Abstract This investigation conducted seven intensive field measurements of PM2.5 and PM2.5–10 at three sites within the Taichung coastal area in Taiwan, between December 1996 and November 1997. Samples were collected for chemical analysis of soluble ionic species, elemental and organic carbon. Comparing these aerosol data with air quality and meteorological data reveals that the fractions of sulfate, nitrate, and ammonia of PM10 on hazy days, are significantly higher than on clear days. Visibility shows a strong negative correlation with sulfate, ammonia, and nitrate and a moderately negative correlation with humidity. The average sulfur oxidation ratio and nitrogen oxidation ratio of PM10 increases from 0.18 and 0.06, respectively, on clear days, to 0.46 and 0.12, respectively, on hazy days. These ratios indicate an enhanced formation of acidic particulates on hazy days. A regression model based on sulfate and humidity has been developed, and the comparison indicates that visibility in the coastal area can be properly simulated by the equation derived herein.