Physico‐Chemical Properties of Aerosols During Fog‐Haze Spell in Delhi: Observational Insights From WiFEX Field Campaign

Abstract Winter fog over Delhi develops within one of the most polluted boundary‐layer environments in the Indo‐Gangetic Plain, where stagnant meteorology and high aerosol loading often evolve together. Under such conditions, it is difficult to determine whether aerosol changes arise from fog itself or simply from weak winter ventilation. We investigated aerosol‐fog interactions during a persistent fog‐haze spell over Delhi from 1 to 11 January 2023 using high‐resolution aerosol mass spectrometry, ceilometer backscatter, liquid water measurements, and surface micrometeorology. The event sequence comprised nine fog episodes and one haze episode within a shallow, weakly ventilated boundary layer. To minimize first‐order dynamical confounding, each fog event was paired with a dry non‐fog window of equal duration under similar low‐level dynamic conditions. Relative to those matched dry periods, fog showed stronger depletion of organics, nitrate, and primary organic aerosol, whereas sulfate did not follow the same dominant depletion pattern. A phase‐wise analysis further showed that median non‐refractory submicron particulate matter, organics, nitrate, ammonium, chloride, and especially primary organic aerosol (POA) declined from pre‐fog to in‐fog conditions, whereas sulfate remained stable to slightly enhanced. The secondary organic aerosol (SOA) fraction increased from 32.9% before fog to 55.9% during fog and 61.5% after fog because POA decreased much more sharply than SOA. Bulk organic aerosol became less oxidized during fog and regained oxygenation after dissipation. These results show that winter fog over Delhi selectively reorganizes aerosol composition through coupled removal, repartitioning, and transformation, with the response varying by species class and pre‐existing aerosol loading.