A Soot Transformation Study: Interactions Between Soot, Sulfuric Acid and Secondary Organic Aerosol (SOA)

Abstract

Atmospheric black carbon (BC), generally called soot, is the most important aerosol component that warms the Earth’s climate significantly, and reducing atmospheric soot level has been proposed as a strategy for near-term climate change mitigation. However, policy development is hampered by large uncertainties in models’ predictions regarding the global warming induced by BC. These uncertainties primarily result from a limited scientific understanding of the transformations soot undergoes upon interacting with other aerosol components such as sulfuric acid and secondary organic aerosol (SOA). Unlike soot, sulfuric acid and SOA are thought to induce cooling effects. However, soot – sulfuric acid – SOA interactions are postulated to amplify soot’s warming effect. Condensed materials such as sulfuric acid and SOA can modify soot’s morphology, i.e. the distribution of soot aggregates and condensates in space, thereby altering its properties and atmospheric life time. The overall aim of this thesis is to characterize freshly emitted soot and the transformations in morphology and optical properties it undergoes upon condensation of sulfuric acid and SOA.

Two frameworks were developed for quantifying the in-situ morphological properties of BC mixed with either primary organic aerosol (POA) during evaporation process or sulfuric acid and/or SOA during condensation process. The morphological transformation of soot particles was quantified with these frameworks in terms of void fractions, effective densities, and in-situ dynamic shape factors. Soot morphological transformation during condensation process was shown to occur via two complementary and sequential processes: the filling of voids within particles and mobility diameter growth.

In addition, the light absorption of soot from two flame types (an industrial flame and three lab-scale flames) was studied. Significant quantities of light absorbing organics (referred to as brown carbon, BrC) were observed in lab-scale flames, but the mature soot in the industrial flame did not contain BrC. The mass absorption cross section (MAC) of BC and BrC from lab-scale flames was quantified, and the values for BrC proved to be comparable to those for BC at a short wavelength (405 nm). The most widely used model for quantifying the optical properties of coated soot at present is core-shell Mie theory, in which the key parameter is the refractive index. This study identified and evaluated alternatives to Mie theory. It was found that the nature of the condensed material can significantly influence the light absorption of coated soot particles, and that reaction between soot and sulfuric acid can have particularly important effects. The absorption cross section of soot was significantly reduced (by up to 26%) upon interaction of the soot surface with sulfuric acid, whereas the absorption cross section increased significantly when soot was coated with SOA or acidity-mediated SOA.

Field studies of soot coated with other aerosol components, i.e. organics, sulfate and nitrate, were conducted in Beijing, and the results were compared to the lab studies. The findings suggested that ambient BC particles during summertime in Beijing was internally mixed and heavily coated since their effective density was similar to the material density of the coatings.