Radiative characteristics of regional hazes dominated by smoke from biomass burning in Brazil: Closure tests and direct radiative forcing

Ground-based Sun photometer measurements, and airborne measurements of the physical and optical properties of regional hazes dominated by smoke from b iomass burning in Brazil, are used for aerosol radiative vertical column an d local radiative closure tests. Optical depths at midvisible wavelengths o f up to 2.5 measured by two independent methods (ground-based Sun photomete rs and airborne measurements of the vertical profiles of aerosol optical ex tinction to the top of the smoke layer) agreed, on average, to within simil ar to 20%. Local aerosol closure tests were carried out using in situ measu rements of particle size distributions as inputs to a computational model o f the aerosol. Calculated aerosol masses, aerosol absorption and scattering coefficients, and the amounts of solar radiation backscattered by the aero sol were generally within 25% of the measured values. The computational mod el was used to calculate a broader range of radiative transfer parameters, including aerosol mass scattering and absorption efficiencies, the asymmetr y parameter, and upscatter fraction, across the solar spectrum. Regional va lues of direct aerosol radiative forcing produced by smoke aerosol in the c errado and primary forest areas of Brazil are derived using the radiative t ransfer parameters as inputs to a radiative transfer model. The resulting n et direct radiative forcing can result in either a cooling or a heating dep ending on the underlying surface albedo. Over a typical tropical forest the change in the daily average net shortwave flux per unit optical depth (at a wavelength of 550 nm) is -20 +/- 7 W m(-2) (where a negative value indica tes cooling). Over the cerrado the forcing is -8 +/- 9 W m(-2), while over a dark surface, such as the ocean, the forcing is -26 +/- 6 W m(-2). Over a reflective surface, such as a desert, we calculate a positive (heating) fo rcing of +25 +/- 12 W m(-2).