Ground based measurements of ozone, CO, HNO3, SO2, HCl, and formic acid were carried out at Oki, a remote island site in the Sea of Japan, during February 26 to March 16, 1994, as the Pacific Exploratory Mission in the Western Pacific/Perturbation by East Asian Continental Air Mass to the Pacific Oceanic Troposphere (PEM-West B/PEACAMPOT B) campaign. According to trajectory analysis, the air mass reaching Oki was classified into four groups: northerly, northwesterly, west-northwesterly, and westerly flows. Clear dependence of gas concentrations on flow direction of air mass was found for all species studied. Lowest concentrations were observed in the northerly airflow originating from the Bering Sea. The mean concentrations of ozone and CO in northerly flow were 37.6¿1.9 and 157¿7 ppbv, respectively. The mean values of HNO3, SO2, HCl, and formic acid in the northerly flow were 34¿10, 41¿5, 107¿23, and 54¿81 pptv, respectively. The highest concentrations of all species were observed in the westerly flow passing through a lower boundary layer over the Yellow Sea and South Korean Peninsula. The mean concentrations of ozone and CO in the westerly flow were 45.9¿4.0 and 292¿44 ppbv, respectively. The mean values of HNO3, SO2, HCl, and formic acid in the westerly flow were 137¿37, 2075¿1307, 515¿214, and 264¿183 pptv, respectively. Clear seasonal variation of the background concentrations of ozone and CO were identified by comparison with the data from the PEM-West A /PEACAMPOT A campaign conducted in the fall. In contrast to other species, strong diurnal variation of formic acid with a daytime maximum was observed. The possibility of photochemical formation of formic acid from HCHO and the HO2 radical is discussed. ¿ 1997 American Geophysical Union