Stage-wise evaluation of the Graded Response Action Plan (GRAP) under meteorological constraints in Delhi-NCR hotspot regions

Delhi-National Capital Region (Delhi-NCR) is prone to repeated episodes of severe air pollution, triggered by high air pollution emissions, unfavorable meteorological conditions and episodic regional air pollution transport. To fight acute episodes of pollution, the Graded Response Action Plan (GRAP) was adopted as a graded emergency response plan with progressively higher levels of intervention based on worsening levels of Air Quality Index (AQI). However, there is yet little systematic evidence of the stage-wise performance of GRAP in real-world atmospheric conditions. This study shows a stage-wise and multi-pollutant assessment of GRAP using continuous monitoring data from nine officially classified pollution hotspots in Delhi-NCR. Concentrations of PM2.5, PM10, NO2, NOx, CO, O3, SO2, NH3 and selected VOCs were analyzed during pre-GRAP, GRAP and post-GRAP (September 2023 - March 2024) phase along with key meteorological parameters. Statistical analyses included descriptive statistics, analysis of variances (ANOVA), Pearson correlation, and principal component analysis. Results indicated that concentrations of PM2.5 were consistently high during higher stages of GRAP despite increases in control measures, with significant linkages (p < 0.05) between pollutant accumulation and weak dispersion conditions with low wind speed and reduced mixing. In contrast, ozone concentrations during and following the enforcement of GRAP showed signs of a non-linear photochemical response in a VOC-limited regime. Overall, the results show that GRAP provides a limited and meteorology-dependent, short-term relief which is limited by reactive activation, the formation of secondary pollutants and unfavorable dispersion conditions. Integrating meteorology-informed forecasting, stage-specific targeting and region wise co-ordination is proposed for improved emergency air quality management in Delhi-NCR.