Extreme Post-Monsoon Air Pollution in Delhi: Aerosol Dynamics, Fire Emissions, and Meteorological Controls

Rapid deterioration of urban air quality poses severe threats to climate, ecosystems, and human health, particularly in megacities such as Delhi, India. This study presents a comprehensive assessment of aerosol dynamics during the post-monsoon season (PMS; October–November) from 2019 to 2025, a period frequently associated with extreme pollution episodes driven by crop residue burning and unfavorable meteorological conditions. We integrated ground-based PM₂.₅ observations, satellite-derived aerosol optical depth at 550 nm (AOD₅₅₀), active fire counts, and key meteorological parameters to examine the drivers of severe air pollution events. The highest mean AOD₅₅₀ (0.79-0.80) and PM₂.₅ concentration (140-150 μg m⁻³) were observed. Across all years, PM₂.₅ levels peaked between mid-October and mid-November, exceeding the WHO 24-hour guideline (15 μg m⁻³) indicating a persistent public health emergency. A moderate to strong correlation was identified between PM₂.₅ and AOD, highlighting the role of columnar aerosol loading in surface pollution. Fire hotspot analysis revealed that 36–58% of total fire events occurred in identified hotspot regions. A statistically significant non-linear negative relationship was observed between wind speed and both AOD and PM₂.₅, underscoring the influence of stagnant meteorological conditions. HYSPLIT back-trajectory and wind rose analyses indicate dominant air mass transport from the north and north-west during PMS. The findings emphasize the urgent need for integrated mitigation strategies, including sustainable residue management, adoption of cleaner agricultural practices in hotspot regions, and stricter emission controls, to reduce pollution exposure and associated health risks.