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RESEARCH PRODUCT

RAPID CONDENSATIONAL GROWTH OF PARTICLES IN THE INLET OF PARTICLE SIZING INSTRUMENTS

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Abstract Rapid particle growth by the condensation of water vapour resulting from expansion in the inlet of particle sizing instruments such as optical particle counters and impactors was modelled. The corrected Mason diffusion growth equation extended to the application for particles beyond the continuum region has been found suitable. The influence of particle acceleration in the nozzle air flow, modelled with the CFX-FLOW3D program, on the condensation process was considered. The study was focused on a typical inlet nozzle of an optical particle counter with an inner diameter of 0.5 mm and a length of 20 mm, connected by an additional 20 mm long conical nozzle at a flow rate of 28.5 ml s −1 . The results show that particles smaller than 1.0 μ m can grow quickly during the very short-time passage through the nozzle as the saturation in the nozzle flow can increase several times. For particles with an initial radius of 0.15 μ m, i.e. the typical minimum detectable size of many commercial optical particle sizing instruments, condensational growth will cause a size increase up to 7% in the viewing volume. The whole particle size distribution will shift to larger radii, which results in an overestimate of both particle size and number in measurements. This condensational over-numbering and sizing effect should be given concern in evaluating measurement results as well as in aerosol instrument design.