Simulated Cyclic Generation of Iron-Based Brown Carbon during the Photochemical Aging in Atmospheric Aqueous Water

Brown carbon (BrC) directly impacts the global radiative balance due to its unique light-absorbing capacity and plays a key role in the atmospheric environment, climate change, and human health. Iron (Fe) has been found to drive the secondary BrC formation in atmospheric aqueous water. Because of the strong photochemical activity, iron-based BrC particles can occur complex physicochemical processes via photoaging reactions, especially at the surface of particles and in the aqueous water. However, the evolution of chemical and optical properties during the photoaging of iron-based BrC is still insufficient. Here, we investigated the photoaging behavior of iron-based BrC, formed by the reaction between fumaric acid (FA) and Fe(III), in both dispersed aqueous systems and particulate phases. Iron-based BrC particles displayed enhanced absorptivity between 200 nm and 800 nm during the photoaging process. Under the atmospheric aqueous water condition, Fe(III) in the BrC particles converted to dissolved Fe(II) and therefore migrated into the aqueous phase during the photoaging process. Reactive oxygen species (ROS) were generated during the photoaging process, potentially enhancing atmospheric oxidative capacity. The photogenerated Fe(II) displayed strong stability in atmospheric aqueous water and can be reoxidized to regenerate iron-based BrC upon addition of FA and H 2 O 2 to simulate nighttime atmospheric oxidation. This work elucidates the photochemical evolution of BrC at multiphase interfaces and simulates its natural cycle of formation, aging, and regeneration in atmospheric aqueous water under daytime photolysis and nighttime chemical replenishment conditions. These results highlight the critical role of atmospheric aqueous water during the photoaging and cyclic generation of iron-based BrC in the real atmosphere.