Premise PM5. Spatial gradients for primary particulate matter may be more pronounced

than those for secondary particulate matter or those for ozone. As previously noted,

secondary particulate matter and ozone result from an interaction of meteorology and chemistry

taking several hours to days. Because of the time scales and mixing required, sharp spatial

gradients in concentrations of these pollutants are unlikely. In contrast, primary particulate

matter is emitted in the form it appears at monitoring sites. It is likely that concentrations of

primary particulate matter are greatest near major source areas of primary particulate matter.

The preceding implies that it may be necessary to estimate concentrations of primary

particulate matter using models with finer spatial resolution than is necessary for secondary

particles. Further, there may be several sources or concentrations of sources of primary

particulate matter within an area designated as “nonattainment” for PM2.5. The guidance will

need to address how to evaluate model performance and how to estimate whether attainment of

the NAAQS is likely in such locations.

Premise PM6. Seasonal differences are likely in emissions of PM2.5 and its precursors, as

well as in meteorological conditions affecting source/receptor relationships. Emissions from

several potentially important sources of PM2.5 , such as residential wood burning, wildfires,

agricultural burning, prescribed burning and biogenic sources have distinctive seasonal patterns.

Further, meteorological factors which may affect PM2.5 or regional haze, such as relative

humidity, sunlight intensity, mixing heights, precipitation and temperature, have marked

seasonal differences in many parts of the United States. The annual NAAQS for PM2.5 and the

regional haze rule address a composite of conditions measured over many days. To understand

how such composites respond to changes in emissions, it will be necessary to model a variety of

days with varying emissions and meteorological conditions. This implies that States will need to

develop base emissions estimates for a full year (or multiple years) or a representative portion of

days may need to be modeled from each season.

 

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