Abstract
We present here the optical properties of humic-like substances (HULIS) isolated from the fine fraction of biomass burning aerosol collected in the Amazon basin during the LBA-SMOCC (Large scale Biosphere atmosphere experiment in Amazonia ? SMOke aerosols, Clouds, rainfall and Climate) experiment in September 2002. From the isolated HULIS, aerosol particles were generated and their scattering and absorption coefficients measured. The size distribution and mass of the particles were also recorded. The value of the index of refraction was derived from "closure'' calculations based on particle size, scattering and absorption measurements. On average, the complex index of refraction at 532 nm of HULIS collected during day and nighttime was 1.65?0.00187i and 1.69?0.00163i, respectively. In addition, the imaginary part of the complex index of refraction was calculated using the measured absorption coefficient of the bulk HULIS. The mass absorption coefficient of the HULIS was found to be quite low at 532 nm (0.031 and 0.029 m2g?1 for the day and night samples, respectively). However, due to the high Ångström exponent of HULIS (6?7) the specific absorption increases substantially towards shorter wavelengths m2g?1 at 300 nm), causing a relatively high (up to 50%) contribution to the absorption at this wavelength. For the relative contribution of HULIS to light absorption in the entire solar spectrum, lower values (6.4?8.6%) are obtained, but those are still not negligible.