For areas which contain one or more speciation sites, and where FRM sites exhibit very

little spatial gradients, it may be appropriate to assume that the speciation site is representative of

the entire area. Areas which exhibit strong spatial gradients in PM2.5 concentrations should

strongly consider using more sophisticated techniques to estimate species concentrations.

Combining ambient data with model output concentrations (number 4 above) may help adjust

concentrations in areas with strong gradients, but limited speciation data. This technique has

great promise but has not been used extensively for PM2.5 analyses. The technique is further

limited by uncertainties in the representativeness of the model outputs and emissions inventories.

5.1.3 New Species Calculations and Definitions

Recent data analyses (Frank, 2006) have noted that the FRM monitors do not measure the

same components and do not retain all of the PM2.5 that is measured by routine speciation

samplers and therefore cannot be directly compared to speciation measurements from the

Speciation Trends Network (STN)40. By design, the FRM mass measurement does not retain all

ammonium nitrate and other semi-volatile materials (negative sampling artifacts) and includes

particle bound water associated with sulfates, nitrates and other hygroscopic species (positive

sampling artifacts). This results in concentrations (and percent contributions to PM2.5 mass)

which may be different than the ambient levels of some PM2.5 chemical constituents.



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