Abstract：A CE-318 automatic sun tracking spectrophotometer was used to measure direct solar radiation at Zhengzhou, China from February to September 2007. The aerosol optical depth (AOD), wavelength exponent (a) were retrieved using Bouguer-Lamber law and their seasonal and diurnal variations were studied. During the study period, the value of AOD at Zhengzhou was 0.49±0.09, 0.60±0.13, 0.76±0.20 and 1.08±0.34 at 1020, 870, 670 and 440nm, respectively. The AOD varied seasonally, with a summer maximum, a winter and autumn second place and a spring minimum. The value of a ranged between 0.37~0.69 in spring and 1.18~1.26 in summer, which indicated that the dust was principal aerosol in half of time in spring and the industry-urban aerosol dominated in summer. The diurnal variability of AOD was consistent with the diurnal variability of surface air pollution, with a maximum at 8:30 and 17:00 and a minimum at 11:30. Another smaller peak value of AOD occurred at 12:30 with the raise of temperature, but which was also lower than the values of other time.
