Research Projects

Project at a Glance

Project Status: complete

Title: Development of an improved source sampling method for polycyclic aromatic compounds and other semi-volatile organic species

Principal Investigator / Author(s): Wall, Stephen

Contractor: CA Department of Health Services

Contract Number: a932-098


Research Program Area: Emissions Monitoring & Control

Topic Areas: Stationary Sources, Toxic Air Contaminants


Abstract:

Benzo(a)pyrene (BaP) and other polycyclic aromatic hydrocarbons (PAH) are classified as toxic air contaminants in California under the classification polycyclic organic matter (POM). More accurate sample collection, extraction and analysis methods are needed in order to make assessments of stationary combustion sources that emit PAH.

To this end, a new Reduced Artifact Dilution Sampler (RADS) has been developed which consists of an isothermal/isokinetic stack probe, an automated dilution system and a reduced artifact sampling system for the collection of both vapor phase and particulate PAH. A primary advantage of dilution volume sampling over the current Modified Method 5 (MM5) system, is that chemical reactions between the condensing constituents occur under physical and chemical conditions similar to those at the stack exit. The RADS system is designed to collect PAH from the dilution system without significant artifact formation from acidic combustion gases or volatility loses from the particulate matter collected on the filter. The reduced artifact sampling train includes an acid gas denuder, and a Teflon filter followed by a series of polyurethane foam (PUP) plugs.

A prototype Photoelectric Aerosol Sensor (PAS) was investigated foruse as a real-time PAH aerosol monitor. The PAS is designed to be small enough for field usage and is equipped with a stack-sampling probe with an internal dilution system. Although the instrument is known to respond to the PAH aerosol concentration in some combustion sources, a performance evaluation was necessary for the mixture of emissions present in different stationary incinerator sources.

A simplified well controlled combustion source for polycyclic aromatic hydrocarbon (PAH) aerosols was constructed in the laboratory to evaluate the performance of a prototype photoelectric Aerosol Sensor (PAS) and to develop the new Reduced Artifact Dilution Sampler (PADS). Fresh combustion generated aerosol provided a realistic particulate matrix to evaluate the collection efficiency of the reduced artifact sampling train, and the recovery efficiency of the associated PAH analytical method. Monitoring the laboratory combustion source with the real-time PAS provided a convenient method for determining the stability, and the relative level of PAH aerosol produced under different combustor operating conditions.

New more efficient analytical methods, including Pulsed Ultrasonic Extraction (PWE) of the combustion particles and Simple Compression Extraction (SCE) of the PUF plugs, were developed to improve the speed and accuracy of PAH determinations by gas chromatography / mass spectroscopy (GC/MS). A new simplified single step clean-up technique was also used to remove interfering substances co-extracted from the sample matrix. The new PAH extraction and single step clean-up techniques, as well as, supercritical fluid extraction (SFE) were evaluated using standard reference materials of urban particulate matter and diesel emissions.

In field trials, the PADS system successfully maintained isokinetic and isothermal sampling conditions for diesel exhaust stack velocities over 11 m/s (36 ft/s) at standard temperature and pressure conditions (STPC, 20C and 760 mm/g) and temperatures exceeding 200oC (392F). A dilution factor of 35:was automatically maintained to reach near ambient temperature conditions for sample collection. Dilution to ambient air temperature conditions was a necessary operating condition, so that chemical reactions between condensing constituents occur under physical and chemical conditions similar to those at the stack exit. For this field study, the RADS was utilized in the simplified high volume dilution sampler configuration without particle size segregation. Intended for routine monitoring applications, this configuration was the most suitable choice for the intermethod comparison with MM5.

Utilizing the PAS as a source survey tool, the Combustion missions Research Laboratory (CERL) at EHLB successfully determined that the optimum time for sampling was relatively short, on the order of 10 minutes, due to the high levels of PAH containing particulate matter in the generator exhaust. The analytical results of the side-by-side comparison indicate that the CERL method using the RADS reported substantially higher PAH levels than found with Method #423 using the MM5 sampler. For example, over six times more BaP was determined by the CERL method than reported for Method #429. A correlation between enhanced PAH collection by the FIADS and the carbon number of the individual PAH species suggests that sampling artifacts which act to reduce the PAH collected in MM5 occur in both the XAD-2 resin bed and on the heated filter.

Together the PAS and PADS provide a promising integrated approach to determining the PAH emissions from stationary combustion sources. Although the PAS cannot be considered to be specific for individual PAH compounds, calibration with combustion aerosol has demonstrated the usefulness of the PAS as a total PAH aerosol monitor. This is ,useful for field screening source emissions to determine the necessity of collecting samples for chemical analysis using the RADS system. Depending on the level of PAH present, the RADS can be used as a high volume dilution sampler without particle size segregation or as a low volume reduced artifact sampler for respirable PAH. In either configuration, the PAS can be used as a RADS loading monitor to ensure sufficient sample is collected for the level of detailed chemical analysis required at a particular source.


 

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