Additional Instrument Design Improvements

In the quarter century since the development of the FSCS, there have been vast technological advances that could be implemented to improve the instrument. The size of many components could be reduced; the time resolution of the measurement and the level of automation could both be increased. Most of these potential improvements are not tied to the design of the growth chamber and are therefore outside the scope of this work. However, more fundamental modifications to the inlet region and to the droplet detector would ensure that the gains achieved by modifying the geometry of the growth chamber are fully realized.

4.7.1

In the FSCS, a single flow enters the growth chamber through a wedge-shaped manifold that widens smoothly from a narrow slit [Fukuta and Saxena, 1979a]. This design successfully produces a uniform parabolic flow across the chamber, but it also introduces into the chamber a significant number of particles outside of the region of maximum supersaturation. These particles compete for the limited water vapor in the chamber, and could potentially reduce the maximum supersaturation reached along the centerline. Their presence also further complicates the role of gravitational settling in a horizontally-oriented instrument. As some growing droplets fall out of the detected streamlines, other droplets may fall into that region.

These problems can be avoided by surrounding the sample region with a particle-free sheath flow. Two well-balanced streams, filtered to remove all particles, could be introduced along the hot and cold wall in such a way that the flow is evenly distributed across the width of the chamber; these flow streams would fill nearly the entire height of the chamber. The sheath flows should be kept separate until the streamlines have stabilized, after which the aerosol sample flow can be introduced through a narrow slit along the centerline of the chamber. The sample flow should be small relative to the sheath flows, so that all particles are exposed to essentially the same supersaturation. Any required flow shaping (e.g., shifting from a rectangular geometry to a trapezoidal one) can be accomplished by gradually narrowing or widening one end of the chamber after the two sheath flows and the sample flow have merged into a single laminar, parabolic flow. The shaped, merged flow would then enter the growth chamber and activation could occur along the centerline as described earlier.

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4.7.2

For an instrument design similar to that of the FSCS to be effective, the activated droplets must be sampled on their growth streamlines; the position of the streamline within the growth chamber is the parameter that determines the supersaturation at which activation occurred. This streamline sampling can be accomplished in several ways. The original FSCS design employed a moving sampling probe at the end of the growth chamber, which pulled a portion of the flow through the chamber into an attached optical particle counter. This apparatus was successful in sampling individual streamlines, but required ~15 seconds to obtain a full CCN spectrum [Fukuta and Saxena, 1979a]. This low time resolution is often insufficient for airborne measurements, where particle concentrations can change by an order of magnitude in a period of several seconds (e.g., Chapter 3). The cycle speed of the sampling probe could be accelerated somewhat, but the presence of moving parts within the flow streams distorts the flow, and this effect would be enhanced if the probe were translated across the chamber more rapidly.

Ideally the droplet detector on an instrument based on the FSCS design would be able to obtain a CCN spectrum more rapidly (on the order of one spectrum per second), and would observe the droplets without distorting the flow through the instrument. These goals are most easily achieved by designing a detector as part of the instrument, rather than attaching the growth chamber to an external particle counter. There are two ways to accomplish this; one is to design a scanning device that would illuminate only a small portion of the flow at any given time, so that a droplet in that fraction could be observed by a single photodetector. As for the original FSCS design, the location of an observed droplet within the chamber would be determined by the time at which the observation was made. The scanning mechanism could easily be made fast enough to achieve sufficient time resolution, but would probably require either moving parts or very sensitive electronics.

Another option is available due to advances in photodetector technology. A small linear array photodetector could be incorporated into a design so that each detector in the array collects light from only a small portion of the sample flow after the flow has exited the growth chamber (Figure 4.12). Such an arrangement would only require a single light source, and all streamlines could be monitored concurrently. The time resolution would be limited only by the counting statistics required for the observed concentrations to be significant, an issue that could be addressed to some degree by modifying the sample flow rate. The design does require that light scattered from a given streamline be observed only by a single channel in the array, but this can

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be accomplished through the use of shutters and shaping optics. The amount of data requiring processing is also significantly greater if an array detector is employed since each channel of the array acts as an individual particle counter, but the analysis of the data would likely be more straightforward, since the supersaturation of the observation would not have to be calculated based on the timing of the measurement.

It is also important to note that if the geometry of the growth chamber were modified to allow longer growth times for those streamlines exposed to lower supersaturations, there would be a corresponding effect on the view volume in the detector region. For an equal sampling period, less flow would pass through the illuminated region on the low velocity, low supersaturation end of the chamber. Since fewer particles activate and grow to droplet size at lower supersaturations, there would also be fewer droplets in a given volume on that same end of the chamber. These effects when compounded might require that the sampling period be extended in cases when the

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CCN concentrations are low. However, the advantages of obtaining CCN concentrations at lower supersaturations outweigh any small reduction in time resolution arising from variations in residence times in the growth chamber.