Long term measurements of aerosol optical properties at a primary forest site in Amazonia
Article, Other literature type
Rizzo, L. V.
Cirino, G. G.
Fors, E. O.
Wiedemann, K. T.
Leal, L. S. M.
- Publisher: Copernicus Gesellschaft Mbh
ATMOSPHERIC CHEMISTRY AND PHYSICS, 13, 5, p. 2391-2413 23 p
(issn: 1680-7324, eissn: 1680-7324)
A long term experiment was conducted in a primary forest area in Amazonia,
with continuous in-situ measurements of aerosol optical properties between
February 2008 and April 2011, comprising, to our knowledge, the longest database
ever in the Amazon Basin. Two major classes of aerosol particles, with
significantly different optical properties were identified: coarse mode
predominant biogenic aerosols in the wet season (January–June), naturally
released by the forest metabolism, and fine mode dominated biomass burning
aerosols in the dry season (July–December), transported from regional fires. Dry
particle median scattering coefficients at the wavelength of 550 nm
increased from 6.3 Mm<sup>−1</sup> to 22 Mm<sup>−1</sup>, whereas absorption at 637 nm
increased from 0.5 Mm<sup>−1</sup> to 2.8 Mm<sup>−1</sup> from wet to dry season. Most
of the scattering in the dry season was attributed to the predominance of
fine mode (PM<sub>2</sub>) particles (40–80% of PM<sub>10</sub> mass), while the enhanced
absorption coefficients are attributed to the presence of light absorbing
aerosols from biomass burning. As both scattering and absorption increased
in the dry season, the single scattering albedo (SSA) did not show a
significant seasonal variability, in average 0.86 ± 0.08 at 637 nm for
dry aerosols. Measured particle optical properties were used to estimate the
aerosol forcing efficiency at the top of the atmosphere. Results indicate
that in this primary forest site the radiative balance was dominated by the
cloud cover, particularly in the wet season. Due to the high cloud
fractions, the aerosol forcing efficiency absolute values were below
−3.5 W m<sup>−2</sup> in 70% of the wet season days and in 46% of the dry season
days. Besides the seasonal variation, the influence of out-of-Basin aerosol
sources was observed occasionally. Periods of influence of the Manaus urban
plume were detected, characterized by a consistent increase on particle
scattering (factor 2.5) and absorption coefficients (factor 5). Episodes of
biomass burning and mineral dust particles advected from Africa were
observed between January and April, characterized by enhanced concentrations
of crustal elements (Al, Si, Ti, Fe) and potassium in the fine mode. During
these episodes, median particle absorption coefficients increased by a
factor of 2, whereas median SSA values decreased by 7%, in comparison to
wet season conditions.