10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Fine Particulate Mass Scattering Efficiency and Refractive Index in the Central Amazonian Basin (ATTO station)
SAMARA CARBONE, Guilherme Santa Cecília, Luciana Rizzo, Joel Brito, Nga Lee Ng, Lu Xu, Jorge Saturno, Bruna A. Holanda, Florian Ditas, Christopher Pöhlker, Meinrat O. Andreae, Paulo Artaxo, Federal University of Uberlândia
Abstract Number: 1271 Working Group: Remote/Regional Atmospheric Aerosol
Abstract To estimate the mass scattering efficiency (MSE) of PM1 and its individual components, linear regression models were applied to the aerosol scattering coefficient (measured by the Nephelometer, 525 nm) and to the PM1 main components (measured by the Aerosol Chemical Speciation Monitor, ACSM). In this study, the scattering coefficient and the PM1 variables represent the dependent and independent variables, respectively. During the dry season (August–December, 2014), in the central Amazonian basin, the average MSE obtained was 4.9 m2 g-1. In this season, the fine PM1 is dominated by chemical compounds from local and regional forest fires. Two specific episodes interfered in the analysis, and therefore were removed and analyzed separately. One of the episodes consisted of a volcanic eruption plume from Africa. The MSE during this episode was 8.9 m2 g-1, suggesting that the fine particles from this source were two times more efficient in scattering the solar radiation at 525 nm. During the wet season (February–May, 2015) the obtained average for MSE was 5.8 m2 g-1, slightly larger than the dry season. The larger value in this season could be due to the presence of chloride in the fine particles. In this time of the year, when the air pollution loading is low, it is possible to observe NaCl-riched particles, likely from the Atlantic Ocean. To study the MSE contribution of individual PM1 components a multi linear regression model (MLR) was used. Different grouping possibilities of the main components (except for BCe) were tested. Although this type of analysis can be biased by the number of chosen variables and presence of collinearity, all the possible groupings presented a similar trend for each season. That is, during the wet season the MSE values were lower for organics (1.2–3.2 m2 g-1), and larger for sulfate (or ammonium sulfate, 12.2–20.7 m2 g-1), while at the dry season the MSE for organics were larger (4.1–5.4 m2 g-1) and lower for sulfate (or ammonium sulfate, 3.2–4.8 m2 g-1). Wider variability of MSE is observed during the wet season, which could be related to the predominance of externally mixed particles and/or presence of more heterogeneous sources during this season.
Moreover, the Mie scattering code was used to estimate the refractive index by means of a closure study using the particle size distribution measured with the Scanning Mobility Particle Sizer (SMPS, TSI, mobility diameter=10-370 nm) and MLR model grouping results. The effective refractive index obtained for the dry season was 1.58-0.019i, and 1.61-0.0008i for organics. The real part of organic refractive index ranged from 1.4 to 1.77 and presented a rather stable and independent behavior as a function O:C, suggesting that at the central Amazonian basin, the PM1 refractive index for organics is not driven by oxidation.