MEASUREMENT OF SIZE, NUMBER CONCENTRATION AND VELOCITY OF AEROSOL-PARTICLES USING AN OPTICAL-PARTICLE COUNTER

An aerosol measurement instrument is presented which allows for the si multaneous measurement of the size distribution, number concentration and velocities of particles. A commercial optical particle counter (OP C) was modified in terms of optics and signal evaluation to provide th e required measurement information. The design of this instrument allo ws the definition of a cubic measuring volume by purely optical means. This is achieved by an aperture/lens system which projects a sharply defined light beam into a stream of aerosol flow. Light scattered from single particles at average angles of 90-degrees is collected by two opposite receiver units, each projecting light on to a separate photom ultiplier. The intensity of the scattered light with this instrument i s found to be an unambiguous function of the particle size. The total number of particles detected per unit time results in the particle flu x. The particle velocity can be calculated, in principle, through the correlation of the signal length and the optical length of the measuri ng volume, provided that the particles have a straight trajectory thro ugh the measuring volume and the measuring volume length in the mean f low direct ion is we de med. The absence of sharpness in real optical projections effects a border zone of definite length, in which the ill umination declines to zero. This leads, together with the low-pass fil tering of the particle signals, to an increase in the length of the si gnal slopes, causing some difficulties in the determination of the sig nal length. A digital signal evaluation technique was developed that r enders possible the clear differentiation between the slope and the ke rnel region of the signal. The latter represents the motion of particl es through the completely illuminated region, which can be a more accu rate parameter to define the signal length. In addition to the signal length determination, a cross-correlation technique was tested for its potential to obtain particle velocity. The instrument has two interla ced measuring volumes of nearly the same size, which are shifted for t his special application in the main flow direction by 20 mum. The phas e difference between the signals from the two photomultipliers, togeth er with the optical distance, yields the particle velocity.