Influence of the dynamic behaviour of impactor surfaces on particulate matter emission measurements with electrical low pressure impactors

Abstract

Cascade impactors, such as the Dekati electrical low pressure impactor (ELPI+), are useful measuring devices as they can classify particles per size range based on their inertial behaviour. However, various mechanisms, such as particle loss and particle bounce, make the measured particle size distribution to diverge from the real distribution. Moreover, some of these mechanisms depend on the amount of particles that have already been collected in an impactor stage and consequently the collection efficiency of a stage may be time-dependent. In this work, such mechanisms are investigated and their impact on the performed measurements is assessed. The use of greased and ungreased aluminium collection substrates and porous sintered impactor plates were extensively tested in the ELPI+. The aluminium substrates, regardless of the initial greasing, show an exponential decrease in total number concentration and an exponential increase in geometric mean particle diameter with increasing particle load on the substrates. This is the result of an exponential concentration decrease of the stages that collect particles smaller than the accumulation peak and a concentration increase of the stages that collect larger particles. The effect is attributed to the decrease in particle bounce with increasing particle loading. On the other hand, the sintered impactor plates show barely any time-dependent collection behaviour.