  Small ions are part of the atmospheric aerosol spectrum, and study of
ion-aerosol interactions is fundamental in atmospheric physics. Air ion physics
and instrumentation are reviewed, including the historical context. A
miniaturised Gerdien condenser for ion measurement, operating in situ to
minimise inlet errors, is described. Two operating modes using independent
current and voltage decay measurements are employed. A more sophisticated
self-calibrating and fully programmable ion mobility spectrometer (PIMS) based
on the same principles, is also discussed. Detailed analysis of error terms and
application of new technology is demonstrated to greatly improve its
capability. Self-consistent experimental approaches, including ionisation and
ion concentration instruments, are used for calibration. In developing and
characterising the individual components of the PIMS, favourable and
unfavourable operating regimes are identified: this approach can also be
applied to other aspiration ion counting techniques. Use of a sophisticated
programmable electrometer permits compensation for leakage terms.
Electrically-charged aerosol particles have been found to complicate the ion
measurements. Consequently, conventional ion-aerosol theory, which neglects the
particulate concentration, is thought to be incomplete. The polymodal ion
mobility spectrum is also found to influence the instrument's operation.
Development of the novel programmable ion instrumentation, in conjunction with
consideration of the ion mobility spectrum yields an improved and flexible
approach to in situ atmospheric ion measurements.
