10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Density, Morphology, and Internal Structure of Biomass Burning Brown Carbon Aerosol
BENJAMIN SUMLIN, Christopher Oxford, Bongjin Seo, Robert Pattison, Brent Williams, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 403 Working Group: Carbonaceous Aerosol
Abstract Recent studies have reported the ubiquitous presence of brown carbon (BrC) aerosol in the atmosphere. These particles, predominantly emitted from biomass burning, strongly absorb light in the near-ultraviolet wavelengths and offset the radiative cooling effects associated with organic aerosols. The density of a particle is an important physical property of aerosols, because it determines the transport of particles in the atmosphere and the human respiratory system, as well as their optical properties such as refractive index, which tends to increase with increasing density [1].
Our current knowledge of this particle property is limited, especially when considering the influence of combustion temperature and fuel type. For non-spherical or inhomogeneous particles, the dynamic shape factor must be included when relating the aerodynamic and mobility diameters. This dimensionless parameter accounts for the effect of non-sphericity on the particle drag force. It is difficult to accurately determine the dynamic shape factor for a single particle. With the advent of mass-based classifiers, an alternative particle property, the effective density (ρeff), has recently been adopted by the atmospheric aerosol community [2-4].
We measured ρeff of primary BrC aerosol emitted from smoldering combustion of Boreal peatlands using a differential mobility analyzer, a centrifugal particle mass analyzer, and a condensation particle counter. We controlled energy transfer into the fuel by altering the combustion ignition temperature, and we find that the particle ρeff ranged from 0.85 to 1.19 g cm-3 with ignition temperatures from 180 to 360 °C. Spherical morphology was observed from electron microscopy analysis, and the mass–mobility exponent was a constant 3.0, indicating no internal microstructure or void spaces. Additionally, variation in ρeff upon partial volatilization by a volatility tandem differential mobility analyzer was analyzed to study the properties of the organic matter constituting these particles. The ρeff of particle phase mass remaining after thermal volatilization was confined to a narrow range between 0.9 and 1.1 g cm-3. These findings lead us to conclude that primary BrC aerosols from biomass burning have homogeneous internal composition, and their ρeff is in fact their actual density.
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