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 biomass burning in Brazil, are used for aerosol radiative vertical column and local radiative closure tests. Optical depths at midvisible wavelengths of up to 2.5 measured by two independent methods (ground‐based Sun photometers and airborne measurements of the vertical profiles of aerosol optical extinction to the top of the smoke layer) agreed, on average, to within ∼20%. Local aerosol closure tests were carried out using in situ measurements of particle size distributions as inputs to a computational model of the aerosol. Calculated aerosol masses, aerosol absorption and scattering coefficients, and the amounts of solar radiation backscattered by the aerosol were generally within 25% of the measured values. The computational model was used to calculate a broader range of radiative transfer parameters, including aerosol mass scattering and absorption efficiencies, the asymmetry parameter, and upscatter fraction, across the solar spectrum. Regional values of direct aerosol radiative forcing produced by smoke aerosol in the cerrado and primary forest areas of Brazil are derived using the radiative transfer parameters as inputs to a radiative transfer model. The resulting net direct radiative forcing can result in either a cooling or a heating depending 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 indicates 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) forcing of +25±12 W m −2 .