Seven-Year Trends in Ambient Air Quality and Ozone Dynamics in Delhi: COVID-19 Lockdown Lessons and Future Pathways from GAINS/ECLIPSE Projections

Delhi air quality experienced a rare disruption during the COVID-19 lockdown, offering a natural experiment to study emission meteorology health linkages. Using data from three Continuous Ambient Air Quality Monitoring Stations (CAAQMS) in Delhi (2017–2023), we assessed long-term variability in PM₂.₅, NO₂, SO₂, and O₃ before, during, and after the lockdown. Lockdown restrictions in 2020 led to sharp reductions in PM₂.₅ (up to −87%), NO₂ (up to −72%), and SO₂ (up to −47%), while ozone increased (up to +66%) due to reduced titration and enhanced photochemistry. A long-term trend analysis showed pollutant levels rebounded post-lockdown, with 2021–2023 concentrations nearing or exceeding pre-lockdown baselines. Meteorological normalization indicated that solar radiation and relative humidity modulated ozone formation, while particulate declines enhanced photochemical activity. Using exposure response functions, we estimated that short term lockdown reductions in PM₂.₅ and NO₂ potentially avoided 1,200–1,500 premature deaths in Delhi. However, these improvements were temporary, underscoring the importance of sustained emission-control strategies. The study highlights how the lockdown serves as a natural experiment, providing insights for targeted policy measures such as traffic restrictions, industrial emission controls, and dust management to secure long-term health benefits.o place these observations in a broader context, GAINS/ECLIPSE (V6b) projections for 2025 were evaluated. Under the Current Legislation (CLE) scenario, NOx and VOC emissions are expected to remain high, sustaining ozone production, whereas the Maximum Feasible Reduction (MFR) scenario indicates substantial declines with strong control measures. These projections reinforce that while the lockdown offered temporary relief, only sustained and stringent emission-control policies can achieve durable air quality and health benefits in Delhi.