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Centre for Atmospheric Science

Impactors and Filters


Impactors, virtual impactors and cyclones are devices which separate particles based on inertia and in general operate to remove particles of a given size range from the sample flow, either collecting them for later analysis or removing them to avoid problems of instrument contamination etc.

The basic principle of an impactor is illustrated in figure 1. Essentially air containing particles is accelerated through an orifice or nozzle towards a plate some distance below the orifice, which causes the airstream to change direction abruptly. Particles which are small enough follow the stream lines remain suspended, while those that cannot hit or impact on the plate. The cut off point of an impactor is the size at particles are collected with 50% efficiency. Collection efficiency increases for particles larger than the cut off and decreases for smaller particles. The cut off diameter for a round jet impactor is dependant on the orifice diameter and flow rate according to the following equation (Baron and Willeke):

Impactor Equation

Similar relationships exist for other impactor designs. Impactor design also influences the sharpness off the cut off (ie how quickly collection efficiency changes from 0-100% as a function of particle size). In a virtual impactor the collection plate is replaced with an orifice through which a small secondary flow is taken. Rather than being impacted onto a surface particles larger than the cut off enter this secondary flow while the majority of smaller particles continue with the primary flow. A cyclone also works on the principle of inertial separation and works by setting up a vortex like flow, where large particles which cannot follow the streamlines exit the flow and are deposited on the walls of the cyclone.


Cascade Impactors

Cascade impactors are generally used to collect particles of different size ranges on different substrates for chemical or gravimetric analysis. A cascade impactor is simply a series of impactors with decreasing cut off diameters, so the substrate placed at each stage collects only particles between the cut off diameter of the current stage on the one immediately up stream.


Particle Bounce

One of the main problems in many types of impactor is particle bounce, this is where particles larger than the cut off diameter hit the impaction surface and then bounce off and re-enter the airstream. This can be a particular problem in cascade impactors where particles which have bounced off earlier stages can be collected on later stages potentially contaminating the analysis. There are a number of techniques used to reduce or eliminate particle bounce which include using sticky substrates (only suitable for certain types of analysis); sampling at high humidity (only helps with particles which deliquesce); using a virtual impactor; and using porous substrates.



Filter systems draw air through pores in the collection substrate collecting particles which are unable to pass through the pores either because they are larger than the pore size or because the impact onto the surface of the filter. Particles collected on filters may be analysed using many of the same techniques as for impactors. Filter systems do not suffer from problems of particle bounce, but it is not possible to obtain the same degree of size segregation as it is with a cascade impactor.


Application of impactors and filters in CAS

Impactor systems have been used by CAS for a number of years, typically to provide chemical composition information which cannot be obtained from online instrumentation, and to complement data from instruments such as the AMS. We currently operate two types of cascade impactor: the Andersen high volume impactor (now manufactured by Thermo) and the Compact Cascade Impactor (CCI) (designed and manufactured at Harvard).

The Andersen impactor is a six stage multiple slot impactor which operates at a flow rates of up to 1000l/min, so collecting large quantities of material in a relatively short space of time. This makes it ideal for use with analysis techniques which require large quantities of material, or where high time resolution information is needed. Cut off sizes for this impactor are typically, and it can operate with cellulose, glass fibre, quartz, or aluminium substrates. We often use pre-baked quartz substrates with this impactor for detailed organic analysis. Samples from this system are also suitable for analysis for inorganics using ion chromatography.

The CCI is a 5 or 6 stage (depending on model) slot impactor which has been specially designed to operate with foam substrates to eliminate particle bounce. This impactor operates with a flow rate of 30l/min and has cut off sizes of . Typically this is used to collect samples over several days which are then analysed for inorganics, including sea salt using ion chromatography. This impactor can also be deployed on the BAe146 aircraft.

Filter samples are collected on many projects, both ground based and airborne. Typically two stage filter packs with nucleopore filters have been used to collect sub and super micron particles for analysis by electron microscopy, this technique is particularly used to identify desert dust and other insoluble material. Samples collected on PTFE and Quartz filters have also been analysed for metals and organics respectively. In the analysis of impactor and filter samples CAS makes extensive use of the chemical analysis facilities available elsewhere in SEAES which include Ion Chromatography, Environmental Scanning Electron Microscopy.

All impactor and filter samples require careful handling and storage to avoid contamination and degredation of samples. Additionally filter holders and impactor components require careful cleaning between samples. This coupled with analysis makes impactor and filter techniques very labour intensive processes.