11. Evaluation and Minimization of Organic Aerosol Sampling Artifacts Using Impactors and Quartz Fiber Filter Denuders

Many constituents of particulate organic carbon (POC) are found in significant fractions in both the gas and particulate phases. The amount in each phase depends on several factors such as volatility, polarity, and the nature of other particulate components to which it might adsorb. 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 be positive or negative compared with the actual concentration found in the air at the time of collection. These collection artifacts have been studied for some time by a number of researchers; quantitatively measuring the particulate content free from gas-phase artifact processes has been proven to be difficult. The objective of this proposed research is to better characterize the magnitude of these processes to assess the contribution and source of POC to PM2.5.

As with particulate nitrate, the most appropriate method for this quantification uses a denuder to remove gas phase interfering species and then samples with a filtration medium capable of quantitatively capturing the particulate phase. Unlike particulate nitrate, 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. While a great deal of indirect evidence exists to support a minor positive artifact as the dominant artifact (typically 15% of the POC), one research group has developed a denuder-based approach which not only gives little support for the positive artifact but shows evidence of a negative artifact, which may result in 20-80% of the POC volatilizing from a quartz filter.

We are conducting a program to evaluate the POC collection artifact under ambient conditions using the best approaches developed to date, all performed simultaneously by a single research group. Our basic approach is to simultaneously collect POC using nine sampling configurations and compare the results with the PM2.5 Federal Reference Method to estimate sampling uncertainty due to collection artifacts. The methods and the rationale are summarized in the attached table. Quartz filters and carbon-impregnated glass fiber filters will be analyzed for organic carbon using a thermal volatilization-oxidation approach.

These results will be analyzed to determine the magnitude of the sampling artifact and the uncertainty caused in PM2.5 measurement. This project will address the uncertainties due to measurement artifacts caused by the collection of particulate organic carbon (POC), which affects both the accuracy of the reference methods and the assignment of sources. A practical approach for minimizing artifacts for routine PM2.5 sampling will be developed and evaluated.

CONFIGURATION	ANALYSIS	PARAMETER/ FUNCTION
1. ____ Q	TOA	PM2.5 POC
2. ____ T	Mass	Federal Method PM2.5 reference
____ Q	TOA	Adsorbed VOC, VPOC
3. /Q	none	Removes VOC that adsorb on quartz filter
____ Q	TOA	POC w/o adsorbed gases
____ Q	TOA	Indicates /Q/ efficiency or adsorbed VPOC
4. ------ I	TOA	POC w/o adsorption or volatilization
____ Q	TOA	Very fine POC, adsorbed VOC, VPOC
____ Q	TOA	Adsorbed VOC
/Q/	none	VOCs that adsorb on quartz filter medium removed
5. ------ I	TOA	POC w/o adsorption or volatilization?
____ Q	TOA	Very fine POC, adsorbed VPOC
____ Q	TOA	Adsorbed VOC
6. ------ I	TOA	POC w/o adsorption or volatilization
/Q/	none	VPOC, VOC that adsorb on quartz removed
____ Q	TOA	Very fine POC
____ Q	TOA	Potentially very little OC
7. ____ T	none	Removes POC
------ I	TOA	Dynamic POC blank
____ Q	TOA	Adsorbed VOC, VPOC
8. /C/	none	Removes VOC
____ Q	TOA	POC, remaining VOC
____ Q	TOA	Adsorbed VPOC, remaining VOC
____ CIF	TPV	Remaining VOC, VPOC
9. ____ T	none	Removes POC
/C/	none	Removes VOC
____ Q	TOA	Absorbed remaining VOC+VPOC
____ Q	TOA	As above, measure of Q breakthrough
____ CIF	TPV	Remaining VOC+VPOC

Q=quartz filter, T=Teflon filter, I=Impactor, /Q/=quartz fiber filter denuder, /C/=carbon denuder, CIF=carbon impregnated filter, VOC=volatile organic carbon, POC=particulate organic carbon, VPOC=volatilized POC, TOA= thermal optical analysis, TPV= temperature programmed volatilization.