| Abstract: | There are four types of PM10 (particulate matter with an aerodynamic diameter <10 μm) episodes that occur frequently in central Taiwan: long-range transport with dust storms (DS), long-range transport with frontal pollution (FP), river dust (RD), and stagnant weather (SW). During the periods of the four episodes, poor visibility usually results. Multiple linear regression was applied to visibility using eight potential influential variables (temperature, relative humidity, wind speed, PM2.5, PM2.5–10, SO2, NO2, and NO) as independent variables. Of the eight variables, PM2.5 showed the greatest explainable percentage of about 48.6% and 58.1% for Taichung and Wuchi stations, respectively. This suggested that PM2.5 was the most important contributor to reduced visibility. Compared with other type of episodes, the aerosols tended to be of fine size during the SW episodes. This is the main reason that the poorest visibility occurred during the SW episodes. Good correlation between visibility and secondary inorganic salts (NH4 +, NO3 −, and SO4 2−) were found at Taichung station (r = 0.71) and Wuchi station (r = 0.81), suggesting that secondary inorganic salts did contribute significantly to the degradation of visibility. The visibility degradation due to the effects of NO3 − was much higher than that due to SO4 2− and NH4 + in the urban area, whereas the visibility degradation due to the effects of NO3 −, SO4 2−, and NH4 + did not show significant difference in the rural area. |
