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The addition of coating to the black carbon (BC) enhances its absorption as more light is focused by the coating "lens." The absorption enhancement factor (Eabs) of BC is difficult to quantify due to an inadequate representation of its mixing structure and the interaction with radiation. Here, by tracking the evolution of the fresh BC particles in the ambient, we found a transitional stage of the particle Eabs with the non‐BC‐to‐BC mass ratio (RBC) at ∼2, below which there were insufficient coating materials to encapsulate the BC core and the absorption enhancement was not significant (∼14%). When the RBC >∼ 2, obvious absorption enhancement occurred as the BC cores were fully covered. Secondary inorganic species played the most critical role in the coating materials to enhance the "lensing effect." We suppose the particle‐resolved core‐shell Mie model can be applied in the Eabs prediction for most cases. Plain Language Summary: Black carbon is the dominant light‐absorbing component of atmospheric aerosols, significantly affecting the global climate. In the atmospheric aging process, such as condensation and oxidation, fresh BC is gradually coated. The light absorption of BC is significantly associated with the amount of its coating material since the coating could act as "lens" to refract more light toward the BC core. However, this "lensing effect" for BC particles is poorly quantified, bringing uncertainties when evaluating the aerosol radiative forcing. In this study, we found there was a transitional behavior for the light absorption by BC particles which was related to the particle mixing states. When the non‐BC‐to‐BC mass ratio for particles is larger than 2, the coating materials are insufficient to encapsulate the BC core and cannot act as the coating "lens." Our study provides a step forward in understanding the global radiation effect of BC. Key Points: A transitional stage of the absorption enhancement (Eabs) during the evolution of black carbon (BC) was directly observed in the ambientBy tracking the evolution of fresh BC, the predicted Eabs based on the particle‐resolved model showed consistent transitional stageThe secondary inorganic species acted the most critical role in the coating materials to enhance the absorption of BC‐containing particles [ABSTRACT FROM AUTHOR]

Black carbon (BC) strongly absorbs solar radiation, contributing to global warming. Absorption enhancement of BC particles is difficult to quantify due to an inadequate representation of their complex morphology and mixing structures, as well as interaction with radiation. Here, we apply a 3D method accounting for detailed BC mixing structures to predict the absorption enhancement of individual BC particles (Eabs) and the total BC particle population (Eabs, bulk). The diverse range of mixing structures in individual BC particles leads to variable Eabs that could hardly be predicted by empirical approximations. We find that the volume proportion of the BC embedded in coating (F) determines Eabs when the particle to BC core diameter ratio (Dp/Dc) is larger than 2.0. Our findings reveal the potential mechanism behind the differences in observed and modeled Eabs, bulk. The framework builds a bridge connecting the microscopic mixing structure of individual BC particle with Eabs, bulk. Plain Language Summary: Absorption by black carbon (BC) in the atmosphere strongly affects radiative balance and global climate. The large discrepancies in observed and modeled BC absorption enhancements raise a hot debate. Through applying a new 3D shape model based on electron microscope observations, we propose a new framework that estimates BC absorption enhancement through accounting for mixing structure diversity in individual particles. Our results reveal that the diverse range of mixing structures in individual particles in ambient air leads to complex absorption enhancement that could hardly be predicted by the empirical approximation. The bulk absorption enhancements based on diverse mixing structures provide an explanation for the globally disparate results from laboratory and field observations. The new framework linking microphysical structures to bulk BC optical properties can be used to improve assessment of climate impact. Key Points: The impact on absorption enhancement by embedded fraction is significantly enhanced when BC particle to core diameter ratio >2.0The embedded fraction and coating thickness in aged BC particles can lead to significant differences in bulk absorption enhancementA new bridge is built to connect the microscopic mixing structure of individual aged BC particles with bulk absorption enhancement of BC [ABSTRACT FROM AUTHOR]

The complex particle morphology of black carbon (BC) induces considerable uncertainties in constraining its optical properties. In particular, the diversity of how the BC core and coating are combined is yet to be resolved due to the lack of direct ambient quantification. In this study, by concurrent measurements of the electrical mobility diameter (Dm) and optical volume‐equivalent diameter (Dve), the dynamic shape factor (χ = Dm/Dve) of ambient BC‐containing particles at different mixing states was quantified for the first time. BC in urban Beijing showed a widely distributed χ of 1.4 ± 0.43. Particles with larger refractory BC (rBC) cores showed higher non‐sphericity and higher χ. A ten‐fold increase in the volume ratio of the coating over rBC (VR) could decrease the fraction of fractal BC by 36%. This study proposed a χ‐VR space to describe the particle shape, which is able to include all ambient quantified BC morphologies in optical model. Plain Language Summary: Black carbon (BC) is an important short‐lived light‐absorbing aerosol component in the atmosphere. Its optical properties are complicated by the complex particle morphology, such as the non‐BC substances associated with it and the combination of BC and non‐BC substances within particle. The previous quantification of particle morphology could not distinguish the information specifically for ambient BC, nor the single BC particle characterization can represent full diversities for all particles. This study used concurrent measurements of volume‐equivalent and mobility diameter of BC‐containing particles (BCc) to characterize the coating content and particle shape online for all BC particles in the ambient environment. As a result, we can include all the measured particle morphology information in optical models. Particles with larger BC cores showed higher non‐sphericity, with a greater discrepancy between the measured absorbing properties and that calculated by the Mie theory. Key Points: The morphology of ambient black carbon‐containing particles was characterized by volume‐equivalent and mobility diameter measurementsBlack carbon with higher coatings showed more spherical shapesA space using coatings and particle shape was developed to incorporate morphology information for all black carbon‐containing particles [ABSTRACT FROM AUTHOR]

Black carbon (BC) particles are responsible for substantial radiative heating of the atmosphere. However, the climate‐relevant properties of BC particles in the free troposphere (FT) are poorly constrained over longer time scales. Here, we report on in situ, intensive field campaigns deploying a single‐particle soot photometer during summer and winter seasons at the Jungfraujoch (3,580 m a.s.l., Switzerland), a site often located in the lower FT. The refractory BC (rBC) mass size distribution was very stable across the two seasons. BC was generally internally mixed (i.e., heavily coated with other aerosol material) in summer. However, against general expectations, BC was predominantly externally mixed in winter, with a high occurrence of negligibly‐to‐thinly coated BC. This strong seasonality in lower‐FT BC mixing state has not been previously observed and would substantially influence the lifetime, loading, and climate effects of free‐tropospheric BC. Future measurements should investigate the generality of these results. Plain Language Summary: Black carbon (BC) aerosols (i.e., soot particles) play an important role in global climate change by causing atmospheric warming. To understand this role, we need to know what types of BC particles are found in the atmosphere; how big are they and how are they mixed with other aerosols? This type of information is lacking for BC particles found in the free troposphere (the atmospheric layer immediately above the mixed boundary layer of air at the Earth's surface). We performed measurements during winter and summer seasons at Jungfraujoch in the Swiss Alps (3,580 m a.s.l.) in order to fill this gap in knowledge. We discovered a large and unexpected seasonal contrast; in summer at this site, BC aerosols are heavily coated with other aerosol material, whereas in winter the BC is largely uncoated. This seasonal contrast has important implications for our understanding of how long BC particles stay in the atmosphere (coated particles are removed faster), and our estimates of BC climate impacts (coated particles absorb more sunlight). Key Points: Laser‐induced‐incandescence measurements revealed no seasonal variability in black carbon (BC) mass size distributions at JungfraujochBC was heavily coated in summer but largely uncoated in winter, a seasonality that has not been reported beforeThese measurements have important implications for the aging timescales and lifetime of BC in the free troposphere [ABSTRACT FROM AUTHOR]