Perchlorate ion (ClO
4-) has been recognized as a new environmental pollutant. The concern about ClO
4- stems from the fact that it displaces iodide in the thyroid gland. Iodine-containing thyroid hormones are essential for proper neural development from the fetal stage through the first years of life. In this study, we determined the concentrations of ClO
4- in aerosols collected at Cape Hedo Atmosphere Aerosol Monitoring Station of National Institute of Environmental Studies (NIES-CHAAMS) Okinawa Island, Japan. Aerosols were collected on quartz filters by using a high-volume air sampler. The ClO
4- concentrations were determined by ion chromatography (ICS-2000, DIONEX). About 60% of the weekly-collected aerosols contained detectable level of ClO
4-, and the highest ClO
4- concentration for the samples collected at NIES-CHAAMS was 1.8ng m
-3. Back trajectory analysis showed that ClO
4- concentrations were higher when air mass came from Asian continent, suggesting that ClO
4- found at NIES-CHAAMS was probably due to long-range transport from Asian continent.
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