How do air ions reflect variations in ionising radiation in the lower atmosphere in a boreal forest? Xuemeng Chen1*, Veli-Matti Kerminen1, Jussi Paatero2, Pauli Paasonen1, Hanna E. Manninen1, Tuomo Nieminen1,3, Tuukka Petäjä1 and Markku Kulmala1 5 1Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland 2Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland 3Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland 10 Correspondence to: Xuemeng Chen (
[email protected]) 1 Abstract. Most of the ion production in the atmosphere is attributed to ionising radiation. In the lower atmosphere, ionising radiation consists mainly of the decay emissions of radon and its progeny, gamma radiation of the terrestrial origin as well as photons and elementary particles of cosmic radiation. These types of radiation produce ion pairs via the ionisation of nitrogen and oxygen as well as trace species in 5 the atmosphere, the rate of which is defined as the ionising capacity. Larger air ions are produced out of the initial charge carriers by processes, such as clustering or attachment to pre-existing aerosol particles. This study aimed 1) to identify the key factors responsible for the variability in ionising radiation and in the observed air ion concentrations, 2) to reveal the linkage between them, and 3) to provide an in-depth analysis into the effects of ionising radiation on air ion formation, based on measurement data collected 10 during 2003-2006 from a boreal forest site in southern Finland. In general, gamma radiation dominated the ion production in the lower atmosphere.