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Journal article

Long term measurements of aerosol optical properties at a primary forest site in Amazonia

Rizzo L, Artaxo P, Mueller T, Wiedensohler A, Paixao M, Cirino G, Arana A, Swietlicki E, Roldin P, Fors E, Wiedemann K, Leal L, Kulmala M ...see all

ATMOSPHERIC CHEMISTRY AND PHYSICS, vol. 13, issue 5 (2013) pp. 2391-2413 Published by COPERNICUS GESELLSCHAFT MBH

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Abstract

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(-1) to 22 Mm(-1), whereas absorption at 637 nm increased
from 0.5 Mm(-1) to 2.8 Mm(-1) from wet to dry season. Most of the
scattering in the dry season was attributed to the predominance of fine
mode (PM2) particles (40-80% of PM10 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 Wm(-2) 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.

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Authors

  • L V Rizzo

  • P Artaxo

  • T Mueller

  • A Wiedensohler

  • M Paixao

  • G G Cirino

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