Measurements of the small-, intermediate- and large-ion concentrations and atmospheric electric conductivity have been made onboard ORV Sagarkanya off the west coast of India during the Arabian Sea Monsoon Experiment (ARMEX) - 2003. On a few days during the pre-monsoon season, the intermediate ion concentration is observed to periodically increase by 20--60% of its ambient value with a periodicity of ~2 hours. An examination of the ion-concentration variations with respect to cruise pattern and the prevailing wind direction reveals that the observed increases in intermediate ion concentration are caused by the ship's exhaust polluting the air over the measurements site. Formation of intermediate ions in the engine's exhaust has been associated with the generation of chemiions within the engine combustors which provide centers for the rapid growth of molecular clusters and the formation of electrically charged sulphuric acid/water aerosols.