The appts. uses an electret membrane (1) to provide an external electric field to the instrument and a central electrode sensor (12) with a system of parallel electrodes (13) measures space charge. The parallel electrodes have an air passage through which is drawn air by an aspiration system. Changes on the electrodes of the electric field voltage (20,21) and current (14,15,16,17) are measured. The current required to restore the original field provides a means for measuring the space charge. A charge control system includes an ion source (22) and a mode controlling cylindrical electrode (25). Two electrodes (34,36) sense the charge carried by large atmospheric particles. A system (38) controlled by a microprocessor (40) varies the humidity and an optical emitter (39) and receiver (41) detects the particles provided. A capture system is also provided having an adiabatic holder (46), a humidifier (47,48) and a particle impacter (50). Particles are then output from a funnel (51) onto a rotary disk (53) where they are analysed.
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