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FIGURE 5.18 Correction factors for the measured uptake coefficient, ymcas, as a function of the ratio of the diffuso-reactive length (/) to the droplet radius (a) (adapted from Hanson et al., 1994).

tion occurs throughout the particle volume. In this case, the correction factor becomes significant and yc is less than ymcas. Figure 5.18 shows the values of this correction factor for various values of l/a (Hanson et al., 1994). Clearly, this correction factor can be quite large for small particle sizes/large diffuso-reactive lengths. Theoretical treatment suggests that the correction term may also vary with the electrical charge on the particle (Aikin and Pesnell, 1998), although it is not clear if this effect is significant for atmospheric droplets.

If the reaction is slow compared to diffusion so that the diffuso-reactive length is about the same or larger than the particle size, reaction takes place throughout the entire volume of the particle. On the other hand, if reaction is very fast compared to diffusion, i.e., the diffuso-reactive length is small compared to the size of the particle, reaction occurs close to the interface; in this case, it is the surface area, rather than the volume, that determines the magnitude of the reactive uptake (e.g., see Fried et al., 1994).

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