Abundance and mixing state of black carbon across atlantic coastal and remote marine surface atmospheres
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Abstract
Black carbon (BC) is a strong absorber of solar radiation and a major contributor to climate warming. It is mainly produced by the incomplete combustion of fossil fuels, biofuels, and biomass. In the atmosphere, BC undergoes aging through condensation, coagulation, and multiphase reactions, during which it becomes coated with other aerosol components. These aging processes substantially enhance BC’s light absorption and hygroscopicity, thereby influencing its direct radiative forcing and aerosol-cloud interactions. However, large uncertainties remain in assessing the climatic impacts of BC mixing state. A key reason is the limited understanding of BC mixing characteristics under real atmospheric conditions. Most existing studies have focused on continental environments, particularly near-surface observations. However, the diversity of BC mixing states across different environments poses challenges for constraining global models. For global climate models, observations of BC loading and mixing state over marine regions are especially important for characterizing remote environments and representing global averages. Nevertheless, available measurements over oceans remain scarce, particularly those concerning BC mixing state. As a result, our understanding of how the marine environment influences BC aging and mixing state remains very limited.
This thesis focuses on shipborne measurements of BC mass concentrations and mixing state over the Atlantic Ocean, conducted using a Single Particle Soot Photometer (SP2) aboard the research sailing yacht S/Y Eugen Seibold. The study investigates the key factors influencing BC loading and mixing state in marine environments. Based on an improved method for BC mixing-state retrieval, this work elucidates the aging processes through which freshly emitted BC externally attached to non-BC aerosols (e.g., sea salt) evolves into thickly coated BC during transport in humid marine atmospheres.
In Chapter 2, to enable rapid and scalable analysis of SP2 datasets collected in marine environments, the SP2 Parallel Accelerated Runtime Kit (SPARK) is developed in this thesis. It is a cross-platform toolkit designed for efficient and accurate analysis of large SP2 datasets. SPARK enhances data accuracy through improved numerical algorithms and ensures long-term data stability by automatically detecting and correcting instrument drifts such as laser misalignment, baseline shifts, and timing variations. Its parallel computing capabilities greatly accelerate data processing, allowing routine analysis of extensive campaign datasets. Moreover, its standardized and physically consistent retrievals of optical sizing and coating properties provide the analytical foundation for all subsequent chapters, ensuring consistent and high-precision characterization of BC mass, size, and mixing state across the Atlantic shipborne measurements.
In Chapter 3, this study examines the transformation of freshly emitted BC particles in marine atmospheres, which tend to attach to marine aerosols and subsequently form thickly coated core-shell structures under high relative humidity. Based on in-situ measurements from ten Atlantic Ocean cruises aboard the S/Y Eugen Seibold, this study compiled a comprehensive dataset (over 1120 hours of observations) of BC concentrations and mixing states across both near-coastal and remote regions (10° N-60° N, 10° E-65° W). The results show that the background BC concentration over the Atlantic Ocean is approximately 100 ng m-3, indicating a well-mixed marine atmosphere extending up to ~1000 km offshore. Non-core-shell BC particles dominate (>50%) in coastal regions, likely due to coagulation with marine aerosols. High humidity (>85%) further transforms BC associated with marine aerosols into thickly coated core-shell particles via hygroscopic growth. These findings provide new insights into BC behavior and transformation processes in marine environments.
In Chapter 4, this study reveals that regional transport in marine environments can produce highly aged BC particles with coating thicknesses of several hundred nanometers. In this work, shipborne measurements from seven cruises (over 660 hours of observations) conducted between June and August 2020 across the Northeast Atlantic (35-66° N, 30° W-10° E) were classified into two groups according to air mass origin: continental-dominated and marine-dominated. Comparisons between these two groups reveal that air masses originating from continental Europe increased BC loadings in the Atlantic marine atmosphere by tens to hundreds of ng cm-3. Marine air masses, in contrast, favored the formation of aged BC with larger cores and thicker coatings, often reaching 100-200 nm. For BC particles emitted from Europe, coating growth in continental environments typically ranges from only a few to tens of nanometers (Laborde et al., 2013). Our observations demonstrate that BC transported through marine environments can reach highly aging levels, likely due to coagulation with marine aerosols (e.g., sea salt) and droplet formation at high humidity. This is consistent with the mechanisms proposed in Chapter 3. Our findings highlight the crucial role of marine environments in enhancing BC aging during regional transport and promoting the formation of thickly coated particles, which have important implications for the global climate system.
In summary, this thesis provides new insights into BC aging processes in the marine atmosphere and their implications for improving climate model representations of aerosol-radiation interactions. It addresses the current knowledge gap regarding how marine conditions shape BC mixing state, which is a key factor controlling its climate impact. Our results demonstrate that BC aging and mixing in marine environments are strongly influenced by coagulation with marine aerosols and high humidity, rather than solely by local emissions or condensation. These findings reveal previously unquantified marine pathways that would enhance BC light absorption and aerosol-cloud interactions. Ultimately, this thesis provides crucial observational evidence to inform and constrain the parameterization of BC mixing state and its associated radiative and microphysical effects in marine regions, thereby improving future climate modeling and predictions of BC’s role in global climate change.