An experimental approach to correcting counting errors in the aerodynamic particle sizer (APS model 3310)

This paper describes the different ways of analyzing the output of a real-t ime device for measuring and counting airborne particles, the aerodynamic p article sizer (APS). This instrument is very widely used in aerosol researc h throughout the world. It is a time-of-flight instrument in which a partic le's measured transit time in the changing flow in a jet passing between tw o laser beams is converted to its aerodynamic diameter. As the particle pas ses between the two laser beams, two signal processors, the small particle processor (SPP) and the large particle processor (LPP), independently provi de measures of the particle's transit time from the light pulses that are p roduced. This information is related to the aerodynamic particle diameter o f 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, resultin g in 'phantom' particle generation. The SPP is known to generate phantom co unts, while the LPP is known to reduce phantom counts. A new method is desc ribed in this paper that gives guidance on how to deal with such coincidenc e problems. The principle is that it relies on additional information to ob tain 'correction factors'. In this case, well-established theory for the as piration efficiencies of thin-walled aerosol sampling probes has been used along with corresponding 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, an d the advantages and limitations of different correction methods are discus sed.