Berner-type impactor

The dichotomous sampler is a virtual impactor with two emerging aerosol flows. The air beyond the virtual interface is withdrawn at a slow rate, thus rendering this impactor a two-flow or dichotomous impactor. In both flows aerosol particles can be collected on a filter or analysed in a real-time aerosol instrument. A continued suspension of the size-fractionated particles in the effluent air flows is the principal advantage of these virtual or dichotomous impactors (Chen et al., 1985). 

The dichotomous sampler operates at a flow rate of 1 cm h (16.7 Lrnin ). The coarse (PM 10 to 2.5) particle receiver tube has a flow rate of 0.1 cm h (1.67 Lmin ) for collection of the coarse aerosol particles, while the fine particles, less than 2.5 pm, follow a flow rate of 0.9 cm h (15 L min ) to the fine particle filter. 

According to the impaction theory, the dimensionless Stokes number, Stk, is the governing parameter for impaction and is defined as follows  

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Fig. 6.25. Berner-type impactor stage (partly cut away in order to show the arrangement of the stage elements) with the orifice plate, OR. The collection aluminium foil (not shown) covers the stagnation plate, SP, completely and is held in place by the spacer, S.

The electrical Itw-pressure impactor (ELPl) has been developed, using the Berner-type multijet low-pressure impactor stages. The cut sizes of the seven channel system range from 0.030 to 1.0 pm. Real-time measurements can be achieved due to the instrument s fast time response. The schematic representation of the impactor construction is shown in Fig 13.44. 

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