|Continuous-Flow Streamwise Thermal Gradient
Cloud Condensation Nuclei (CCN) Counter (CFSTGC)
Last Updated: October 9, 2004
|Developed by Droplet Measurement
Technologies, the CFSTGC is based on a concept by
Roberts and Nenes . The instrument counts the
fraction of aerosol particles that become droplets when
exposed to a given water vapor supersaturation (RH >
|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
|With laminar flow, heat and water vapor are transferred to
the centerline of the column from the walls only by
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
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
Click here to access
a study on the DMT instrument design
1) Roberts, G., and A. Nenes (2005). "A Continuous-Flow Streamwise
Thermal-Gradient CCN Chamber for Atmospheric Measurements", Aeros.
Sci. Tech., 39, 206−221.