Collection characteristics of low-pressure impactors with various impaction substrate materials

The effects of types of low-pressure impactors (DLPI; Dekati; referred to hereinafter as “LPI”; and NanoMOUDI-II; MSP) and impaction substrate materials (aluminum foils, quartz fiber filters, and polycarbonate filters) on mass size distribution were studied for diesel exhaust particles from dynamometer test (DEP) and roadside atmospheric particles for a better understanding in terms of chemical analysis of impactor sample. Particle mass size distribution measured with LPI using aluminum foils and polycarbonate filters was distorted toward smaller mode diameters than those measured with NanoMOUDI using aluminum foils in sampling DEP and roadside atmospheric particles. This difference is explained by the occurrence of particle blow off in LPI. Heavy loading due to high concentrations in a specific size range results in greater particle loading height, from which particles are easily blown off by high jet velocities. Mass size distribution of DEP measured with LPI using a quartz fiber filter was modified to larger mode diameter than that measured with NanoMOUDI using aluminum foils. Higher particle collection efficiency than the ideal condition and smaller particles than the calibrated size, collected on upper stages, resulted in larger mode diameter, owing to the occurrence of filtration as well as impaction, and a shorter jet-to-plate distance, owing to the thickness of the filter. On the other hand, mass size distribution of roadside atmospheric particles measured with LPI using quartz fiber filters was similar to that measured with NanoMOUDI using aluminum foils. It is possible that the artifact of filtration is not important because of lower particle concentrations in the roadside atmosphere than in DEP. This experimental study of mass size distributions with various impactor types and substrate materials will enhance our understanding of the size distribution of chemical components, because impaction substrate material is restricted from methods of chemical analysis even though each substrate material has different collection characteristics.