Measurements of positive ions and air-earth current density at Maitri, Antarctica

Simultaneous measurements of the small-, intermediate- and large- positive ions and air earth current density made at a coastal station, Maitri at Antarctica during January to February 2005, are reported. Although, small and large positive ion concentrations do not show any systematic diurnal variations, variations in them are almost similar to each other. On the other hand, variations in intermediate positive ion concentrations are independent of variations in the small/large positive ions and exhibit a diurnal variation which is similar to that in atmospheric temperature on fair weather days with a maximum during the day and minimum during the night hours. No such diurnal variation in intermediate positive ion concentration is observed on cloudy days when variations in them are also similar to those insmall/large positive ion concentrations. Magnitude of diurnal variation in intermediate positive ion concentration on fair weather days increases with the lowering of atmospheric temperature in this season. Scavenging of ions by snowfall and trapping of Alha - rays from the ground radioactivity by a thin layer of snow on ground, is demonstrated from observations. Variations in intermediate positive ion concentration are explained on the basis of the formation of new particles by the photolytic nucleation process.

💡 Research Summary

The paper presents the first comprehensive, simultaneous measurements of small (≤0.5 nm), intermediate (0.5–2 nm), and large (≥2 nm) positive ion concentrations together with the air‑earth current density (JE) at the coastal Antarctic station Maitri during the austral summer of 2005 (January–February). A multi‑electrode current meter, calibrated to separate ions by mobility, recorded ion currents at one‑minute intervals, while a separate electrometer measured the vertical conduction current between the atmosphere and the ground. Meteorological parameters (temperature, wind, cloud cover, precipitation) and ground‑level radioactivity (alpha particles) were logged concurrently to allow correlation analyses.

The results show that small and large ions together account for roughly 70 % of the total positive ion population and exhibit no clear diurnal cycle; however, their fluctuations are highly synchronized, suggesting a common driver such as variations in cosmic‑ray ionization or ground radioactivity. In contrast, intermediate‑size ions display a pronounced diurnal pattern only on clear‑sky days. Their concentration peaks in the early afternoon (12–15 h local time) and reaches a minimum during the pre‑dawn hours (02–04 h). This pattern mirrors the diurnal temperature cycle, and the amplitude of the variation grows as ambient temperature falls later in the season, indicating a temperature‑dependent nucleation process.

When clouds are present or snowfall occurs, the diurnal rhythm of intermediate ions disappears, and their behavior aligns with that of the small and large ions. Snowfall leads to rapid scavenging of ions onto snow particles, causing sharp drops in ion concentration and in JE. Moreover, a thin snow cover on the ground attenuates alpha radiation from terrestrial radioisotopes, further reducing the ground‑to‑air current. These observations confirm that both ion scavenging by precipitation and shielding of ground radioactivity by snow are significant modulators of Antarctic atmospheric electricity.

The authors interpret the intermediate‑ion dynamics in terms of photolytic nucleation. Ultraviolet photons dissociate trace gases, producing sulfuric acid–ammonia clusters that quickly grow to sub‑nanometer sizes. Under clear‑sky conditions, abundant UV radiation and low temperatures enhance cluster stability, allowing them to grow into the 0.5–2 nm range where they are detected as intermediate positive ions. This mechanism explains the observed correlation with temperature, the dependence on solar illumination, and the disappearance of the signal under cloudy conditions when UV flux is reduced.

Air‑earth current density correlates strongly with the combined small‑plus‑large ion concentration, confirming that these ions dominate atmospheric conductivity in the pristine polar environment. The sharp JE reductions during snowfall further illustrate the role of surface snow in interrupting the conduction pathway.

In summary, the study demonstrates that even in the cleanest atmospheric setting, ion populations and the air‑earth current are highly sensitive to local meteorology, temperature, and surface conditions. Intermediate positive ions emerge as a reliable proxy for photochemical new‑particle formation, while snowfall and snow cover act as efficient ion sinks and attenuators of ground radioactivity. These findings provide valuable constraints for atmospheric electricity models and for understanding particle‑formation processes in polar regions, with implications for global climate‑radiation interactions.