Developed by Droplet Measurement Technologies, the CFSTGC is based on a concept by Roberts and Nenes [2005]. The instrument counts the fraction of aerosol particles that become droplets when exposed to a given water vapor supersaturation (RH > 100%).

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.

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.

ATHOS uses laser-induced fluorescence (LIF) to measure OH and HO2 simultaneously. OH is both excited and detected with the A2Σ+ (v’=0) → X2π (v”=0) transition near 308 nm. HO2 is reacted with reagent NO to form OH and is then detected with LIF. The laser is tuned on and off the OH wavelength to determine the fluorescence and background signals. ATHOS can detect OH and HO2 in clear air and light clouds from Earth's surface to the lower stratosphere. The ambient air is slowed from the aircraft speed of 240 m/s to 8-40 m/s in an aerodynamic nacelle. It is then pulled by a vacuum pump through a small inlet, up a sampling tube, and into two low-pressure detection cells - the first for OH and the second for HO2. Detection occurs in each cell at the intersection of the airflow, the laser beam, and the detector field-of-view.