The sampling head for the unit is shown
in Fig. 1. The air enters the unit through an
impactor stage designed to have a 50%
collection efficiency at 10 /Lm equivalent
aerodynamic diameter. It then is drawn
through a stacked-filter unit (SFU). The
SFU consists of.. a holder for two sequential
filters construc'ted by the Norwegian Institute
for Air Research (NILU). The initial
filter is an 8 /Lm pore 47 mm Nuclepore@
filter and the second filter is an 0.4 /Lm
pore Nuclepore@ filter. At a flow rate of 16
lpm, the unit should act as a dichotomous
sampler. The flow through the 8 /Lm pores
will result in collection of ~ 2.2 /Lm particles
with 50% efficiency (Cahill et aI., 1979).
The < 2.2 /Lm particules are then collected
on the 0.4 /Lm filter (Cahill et aI., 1979).
The complete unit design is shown in Fig. 2.