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THE POTENTIAL INFLUENCES OF FACE VELOCITY ON PM ARTIFACT LOSSES FOR EXPOSURE SAMPLERS USING TEFLON FILTER COLLECTION SUBSTRATES
Citation:
Rodes, C. E., P. A. Lawless, J. Thornburg, R B. Zweidinger, G A. Norris, R W. Williams, G F. Evans, L A. Wallace, L S. Sheldon, AND S R. McDow. THE POTENTIAL INFLUENCES OF FACE VELOCITY ON PM ARTIFACT LOSSES FOR EXPOSURE SAMPLERS USING TEFLON FILTER COLLECTION SUBSTRATES. Presented at 11th Annual Meeting of the International Society of Exposure Analysis, Charleston, SC, November 4-8, 2001.
Impact/Purpose:
The primary study objectives are:
1.To quantify personal exposures and indoor air concentrations for PM/gases for potentially sensitive individuals (cross sectional, inter- and intrapersonal).
2.To describe (magnitude and variability) the relationships between personal exposure, and indoor, outdoor and ambient air concentrations for PM/gases for different sensitive cohorts. These cohorts represent subjects of opportunity and relationships established will not be used to extrapolate to the general population.
3.To examine the inter- and intrapersonal variability in the relationship between personal exposures, and indoor, outdoor, and ambient air concentrations for PM/gases for sensitive individuals.
4.To identify and model the factors that contribute to the inter- and intrapersonal variability in the relationships between personal exposures and indoor, outdoor, and ambient air concentrations for PM/gases.
5.To determine the contribution of ambient concentrations to indoor air/personal exposures for PM/gases.
6.To examine the effects of air shed (location, season), population demographics, and residential setting (apartment vs stand-alone homes) on the relationship between personal exposure and indoor, outdoor, and ambient air concentrations for PM/gases.
Description:
The influences of artifact formations and losses on Particulate Matter (PM) sampler collection surfaces are well documented, especially for nitrates (Hering and Cass, 1999), and SVOC's (McDow, 1999), and more recently for speciated carbon (Turpin and Lim, 2001). These artifact biases not only affect speciation analyses, but also importantly, the total PM mass concentration. An intercomparison of methods for PM mass concentration for the PM2.5 fraction in four exposure panel studies evaluating the associations between PM and selected health indicators suggests that the bias relationships between samplers appear to be partially dependent upon the relative face velocities through the Teflon filter deposits. This paper will address the hypothesis that face velocity through Teflon filters is related to "blow-off" (negative) losses for volatile species, based on two assumptions: (a) the volatile portion exists in sufficient quantities for the effect to be observed, and (b) the influence is not obscured by the simultaneous occurrence of positive artifacts (chemical or physical).
Face velocity biases could prove to be important for exposure studies where associations of ambient, indoor, and personal exposure samplers with cohort health indicators are being investigated. For example, ambient PM2.5 samplers such as the EPA FRM sampler have face velocities (79 cm/s) that can be factors 5 to 10 greater than personal exposure samplers (MSP 2 lpm PEM, 15 cm/s). The effective filter face velocity must take into account the porosity of the support matrix, which for the FRM sampler, has only 29% of the surface perforated. Large day-to-day variations can occur in the ambient concentrations of semi-volatile species (e.g. West Coast nitrates, Winter wood smoke PM), biasing the sampling methods differently on a daily basis, and confounding associations with the health measures. Importantly, PM chemistry may vary between outdoor and indoor sampling environments, for example, given the much lower acidity levels, and often higher ammonia concentrations indoors.
Between-method comparison ratios will be presented for 2 and 4 lpm MSP PEM's, Harvard Impactor indoor and outdoor samplers, Andersen FRM's, URG DFPSS and VAPS samplers, and Andersen dichotomous samplers, showing that in the majority of the cases, the sampler collecting the greatest mass had the lowest face velocity. Significant biases tended to occur in scenarios where volatile species were known to exist in sufficient quantities. Concurrent artifact phenomena/interventions explaining the exceptions were noted to include the presence of sodium carbonate denuders which limited positive nitrate formation, and excessive loadings on impactor surfaces which resulted in crustal particle bounce.
This work has been funded in part by the US EPA under Contract No. 68-D5-0040 and EPA cooperative agreement CR822981-01 to the Research Triangle Institute. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.