surface
or mass distributions. In particle number plots
a large peak is observed at ~0.02 µm and a slight peak around 0.1 µm. Particle concentration distributions based on volume show two peaks
one in the 0.1- to 1.0-µm range and the second in the 1- to 2-µm range. In surface area distribution plots
a major peak is observed near 0.1 (µm
with smaller peaks between 0.01 and 0.1 µm
and between 1 and 10 µm. These particle distribution plots indicate that urban aerosol concentrations are multimodal. Until recently
it was widely accepted that atmospheric aerosols consisted of three distinct particle modes. With increasing scientific attention being given to ultrafine particles (UFPs) (diameters of <0.01 µm)
atmospheric particles are now being described in terms of the four particle modes illustrated in Figure 2.9. Though no vertical-axis label is shown
it could represent particle distribution based on number
mass
surface area
or volume distribution. Figure 2.9 also shows major sources and particle removal processes for each mode. The UFP mode and the two peaks sometimes observed in the accumulation mode are illustrated by dashed lines. As can be seen (Figure 2.9)
atmospheric aerosols are divided into two major fractions based on their size
origin
and physical properties. Coarse particles have diameters of >2.5 µm
fine particles of <2.5 µm. These particle modes or fractions are distinct relative to (1) their origin
(2) how they are affected by atmospheric Air quality 62
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