How do air ions reflect variations in ionising radiation in the lower atmosphere in a boreal forest?

Abstract

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.

Features in air ions measured by an air ion spectrometer (AIS) at Dome C

“…1), which is typically known as the cluster ion band. The upper boundary of this band typically lies at around 1.7 nm in the mobility diameter (∼ 1.3 nm in the mass diameter; Mirme et al, 2007) under mid-latitude ambient conditions, representing critical cluster sizes Chen et al, 2016). After applying the flow and ambient condition correction to the AIS data, the cluster ion size range at Dome C was found to be between 0.9 and 1.9 nm in the positive polarity (Fig.…”

“…The median cluster ion concentration was typically below 100 cm −3 during the winter. This seasonality is related to variations in the natural ionizing radiation, which produces initial charge carriers via the ionization of air molecules, as well as to the availability of vapours capable of forming cluster ions (Chen et al, 2016). The variability in the ionizing radiation originates, in general, from changes in the atmospheric radon concentration, terrestrial gamma, and cosmic radiation (Chen et al, 2016).…”

“…This seasonality is related to variations in the natural ionizing radiation, which produces initial charge carriers via the ionization of air molecules, as well as to the availability of vapours capable of forming cluster ions (Chen et al, 2016). The variability in the ionizing radiation originates, in general, from changes in the atmospheric radon concentration, terrestrial gamma, and cosmic radiation (Chen et al, 2016). Owing to the presence of the thick ice and snowpack (over 3000 m in depth; Augustin et al, 2004), the contribution of radon exhalation or terrestrial gamma emissions from ground at Dome C to the ionization of air molecules can be neglected.…”

“…After undergoing a series of chemical and dynamic processes with trace gases and preexisting aerosol particles, electric charges that survive the initial recombination and other loss mechanisms are stabilized in the form of charged aerosol particles (Chen et al, 2016). Charged nanoparticles in the mobility size range of 1.7-7 nm are typically observed during new particle formation (NPF) events (Manninen et al, 2009;Hirsikko et al, 2011;Leino et al, 2016).…”

“…dred nanometres) (e.g. Harrison and Tammet, 2008;Hirsikko et al, 2011;Chen et al, 2016). Scientific interest in air ions has persisted for over a century.…”