A NEW ELECTROMOBILITY SPECTROMETER FOR THE MEASUREMENT OF AEROSOL SIZE DISTRIBUTIONS IN THE SIZE RANGE FROM 1 TO 1000 NM

The Electromobility Spectrometer is an automated measurement system for the size analysis of fine and ultrafine aerosols using Differential Mobility Analysers (DMA) for the classification of particles and an electrical sensor for their detection. To cover a particle size range from 1 to 1000 nm, new DMAs with different geometries have been designed to optimize performance for small and large particles. Key problems in the size characterization of particles smaller than 20 nm have been addressed. A Faraday cup electrometer with a sensitivity of 10(-16) A is used as a particle sensor to avoid the deficiency of condensation nucleus counters with counting efficiencies decreasing with particle size. A new flow control technique allows the generation of stable air flows to further improve system performance. Additional improvements include the use of a multi-stage cascade impactor at the aerosol inlet, an instrument background correction mechanism and a refined data reduction algorithm. The computer controlled measurement program allows for variable size resolution, parallel operation of two DMAs and a time resolution for the measurement of size distributions of the order of 1 min.