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GRIMM Differential Mobility Analyser


The DMA differentiates particles by their electrical mobility and is based on the principle of electrostatic classification. Aerosol particles pass between electrodes in the DMA and are subject to an electric field. These particles will all have different electrical mobilities and this allows the DMA to separate the particles. Aerosol particles within a certain size range are removed and passed on to the CPC. Since the voltage size will determine the size range of particles selected, the DMA may be used to produce a monodisperse aerosol of specific size.



Sheath Air

Clean air is supplied from the CPC and enters the top of the DMA via a yellow tube. This sheath air provides a laminar flow close around the high voltage rod in the centre of the DMA. The excess air later exits the DMA via a black tube and is recirculated.

Impactor Nozzle

The sample enters the device and meets an impactor. This impactor is designed to remove larger particles at the inlet before they enter the DMA cylinder. Different sizes of impactor are available so that different sizes of cut-off may be used.


The neutraliser is a radioactive source that provides a known (Boltzmann) distribution of charge to the incoming aerosol particles. There are a number of different sources that may be used as neutralisers, the most common of which is Am-241. This is an alpha-emitter. It is a bipolar charger that usually consists of gold-coloured foil strips, permanently integrated into a steel housing. The intensity of the source is 3.7 Bq (1 Bq is one disintegration per second), which is ~100 µCi (1 Ci is approx. 3.1×1010 Bq). A licence is required for its posession in the UK.

The aerosol particles are ionised after passing over the alpha-source. The charge of the aerosol leaving the neutraliser will be described by a Boltzmann distribution; however, to be sure of this, a value of N>~1×107 is required.

-NB- It is generally advisable to obtain a licence for as high an intensity of radiation as possible, as the intensity value is cumulative when dealing with multiple sources, for the purposes of the licence.

DMA Cylinder

The sample air then enters the top of the DMA cylinder.

The DMA consists of two concentric cylinders. These are the electrodes and are connected to a high voltage power supply. The electrodes are charged positively and negatively, so the charged aerosol particles are attracted to them (neutral particles will pass straight through and exit as sheath air). The differing mobilities of the (appropriately charged) particles will determine how quickly they reach the central rod. Only particles of a specific size and charge will travel to the rod and escape via a slot to enter the CPC.

The voltage determines the size of the particles selected, and this may be varied in fixed steps of specific sizes. A scan will involve a 10 second pause after each voltage change to ensure that the DMA is empty before the next readings are taken. It is important to note that the DMA differentiates particles according to mobility and not size. Also, not all particles of the relevant size will be appropriately charged to exit via the central rod.


  • The yellow tube is for the sheath air inlet at the top of the DMA.
  • The black tube is for the excess air outlet. This eventually goes back into the inlet.
  • The grey tube is the electrical connection.
  • The red wire is the high voltage supply.

Attach all of the above to their appropriate points on the DMA.

Remove the nozzle of the impactor to check it is the correct device for the DMA in use, and check that it is clean.


The instrument will NOT work if the impactor is dirty/blocked, the flow rate is wrong etc.

The charges on aerosol particles will vary. The charge distributions are modelled in the software, and attempts are made to control the charge distribution using the neutraliser. Nevertheless, particles of opposite charge will go to a different electrode - so only those particles of the relevant size AND CHARGE will exit via the central rod. Neutral particles travel straight through the DMA and exit as sheath air. Also, particles may not necessarily have the same size of charge, and this will affect their mobility, even if they are of the correct size to be selected.

There is an ambient humidity operating limit of 95% not condensing - otherwise there may be spark discharge within the high voltage DMA.

The sampling relative humidity may also be ~95% but this may be dependent on the specific gas mixture samples.

The long classifier (DMA) is designed for the particle range 10 - 1100 nm (considered particularly suitable for environmental purposes).

The SMPS+C is not designed to be operated when the ambient temperature is greater than 30 C. There is a separate adapter for high temperature use but this requires more energy.


DO make sure all connections are tight.

DO attach the DMA first, then switch on the CPC. If attachments are made with the CPC turned on, it won't recognise the connection to the DMA until the Windows software provides a reboot instruction.

DO remove the nozzle of the impactor with the nozzle tool (situated above the impactor on the DMA) before use to check it is clean and that it is the correct device for the DMA. If the valve is blocked, it should be cleaned by having clean air blown through it, or by ultrasonic methods.

DO lightly grease the impactor before use with a thin layer of the lubricant provided to ensure the larger particles adhere to it.

DO handle the electrodes of the DMA with considerable care. They are very precisely and accurately built, and must not be dented or distorted as this will in turn distort the electric field produced within the DMA

DO remember that the DMA classifies according to mobility (which is proportional to particle size AND charge) and not simply to the size of the particles.

DON'T forget that the charges on aerosol particles will vary. The charge distribution is modelled but this model may not exactly reflect the actual charge distribution. This will affect the final reading and must be taken into consideration.

DON'T forget to check the instrument regularly to ensure it is clean and working correctly. Go through the points outlined in the Care and Maintenance section regularly to keep the instrument in good condition and ensure accurate readings are taken.

DON'T assume the instrument is always working correctly. When the CPC is in Service Mode, errors will not be reported and the instrument will continue to work even if conditions there is a fault in the system. If in doubt, check the instrument is functioning correctly by taking a reading without applying a voltage. When zero voltage is applied to the electrodes, any readings should give a zero value for the sample aerosol.


To ensure the instrument remains in good condition and provides accurate results, take careful note of the following points:

Regularly remove the nozzle of the impactor to check it is clean and that it is the correct device for the DMA in use. If the valve is blocked, it should be cleaned by having clean air blown through it, or by ultrasonic methods.

The impactor should be lightly greased before use, with a thin layer of the lubricant provided, to ensure the larger particles adhere to it.

The electrodes will require regular cleaning to ensure accurate readings from the CPC. This should be done every 3-6 months, or the distributions outputs will be inaccurate. The electrodes should be wiped down with a soft, clean tissue/cloth, and have clean air blown through between them.

Remember that the electrodes of the DMA are very precisely and accurately built. Thus they need to be handled with considerable care in order to ensure that they are not dented or distorted, as this will in turn distort the electric field produced within the DMA.

Earth Observation Data Group, Department of Physics, University of Oxford. Page last updated: @10:10 GMT 11-Nov-2022