Number size distribution of ambient aerosols was measured using a wide-range particle spectrometer (WPS) at the Cape Hedo Atmosphere and Aerosol Monitoring Station in Okinawa and at the Fukue Atmosphere Monitoring Station in Nagasaki, Japan. Both sites are located in westernmost Japan, but in different proximity to the north-eastern region of China, which air masses arriving at these sites often pass through. At each site field measurements were conducted in February 2012 and 2013, respectively. The measurement results at Cape Hedo showed a bimodal distribution with maximum at 50 nm and 200 nm throughout the observation period. In contrast, the size distribution at Fukue Island varied: a bimodal distribution with maximum at 50 nm and 150 nm was observed between 1:00 a.m. and 4:00 a.m. local time on February 23rd, while a monomodal distribution with maximum at ~15 nm, which was very likely new particle formation, was observed at 12:00 p.m. local time on February 24th. During the latter episode the number concentration exceeded 40000 cm^-3, and growth of these nucleation mode particles was also observed. Such particle formation and growth were not observed at Cape Hedo. The study here demonstrates difference in size distribution of ambient aerosols contained in transboundary air masses and provides valuable information on aerosol size distribution in East Asian region.

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