Monsoon-driven variability in carbonaceous aerosol sources, chemistry, and optical properties over the equatorial Indian Ocean

Abstract. 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.

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.

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.