Performance evaluation of a semivolatile aerosol dichotomous sampler (SADS) for exposure assessment: impact of design issues

Abstract

Aerosols of semivolatile organic compounds (SVOCs) pose significant health risks to workers in various occupational settings. Measuring human exposure to these aerosols requires a separate assessment of the contribution of particles and gases, which is not resolved by existing sampling techniques. Here, we investigate experimentally the performance of a semivolatile aerosol dichotomous sampler (SADS), proposed in previous studies, for sampling monodisperse liquid particles with aerodynamic diameters between 0.15 and 4.5 µm, corresponding to workplace aerosols. The measured sampling performances are compared to their theoretical counterparts computed by computational fluid dynamics. The effects of leakage rate, repeatability of the assembly, imprecision of the actually machined nozzle diameters, and SADS part misalignment are examined. The SADS assembly is found to be easily leaky, but consequences on sampling can be overcome when a prior leak test with a leakage rate below 4 Pa s−1 is passed. Variation of nozzle diameters in the range (−4.5 %, +3.7 %) with respect to nominal values affects marginally (< 3 %) aerosol transmission efficiency, but sampling performance is little reproducible during successive SADS assemblies (CV = 22.1 % for wall losses). Theoretically unpredicted large (40 %–46 %) wall losses are measured for particles larger than 2 µm, located mostly (80 %) on the external walls of the collection nozzle. Assembly repeatability issues and simulations of SADS parts misalignment effect by computational fluid dynamics (CFD) suggest that these undesirable particle deposits are due to the mechanical backlashes of the assembly. Thus, the current design does not guarantee a nozzle misalignment of less than 5 % of the acceleration nozzle diameter, and other important geometric parameters are not further constrained. The promising theoretical sampling performance of the SADS for SVOC aerosols larger than 1 µm thus falls short of expectations due to mechanical design issues that can be improved before possible field use.