Cloud Condensation Nuclei (CCN) Counter (CFSTGC)
CURRENT RESEARCH
Last Updated: October 9, 2004
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Developed by Droplet Measurement Technologies, the CFSTGC is based on a concept by Roberts and Nenes [2004]. The instrument counts the fraction of aerosol particles that become droplets when exposed to a given water vapor supersaturation (RH > 100%). |
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Schematic of the CCN Counter: The ambient inlet is split into aerosol and sheath flow. The sheath flow is filtered, humidified and heated. The two flows meet at the top of the CCN column. The CCN column is where supersaturation is generated and particles grow to become droplets, which are then large enough to be detected by an Optical Particle Counter (OPC). The inner walls of the CCN column are maintained moist. The aerosol flow is directed through the centerline of the column, and is surrounded by an annular flow of particle free sheath air. |
As with all CCN counters, a temperature gradient is applied to produce a supersaturation of water vapor. However, the mechanism for generating supersaturation is not the same for all CCN counters. For example, for continuous flow parallel plate diffusion chambers, the temperature gradient is perpendicular to the flow, and supersaturation is a result of the nonlinear dependence of vapor pressure upon temperature. The same mechanism applies for static diffusion cloud chambers, where there is no flow at all.
However, as the name implies, for the Continuous Flow Streamwise Thermal Gradient CCN Counter, the temperature gradient is in the streamwise direction (maintained by thermoelectric coolers). In this case, supersaturation results as a consequence of the greater rate of mass transfer over heat transfer, as explained below.
Thermal Diffusivity < Molecular Diffusivity of Water Vapor
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With laminar flow, heat and water vapor are transferred to
the centerline of the column from the walls only by
diffusion. Since molecular diffusivity is greater than thermal diffusivity, the distance downstream that a water molecule travels before reaching the centerline is less than the distance the heat travels downstream before reaching the centerline. If you pick a point at the centerline, the heat originated from a greater distance upstream than the water vapor. There are four facts that are necessary to explain how supersaturation is generated within the CFSTGC: 1) Assuming that the inner surface of the column is saturated with water vapor at all points, since the temperature is greater at point B than at point A, the water vapor partial pressure is also greater at point B than at point A. 2) The actual partial pressure of water vapor at point C is equal to the partial pressure of water vapor at point B. 3) However, since the temperature at point C is the same as at point A, the equilibrium water vapor pressure at point C is equal to the water vapor partial pressure at point A. 4) The saturation ratio is the ratio between the actual partial pressure of water vapor and the equilibrium vapor pressure. This is equivalent to the partial pressure at point B divided by the partial pressure at point A, which is always greater than one. Thus supersaturation is generated through a dynamic equilibrium. |
Click here to access a study on the DMT instrument design
Citations:
1) Roberts, G., and A. Nenes (2005). "A Continuous-Flow Streamwise Thermal-Gradient CCN Chamber for Atmospheric Measurements", Aeros. Sci. Tech., 39, 206−221.