Rawatlal, MSpence, KAMSmith, Marie ELabuschagne, CAltieri, KE2026-07-272026-07-272026-072472-3452https://doi.org/10.1021/acsearthspacechem.6c00101http://hdl.handle.net/10204/14865Aerosols play a major role in climate by influencing both radiative forcing and cloud condensation nuclei (CCN) formation, with organic aerosols comprising a large fraction of the global natural aerosol burden (∼20–70%). Despite this importance, long-term observations of water-soluble organic carbon (WSOC), particularly in size-segregated aerosols from natural sources, remain scarce in the southern hemisphere. Here, we present size-resolved WSOC measurements from an eight-month observational campaign (April to November 2018) at the Cape Point Global Atmosphere Watch (GAW) station, a remote coastal site at the southern tip of Africa, ideally suited for elucidating marine, continental, and mixed aerosol sources. Weekly aerosol samples were classified using air mass back trajectories, radon-222 and carbon monoxide concentrations, wildfire observations, and satellite-derived chlorophyll-a to assess source influences. Across all air mass classifications, WSOC concentrations were consistently higher in the fine mode (<1 μm) than in the coarse mode (>1 μm) aerosols (0.58 ± 0.55 μg/m3 vs 0.20 ± 0.22 μg/m3, respectively). Fine mode WSOC concentrations were highest during periods influenced by biomass burning (0.87 ± 0.49 μg/m3) and continental outflow, while marine and modified marine air masses exhibited lower fine mode WSOC concentrations. However, periods of enhanced marine biological activity were associated with increased fine-mode WSOC concentrations. The observed predominance of WSOC in the fine mode across marine, modified marine, and continental air masses highlights the importance of secondary formation and atmospheric aging processes, rather than direct primary emissions, in controlling WSOC concentrations at Cape Point. Overall, WSOC levels at this coastal site reflect an integration of natural and anthropogenic sources, transport history, and atmospheric processing. These results establish a regional baseline for WSOC aerosols in southern Africa and contribute to a broader understanding of organic aerosol formation in a data-sparse southern hemisphere coastal environment.FulltextenWater-soluble organic carbonSize-segregated aerosolsNatural aerosol sourcesSouthern hemisphereGlobal Atmosphere WatchArticlen/a