Frequently Asked Questions About Area Source Testing
Q: What qualifies as an area source?
A: There are point sources (stacks), mobile sources (cars), fugitive sources (pipeline/process leaks), and area sources. Area sources include landfills, land farms, surface spills, land application, lagoons, liquid treatment processes (municipal and industrial), open tanks, floating lid tanks, buried waste, subsurface contaminated soils, contaminated groundwater, and surfaces or materials off-gassing compounds.
Q: Is it better to model or measure emissions from an area source?
A: Both! It is always best to estimate area source emissions using multiple approaches. Modeling is recommended as a screening-level assessment. If done properly and if the estimate meets the project objective, emissions modeling may provide the assessment needed. Measurement approaches are typically used to provide a more representative emission estimate.
Q: What are the limitations of modeling emissions from an area source?
A: Predictive emissions modeling requires adequate site data or source-term data as well as a model that adequately represents the area source. Predictive modeling is typically conservative resulting in an over-estimate of area source emissions. As such, the predictive modeling approach is typically used as a "first cut" estimate or a screening-level assessment.
Q: What are the limitations of measuring emissions from an area source?
A: Measuring area source emissions requires an adequate data set to describe the source. Most area sources are heterogeneous and source data must be collected to represent the source. Proper sampling equipment, source-test protocols, and analytical methodology are required in order to generate useful measurement data.
Q: Is assessing emissions from an area source expensive?
A: Compared to what? If predictive emission models and site characterization data are available, predictive modeling is probably the most cost-effective approach. However, if in-depth area source emission data are needed, direct measurement technologies are probably the most cost-effective approach. Direct emission measurement is usually the most cost-effective measurement approach as compared to indirect assessment or fence line measurement/modeling. Direct flux measurement using the surface flux chamber has been reported to be the most cost-effective measurement approach. So if you need measured data, the flux chamber is probably the way to go.
Q: What is the best way to measure emissions from an area source?
A: Typically the surface flux chamber technology is the preferred area source assessment technology. It is generally applicable to most area sources and flux chamber data meets DQO's for compliance testing and are acceptable for human health risk assessment.
Q: How do you estimate an emissions from an area source with a flux?
A: An emissions estimate is expressed as a mass released per time (ug/minute for the entire source). A flux measurement is expressed as a mass per time and area source (ug/m2,min-1). Emission are estimated by multiplying the surface area (m2) of the area source by the representative flux (ug/m2,min-1) for the area source emission (ug/minute).
Q: What affects surface flux?
A: Everything! Each type of surface (land, liquid, sludge, process unit, etc.) has unique considerations which are addressed in the work plan. Weather can have a significant affect on surface flux but it is typically second to source and process variability. Application to land surfaces is usually limited to less than 0.1" of precipitation in the prior 48 hours when an annual estimate of emissions is the objective. Daily variability in barometric pressure is typically insignificant unless a major storm and low pressure cell passes through. Spatial, process, and seasonal affects can typically be measured and should be addressed in the sample collection plan.
Q: How can I be sure that the data from the surface flux measurement is representative of the source?
A:QA studies performed by EPA have demonstrated the precision and accuracy of the technology. Project specific QC data demonstrate that the application of the technology is correct. Verification studies have also been conducted and reported demonstrating representativeness of source characterization data. Verification data can also be included in a project-specific testing effort to demonstrate the utility of the technology.
Q: Is the design and operation of the flux chamber important?
A: Yes! It is essential that the EPA technology be used as recommended which includes using the proper flux chamber (EPA design) and following the test protocol as per the EPA user's guide.
Q: Is the data from the flux chamber adequate for a health risk assessment or compliance reporting?
A: Yes! The EPA flux chamber is considered an "in-depth" area source assessment technology. If used properly, these data can be used for compliance reporting and health risk assessment.
Q: What compounds can be measured using the flux chamber?
A: Any compound that has a standard method or protocol can be tested from the flux chamber. The chamber provides the "environment" where a compound can be sampled and the concentration data used to calculate a compound flux. Both grab and integrated samples can be collected from the chamber at flow rates of up to 2 liters per minute.
Q: Can odor be measured using the flux chamber?
A: Yes! Odorous area sources are tested by collecting a grab sample in a tedlar bag for analysis by ASTM E-679-91 (human olfactory analysis generating intensity and concentration data- D/T) from the chamber. Given that ambient air odor measurements are difficult to perform, odor flux data used in an appropriate dispersion model provide very useful odor impact assessment data.
Q: Can the surface flux chamber be used to estimate emissions into a structure?
A: Indoor studies are performed in one of two ways. A first estimate is made by collecting outdoor surface flux data along the structure foundation and applying an infiltration factor to the outdoor flux data. A more detailed assessment involves measuring flux on points of infiltration indoors and calculating an indoor flux. Indoor air concentration (or exposure) is estimated by using the indoor flux and a box model representing the structure ventilation. This approach is particularly useful if the study compounds are also found in the indoor air from process sources.