Acondensation particle counter orCPC is aparticle counter that detects and countsaerosol particles by first enlarging them by using the particles as nucleation centers to create droplets in asupersaturated gas.[2]
Three techniques have been used to producenucleation:
The most usually used (also the most efficient) method is cooling bythermal diffusion. Most abundantly used working fluid isn-butanol; during last yearswater is also encountered in this use.[4]
Condensation particle counters are able to detect particles with dimensions from 2nm and larger. This is of special importance because particles sized down from 50 nm are generally undetectable with conventionaloptical techniques. Usually thesupersaturation is ca. 100…200 % in condensation chamber, despite the fact thatheterogeneous nucleation (droplet growth on surface of a suspended solid particle) can occur at supersaturation as small as 1%. The greater vapour content is needed because, according tosurface science laws, thevapour pressure over aconvex surface is less than over a plane, thus greater content of vapor in air is required to meetactual supersaturation criteria. This amount grows (vapor pressure decreases) along with decrease in particle size, the critical diameter for which condensation can occur at the present saturation level is calledKelvin diameter. The supersaturation level must, however, be small enough to preventhomogeneous nucleation (when liquid molecules collide so often that they formclusters – stable enough to ensure further growth is possible), which will produce false counts. This usually starts at ca. 300% supersaturation.[4]
On the right, a diffusional thermal cooling CPC is shown in operation. In order to ensure a high vapour content, the working liquid is in contact with a hollow block of porous material that is heated. Then the humified air enters thecooler where nucleation occur. Temperature difference between the heater and the cooler determines the supersaturation, which in its turn determines the minimal size of particles that will be detected (the greater the difference, the smaller particles get counted). As proper nucleation conditions occur in the center of the flow, sometimes incoming flow is divided: most of it undergoes filtering and forms the sheath flow, which the rest of flow, still containing particles, is inserted into via acapillary. The more uniform is obtained supersaturation, the sharper is particle minimal size cutoff. During the heterogeneous nucleation process in the nucleation chamber, particles grow up to 10…12 μm large and so are conveniently detected by usual techniques, such as laser nephelometry (measurement of light pulses scattered by the grown-up particles).[4]
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