Chemical characteristics of precipitation in Okinawa Island, Japan

This study examined the chemical composition and characteristics of precipitation collected from March 2003 to February 2005 in the central part of Okinawa Island, Japan. Chloride ions contributed most to the total ion equivalent concentration, and followed in order by Na+>Mg2+, SO42−>Ca2+>H+, NH4+, K+>NO3−, HCO3−. Concentrations of Na+, K+, Ca2+, Mg2+, Cl−, and sea salt SO42− (ss-SO42−) increased from summer to autumn and decreased from winter to spring. In contrast, concentrations of NH4+, NO3− (except for July 2004), non-sea salt SO42− (nss-SO42−), and H+ were lower in summer and higher in winter. During periods with typhoons, concentrations of sea salt components, such as Na+ and Cl−, increased while NO3− and nss-SO42− concentrations decreased. Wet deposition driven by typhoons accounted for about 77% of the total annual wet deposition. The pH values ranged from 3.89 to 7.61. Acid rain (pH<5.6) occurred in 72% of the collected samples, even though Okinawa Island is considered to be an unpolluted area. Principal component analysis indicated three main origins of the chemical components in precipitation: (1) sea salt generated from local surrounding ocean (Na+, K+, ss-Ca2+, Mg2+, Cl−, and ss-SO42−), (2) soil generated from local land (nss-Ca2+ and D-SiO2), and (3) anthropogenic source of Asian Continent (NH4+, NO3−, and nss-SO42−).