Linking chemical composition, WSOC and optical behaviour of PM_2.5 pollution sources in coastal atmosphere at Kochi,...

Coastal regions receive aerosols from both marine and terrestrial sources. While sea-salt particles and marine biogenic emissions are part of the natural background, PM_2.5 in urbanised coastal areas is primarily controlled by vehicular emissions, industry, biomass burning, and resuspended dust. The optical properties and sources of water-soluble organic matter in coastal PM_2.5 remain poorly constrained. To address this gap, PM_2.5 samples were collected in Kochi from November 2024 to May 2025, covering post-monsoon and pre-monsoon seasons. PM_2.5 concentrations showed strong seasonality, with higher levels during post-monsoon (126.53 ± 90 μg m^-3) than pre-monsoon (82 ± 66 μg m^-3), reflecting pollutant accumulation under stagnant conditions. Water-soluble organic carbon (WSOC) is higher in post-monsoon (13.60 ± 4.88 μg m^-3) than in pre-monsoon (8.54 ± 4.31 μg m^-3), consistent with enhanced secondary production under humid conditions. Secondary inorganic ions (NO₃^-, NH₄⁺, SO₄^2-) dominated the water-soluble ion mass (∼25-55%), while the NO₃^-/SO₄^2- ratio (0.25-5.87; mean: 1.97 ± 2.04) indicated substantial influence from NO_x-related emissions, particularly mobile sources. Optical analyses revealed strong wavelength-dependent absorption typical of brown carbon (AAE_330-400 6.47 ± 1.56; MAE₃₆₅ 6.25 ± 4.65 m² g^-1). EEM-PARAFAC resolved four humic-like components, with post-monsoon samples dominated by terrestrial humic-like substances (M) and pre-monsoon samples showing enhanced marine-associated and photochemically processed components (C, B). Fluorescence indices (FI 1.20 ± 0.05), higher BIX (pre-monsoon), higher HIX (post-monsoon), and non-sea-salt fractions (∼99-100%) further confirm dominant terrestrial control. Thus, despite the coastal setting, PM_2.5 and WSOC over Kochi are primarily driven by terrestrial emissions and secondary production.