Assessment of smoke aerosol impact on surface solar irradiance measured in the Rondônia region of Brazil during Smoke, Clouds, and Radiation - Brazil

Abstract

The impact of smoke aerosols on the surface solar irradiance in the Rondônia region of Brazil is examined. The climate of this region is subequatorial with two distinct seasons. There is a sharp decline of the precipitation and cloudiness during the dry season which lasts from June to September accompanied by anthropogenic burning of savanna and forest in August and September with the resultant strong emission of smoke into the troposphere. Thus the magnitude of the column aerosol optical depth increases in these months while both precipitable water and cloud amount decrease in accordance with the seasonal cycle of humidity. All these atmospheric constituents influence the magnitude of the solar irradiance at the surface. In order to assess the aerosol and gaseous effects on the surface irradiance as compared with impact of cloudiness, we performed the clear-sky radiative transfer model calculations by employing a broadband radiation code and both aerosol optical depth and precipitable water retrieved from Sun photometer measurements. Calculation results show that the elevated aerosol optical depth observed in Rondônia during August and September causes a negative trend in the daily mean clear-sky surface solar irradiance during this period. Since the daily mean solar irradiance measured at the surface under all-sky conditions demonstrates a similar negative trend, it can be explained by the aerosol influence while the effect of clouds appears in the daily variations of the irradiance. It was also shown that smoke aerosols cause a decrease of the monthly mean values of all-sky surface irradiance in August and September as compared with July and October. Corresponding decreases in the near surface air temperature due to the smoke aerosols were not found.