Techniques for high-quality ambient coarse particle mass measurements

Several recent studies have shown associations between ambient concentratio ns of particle mass (PM) and rates of morbidity and mortality in the genera l population. These studies have raised the issue of quality of coarse mass (CM, PM between 2.5 and 10 mu m) data used for these purposes. CM data may have precision three or more times worse than the associated PM2.5 or PM10 data, depending on the measurement method, PM2.5 to PM10 ratios, and CM co ncentrations. CM is measured either as the difference between collocated PM 10 and PM2.5 samplers or more directly with a dichotomous (virtual impactor ) sampler. CM precision for the difference method is degraded due to the in creased errors inherent with using the difference between two independent m easurements, as well as the high PM2.5 to PM10 ratios (and low CM concentra tions) typical of the eastern United States. The dichotomous sampler (dicho t) makes a more direct measurement of CM, but there is a potential for sign ificant postexposure loss of particles from unoiled CM dichot filters, as w ell as uncertainties in the dichot's CM channel enrichment factor. Compared to the dichot, low-volume inertial impactor samplers such as the Harvard I mpactor (HI) or PM2.5 Federal Reference Method (FRM) are simpler to operate and maintain, provide sharper cut points, and do not require oiled filters to prevent loss of CM from the filter during transport. With the recent in terest in CM spatial and temporal variability with respect to PM health eff ects, we have developed modifications to the HI PM method to provide measur ements of 24-hour PM with estimated CM precision of better than 5% CV and r (2) higher than 0.95, primarily by lowering field blank variability and inc reasing gravimetric analytical precision. These high-precision PM technique s are not limited to the HI sampler; they can also be applied to the PM2.5 FRM sampler. The measurement methods described here can be applied to futur e PM studies to avoid the potential problems with exposure assessment cause d by CM measurements that have poor precision.