Observational investigations on air ions in the lower troposphere

Air ions are constantly generated throughout the atmosphere by natural ionising radiation and they participate in the formation and dynamic processes of atmospheric aerosol particles. Their flow in the atmosphere is the cause for the air conductivity. However, there is a gap in understanding how variations in ionising radiation levels are reflected in air ion properties. Besides, observations related to air ions have been conducted at many sites around the globe, but the knowledge on features in air ions in vapour-limited environments is missing. The work in this thesis is dedicated to fill in these voids in knowledge related to air ions using the observational approach. Factors influencing natural radioactivity in the atmosphere in a boreal forest were identified at the Hyytiälä SMEAR II Station in Finland. We found that the diurnal and seasonal patterns in the natural ionising radiation level were mainly introduced by boundary layer dynamics as well as snow cover and soil conditions. Current instrumentations for measuring the number size distribution of air ions have a lower size limit of 0.8 nm in the Millikan mobility diameter. Based on our analysis, the concentrations of 0.8-1 nm ions were generally seen varying similarly to the natural ionising radiation level. We found a clear enhancement of ionising radiation on 0.8-1.7 nm ion production on atmospheric NPF event days but not on non-event days, which suggests that 0.8-1.7 nm ions undergo less dynamic modifications and are possibly formed over shorter time scale on NPF event days than on non-event days. To study features in air ions under conditions of limited vapours, the Concordia Station at Dome C, Antarctica, was chosen as the measurement site. Air ion processes were found to be active at the Concordia Station, including atmospheric new particle formation (NPF), ion production and loss in relation to cloud formation and wind-induced ion formation. Overall, these results advance our understanding on the ion processes in the atmosphere, which can assist obtaining further insights into atmospheric aerosol formation mechanisms and ultimately finding solutions to air pollution issues and understanding climate variability