Abstract
There is a need to better characterize aerosol absorption due to its
remarkable radiative effects on climate. In particular, we need to
understand the separation of total absorption between the two main
components, the black carbon (BC) and brown carbon (BrC), especially in
places where the anthropic influence is little, such as in pristine
regions like Central Amazonia. The mechanisms that control the formation
and evolution of BC and BrC in tropical forests remain unclear. In this
study, we have performed detailed measurements at the Amazon Tall Tower
Observatory (ATTO) tower on aerosols collected in Nuclepore filters and
analyzed them with high-resolution optical spectrometers with a wide
spectral range (300 to 2500 nm). Thus, we determined the absorption
characteristics of BrC as a function of wavelength. The results show
that BrC absorption is spectrally significant below 660 nm and is
maximum at wavelengths close to 370 nm. Combining the measured spectral
dependency with MIE modeling of the BC contribution, we determined that
the BrC accounts for 14.8% of the total absorption. A similar fraction
of BrC to total absorption was obtained through a similar analysis of
in-situ measurements. Elemental chemical analysis of the filters,
cluster, and factor analysis shows that the BrC is associated with
airborne dust. The different methods to quantify BC and BrC are
consistent and show similar results. This study will allow the
quantification of the role of BrC and BC in aerosol absorption of
radiation in Amazonia.