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103 downwind

Dp > 2 ¿im; smaller mode contains gas-to-particle conversion products (sulfate, nitrate, ammonium, organics) while larger mode has soil and ash particles

" From Pandis et al. (1995), Whitby and Sverdrup (1979), and Fitzgerald (1991).

" From Pandis et al. (1995), Whitby and Sverdrup (1979), and Fitzgerald (1991).

rithms to the base e can be used, but since the latter is more common, we follow through the discussion using natural logarithms, In.) This distribution is expressed as dN d\nD

FIGURE 9.12 Meaning of standard deviation for a normal distribution. The hatched area represents 68% of total area under curve.

where N is the number of particles having diameters whose logarithms are between In D and In D d In D, NT is the total number of particles, ag is known as the geometric standard deviation, and DgN is the geometric number mean diameter, which for this distribution is equal to the number median diameter, defined as the diameter for which half the number of particles are smaller and half are larger. A typical plot of Eq. (B) is shown in Fig. 9.13, where (dN NT) din D is plotted against the diameter on a logarithmic scale.

The geometric number mean diameter, D N, is related to the arithmetic mean of In (diameter):

In V

Here w • is the number of particles in a group whose diameters are centered around dj. Thus In DgN is really a weighted value of In d, where the weighting is by the number of particles in that size interval.

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