On the charging state of atmospheric aerosols and ion-induced nucleation

Atmospheric aerosol particles (particles suspended in the air) have significant effects on the Earth’s climate. They affect the Earth’s radiation budget directly by scattering radiation, and indirectly by acting as cloud condensation nuclei, influencing the brightness and lifetime of clouds. Nucleation (new particle formation and growth) accounts for about half of the cloud condensation nuclei. Several nucleation mechanisms have been proposed. We can divide these mechanisms into two main groups: ion-induced and neutral mechanisms. The charged fraction of an aerosol population is needed in order to determine the fraction of particles formed through ion-induced nucleation (IIN). The charging state is a quantity directly related to the charged fraction, but which conveys more physical meaning in a dynamical aerosol particle population. The charging state is defined as the ratio of the ambient charged fraction to the charged fraction of the same sample at bipolar equilibrium. The Ion-DMPS is an instrument that was specifically designed to measure the charging state of aerosol populations as a function of size and time. Its switchable neutralizer and bi-polar DMA combination allows for the measurements of the aerosol size distribution in four modes with the very same inlet and losses. The ratio of the neutralized and ambient modes, for each polarity, gives us the charging state. In this thesis, the behavior of the charging state as a function of particle diameter is theoretically described. The Ion-DMPS is presented and used to estimate the IIN fraction for the first time. Methods to analyze the charging state are developed, described and evaluated. The IIN fraction, derived from extrapolated charging states, was found to be around 6% in rural background Hyytiala and around 1% in urban Helsinki, on average. IIN varies from a place to another and from one day to another. It is more important when the ion-pair production rate is high or when neutral nucleation is limited by too low vapor saturation ratios or too high temperatures. In Hyytiala, where the IIN fraction varies from day to day, the IIN fraction was higher in summer and very small in winter, and negative IIN was more common than positive IIN. In Helsinki, no seasonal tendency appeared. Days with an important neutral contribution yielded higher nucleation mode particle concentrations. IIN seems to survive in conditions in which neutral nucleation takes place difficultly. However, in favorable conditions, neutral nucleation thrives.