The light-scattering coefficient of aerosols from biomass burning was measu
red as a function of relative humidity (RH) and wavelength in four regions
of Brazil extending from the cerrado to the primary forests of the Amazon B
asin. At a wavelength of 550 nm the ratio of the light-scattering coefficie
nt at a RH of 80% to that at an RH of 30% ranged from 1.01 to 1.51, with an
average value of 1.16. The aerosol backscatter ratio as a function of RH d
ecreased by 10 to 20% as the RH was increased from 30 to 80%. Measurements
of the geometric growth of the aerosol with RH were consistent with the mea
sured changes in light scattering. Use of these new measurements yields an
estimate for the globally averaged direct radiative forcing by biomass burn
ing aerosol of about 0.25 W m(-2).