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

Abstract

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 premonsoon seasons of 2002 and 2003. Seasonally averaged values of the total as well as polar conductivity are much higher during the monsoon than premonsoon season. Surprisingly, however, the concentration of small ions is less and those of large and intermediate ions are more during the monsoon than premonsoon season. The diurnal variations observed during the premonsoon 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 premonsoon 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 (10-12 m s^-1) over the Arabian Sea is postulated to explain the monsoon season observations.