Relative Humidity (%)

5 10 15

Relative Humidity (%)

Heterogeneous condensation is secondary aerosol formation by the scavenging of the low-vapor-pressure products onto preexisting particles. If the concentration of particles is sufficiently high, this dominates over the formation of new nuclei via homogeneous nucleation (e.g., Friedlander, f978, 1980).

The condensation of low-volatility vapors on preexisting particles depends on a number of factors, including the rate of collisions of the gas with the surface, the probability of uptake per collision with the surface, i.e., the mass accommodation coefficient (see Chapter 5.E.I), the size of existing particles, and the difference in partial pressure of the condensing species between the air mass and the particle surface. While some of these parameters are reasonably well known, others are not. For example, mass accommodation coefficients for the complex surfaces found in the atmosphere are not well known. Indeed, the exact nature of the surfaces themselves, which determines the uptake and the partial pressures of gases at the surface, remains a research challenge.

However, despite the uncertainties in quantifying homogeneous new particle formation compared to condensation on preexisting particles, there are a variety of field data supporting the occurrence of both in the troposphere. Figure 9.32, for example, shows some particle measurements made from a ship off the coast of Washington State by Covert and co-workers (f992). Figure 9.32a shows the total particle surface area for particles with diameters >0.02 pm; this surface is needed for condensation of low-volatility vapors onto preexisting particles. It is seen that at about 1430 hours, the surface area decreased significantly, from about 22 to less than 5 ¡xm2 cm 3 of air. As the surface area fell to low values, the relative numbers of ultrafine particles, defined in this case as those with diameters

0.015 /am, increased (Fig. 9.32b). This is consistent with the formation of new particles by homogeneous condensation, since it occurred in the smallest size range and when the available surface area for condensation on existing particles was small. The subsequent growth in the number of slightly larger particles, with diameters in the 0.02- to 0.024-/j,m range (Fig. 9.32c) may then be due to heterogeneous condensation on the ultrafine particles. This causes an increase in these larger particles accompanied by a decrease in the ultrafine range, as seen in Fig. 9.32b.

Smog chamber studies have documented similar aerosol growth mechanisms. For example, in the photochemical oxidation of dimethyl sulfide, the formation and growth of particles in an initially particle-free system was observed. However, if seed particles with 34-nm mean size were present, an oscillation in the

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