Temporal variability of atmospheric turbidity and DNI attenuation in the sugarcane region, Botucatu/São Paulo/Brazil

Abstract

In this study, attenuation of direct normal solar irradiance (DNI) in Botucatu / São Paulo, an area under the influence of local and adjacent agricultural burning, is expressed using the Linke's turbidity factor (TL) in the period from 1996 to 2008. Two methodologies represented as TLDj and TLLi were used. Temporal variability (hourly average for the season and monthly average) is presented. Turbidity was correlated with wind speed and air temperature. Frequency distribution and cumulative frequency are analyzed to determine turbidity predominance levels in the local atmosphere. Optical depth information of aerosols at 550nm (AOD550nm) and water vapor were obtained by the Terra satellite using the MODIS sensor. The highest degree of DNI transmission is observed in the morning. Close to solar noon, transmission is smaller (greatest TL value). Diurnal TL variability is more evident in the hot period than in the cold period. May and June were the months of lowest DNI attenuation (highest atmospheric transparency). The highest DNI attenuation occurs in spring (TLDj=4.22±0.05 and TLLi=4.65±0.06) and summer (TLDj=4.27±0.14 and TLLi=4.69±0.15). Wind speed and air temperature were positively correlated with TL. In >28% of hours of clear sky, turbidity exceeded the value of 4.0. The region of Botucatu seems to be influenced by water vapor and aerosols from different origins. This study concludes that these factors significantly reduce DNI incidence on the surface, with higher atmospheric transparency in the cold period and lower atmospheric transparency in the warm period.