| Condensation nuclei in the atmosphere are known to be an important factor in the development of clouds, the occurrence of rainfall, and the formation of particulate air pollutants that can cause undesirable effects on man and his environment. Condensation nuclei are invisible and numerous, and their number concentration has become the characteristic of interest and has been widely studied since the development of the first condensation nuclei counter by Aitken in 1888.;According to Kyle, the number concentration of particles in the sensitive volume of an instrument can be determined by knowing the distribution of signal output from the instrument. He assumed that each signal comes from a different random sample taken from a volume in which particles are distributed according to Poisson statistics and that each particle contributes equally to the signal. A General Electric Condensation Nuclei Counter has been adapted to test the feasibility of Kyle's theory under laboratory conditions. As a result, the theory has been modified and a correction factor is required for concentration measurement.;Based on the modified theory, a new condensation nuclei counter was designed to have the following advantages over the widely-used counters: (a) It provides an absolute concentration measurement. (b) Even a small random fluctuation of nuclei concentration can immediately be detected.;A conventional nuclei counter employs the so-called condensation technique which enables the minute nuclei to grow, in a supersaturated environment, to (mu)m-sized droplets; the number concentration of the visible droplets is then measured. Since each nucleus grows to a droplet, the number concentration of droplets and nuclei remains the same. The number of droplets is measured by (1) direct observation with a microscope (direct counter), (2) counting from photographs of the droplets (photographic counter), (3) suitably calibrated light transmission (or scattering) measurement (relative photoelectric counter). Most of the widely-used counters are relative counters in which the instrument reading must be calibrated against a direct or photographic counter. |
