Organic arsenic compounds, which were delivered in the atmosphere as a result of the activity of biomethylation in the environment exist in a variety of forms, such as methylar-sonic acid (MMAA), dimethylarsinic acid (DMAA) and trimethylarsine,oxide (TMAQ). In order to understand the chemical form of arsenic compounds and distribution of them in the unpolluted area, their concentrations in the marine atmosphere were measured at several sites in the North Pacific Ocean. The arsenic compounds were collected by high volume air sampler with a quartz fiber filter (Pallflex 2500 QAST) for 5 or 6 days at1000dm3.min-1 air flow rate. Aerosol samples were extracted with nitric acid and hydrogen peroxide for inorganic arsenic analysis, with distilled water for organic arsenic analysis. The use of Chelex 100 resin was very effective for the elimination of foreign ions which interfered with organic arsenic analysis; Analysis of arsepic compounds in the sample solution was used by hydride generation atomic absorption spectrophotometry combined with a cold trap of liquid nitrogen for trapping of generated arsines. This method can determine arsenic compounds separately by using the difference of their boiling points of arsines generated from a sample solution, and it has a high sensitivity for arsenic analysis. The detection limits by this method were 0.7 pg/m3 for MMAA, 0.8 pg/m3 for DMAA, and 0.2 pg/m3 for TMAO in case of 400 m3 sampling volume. In Oahu, Hawaii, the concentration range of arsenic compounds were 40~520 pg/m3 for inorganic arsenic, 0.8~5. 5 pg/m3 for DMAA, and 0.2~8.8 pg/m3 for TMAO. Methylarsonic acid was not found in any sample in Oahu, Hawaii. On the other hand, the concentration range of arsenic compounds in Yokohama, Japan were 920~5900 pg/m3 for inorganic arsenic, 1.1~22.8 pg/m3 for MMAA, 0.8~5.3 pg/m3 for DMAA, and 1.6~4.8 pg/m3 for TMAO; the concentration of inorganic arsenic in Yokohama was about 15 times higher than that in Oahu, Hawaii. The inorganic arsenic concentration in other remote islands, Midway, Fanning and Chichijima in the North Pacific Ocean was almost in the same level (13~520 pg/m3) as Oahu, Hawaii. It is considered that these values are the background concentration of inorganic arsenic in the marine atmosphere. Only a slight difference of organic arsenic concentration except MMAA was found between Oahu, Hawaii and Yokohama, compared with a large difference of inorganic arsenic concentration. This fact indicates that organic arsenic, such as DMAA and TMAO, could be produced by biomethylation in the environment and have a homegeneous distribution in the atmosphere.
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