An Experimental Approach to Correcting Counting Errors in the Aerodynamic Particle Sizer (APS Model 3310) Avula Sreenath*, Gurumurthy Ramachandran**, James H. Vincent*** (Received: 15 January 1999; resubmitted: 3 September 1999) Abstract This paper describes the different ways of analyzing the output of a real-time device for measuring and counting airborne particles, the aerodynamic particle sizer (APS). This instrument is very widely used in aerosol research throughout the world. It is a time- of-¯ight instrument in which a particle's measured transit time in the changing ¯ow in a jet passing between two laser beams is converted to its aerodynamic diameter. As the particle passes between the two laser beams, two signal processors, the small particle processor (SPP) and the large particle processor (LPP), independently provide measures of the particle's transit time from the light pulses that are produced. This information is related to the aerodynamic particle diameter of the particle (d ae ) by means of calibration against `unit' density (1000 kg=m 3 ) spheres. If more than one particle is involved in the analysis of particle transit time, then it gives rise to coincidence effects, resulting in `phantom' particle generation. The SPP is known to generate phantom counts, while the LPP is known to reduce phantom counts. A new method is described in this paper that gives guidance on how to deal with such coincidence problems. The principle is that it relies on additional information to obtain `correction factors'. In this case, well-established theory for the aspiration ef®ciencies of thin- walled aerosol sampling probes has been used along with corre- sponding experimental data obtained in a wind tunnel using the APS. Results using this method are compared with various other methods that have been tried in the past. The paper provides insights on to how the user can operate the APS to avoid counting errors like those described, and the advantages and limitations of different correction methods are discussed. 1 Introduction The aerodynamic particle sizer (APS # , TSI Inc., St. Paul, MN) is a direct-reading instrument for counting and measuring the aerodynamic diameters (d ae ) of sampled aerosol particles. It is a `time-of-¯ight' instrument that detects the velocity of particles relative to the air¯ow in an accelerating nozzle by determining the particle transit time between two laser beams. As the particle passes through the sensing zone, two light pulses are produced and detected by photodetectors. The resultant electronic pulses trigger a timer and so provide the basis of the measurement of the transit time. By calibration using polystyrene latex (PSL) spheres of known density, the transit time provides a direct measure of d ae (Wang and John [1]). The major factors in¯uencing the accuracy of the APS are particle density, particle shape factor, particle deformation and the particle counting process (Wang and John [1], Baron [2], Ananth and Wilson [3], Brockmann et al. [4], Brockmann and Rader [5], Chen et al. [6±8], Cheng et al. [9], Grif®ths et al. [10], Heitbrink et al. [11] and Heitbrink and Baron [12]). The last factor relates to a class of situations in which particle count errors arise from the way in which the particles are detected and timed, and may be both statistical and physical in nature. Despite the widespread use of this version of the APS instrument in aerosol research throughout the world, such errors have previously been addressed by only a few researchers (e.g., Heitbrink et al. [11] and Heitbrink and Baron [12]) and remain poorly understood. This paper presents a new approach to the development of cor- rection methods. The key ingredient is the application of addi- tional external information to how the raw data from the APS Model 3310 are processed and interpreted. In the case in point, that additional information is provided in the form of what is well- known theoretically about the aspiration ef®ciencies of certain simple aerosol samplers (i.e., thin-walled probes). This then provides a means by which to correct data, using the APS, obtained from experimental studies of other aerosol samplers. 2 Coincidence Effects Accurate measurement of d ae for a single particle by the APS Model 3310 requires detection of two light pulses from the same particle. The temporal difference between these two pulses of light is determined by means of two timers, the ®rst referred to as the `small particle processor' (SPP) and the second as the `large particle processor' (LPP). The SPP is speci®cally designed to count small particles (that have small transit times) and LPP to * Dr. A. Sreenath, TSI Incorporated, Shoreview, MN, 55126 (USA). ** Dr. G. Ramachandran, (to whom correspondence should be addres- sed), Assistant Professor, Division of Environmental and Occupa- tional Health, School of Public Health, University of Minnesota, Minneapolis, Box 807 Mayo, 420 Delaware Street, S.E., MN 55455 (USA). *** Dr. J. H. Vincent, Professor and Chair, Department of Environmental and Industrial Health, School of Public Health, University of Michigan, 109 S. Observatory Street, Ann Arbor, MI 48109-2029 (USA). # WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 0934-0866/99/257±265 $17.50:50=0 Part. Part. Syst. Charact. 16 (1999) 257±265 257