Size distributions of atmospheric ions inside clouds and in cloud-free air at a remote continental site

During the late autumn 2004, aerosol and air ion number size distributions inside and outside clouds and cloud droplet number size spectra were measured in Pallas, northern Finland. The concentrations of cluster ions (air ions with a diameter < 1.6 nm) were substantially lower, roughly by an order of magnitude, inside clouds as compared with cloud-free air. The observed concentration levels of cluster ions could be explained by a source rate of a few ion pairs per second. The main sink for cluster ions was the cloud droplet population during the cloudy periods and the ion-ion recombination in cloud-free air. Very few intermediate ions (1.6-7.4 nm diameter) were present during the cloudy periods, indicating that processes capable of generating intermediate ions were rather inactive inside clouds during the measurement campaign.