Delhi cannot clean its air alone: airshed-scale mitigation outperforms local controls even under unfavourable winter meteorology

Abstract Delhi’s persistent wintertime pollution cannot be addressed through local measures alone. Using a high-resolution, online-coupled atmospheric chemistry-transport model (WRF-Chem), together with India-specific anthropogenic emissions and near-real-time biomass-burning inputs, we quantify how existing and realistically strengthened mitigation policies could improve air quality over NCT-Delhi when implemented within Delhi-NCR versus across the broader regional airshed. Simulations for September 2019-January 2020 show that restricting crop-residue-burning (CRB) controls only within Delhi-NCR reduces PM 2.5 over NCT-Delhi by merely 2–3%. Extending the same control across the full airshed yields a three- to four-fold larger benefit (~10%), highlighting the strong influence of upwind emissions. A 50% reduction in residential emissions, representing strengthened implementation of existing government policies, combined with an airshed-wide CRB ban, lowers PM 2.5 by ~30%. Additional transport-sector interventions provide a further ~13% reduction, while industrial-sector measures contribute ~11% during the winter months. Mitigation benefits persist even under highly unfavourable meteorological conditions, including shallow boundary layers and dry atmospheres that typically intensify winter pollution. Overall, the results demonstrate that city-centric actions alone cannot deliver sustained air-quality improvements for NCT-Delhi. Coordinated, multi-state, airshed-scale mitigation combining residential, transport, and industrial emission controls with an effective CRB ban is essential to substantially reduce post-monsoon and winter PM 2.5 levels.