IN-SITU OPTICAL-PARTICLE COUNTER WITH IMPROVED COINCIDENCE ERROR-CORRECTION FOR NUMBER CONCENTRATIONS UP TO 10(7) PARTICLES CM(-3)

An optical particle counter of the type first described by Umhauer (19 83) was modified for the purpose of in situ measurements at high conce ntrations to achieve very low coincidence errors. Therefore, the optic ally defined measuring volume was reduced to 36 x 52 x 35 mu m(3) and additionally a digital signal processing (DSP) as described by Sachweh (1991) was applied to measure the fraction of coincident signals for different aerosols (quartz, Monospheres and glycerin) and number conce ntrations up to 2 x 10(6) particles cm(-3). Thus, errors from coincide nce events in particle size distribution and number concentration coul d be corrected successfully. The fraction of coincident signals could directly be measured by the DSP. The results were found to be in good agreement with the theoretical model derived by Raasch and Umhauer (19 84). From these experiments the upper concentration limit could be ass essed to be approximately 10(7) particles cm(-3). A lower 50% size det ection limit of 0.2 mu m was also determined for PSL spheres in compar ison with a condensation particle counter. Performance comparisons are also reported for a commercial HC15 counter (Polytec) based on the sa me operating principle but ''normal'' sensing volume of 110 x 220 x 22 0 mu m(3). (C) 1998 Elsevier Science Ltd. All rights reserved.