Articles | Volume 16, issue 22
Atmos. Chem. Phys., 16, 14297–14315, 2016

Special issue: Pan-Eurasian Experiment (PEEX)

Atmos. Chem. Phys., 16, 14297–14315, 2016

Research article 16 Nov 2016

Research article | 16 Nov 2016

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 Xuemeng Chen et al.
  • 1Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 2Finnish Meteorological Institute, Observation services, P.O. Box 503, 00101 Helsinki, Finland
  • 3Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland

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 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 during 2003–2006 from a boreal forest site in southern Finland. In general, gamma radiation dominated the ion production in the lower atmosphere. Variations in the ionising capacity came from mixing layer dynamics, soil type and moisture content, meteorological conditions, long-distance transportation, snow cover attenuation and precipitation. Slightly similar diurnal patterns to variations in the ionising capacity were observed in air ion concentrations of the cluster size (0.8–1.7 nm in mobility diameters). However, features observed in the 0.8–1 nm ion concentration were in good connection to variations of the ionising capacity. Further, by carefully constraining perturbing variables, a strong dependency of the cluster ion concentration on the ionising capacity was identified, proving the functionality of ionising radiation in air ion production in the lower atmosphere. This relationship, however, was only clearly observed on new particle formation (NPF) days, possibly indicating that charges after being born underwent different processes on NPF days and non-event days and also that the transformation of newly formed charges to cluster ions occurred in a shorter timescale on NPF days than on non-event days.

Short summary
Ionising radiation is responsible for air ion production. However, minor efforts have been invested in understanding the connection of observed air ions to ionising radiation in the lower atmosphere and underlying processes therein. In this work, we analysed 4 years of ambient data collected in a Finnish boreal forest and found that gamma radiation dominates air ion production in the lower atmosphere and demonstrated clear promotion effects of the ionising radiation on air ion production.
Final-revised paper