In general, the speciation data can be directly compared to the model outputs. Of note,

care should be taken in examining issues with blank corrections for organic carbon (especially

when measured OC is low) as well as the EC/OC split. The STN and IMPROVE networks use

different analysis techniques when estimating EC and OC. This will generally have a larger

impact on EC concentrations. It may not be appropriate to compare EC concentrations between

networks. The Clean Air Scientific Advisory Committee (CASAC) has recommended to

transition the STN network to the IMPROVE carbon protocol (TOR) in order to provide data

consistency among the two networks (U.S. EPA, 2005f). Over the next two to three years (2007-

2009), EPA plans to convert all of the STN sites (approximately 200 trends and supplemental

sites) to IMPROVE carbon sampling and analysis protocols.

Due to the previously stated positive and negative artifacts, the FRM data may not be

directly comparable with model outputs without some adjustments. The models predict PM

species as they might be measured in the ambient air. But the FRM measurements may not

always measure what is in the air (due to known positive and negative artifacts). As part of the

PM2.5 attainment test in Section 5 we recommended default methodologies to adjust speciation

(STN and IMPROVE) data to better approximate the FRM data. A similar adjustment could be

made to the model output data to allow the direct comparison to FRM measurements. But due to

the relatively limited information to be gained by evaluating daily average total measured PM2.5

mass (as compared to PM2.5 components), it may not be worth the effort. More meaningful

information may be gleaned from continuous PM2.5 ambient data. Comparisons between model

output and continuous data can be made on an hourly basis. This may provide important

information with regards to the daily temporal patterns predicted by the model.


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