A fast mobility spectrometer for atmospheric ions

Abstract

The development of a mobility spectrometer which yields a complete spectrum every 25 seconds is described. The spectrometer uses a Differential Gerdien Chamber of the First Order with 8 collecting electrodes, giving a set of 8 "electrode charges" corresponding to each observed spectrum. A Least Squares iteration is used to fit the parameters of a function which describes the complete mobility spectrum. The spectrum function's parameters can be fitted reliably with standard deviations of 3% or better. The spectrometer was used for observations of temporal variations in the atmospheric ion mobility spectrum at Auckland. These show a clear diurnal variation, and demonstrate that the spectrum can change significantly in an interval of 100 seconds. Atmospheric polar conductivities calculated from the mobility spectrum (which agreed well with independently measured conductivities) indicate that large ions can contribute as much as 40% of the total conductivity.