In this paper, the field observation data collected by the research team of the Climate and Air-quality Taiwan Station (CATS) from June 1991-October 1991 are analyzed. During this period, a mobile laboratory equipped with highly sensitive instruments was used to monitor the level of chemical species at Hsinwu, located on the northwestern coast, and at Kenting, at the southern tip of Taiwan. The results show that the mean hourly-averaged SO2 level at the rural northwestern coast was about 1-4 ppbv. When the southwesterly prevailed, the background mean SO2 level in the Taiwan Strait was about 0.1-1 ppbv, which represented a well-diluted air of anthropogenic origin possibly form southern Taiwan. The CO level was about 220-260 ppbv during the later period. Meanwhile, at the southern tip of Taiwan, the measured ozone level varied with the movement of the large-scale airmasses. In September, each time a typhoon or a tropical depression approached, both the surface pressure and the ozone level dropped. The lowest hourly-averaged ozone level recorded was about 7 ppbv. Meanwhile, the peroxyacetyl nitrate (PAN) level dropped with the decrease in ozone concentration, with the lowest level about 7 ppbv. On the other hand, in October, the continental High dominated, and the northeasterly prevailed. The ozone level varied between 38 and 50 ppbv with an insignificant diurnal variation. Backward air-parcel tracing suggested that the observed high ozone level was associated with the airmass originating from regions of higher latitude near northern China, Korea and Japan. In all, Taiwan, being at the edge of eastern Asia and the western Pacific, is affected simultaneously by the polluted continental airmass and the clean maritime air. Current measurements have clearly supported this concept.