Dark aqueous processing of organics in aerosols and size-resolved fog droplets: Variations in levels, characteristics...

Fog events are frequent during winter in the Indo-Gangetic Plain (IGP), yet their role in modifying the chemical and optical properties of brown carbon (BrC) remains poorly constrained. This study provides the first field-based evidence on the comparative optical and chemical characteristics and evolution in ambient water-soluble interstitial aerosols (WSIA) and size-segregated fog droplet residues (DR) to understand how dark, aqueous-phase processing shapes BrC characteristics. Concurrent measurements using a High-Resolution Aerosol Mass Spectrometer (HR-ToF-AMS) and a liquid wavelength capillary cell (LWCC) spectrophotometer. Distinct trends with significant differences were observed between WSIA and DR characteristics. WSIA exhibited strong near-UV absorption (b_abs365 = 35.4 ± 6.6 Mm^-1) and high intrinsic absorptivity (MAC₃₆₅ = 1.50 ± 0.39 m² g^-1), whereas fog droplet residues showed up to 90 % lower values with a distinct size dependence. Despite substantial increases in oxidation markers (O: C = 0.68-0.92; fCO₂⁺ = 19-24 %), droplet residues displayed significantly lower MAC and SFE_abs values, demonstrating that fog processing reduces intrinsic BrC absorptivity rather than enhancing it. The chemical composition shifted systematically, with organic aerosol dominating WSIA (66 %) but decreasing in fine droplets (46 %), alongside an increase in the nitrate and ammonium fractions. Oxidation increased with droplet size reduction (O:C = 0.79 in WSIA vs. 0.92 in fine residues), accompanied by enrichment of organosulfate fragments and elevated SO₂⁺:SO₃⁺ ratios, indicating fog-phase sulfur processing. Differences in elemental ratios, tracer fragments, and Van-Krevelen trend indicate variations in the mechanisms and/or efficiency of aqueous phase oxidation from WSIA to DR. Collectively, these results offer field-based evidence of fog as a chemically reactive yet optically suppressive environment, with implications for aerosol-radiation interactions and visibility in polluted regions.