Characteristics of sea-salt aerosols collected during the typhoons in Okinawa, Japan

In Tropical Ocean, when seawater surface is warmed by sun light, seawater evaporates, and ascending current of air forms. In addition, when water vapor in the upper sky condenses, it releases heat, which further facilitates upward movement of air. As this cycle is repeated, a tropical cyclone eventually becomes a typhoon. When water vapor condenses, aerosols, especially sea-salt aerosols, work as condensation nuclei, but their quantity and property are not well understood for the growth of a typhoon. We have initiated a study to explore relationships between strength of typhoons and aerosols. We have collected aerosols and investigated chemical compositions and the shape of the sea-salt aerosol particles present when two typhoons approached Okinawa in 2018. Okinawa, Japan is located in the west of the Pacific Ocean, and several typhoons pass by every year. Since Okinawa is a small island and there are not any big mountains, when typhoons approach Okinawa, they are still in the process of growing phase, receiving water vapor from the ocean as "fuel" for the growth. We have analyzed the aerosols collected before, during and after the two typhoons (No. 24 and No. 25) approached Okinawa in 2018. Two typhoons, typhoon No.24 (950 hPa) on September 29, 2018 and typhoon No.25 (975 hPa) on October 4, 2018, passed by the Okinawa island. We found that when typhoons approached, a large amount of aerosols were present, ca. 2-4 times of the non-typhoon periods, and sea-salt was dominant. The quantity of sea-salt was ca. 6-8 times compared to during the non-typhoon periods. It was also interesting to note that shape of sea-salt, observed by SEM-EDX, during the stronger typhoon (No. 24) was much smaller than those of typhoon No. 25. Typically, all the electric power driven instruments were turned off when typhoons approach because of the possible power outage. To our knowledge, our study is the first to report on sea-salt aerosol collection and characterization during the typhoons.