Carbonaceous aerosols over the equatorial Indian Ocean influence regional radiative forcing and long-range transport, yet integrated observations linking aerosol chemistry, optical properties, transport pathways, and source characteristics remain scarce. Here, we present year-round measurements of PM₂.₅ chemical composition, aerosol optical properties, air-mass transport, and representative dual-carbon isotope signatures from the Maldives Climate Observatory at Gan (MCOG; 0.69° S, 73.15° E) to investigate how seasonal monsoon circulation regulates aerosol transport and atmospheric processing.</p> <p>During the northeast monsoon (NEM), continental outflow from South and Southeast Asia resulted in significantly higher PM₂.₅ concentrations (5.6 ± 2.7 μg m⁻³), aerosol optical depth (AOD₅₀₀ = 0.17 ± 0.09), and elevated concentrations of black carbon (BC), organic carbon (OC), and non-sea-salt sulfate (nss-SO₄²⁻). In contrast, marine inflow during the southwest monsoon (SWM), together with enhanced precipitation, substantially reduced aerosol loading while increasing the relative contribution of sea-salt aerosol. Diagnostic aerosol ratios revealed significant seasonal shifts in aerosol source characteristics and chemical composition. Although BC mass absorption cross-sections at 658 nm were slightly higher during the SWM, the seasonal difference was not statistically significant. Representative dual-carbon isotope measurements provided independent evidence for mixed fossil and contemporary carbon sources.</p> <p>These observations demonstrate that monsoon circulation governs aerosol transport, loading, and chemical composition over the equatorial Indian Ocean and provide observational benchmarks for improving the representation of long-range aerosol transport, atmospheric processing, and aerosol–climate interactions in regional chemical transport and Earth system models.