Because of the size distribution of ambient particulate matter, most measured PM2.5 is

likely to be fine mode particulate matter. As a result, it is dominated to a much larger extent

than PM10 by “secondary” particulate matter and primary particulate emissions arising from

combustion. Some of the physicochemical processes leading to secondary particulate matter

formation may take hours or days, as do some of the removal processes. Thus, many of the

sources of measured secondary particulate matter may not be local emitted sources. This implies

that modeling to support attainment demonstrations for PM2.5 (and, as we will discuss later,

regional haze-related applications) will need to cover a very large domain, and will need to

include chemical/physical mechanisms important in formation/removal of secondary particulate

matter. Because several of the processes are slow and first require thorough mixing with the

environment, spatially detailed treatment near sources of precursors may not be necessary.

Individual treatment of precursor emissions from relatively nearby large sources of primary

PM2.5 may be needed on a case by case basis.


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