Premise PM2. In most parts of the country, “Secondary” PM is a more important part of

PM2.5 than it is of PM10. Size-differentiated ambient particulate data suggest that mass of

particulate matter follows a bimodal distribution, with one peak (fine mode) reflecting particles

with aerodynamic diameters ~ 0.1-1.0 micrometers arising from nucleation and accumulation

phenomena, and a second (coarse mode) occurring with aerodynamic diameters in the range of

1.0-20 micrometers. As shown in Figure 1.1, derived from Wilson and Suh, (1997), mass of fine

particulate matter (i.e., PM2.5) attributable to coarse mode particulate matter < 2.5 micrometers is

relatively small. Mass attributable to fine mode particulate matter with aerodynamic diameters >

2.5 micrometers (i.e., coarse particulate matter) is also relatively small.

Origins of coarse and fine mode particulate matter are usually quite different. The former

results mostly from physical types of activities (e.g., crushing, grinding, resuspension due to

motions, etc.). Nearly all these activities result in particulate matter, with little subsequent

chemical change. We call such emissions of particulate matter “primary emissions”, because

they are measured in more or less the same form in which they are emitted. In contrast, origins

of fine mode particulate matter are more diverse. For example, some fine mode particulate

matter is directly emitted to the atmosphere as a result of combustion. Such emissions occur

either directly as particles or as a result of condensation which occurs very shortly after the

emissions occur. These are primary emissions, because what is measured in the ambient air is

essentially unchanged (chemically) from what is released. However, many fine mode particles

are the result of physicochemical reactions which occur in the atmosphere among gaseous

precursors or through absorption or adsorption onto previously existing aerosols. Such particles

constitute “secondary” particulate matter, because they undergo transformations in the

atmosphere causing the chemical and/or physical nature of what is measured to be different from

what is emitted.


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