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FIGURE 9.30 Theoretically predicted and experimentally measured concentrations of H2S04 required for homogeneous nucleation of sulfuric acid at a rate of 1 particle cm 3 s 1 (adapted from Hoppel et al., 1994; based on theoretical calculations of Jaecker-Voirol and Mirabel (1989) and experimental data ol Wyslouzil et al. (1991).

was consistent with homogeneous nucleation when the air was contaminated with S02 from upwind smelters; however, it was much greater than predicted theoretically for clean marine air.)

As discussed by Hoppel et al. (1994), it is likely that under atmospheric conditions, nucleation involves condensation onto preexisting clusters of molecules, or "prenucleation embryos," which are so small in size and number that they have not been measured. Ionic clusters, or neutral clusters formed from the recombination of positively and negatively charged clusters, could serve as embryos for nucleation of sulfuric acid and other species. For example, Turco et al. (1998) propose that recombination of positive ion clusters such as H30 (H20)(, with negative ion clusters such as HS04 (H2S04)m(H20)£/ (which are known from mass spectrometric measurements to be ubiquitous in the atmosphere; see Chapter 11.A.2) may lead to stable large neutral cluster embryos. Hoppel and co-workers predict, for example, that a cluster of radius f nm could act as a nucleus for the condensation of sulfuric acid and water at 60% RH and an H2S04 concentration of only 1 ppt, far below the concentrations shown in Fig. 9.30 for homogeneous nucleation.

The development of techniques to measure particles down to 3 nm in diameter (see Chapter 11.B) has provided some new insights into nucleation in the atmosphere. For example, McMurry, Eisele, and coworkers have measured simultaneously ultrafine particles and gaseous H2S04 in a number of locations (e.g., Weber et al., 1995, 1996, 1997, 1998; Eisele and McMurry, 1997). Formation of ultrafine particles occurs at much smaller concentrations of gaseous H2S04 than expected based on classical binary nucleation theory for sulfuric acid and water. In addition, the dependence on the concentration of gaseous H2S04 is much smaller than expected theoretically. They propose that ammonia may assist in the nucleation process. Figure 9.31 shows the H2S04 vapor pressure above the pure liquid and above a 1:1 mixture of NH3 and H2S04 (Marti et al., 1997a; Eisele and McMurry, 1997). The vapor pressure of H2S04 is reduced by two to three orders of magnitude when NH3 is present at equimolar levels. This large reduction in vapor pressure suggests that the reaction of NH3, which is ubiquitous in the troposphere (see Chapter ff.A.4a), with H2S04 on a molecular level may play a key role in the high observed rates of nucleation. This is consistent with the observation of high concentrations of the smallest measurable ultrafine particles downwind of a large penguin colony, which would be expected to be a significant source of NH3 (Weber et al., 1998). Interestingly, the growth rate of ultrafine particles is often an order of magnitude

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FIGURE 9.31 Vapor pressure of H2S04 as a function of relative humidity for pure H2S04 (•) and a 1:1 mixture of NH3 and H2S04 ( ) (adapted from Marti et al., 1997a; and Eisele and McMurry, 1997).

larger than can be explained based solely on the uptake of gaseous H2S04 by the particles, suggesting that other species such as organics contribute to the growth of ultrafine particles once nucleation to form new particles has occurred (e.g., see Marti et al., 1997b; and Weber et al., 1997). This is consistent with observations that organics can contribute up to 80% of the concentrations of condensation nuclei (CN) under some conditions (e.g., Rivera-Carpio et al., 1996). The uptake of inorganics such as HN03 and HC1 combined with NH3 has also been proposed to contribute to particle growth in some cases (e.g., Kerminen et al., 1997).

Similar observations have been made in the marine boundary layer where biogenically emitted sulfur compounds such as dimethyl sulfide (DMS) may serve as the source of gaseous H2S04 and other low-volatility species, e.g., methanesulfonic acid, MSA (see Chapter 8.E). For example, Clarke et al. (1998) report measurements of ultrafine particle formation, DMS, S02, and gaseous H2S04 in the tropical marine boundary layer that are consistent with the oxidation of DMS to S02 and then H2S04, followed by nucleation. As in many other studies, the nucleation rate of H2S04 was much larger than expected from classical nucleation theory. Interestingly, Weber et al. (1995) conclude from measurements of H2S04, MSA, and ultrafine particles at the Mauna Loa Observatory that H2S04 was the major precursor to ultrafine particles and that the contribution of MSA was small.

For a review of nucleation in the atmosphere, the reader is referred to Nucleation and Atmospheric Aerosols (Fukuta and Wagner, 1992; Kulmala and Wagner, 1996) and Microphysics of Clouds and Precipitation (Pruppacher and Klett, 1997).

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