Statistical signal analysis for optical aerosol spectrometers: Closing the gap between single particle counting and signal fluctuation analysis

Abstract

Light scattering aerosol spectrometers (also known as optical particle counters, OPCs) are widely used for aerosol quantification. The single particle counting method, which is based on light scattering, can measure the size distribution and the number concentration of the sampled aerosol. However, this method is limited to low concentrations due to coincidence error. At higher concentrations, the particle pulses overlap and cannot be counted individually. It was recently shown that the detector signal of an optical aerosol spectrometer can also be evaluated by fluctuation analysis if the concentration is significantly higher than the coincidence limit of the device. This new mode of operation cannot yet provide a detailed size distribution but itis feasible to measure the median particle size and number concentration independently. The measurement information required for fluctuation analysis is drawn from the intensity distribution of the detector signal instead of individual pulses. Therefore, fluctuation analysis requires a certain average number of particles inside the measuring volume so that the detector output continuously leaves the baseline. Theminimum number concentration of the fluctuation analysis is around a factor of20 higher than the coincidence limit for single particle counting. Consequently, there is a concentration range where neither single particle counting, nor fluctuation analysis can be used.

This work introduces a new statistical signal analysis to bridge this gap. The new measurement method was experimentally verified using a monodisperse di-ethyl-hexyl-sebacat aerosol with a particle size range of 0.3 µm to 2.2 µm and a number concentration range of 1 $\times$ 10⁴ cm⁻³ to 2 $\times$ 10⁵ cm⁻³. An accuracy of 2 % with respect to median particle size and 5 % with respect to number concentration was achieved. The new method finally closes the gap between single particle counting and fluctuation analysis, enabling light scattering aerosol spectrometers to quantify aerosols at any given concentration.