With the method of differential optical absorption spectroscopy (DOAS), aerosolalong the light path is monitored and its size distribution is measured with double optical path method. By using double optical path method, the absolute intensity of the source can be determined and the influence of source variation is reduced greatly. Aerosol extinction coefficient from 250nm to 650nm and size distribution from 0.1μm to 1.75μm were obtained. By comparison with optical particle counting, good correlations are found between the DOAS and optical particle counting resulcs.