assuming the vapor phase and particle bound partitioning at the point of release. There is no consistent pattern to the interpretation of V/P based on where the CDD/CDF segregates in the instrument, e.g., the glass fiber filter or the XAD resin.

Factors that may contribute to this are:
the relatively long residence time spent traversing the stack interior; the probe to the instrument is inserted into a relatively hostile environment of the hot combustion gas; the static temperature of the particulate filter caused by heating the particulate filter housing; the fact that located between the particulate trap and the vapor trap is a condensing section consisting of glass tubing surrounded by an ice bath.
expand table Table VX X-X

b. Ambient air sampling:
On the other hand, the ambient air sampling methods do give an approximate indication of the V/P ratio that seems to be responsive to changes in temperature, and degree of chlorination of the CDDs/CDFs. This is in accordance with what would be expected from their individual vapor pressures.

There is no artificial heating or cooling of any component of the sampler. The sampler is exposed to actual temperature, pressure, and humidity of the ambient air. This reduces the possibility that the vapor phase-particle bound partitioning operationally defined as the compound segregating to the particulate trap and vapor trap is actually an artifact induced by artificial heating and cooling within the system. Therefore, the methods present a realistic picture of partitioning under variable ambient conditions.
However, the method has certain limitations that currently prevent deriving a true measurement of V/P partitioning in the ambient air:

The glass fiber filter is designed to capture and retain particulate matter greater than or equal to 0.1 µm diameter. Particles less than this diameter may pass through the filter and be retained in the polyurethane foam vapor trap downstream. If this is the case, the amount of CDDs/CDFs observed to be particle bound would be underestimated, and the amount observed to be in vapor phase would be overestimated.

The relatively high sampled volume of air passed through the system (200 to 400 m3 of air per 24 hours) may redistribute the more volatile congeners from the filter to the adsorbent trap by a process known as 'blow-off'.

c. Theoretical partitioning:
Until sampling methods are improved and modified such that they give results that indicate the true V/P ratio of CDDs/CDFs in ambient air, the theoretical construct described by Bidleman (1988) is used to calculate the V/P ratio for purposes of air dispersion and deposition modeling of emissions from the hypothetical case demonstrated in Chapter 5 of Volume III.

Key advantages to the theoretical approach are that the theoretical construct relies on current adsorption theory, considers the molecular weight and the degree of halogenation of the congeners, uses the boiling points and vapor pressures of the congeners, and uses the availability of surface area for adsorption of atmospheric particles that correspond to a variety of ambient air shed classifications having variable particulate matter densities.

Four air shed classifications are described in Bidleman (1988): "clean continental", "background", "background plus local sources", and "urban". The classification used for the example scenarios in Chapter 5 of Volume III, and shown in Table III-1, is "background plus local sources".

3. Two runs of the COMPDEP model:

In order to provide estimates of vapor and particle phase concentrations of dioxin-like compounds, as well as estimates of wet/dry particle deposition flux, it is necessary that to run the COMPDEP model twice. Both model runs should assume a "unit emissions release rate", e.g., 1 g/s.

Results from these unit runs can easily be transformed to final outputs given assumptions on emissions in vapor and particle forms. A vapor phase run involves turning wet/dry deposition switches to the "off" position. This inactivates a plume depletion equation that subtracts out losses in ambient air concentration due to particle deposition.

What is left are the Gaussian dispersion algorithms. The vapor phase concentrations are used for inhalation exposures and also for vapor transfers onto vegetation for food chain modeling. A second run of COMPDEP with wet/dry deposition switches turned to the "on" position is considered a simulation of particle-bound contaminant. Outputs from this run include wet and dry deposition rates, and air concentrations of contaminants in the particulate phase.

The depositions are used in soil and food chain modeling, and the concentrations are added to the vapor phase concentrations from the first COMPDEP run to arrive at the total air-borne reservoir for inhalation exposures.

4. Assumed particle size distributions of emitted particles:

In order to estimate deposition flux, certain inferences must be made concerning the distribution of particulates according to particle diameter (µm). The distribution of particulate matter by particle diameter will differ from one combustion process to another, and is greatly dependent on the type of feed material, conditions of combustion, and the efficiency of various air pollution control devices. For purposes of demonstration, three particle size categories were generalized from available data on particle fractionation:

• Category 1: < 2 µm,
• Category 2: 2 to 10 µm,
• Category 3: > 10 µm.

By using data on the proportion of total particles emitted per size category, and conducting a surface area to volume calculation, it was estimated that 87.5% of the emission rate of particle-bound dioxin-like congener is associated with particles less than 2 µm in diameter, 9.5% is associated with the particle size of 2 to 10 µm, and only 3% is associated with particles greater than 10 µm. Finally, the particle size distribution is further simplified by assuming a median particle diameter to represent each broad particle size category, as follows:

Particulate category 1 = 1 µm particle diameter
Particulate category 2 = 6.78 µm particle diameter
Particulate category 3 = 20 µm particle diameter

5. Dry deposition:

The COMPDEP estimates dry deposition flux based on the model developed by Dumbauld, et al. (1976). This model assumes that a fraction of the particulate comes into contact with the ground surface by the combined processes of gravitational settling, atmospheric turbulence, and Brownian diffusion.

The COMPDEP model contains enhancements to calculate dry deposition flux using a computerized routine developed by the State of California Air Resources Board (CARB, 1986).

The routine is based on a summary of dry deposition velocity curves developed by Sehmel (1980) for a broad range of particle diameters. For the example application of the COMPDEP model in Chapter 5 of Volume III, particles less than 2 m m, represented by a 1 m m size, were assumed to deposit at a velocity of 0.00711 cm/sec. Particles between 2 and 10 m m, represented by a 6.78 m m size, were assumed to deposit at 0.287 cm/sec.

Finally, particles greater than 10 m m, represented by a 20 m m size, were assumed to deposit at a velocity of 2.47 cm/sec.

6. Wet deposition:

Wet deposition flux depends primarily on the fraction of the time precipitation occurs and thefraction of material removed by precipitation per unit of time by particle size.

Based on these relationships, scavenging coefficients were developed by Cramer (EPA, 1986) for varying types and intensities of precipitation relative to different particle diameters by incorporating the observations of Radke, et al. (1980) in a study of scavenging of aerosol particles by precipitation. The principal assumptions made in computing wet deposition flux are:

1. The intensity of precipitation is constant over the entire path between the source and the receptor;
2. The precipitation originates at a level above the top of the emission plume so that the precipitation passes vertically through the entire plume;
3. The flux is computed on the bases of fraction of the hour precipitation occurs as determined by hourly precipitation measurements compiled by the National Weather Service. The remaining fraction (1-f) is subject only to dry deposition processes. Thus no dry deposition occurs during hours of steady precipitation, and dry deposition occurs between the periods of precipitation.

Simple bioconcentration/biotransfer approaches are used to estimate biota concentrations in this assessment. Specifics for each biota considered are:

1. Fish -
The soil contamination and stack emission source categories estimate the concentration of contaminant on bottom sediments of water bodies.

A fish lipid concentration is estimated based the organic carbon normalized bottom sediment concentration and a BSAF, or Biota Sediment Accumulation Factor. Whole fish concentrations for exposure estimation then equal this lipid concentrations times a whole fish lipid content (or a fillet lipid content). For the effluent discharge source category, fish lipid concentrations are estimated as a function of organic carbon normalized concentrations and the closely related BSSAF, or Biota Suspended Solids Accumulation Factor.

This recently introduced bioaccumulation factor (EPA, 1993) is analogous to the BSAF, and it is suggested in EPA (1993) that, as a first estimate, it take on the same chemical-specific numerical value as the BSAF.

2. Vegetation -
Concentrations in three types of vegetation are considered in this assessment: below ground vegetables (carrots, potatoes, e.g.), above ground vegetables/fruits (tomatoes, apples), and above ground grass and cattle feed which are required for estimation of beef and milk concentrations.

Assumptions critical to all three include: above ground vegetation is impacted by vapor phase transfers and particle deposition - there is no root to shoot translocation, outer portions of the vegetation are only impacted with minimal within plant translocation, a steady state is reached between vapor phase contaminants in air and vegetation, particle bound contaminants deposit onto and mix in a vegetative reservoir and are subject to a fourteen-day dissipation half-life which represents particle washoff, and vegetables/fruits which have an outer protective layer (peas, citrus e.g.) are unimpacted by dioxin-like compounds.

Below ground vegetable concentrations are estimated from soil water concentrations and a Root Concentration Factor, or RCF. Above ground concentrations due to vapor phase transfers are a function of the vapor phase air-borne reservoir, an air-to-leaf transfer factor, Bvpa, and a surface area to volume reduction factor, VG, which is equal to 1.00 for grasses and other leafy vegetation and less than 1.00 for bulky vegetation

3. Beef and Milk -
Weighted average concentrations of dioxin-like compounds in the diets of cattle raised for beef or lactating cattle are multiplied by a congener-specific bioconcentration factor, BCF, which yields the concentrations in the fat of beef or milk.

The same congener-specific BCF is used for beef and milk. This presumes that dioxin-like compounds bioaccumulate equally in body fat and milk fat of beef and dairy cattle. While there is expected to be some difference in bioaccumulation tendencies, the literature was not clear on this issue.

Fries and Paustenbach (1990) discuss the importance of the dietary habits of cattle raised for beef versus those raised for dairy products; beef cattle tend to be grazed substantially more, while dairy cattle tend to be barn-fed for a greater proportion of their dietary intake. Like this assessment, Fries and Paustenbach (1990) model beef and milk concentrations using a single BCF for 2,3,7,8-TCDD.

They used a BCF of 5.0 for 2,3,7,8-TCDD. A set of BCFs for all dioxin-like congeners for this assessment were based on a set of data on a lactating cow (i.e., dietary intakes of dioxin congeners, concentrations in milk, and other pertinent quantities; McLachlan, et al., 1990). The BCF for 2,3,7,8-TCDD from this data set was 4.32. Beef and dairy cattle diets are described in terms of proportions in pasture grass, cattle feed (silage, grains), and soil. Models described above estimate concentrations in these cattle intakes.


EPA (1992a) states, "In exposure scenario evaluation, the assessor attempts to determine the concentrations of chemicals in a medium or location and link this information with the time that individuals or populations contact the chemical. The set of assumptions about how this contact takes place is an exposure scenario."

These assumptions can be made many different ways producing a wide variety of scenarios and associated exposure levels. The number of people exposed at different levels form a distribution of exposures. Ideally assessors would develop this entire distribution to fully describe the exposed population. Since the necessary information for developing a population distribution is rarely available, EPA (1992a) recommends developing a central and high end scenario to provide some idea of the possible range of exposure levels.

The basic setting for which the methodologies are demonstrated is a rural setting which contains both farms and non-farm residences. The three principal sources of contamination, the soil (both on-site and off-site), stack emission, and effluent discharge, categories, are assumed to exist in such a setting. "Central" scenarios are based on typical behavior at a residence and "high end" scenarios are comprised of a farm family that raises a portion of its own food. Key distinguishing features between the high end and central scenarios include:

1) individuals in high end scenarios are assumed to be at their home a greater proportion of the day than the central scenarios (which impacts assignment of contact fraction),
2) individuals in high end scenarios are exposed to impacted beef and milk which they raise on their farm while these exposures are not considered for the central scenarios,
3) the exposure duration for individuals in the high end scenario is 20 years compared to 9 years for the central scenario, and
4) certain exposure parameters, such as water ingestion rate which is 1.4 L/day for the central scenarios and 2 L/day for the high end scenario, are different.

The example scenarios were carefully crafted to be plausible and meaningful, considering key factors such as source strength, fate and transport parameterization, exposure parameters, and selection of exposure pathways. However, it should be clearly understood that the purpose of the demonstration scenarios is to provide users of this methodologies with a comprehensive example of their application. The demonstration exposure scenarios were: