Grantee Research Project Results
Final Report: Evaluation and Minimization of Organic Aerosol Sampling Artifacts Using Impactors and Quartz Fiber Filter Denuders
EPA Grant Number: R831087Title: Evaluation and Minimization of Organic Aerosol Sampling Artifacts Using Impactors and Quartz Fiber Filter Denuders
Investigators: Fitz, Dennis R.
Institution: University of California - Riverside
EPA Project Officer: Chung, Serena
Project Period: October 1, 2003 through October 31, 2007
Project Amount: $187,030
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text | Recipients Lists
Research Category: Particulate Matter , Air Quality and Air Toxics , Air
Objective:
Particulate matter (PM) differs from other criteria pollutants in that it consists of a complex chemical mixture. Particulate organic carbon (POC) is a major contributor to PM and consists of thousands of individual compounds covering a full range of chemical and physical characteristics. Many POC constituents are found in significant fractions in both the gas and particulate phases. The amount in each phase depends largely on volatility and polarity. When PM is collected on a fiber filter, POC may volatilize once collected, or gaseous components may adsorb to either the filtration medium or previously collected particulate matter and therefore become apparent POC. These processes, known as collection artifacts, therefore, may cause a positive or negative measurement biases compared with the actual concentration found in the air at the time of collection. The objective of this research was to characterize the magnitude of these collection artifacts to assess the contribution and sources of POC to PM2.5, and to devise a practical method of minimizing these artifacts based on diffusion denuding and impaction.
Summary/Accomplishments (Outputs/Outcomes):
The most appropriate method to minimize collection artifacts uses a denuder to remove gas phase interfering species and then samples with a medium capable of quantitatively capturing the particulate phase. The denuder must be capable of removing hundreds of different potentially interfering species while the collection medium must be able to effectively trap as many. This project evaluated, under ambient sampling conditions, the best methods to evaluate and minimize the POC collection artifact. The approach is to simultaneously collect POC using nine sampling configurations and compare the results with the PM2.5 Federal Reference Method. The methods are (1) a dual quartz filter pack (2) a Teflon-quartz-quartz filter pack, (3) a quartz fiber denuder followed by a quartz-quartz filter pack, (4) an impactor to remove particles greater than 0.05µm diameter followed by a quartz-quartz filter pack, (5) an impactor preceded by a quartz fiber denuder and followed by a quartz-quartz filter pack, (6) an impactor followed by a quartz fiber denuder and quartz-quartz filter pack, (7) a Teflon filter followed by an impactor, followed by a quartz-quartz filter pack, (8) a carbon-based parallel plate denuder followed by a quartz-quartz filter pack followed by a carbon-impregnated glass fiber filter and, (9) a Teflon filter followed by a carbon-based parallel plate denuder followed a quartz-quartz filter pack followed by a carbon-impregnated glass fiber filter. Collection substrates were analyzed for organic and elemental carbon using a thermal volatilization-oxidation approach.
Figure 1. Schematic drawing of the nine channel POC sampler
CONFIGURATION ANALYSIS PARAMETER / FUNCTION 1. ____ Q TOA PM2.5 POC -------------------------------------------------------------------------------- |
| Q=quartz filter, T=Teflon filter, I=Impactor, /Q/=quartz fiber filter denuder, /C/=carbon impregnated filter denuder, CIF=carbon impregnated filter, VOC=volatile organic carbon, POC=particulate organic carbon, VPOC=volatilized POC, TOA= thermal optical analysis, TPV= temperature programmed volatilization |
Conclusions:
A nine channel sampler shown in Figure 1 was used to evaluate POC sampling artifacts when sampling both ambient air and diesel exhaust. Samples were collected on quartz filters, aluminum impactor substrates, and carbon impregnated glass fiber filters and analyzed for organic and elemental carbon content by thermal volatilization methods. Parallel plate denuders with quartz filter strips were evaluated for removing VOC that would deposit on quartz filters while similar denuders using strips of carbon impregnated cellulose filter were used to remove all VOC.
A significant (30-40%) positive POC collection artifact was found for both sample types when using quartz fiber filters. Quartz filters appear to have a strong affinity to collect VOC. Correction for this sampling artifact can be made by subtracting the OC found on a quartz filter that samples air from which the particulate matter has been removed by a Teflon filter. Similar results were obtained from a cascade impactor with aluminum substrates followed by a quartz after-filter. Since the impactor stages collect most of the particulate matter and little VOC, the correction for the after-filter (using the OC on a separate quartz filter following a Teflon filter) is quite large, approximately 90% of the OC on this filter is an adsorption artifact. Given this large correction, sampling with quartz filters is preferred if a particle-size distribution is not needed. Good comparability found between the two sampling approaches gave a strong indication that the collection artifact is due to adsorption of VOC by quartz filters and not volatilization of collected particulate matter followed by capture on the quartz filter. If the latter were significant, more volatilization would be expected from a quartz filter than from an impaction substrates in which the particles collected are almost immediately covered with other particles.
If a cascade impactor is to be used then the OC/EC split on the aluminum impactor substrates should be verified (since no correction can be made for charring) by summing the EC on the impactor stages and comparing with the difference between the EC on a collocated quartz filter less the EC on the after-filter.
The denuders used were found to be too ineffective to evaluate sampling artifacts. Since the positive collection artifact on quartz fiber filter may be corrected using a quartz back filter, the expensive and handling of denuders is not warranted for routine sampling.
Since the collection artifact for POC was found to be due primarily to the affinity of quartz filter to adsorb VOC and not due to particle volatilization, Teflon filters used for the EPA’s PM2.5 reference methods should not be affected to a significant degree.
Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
ambient air, monitoring, measurement methods, adsorption, particulates,, RFA, Air, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Waste, chemical mixtures, particulate matter, Air Quality, Environmental Chemistry, Engineering, Chemistry, & Physics, Monitoring/Modeling, Air Pollution Effects, Atmospheric Sciences, air toxics, Environmental Engineering, Environmental Monitoring, aerosol particles, health effects, particle size, air sampling, gas chromatography, mass spectrometry, carbon particles, emissions, chemical characteristics, air quality modeling, measurement methods, airborne particulate matter, atmospheric particles, quartz fiber filter denuders, modeling studies, thermal desorption, particulate matter mass, air quality model, particulate organic carbon, carbon aerosols, aerosol analyzers, air quality models, atmospheric particulate matter, impactors, ultrafine particulate matter, chemical speciation sampling, particle size measurement, air pollutants, particle dispersion, air modelingProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.