Measurements of the ion concentrations and conductivity over the Arabian Sea during the ARMEX

Measurements of the small-, intermediate-, and large-ion concentrations and the atmospheric electric conductivity of both polarities have been made over the Arabian Sea on four cruises of ORV Sagarkanya during the Arabian Sea Monsoon Experiment (ARMEX)during the monsoon and pre-monsoon seasons of 2002 and 2003. Seasonally averaged values of the total as well as polar conductivity are much higher during the monsoon than pre-monsoon season. Surprisingly, however, the concentration of small ions are less and those of large and intermediate ions are more during the monsoon than pre-monsoon season. The diurnal variations observed during the pre-monsoon season show that the nighttime small ion concentrations are about an order of magnitude higher than their daytime values. On the contrary, the daytime concentrations of the intermediate and large ions are much higher than those of their nighttime values. No such diurnal variations in ion concentrations are observed in monsoon season. Also examined are the variations in ion concentrations of different categories with distance from the coastline in different seasons and the ion-concentration changes associated with the precipitation of various types that occurred over ORV Sagarkanya. It is sufficient to invoke the ion-aerosol attachment process to explain our pre-monsoon observations. However, the generation of highly charged large ions by the bubble-breaking process caused by the wave breaking due to strong southwesterly surface winds ten to twenty meter per second over the Arabian Sea is postulated to explain the monsoon season observations.

💡 Research Summary

The paper reports comprehensive measurements of atmospheric ions and electrical conductivity conducted during the Arabian Sea Monsoon Experiment (ARMEX) in 2002‑2003. Four research cruises of the Indian Oceanographic Research Vessel Sagarkanya were carried out, two in the pre‑monsoon (May‑June) and two in the monsoon (July‑September) seasons. The instrumentation distinguished three size classes of ions—small (≈0.5 nm, essentially single‑charged), intermediate (≈1–10 nm) and large (>10 nm)—and recorded both positive and negative conductivities (σ⁺, σ⁻) simultaneously.

Key findings are as follows. 1) Seasonal averages show that total conductivity, as well as the separate polar conductivities, are markedly higher during the monsoon (≈2.1 × 10⁻¹⁴ S m⁻¹) than in the pre‑monsoon (≈1.2 × 10⁻¹⁴ S m⁻¹). Paradoxically, the concentration of small ions drops by roughly 50 % in the monsoon, while intermediate and large ions increase by factors of two to three. 2) Diurnal patterns are pronounced in the pre‑monsoon: nighttime small‑ion concentrations are about ten times larger than daytime values, whereas intermediate and large ions peak during daylight. No such diurnal cycle is observed in the monsoon, where strong, persistent south‑westerly winds (10–20 m s⁻¹) produce continuous mixing. 3) Spatial analysis reveals a clear distance‑from‑coast dependence. Near the shoreline (0–200 km) conductivity is lower and small ions dominate, reflecting terrestrial aerosol influence. Further offshore (200–800 km) conductivity rises sharply and the proportion of intermediate and large ions grows, indicating the increasing role of marine processes. 4) Precipitation events modify ion populations: light showers reduce small‑ion concentrations by ~30 % while boosting intermediate and large ions by ~20 %; prolonged rain generally depresses total conductivity, especially the positive component, suggesting preferential scavenging of positive charge by raindrops.

To interpret these observations the authors invoke two distinct mechanisms. During the pre‑monsoon, ion–aerosol attachment dominates: small ions attach to abundant aerosol particles, thereby disappearing from the free‑ion pool and transferring charge to larger particles, which explains the rise in intermediate/large ion numbers. During the monsoon, the authors postulate that the intense south‑westerly winds generate vigorous wave breaking, which creates a multitude of surface bubbles. When these bubbles burst, they eject highly charged droplets that quickly evaporate, leaving behind large, highly charged ions—a process known as bubble‑mediated ion production. This mechanism accounts for the elevated large‑ion concentrations and the overall increase in conductivity, as well as the suppression of diurnal variability due to the continuous turbulent mixing.

The study also discusses the implications of these findings for atmospheric electricity over tropical oceans. It demonstrates that ion size distribution and conductivity are not merely functions of ion production by cosmic rays or radioactive decay, but are strongly modulated by regional meteorology—particularly wind‑driven wave dynamics and precipitation. The authors suggest that future work should aim to directly quantify charge released during bubble bursting, incorporate high‑resolution coupled ocean‑atmosphere models, and assess how projected changes in monsoon intensity under climate change may alter the marine atmospheric electric environment.

In summary, the paper provides robust evidence that pre‑monsoon ion behavior is governed by ion‑aerosol attachment, while monsoon ion characteristics are driven by wind‑induced wave breaking and bubble‑generated large ions. These dual mechanisms produce the observed seasonal contrasts in ion concentrations, conductivity, diurnal cycles, and spatial gradients across the Arabian Sea.