Shapes of soot aerosol particles and implications for their effects on climate

Abstract

[1] Soot aerosol particles (also called light-absorbing, black, or elemental carbon) are major contributors to global warming through their absorption of solar radiation. When embedded in organic matter or sulfate, as is common in polluted areas such as over Mexico City (MC) and other megacities, their optical properties are affected by their shapes and positions within their host particles. However, large uncertainties remain regarding those variables and how they affect warming by soot. Using electron tomography with a transmission electron microscope, three-dimensional (3-D) images of individual soot particles embedded within host particles collected from MC and its surroundings were obtained. From those 3-D images, we calculated the optical properties using a discrete dipole approximation. Many soot particles have open, chainlike shapes even after being surrounded by organic matter and are located in off-center positions within their host materials. Such embedded soot absorbs sunlight less efficiently than if compact and located near the center of its host particle. In the case of our MC samples, their contribution to direct radiative forcing is ∼20% less than if they had a simple core-shell shape, which is the shape assumed in many climate models. This study shows that the shapes and positions of soot within its host particles have an important effect on particle optical properties and should be recognized as potentially important variables when evaluating global climate change.

Organic Emissions from a Wood Stove and a Pellet Stove Before and After Simulated Atmospheric Aging

“…The fact that our OOM particles (after being coated with a similar mass of OM as observed in previous studies; Section 3.2) remained highly aspherical after aging indicates that the particles produced by flaming wood combustion in such a stove may also remain highly aspherical, unless mixed with aerosols from other sources. This conclusion is consistent with atmospheric observations suggesting that such external mixing is often insufficient for BC-containing particles to appear spherical (Johnson et al 2005;Adachi et al 2010). Models that represent aged soot as coated spheres (Jacobson 2001) may therefore obtain biased estimates of light absorption by BC for well-operated wood stoves and other BC sources producing aerosols with similar fractions of organic mass.…”

Ns-Soot: A Material-Based Term for Strongly Light-Absorbing Carbonaceous Particles

“…In some instances, it includes the embedding material (Salamanca et al 2012), and in other instances it does not (Virtanen et al 2004). Consistent with our definition of ns-soot above, we propose that only the aggregates of carbon nanospheres should be termed ns-soot, although many ns-soot particles are coated with or embedded within other materials (e.g., organic matter, sulfate), forming internally mixed particles (Adachi et al 2010). Since the coating and embedding materials are chemically and structurally distinct from the ns-soot, they can be readily distinguished from ns-soot in TEM images (Figure 4).…”

“…The definition used by IPCC indicates that soot is a part of BC. The usage of soot in electron microscopy studies (Table 1) commonly agrees with that of the IPCC (Adachi et al 2010). In contrast, in other papers soot consists of BC and other combustion-generated carbonaceous materials (Bond and Bergstrom 2006;Seinfeld and Pandis 2006;Ramanathan and Carmichael 2008;Salamanca et al 2012).…”

“…For example, ns-soot from East Asia had 139 nanospheres on average (range: 7 to 509) and that from Arizona traffic had 203 (range: 47 to 792; Adachi et al 2007). Samples from Mexico City had 40 § 32 nanospheres in nssoot (Adachi et al 2010).…”

“…Geometric sizes or equivalent diameters can be determined for the aggregate particles. The easiest way to determine values is directly from TEM images (Katrinak et al 1993;Adachi and Buseck 2008;Adachi et al 2010). If desired, far more accurate three-dimensional (3D) values can be obtained using electron tomography (e.g., 138 § 50 nm in volumeequivalent diameter for ns-soot particles from Mexico City; Adachi et al 2010).…”