Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model–measurement differences. We show that accounting for these two effects—variability in per-particle composition and deviations from the core-shell approximation—reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC’s radiative effect on climate.

Soot particles form during combustion of carbonaceous materials and impact climate and air quality. When freshly emitted, they are typically fractal-like aggregates. After atmospheric aging, they can act as cloud condensation nuclei, and water condensation or evaporation restructure them to more compact aggregates, affecting their optical, aerodynamic, and surface properties. Here we survey the morphology of ambient soot particles from various locations and different environmental and aging conditions. We used electron microscopy and show extensive soot compaction after cloud processing. We further performed laboratory experiments to simulate atmospheric cloud processing under controlled conditions. We find that soot particles sampled after evaporating the cloud droplets, are significantly more compact than freshly emitted and interstitial soot, confirming that cloud processing, not just exposure to high humidity, compacts soot. Our findings have implications for how the radiative, surface, and aerodynamic properties, and the fate of soot particles are represented in numerical models.

Soot and black carbon (BC) particles are generated in the incomplete combustion of fossil fuels, biomass, and biofuels. These airborne particles affect air quality, human health, aerosol–cloud interactions, precipitation formation, and climate. At present, the climate effects of BC particles are not well understood. Their role in cloud formation is obscured by their chemical and physical variability and by the internal mixing states of these particles with other compounds. Ice nucleation in field studies is often difficult to interpret. Nonetheless, most field studies seem to suggest that BC particles are not efficient ice-nucleating particles (INPs). On the other hand, laboratory measurements show that in some cases, BC particles can be highly active INPs under certain conditions. By working with well-characterized BC particles, our aim is to systematically establish the factors that govern the ice nucleation activity of BC. The current study focuses on laboratory measurements of the effectiveness of BC-containing aerosol in the formation of ice crystals in temperature and ice supersaturation conditions relevant to cirrus clouds. We examine ice nucleation on BC particles under water-subsaturated cirrus cloud conditions, commonly understood as deposition-mode ice nucleation. We study a series of well-characterized commercial carbon black particles with varying morphologies and surface chemistries as well as ethylene flame-generated combustion soot. The carbon black particles used in this study are proxies for atmospherically relevant BC aerosols. These samples were characterized by electron microscopy, mass spectrometry, and optical scattering measurements. Ice nucleation activity was systematically examined in temperature and saturation conditions in the ranges of $\mathrm{217}\le T\le \mathrm{235}$ K and $\mathrm{1.0}\le S_{\mathrm{ice}}\le \mathrm{1.5}$ and $\mathrm{0.59}\le S_{\mathrm{water}}\le \mathrm{0.98}$ , respectively, using a SPectrometer for Ice Nuclei (SPIN) instrument, which is a continuous-flow diffusion chamber coupled with instrumentation to measure light scattering and polarization. To study the effect of coatings on INPs, the BC-containing particles were coated with organic acids found in the atmosphere, namely stearic acid, cis-pinonic acid, and oxalic acid. The results show significant variations in ice nucleation activity as a function of size, morphology, and surface chemistry of the BC particles. The measured ice nucleation activity dependencies on temperature, supersaturation conditions, and the physicochemical properties of the BC particles are consistent with an ice nucleation mechanism of pore condensation followed by freezing. Coatings and surface oxidation modify the initial formation efficiency of pristine ice crystals on BC-containing aerosol. Depending on the BC material and the coating, both inhibition and enhancement in INP activity were observed. Our measurements at low temperatures complement published data and highlight the capability of some BC particles to nucleate ice under low ice supersaturation conditions. These results are expected to help refine theories relating to soot INP activation in the atmosphere.

Cookstoves are a major source of black carbon (BC) particles and associated organic compounds, which influence the atmospheric radiative balance. We present results from experiments that characterize BC emissions from a rocket stove coated with secondary organic aerosol. Optical properties, namely, BC mass absorption cross-section (MAC_BC) and mass scattering cross-section (MSC), as a function of the organic-to-black carbon ratio (OA:BC) of fresh and aged cookstove emissions were compared with Mie and Rayleigh–Debye–Gans (RDG) calculations. Mie theory reproduced the measured MAC_BC across the entire OA:BC range. However, Mie theory failed to capture the MSC at low OA:BC, where the data agreed better with RDG, consistent with a fractal morphology of fresh BC aggregates. As the OA:BC increased, the MSC approached Mie predictions indicating that BC-containing particles approach a core–shell structure as BC cores become heavily coated. To gain insight into the implications of our findings, we calculated the spectral simple forcing efficiency (dSFE) using measured and modeled optical properties as inputs. Good agreement between dSFE estimates calculated from measurements and Mie-modeled dSFE across the entire OA:BC range suggests that Mie theory can be used to simulate the optical properties of aged cookstove emissions. Copyright © 2016 American Association for Aerosol Research

The peroxisomal very long chain fatty acid (VLCFA) transporter ABCD1 is central to fatty acid catabolism and lipid biosynthesis. Its dysfunction underlies toxic cytosolic accumulation of VLCFAs, progressive demyelination, and neurological impairments including X-linked adrenoleukodystrophy (X-ALD). We present cryo-EM structures of ABCD1 in phospholipid nanodiscs in a nucleotide bound conformation open to the peroxisomal lumen and an inward facing conformation open to the cytosol at up to 3.5 Å resolution, revealing details of its transmembrane cavity and ATP dependent conformational spectrum. We identify features distinguishing ABCD1 from its closest homologs and show that coenzyme A (CoA) esters of VLCFAs modulate ABCD1 activity in a species dependent manner. Our data suggest a transport mechanism where the CoA moieties of VLCFA-CoAs enter the hydrophilic transmembrane domain while the acyl chains extend out into the surrounding membrane bilayer. The structures help rationalize disease causing mutations and may aid ABCD1 targeted structure-based drug design.