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Alternative Working Fluids for Condensation Particle Counters: Towards Material-Independent Aerosol Particle Number Measurement
Commercial condensation particle counters (CPCs) used in measuring nanometer sized particles like exhaust particulate matter (PM) work by condensing a working fluid (WF), commonly n-butanol, on particles and detecting the emerging droplets optically. The response of butanol CPCs typically strongly depends on the aerosol particle material. This particle dependence makes a reliable measurement of particles impossible if no prior knowledge of the tested aerosol is available. Furthermore, when sampling an aerosol with a broad range of different particle species, like in the case of measuring non-exhaust PM, an unequal activation of the particles inevitably prohibits accurate particle number measurements. To overcome the problem of particle dependent activation in CPCs, numerical simulations of the condensation process in a CPC based on Fletchers heterogeneous nucleation theory were performed to identify alternative working fluids. Three WF candidates, namely n-decane and two engineered high-tech fluids (HTF1 and HTF2), were identified and then tested in a CPC on a set of five different aerosol materials with particle sizes ranging from 15 nm to 200 nm. The aerosols tested are: Emery Oil, Tungsten Oxide (WOx), spark discharge generated Carbon, combustion generated Carbon soot and Sodium Chloride (NaCl). Using n-decane reduced the particle dependence by approximately 70% when testing insoluble particles (WOx, NaCl and Carbon). While HTF1 was unsuitable for the efficient utilization in the used CPC setup, HTF2 was able to reduce the particle dependence by almost 90% for all tested particles as compared to a standard butanol CPC.