Observation on visibility as well as sampling and analysis of particulate matters, including PM2.5 and PM1, are carried out at Dali in this study to investigate the reason for poor visibility in Taichung. The result shows that the visibility in summer is usually good while that in autumn is relatively poor. The diurnal variations of visibility in the two seasons, summer and autumn, are quite different. In summer, the visibility is better at night and at dawn while that at noon is reduced. The conditions are reversed in autumn. The correlation of visibility with the mass concentration of PM1 is higher than that of PM2.5. The visibility reduces 10 km with the increase of 20 μg/m3 in PM1 mass concentration. For the chemical composition of PM1, ammonium sulfate is the major component followed by organic matters and elemental carbon. The chemical composition of PM1 changes when the visibility is impaired. Ammonium nitrate increases from 1% to 6 %, and organic matters reduce from 40% to 25 % while ammonium sulfate and elemental carbon remain approximately the same relative amount. In order to tackle with the problem of visibility impairment in Taichung, Primary sulfate and secondary sulfate are the first to be considered and can be cut down through the reduction of SO2 emissions from major pollution sources. Policy rewords factories to use gas boilers to replace oil-fired boilers and coal-fired boilers in Taichung, it is recommended to limit ships and fishing boats to use the 0.3 % low-sulfur heavy oil as their fuels in the future. During the warning period of poor air quality before the visibility impairment, the travelling areas of Phase 1 to Phase 3 diesel trucks should be limited. For the areas of visibility prone to be impaired, such as Dali, diesel trucks of only Phase 5 or Phase 6 should be allowed to enter. The government can provide exemptions of tax or subsidy purchase as the encouragement.

Visibility、Composition of PM1，Sulfate、Nitrate、Extinction efficiency