Nocturnal evolution of physicochemical characteristics of water-soluble and insoluble organic aerosols in a polluted environment: New insights from a combined online and offline study

The Indo-Gangetic Plain faces severe winter pollution, driven largely by combustion emissions of black carbon and organic carbon. However, the evolution of light-absorbing organic carbon, or brown carbon, remains poorly constrained. In this study, the chemical and optical properties of water-soluble and insoluble organic aerosol fractions in Kanpur, India, were characterized using online aerosol mass spectrometry combined with extraction and optical analysis. The water-soluble fraction was more oxidized (O:C = 0.60–0.85; H:C = 1.38–1.22) and dominated by oxygenated compounds (CHO ≈ 66% of organics), including organosulfates (≈4.9%). The insoluble fraction retained hydrocarbon-like and nitrogen-rich character (CH = 35.4%; CHN = 18%; N:C ≈ 0.12; f₆₀ = 1.30%). These differences produced contrasting optical behavior: the water-soluble fraction showed moderate absorption (16.0 Mm−1 at 370 nm; mass absorption efficiency 1.23 m2 g−1), while the insoluble fraction exhibited stronger absorption (50.8 Mm−1 at 370 nm; mass absorption efficiency 1.43 m2 g−1), attributed to hydrophobic nitrogen-containing aromatics. Across the nocturnal period (21:00–02:00 to 02:00–07:00), relative humidity increased from 95% to 99%, aerosol liquid water content from 703 to 4398 μg m−3, and pH from ≈4.0 to 4.8. This coincided with increases in nitrogen-containing species, including organonitrates (2.29 → 2.58 μg m−3; ON/OA 0.21 → 0.23), CHON (+9%), CHO>1N (+37%), and oxidation (O:C 0.72 → 0.75). Simultaneously, the water-soluble fraction underwent wavelength-dependent bleaching (−16% at 370 nm, −29% at 420 nm, and −64% at 470 nm). These results demonstrate that solubility governs nighttime brown carbon chemistry: the water-soluble fraction transforms rapidly, whereas the insoluble fraction retains absorbing chromophores.