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Differential Mobility Analyser


HOW IT WORKS

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.

Neutraliser

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 3.7Bq, which is ~100uCi (1Ci is approx. 3.1e10 Bq; 1Bq is one disintegration per second), and 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>~1e7 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, and this issue will be discussed further later (See Instrument Limitations).

Earth Observation Data Group, Department of Physics, University of Oxford. Page last updated: @21:11 GMT 13-Apr-2014