This study analyzes direct-normal solar irradiances received at the ground level in cloudless conditions, in São Paulo (SP, Brazil; 23°34'S, 46°44'W, 730 meters above the mean sea level), with the objective of evaluating the importance of the particulate matter in suspension (aerosol) on the atmospheric transparency in the visible spectrum. Such influence is assessed in terms of the values assumed by the aerosol optical depth, which is obtained from pyrheliometric measurements performed with different broadband filters. Sensitivity tests assess the importance of some sources of systematic errors such as the filter characteristics and the instrument calibration. The reliability of aerosol optical depth estimates increases with the own turbidity level, being usually low (relative uncertainties greater than 100 %) under clean atmospheric conditions. The analysis of the entire data set collected between March 1995 and January 1996 shows that aerosol optical depths are very low after the occurrence of rain. Also, aerosol optical depths may exhibit important variations in few hours; this might be the consequence of the wind field and/or of the daily cycle of the relative humidity. Future studies in this subject must employ more accurate instrument installed in the same site with meteorological routine observations.
Meteorology; Solar Radiation; São Paulo; Aerosol; Pyrheliometers; Atmospheric Turbidity
