Effect of typhoon on chemical composition of rainwater in Okinawa Island, Japan

We have studied the effects of typhoons on the behavior of chemical components of rainwater. Samples were collected at Yona in the northern part of Okinawa Island and analyzed for major cations and anions. The chemical components in rainwater could be classified into three types. Components belonging to type I originated from sea salt (ss-) in which their concentrations in rainwater increased with an increase in maximum wind speed (MWS). Those belonging to type II were non-sea salt (nss-) components in which their concentrations in rainwater decreased with an increase in MWS. Ca2+ and SO42− belonging to type III in which their concentrations in rainwater had no relationship with MWS. To clarify the behaviors of type III components, they were separated into the two fractions: ss- and nss- by assuming that all Na+ ions were supplied from ocean. Each fraction of Ca2+ and SO42− except nss-Ca2+ showed a significant correlation with MWS. Moreover, When MWS was slower than 10–13 ms−1, the concentration of nss-SO42− was dominant in rainwater. But, the concentration of ss-SO42− in rainwater became dominant when MWS was faster than 13 ms−1. The chemical compositions in rainwater also changed drastically with MWS. Values of Σacid–Σbase (D value) in rainwater ranged from 1.11 to 62.4 μeq l−1 and correlated strongly with pH (5.92 to 4.17). Values of pH during typhoon were higher in contrast to those during non-typhoon weather, which is attributed to the higher decreasing rate of Σacid than Σbase. This result suggests that a typhoon has the ability to remove or dilute pollutants such as NO3− and nss-SO42−.