So, is there a need to upgrade the PM2.5 FRM? The FRM method is a gravimetric, filter-based method. The answer, in part, may depend on the needs of the specific monitoring agency. The FRM program has high operational costs associated with the necessary field service and laboratory weighing, and a long delay between ambient measurements and when data are available. Newer automated instruments have a high initial cost, but they have the advantage of much lower operational costs and can provide data in near-real-time. Automated instruments designated as Federal Equivalent Methods (FEMs) can replace the instruments used for compliance monitoring and the instruments used to provide hourly data. Agencies use hourly data for public health messaging, source attribution studies, and PM2.5 forecasting.
The use of FEMs can offer significant cost savings during these fiscally tight times. It is likely that FRMs will continue
to have a place in monitoring networks where filter samples are necessary for component chemical analysis.
Is There a Bias between the FRM and FEMs?
One of the main problems with the current PM2.5 FRM is the lack of a physical standard to determine the accuracy
of the measurement. Actual atmospheric PM2.5 is a combination of solids and liquids ranging from relatively nonvolatile species, such as ammonium sulfate, elemental carbon, and metallic compounds, to much more volatile components, such as organic carbon, water associated with hygroscopic aerosol species, and ammonium nitrate. The filter-based FRM measurement does not capture all of the volatile components of PM2.5. Some of this mass is lost from the filter due to evaporation during sampling, after sampling, and during the post-weighing process. In side-by-side comparisons, the FRM is sometimes biased high, but in the majority of cases, it is biased low in comparison to other measurement technologies.1 The new continuous measurement technologies have an advantage over the FRM because they provide near-real-time hourly concentrations of PM2.5. This significantly reduces the opportunity for adsorption, or more likely, evaporation from the collection media before a measurement is made.