From Haze to Fog: Investigating Aerosol-Fog Interactions and Source Shifts in Wintertime Delhi

Severe wintertime air pollution episodes in Delhi, India, often coincide with fog events, amplifying their environmental and health impacts. This study investigates the complicated interactions between aerosols and fog during the winter season of 2023-2024, leveraging high-resolution, time-resolved measurements of non-refractory PM₁ (NR-PM₁) using High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-TOF-AMS). The study highlights the temporal variability of NR-PM₁ chemical composition and its transformation during fog events, focusing on processes influencing aerosol acidity, hygroscopicity, and secondary formation.

NR-PM₁ mass concentrations varied widely, with organics (OA) constituting the dominant fraction (~65%), followed by nitrate, sulfate, ammonium, and chloride. Ammonium acted as the primary neutralizing agent, with an average aerosol neutralization ratio (ANR) of 0.95 ± 0.12, indicating near-neutral aerosol conditions. Significant shifts in OA composition were observed between fog and non-fog periods, with fog events promoting enhanced oxidation of organic aerosols. Elemental analysis revealed changes in the OA oxidation state. The oxygen-to-carbon (O/C) ratio increased for LV-OOA (from 0.914 to 1.016) and SV-OOA (from 0.920 to 0.838) during fog, indicating enhanced oxidation within fog droplets. The carbon oxidation state (OSc) also increased for these factors during fog, further confirming this observation.

The Positive Matrix Factorization (PMF) method identified six primary OA factors: Hydrocarbon-like OA (HOA), Nitrogen-rich Hydrocarbon OA (NHOA), Biomass Burning OA (BBOA), Solid Fuel OA (SFOA), Low-Volatile Oxygenated OA (LV-OOA), and Semi-volatile Oxygenated OA (SV-OOA). Fog events led to a ~34% increase in LV-OOA, while SV-OOA decreased correspondingly, suggesting a shift towards more oxidized, low-volatility species,  reflects the significant role of fog in promoting secondary organic aerosol (SOA) formation through heterogeneous and aqueous-phase reactions.

The findings emphasize the dual role of fog as both a sink and a facilitator of aerosol transformation, with implications for regional air quality and visibility. This study provides crucial insights into aerosol evolution mechanisms during fog events, emphasizing the need for integrated observational and modeling approaches to mitigate wintertime pollution episodes in megacities like Delhi.