Apims

Baratron

Gas Manifold

Reactants

FIGURE 5.27 Top view of aerosol apparatus (De Haan et al., 1999).

through an atmosphere of air containing the gas of interest, which is taken up by the liquid stream. The liquid is collected and analyzed to obtain the gas uptake. The reaction time, typically 0.1—1 ms, is varied by varying the length of the liquid jet exposed to the gas prior to collection. This type of experiment is related to the falling-droplet approach except that it utilizes the first millimeter of flow before the breakup into droplets. While the experimental approach is again relatively simple, the analysis is quite complex in that the time-

dependent modeling must be carried out which incorporates the gas- and liquid-phase diffusion processes and chemical reactions in the liquid phase as well as the uptake of the gas at the interface. The value of the mass accommodation coefficient giving the best fit to the data is then obtained.

Another promising approach is the use of thin liquid jets a few micrometers in diameter combined with vacuum techniques that allow the application of surface science techniques such as photoelectron spec-

FIGURE 5.28 Schematic diagram of the liquid jet apparatus (adapted from Bongartz et al., 1994).

troscopy to liquid surfaces (e.g., Faubel et al., 1988, 1997). The use of high-vacuum electron spectroscopies on volatile liquids is limited by their vapor pressure, since the liquid evaporates during the measurement. In addition, the distance between the liquid surface and the spectrometer slit has to be sufficiently small that the electrons can be appropriately sampled. As a result, there are practical limits on the combination of (vapor pressure X distance) to < f Torr mm for X-ray photo-electron spectroscopy (XPS) and <0.1 Torr mm for ultraviolet photoelectron spectroscopy (UPS) (Faubel et al., 1997). A liquid jet apparatus using UPS that meets these limitations has been applied to liquid water, with the spectra suggesting orientation of surface water molecules with the hydrogen atoms pointing outward (Faubel et al., 1997). Although such techniques have not yet been applied to systems of atmospheric interest, they could potentially be quite useful in exploring such surfaces.

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