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

by L V Rizzo, P Artaxo, T Mueller, A Wiedensohler, M Paixao, G G Cirino, A Arana, E Swietlicki, P Roldin, E O Fors, K T Wiedemann, L S M Leal, M Kulmala show all authors
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A long term experiment was conducted in a primary forest area in\nAmazonia, with continuous in-situ measurements of aerosol optical\nproperties between February 2008 and April 2011, comprising, to our\nknowledge, the longest database ever in the Amazon Basin. Two major\nclasses of aerosol particles, with significantly different optical\nproperties were identified: coarse mode predominant biogenic aerosols in\nthe wet season (January-June), naturally released by the forest\nmetabolism, and fine mode dominated biomass burning aerosols in the dry\nseason (July-December), transported from regional fires. Dry particle\nmedian scattering coefficients at the wavelength of 550 nm increased\nfrom 6.3 Mm(-1) to 22 Mm(-1), whereas absorption at 637 nm increased\nfrom 0.5 Mm(-1) to 2.8 Mm(-1) from wet to dry season. Most of the\nscattering in the dry season was attributed to the predominance of fine\nmode (PM2) particles (40-80% of PM10 mass), while the enhanced\nabsorption coefficients are attributed to the presence of light\nabsorbing aerosols from biomass burning. As both scattering and\nabsorption increased in the dry season, the single scattering albedo\n(SSA) did not show a significant seasonal variability, in average 0.86\n+/- 0.08 at 637 nm for dry aerosols. Measured particle optical\nproperties were used to estimate the aerosol forcing efficiency at the\ntop of the atmosphere. Results indicate that in this primary forest site\nthe radiative balance was dominated by the cloud cover, particularly in\nthe wet season. Due to the high cloud fractions, the aerosol forcing\nefficiency absolute values were below -3.5 Wm(-2) in 70% of the wet\nseason days and in 46% of the dry season days. Besides the seasonal\nvariation, the influence of out-of-Basin aerosol sources was observed\noccasionally. Periods of influence of the Manaus urban plume were\ndetected, characterized by a consistent increase on particle scattering\n(factor 2.5) and absorption coefficients (factor 5). Episodes of biomass\nburning and mineral dust particles advected from Africa were observed\nbetween January and April, characterized by enhanced concentrations of\ncrustal elements (Al, Si, Ti, Fe) and potassium in the fine mode. During\nthese episodes, median particle absorption coefficients increased by a\nfactor of 2, whereas median SSA values decreased by 7 %, in comparison\nto wet season conditions.

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