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

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 W m⁻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

Long-term in situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and interannual variability

“…Recent studies showed an increase of b abs from the wet to the dry season from 0.5 Mm −1 to 2.8 Mm −1 (factor 5.6) in the Amazon rain forest at 110 m a.s.l. (Rizzo et al, 2013). This is twice the increase of b abs compared to our observations of biomass burning in the savannah.…”

“…Approximately 40 % of the total particulate mass which is released by deforestation activities in the Tropics originates from biomass burning in the Amazon region (Longo et al, 2009). In addition, long-range transport of biomass burning aerosol and dust from northern and tropical Africa to South America via the Atlantic was observed by foregoing studies (Edwards et al, 2006;Ansmann et al, 2009;Adams et al, 2012;Rizzo et al, 2013). We used HYSPLIT trajectories in combination with the MODIS Global Monthly Fire Location Product to detect possible source regions during the dry and wet season.…”

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…Rizzo et al (2013), however, pointed out that this relationship is only evident for surface and volume mean diameters and was not clearly valid between å sca and count mean diameters. We obtained higher å sca values during the dry season (1.71 ± 0.24) compared to the wet season (1.29 ± 0.50) as shown in Fig.…”

“…3). To have a broader view on how this phenomenon affected the Amazon rain forest aerosol, we added scattering and absorption data from ZF2 published in Rizzo et al (2013) and extended with recent data to the current ATTO time series in Fig. 7a-b.…”

“…Tropical Amazonian forest fires have moderately high ω 0 values (0.93 ± 0.02 at 670 nm), given the high amount of scattering aerosols which are co-emitted with LAC, compared to African savanna fires that have lower ω 0 values (0.84 ± 0.015 at 670 nm) (Reid et al, 2005). In the Amazon rain forest, long-term measurements by Rizzo et al (2013) have found similar values for ω 0 during the dry and the wet season, 0.87 ± 0.06 and 0.86 ± 0.09, respectively. The low ω 0 in the wet season is attributed to long-range transport aerosol masses that include mineral dust and aged BB aerosol particles.…”