       amount that would be derived directly from speciation measurements. Because of uncertainties

in speciation measurements and their estimates from interpolated surfaces, setting a lower limit

(floor) may be necessary so that the OCMmb is not unreasonably low. In the CAIR analysis, the

floor was set so that OCMmb could not be more than 30% lower than measured OCM (OC*1.4).

We used interpolated measured values of OCM to calculate the floor. The lower limit was equal

to interpolated OC * 1.4 * 0.7. If the OCMmb concentration was less than the lower limit, it was

set equal to the lower limit. The recommended approach is to simply set the OC floor equal to

measured OC.

There may also be situations where an OCM “ceiling” is needed. In remote urban areas

with relatively high FRM concentrations that may be surrounded by rural background

concentrations, the OC by mass balance technique may apportion 95% or more of the PM2.5 mass

to OCM. If this is not a reasonable assumption, then a ceiling may be needed to cap the OCM as

a percentage of PM2.5 mass. Based on measured data (FRM sites with co-located speciation

data), it appears that on a quarterly average basis, OCM is rarely more than 80% of total PM2.5

mass. This may be a reasonable default ceiling, but a lower value (or in rare circumstances a

higher value) may be more appropriate in many regions of the country.

Summary of PM2.5 Composition Calculations

Summary of PM2.5 Composition Calculations

The terms of equation 5.6 reflect the final estimated composition of the particles

measured by the FRM (for each quarter). Quarterly average FRM mass is equal to the sum of

the seven species plus blank mass.

PM2.5FRM = { [OCMmb] + [EC] + [SO4] + [NO3FRM] + [NH4FRM] + [water] + [OPP] + [0.5] }

[5.6]

The recommended order to generate the data is as follows:

n651 - n652 - n653 - n654 - n655 - n656 - n657 - n658 - n659 - n660 - n661 - n662 - n663 - n664 - n665 - n666 - n667 - n668 - n669 - n670 - n671 - n672 - n673 - n674 - n675 - n676 - n677 - n678 - n679 - n680 - n681 - n682 - n683 - n684 - n685 - n686 - n687 - n688 - n689 - n690 - n691 - n692 - n693 - n694 - n695 - n696 - n697 - n698 - n699 - n700