Contradictions in dissolved black carbon research: A critical review of its sources, structures, analytical methods,...

Dissolved black carbon (DBC) represents the most active component within the black carbon (BC) continuum and plays a vital role in the global carbon cycle and the removal of inorganic and organic contaminants due to its prolonged residence time and unique condensed aromatic structure. Significant progress has been made in understanding DBC source, molecular structure, analytical methods, stability, and environmental behavior, particularly its photochemical and microbial transformation. However, substantial uncertainties persist, including ambiguities in its definition, limitations in isolation and quantification methods, and unidentified sources. These limitations have led to lots of inconsistencies regarding its stability, environmental transport pathways, and transformation mechanisms. This review critically examines the current landscape of DBC research, with a focus on: (1) key contradictions in DBC cycling processes, including debates over its recalcitrance, mismatched isotopic signatures, and imbalances in the marine DBC budget; (2) limitations for DBC isolation and quantification methods in natural environments; and (3) photochemical and microbial transformation processes, and its interactions with environmental pollutants. By synthesizing recent insights, this review aims to enhance the understanding of DBC's structures, turnover, and environmental behavior, as well as its implications for the global carbon cycle. To address existing challenges, future studies are suggested to prioritize resolving these contradictions, developing standardized analytical approaches, and achieving a clearer elucidation of DBC cycling processes across diverse environments.