Oxygenated Interface on Biomass Burn Tar Balls Determined by Single Particle Scanning Transmission X-ray Microscopy

Abstract

Carbonaceous particles originating from biomass burning can account for a large fraction of organic aerosols in a local environment. Presently, their composition, physical and chemical properties, as well as their environmental effects are largely unknown. Tar balls, a distinct type of highly spherical carbonaceous biomass burn particles, have been observed in a number of field campaigns. The Yosemite Aerosol Characterization Study that took place in summer 2002 occurred during an active fire season in the western United States; tar balls collected during this field campaign are described in this article. Scanning transmission X-ray microscopy and near-edge X-ray absorption fine structure spectroscopy are used to determine the shape, structure, and size-dependent chemical composition of ∼150 individual spherical particles ranging in size from 0.15 to 1.2 μm. The elemental composition of tar balls is ∼55% atomic carbon and ∼45% atomic oxygen. Oxygen is present primarily as carboxylic carbonyls and oxygen-substituted alkyl (O-alkyl-C) functional groups, followed by moderate amounts of ketonic carbonyls. The observed chemical composition, density, and carbon functional groups are distinctly different from soot or black carbon and more closely resemble high molecular weight polymeric humic-like substances, which could account for their reported optical properties. A detailed examination of the carboxylic carbonyl and O-alkyl-C functional groups as a function of particle size reveals a thin oxygenated interface layer. The high oxygen content, as well as the presence of water-soluble carboxylic carbonyl groups, could account for the reported hygroscopic properties of tar balls. The presence of the oxygenated layer is attributed to atmospheric processing of biomass burn particles.

Airborne soil organic particles generated by precipitation

“…Particle elemental composition was quantified and is reported in units of atomic fractions. More details on CCSEM/EDX analysis and X-ray mapping of particles are reported elsewhere 11,12,31,34,35 . An FEI Titan environmental transmission electron microscope (TEM, FEI) operated at 300 kV was used to perform high-resolution TEM imaging.…”

Direct observation of ice nucleation events on individual atmospheric particles

“…51 Analysis of NEXAFS spectra provide chemical composition and mixing states of individual particles with spatial resolution of B30 nm. [52][53][54][55][56] Acquired carbon K-edge spectra allow identification of the organic carbon, elemental carbon (i.e. soot), and overall contribution of inorganic components within individual particles.…”

Atmospheric tar balls: aged primary droplets from biomass burning?

“…In the literature the C / O atomic ratio in atmospheric tar balls varies widely. Several studies established high C / O ratios (7-10) (Pósfai et al, 2003(Pósfai et al, , 2004Hand et al, 2005;Niemi et al, 2006;Adachi and Buseck, 2011), whereas others found some tar balls with significantly lower C / O ratios in the range of 1-2 (Tivanski et al, 2007;China et al, 2013).…”

“…It should be stressed that tar-ball particles might undergo aging processes in the atmosphere resulting in a significantly enhanced oxygen ratio in the outermost layer of the particles at a thickness of 30-40 nm (Hand et al, 2005;Tivanski et al, 2007;China et al 2013). During their long range atmospheric transport tar-ball particles occasionally form aggregates with up to 10 particles, including coagulation with dust particles (Hand et al, 2005;Deboudt et al, 2010).…”