An electrostatic instrument for measuring particle concentrations and possibly sizes in aerosols, such as an Electrostatic Low Pressure Impactor or Differential Mobility Analyser suffers from errors which limit the useful response bandwidth of the device. The invention minimises or eliminates these transient errors which are caused by changing particle concentrations in the aerosol. A system may be added to an otherwise conventional instrument to compensate for the transient effects based on a model of the charge production mechanism. Alternatively, a screening electrode placed over the sense electrodes in the instrument, and held at controlled electrical potential difference, is added to the instrument to eliminate the effect. A third embodiment adds compensating electrodes which provide a direct measurement of the transient effect which can be subtracted from the signal.