Deciphering Seasonal Variability and Source Dynamics of Urban Pollutants Over Delhi Under Surface Meteorological Influence Using Ground-Based and Trajectory Modeling Techniques

This study undertakes the spatio-temporal analysis of the concentrations of criteria air pollutants—NO₂, SO₂, PM₂.₅, and CO—utilizing ground-based monitoring datasets from Delhi for the period 2021-22. The investigation meticulously assesses pollutant variability in relation to meteorological variables including wind velocity, wind direction, near-surface air temperature, planetary boundary layer (PBL) dynamics, and relative humidity. Notably, the average NO₂ concentration is observed to be marginally elevated in the post-monsoon season (45.63 ± 17.84 µg/m³), with winter concentrations closely trailing (44.53 ± 15.95 µg/m³). CO levels exhibit a pronounced peak in winter (1.56 ± 1.05 mg/m³), correlating with the agricultural residue combustion in Punjab and Haryana. The highest SO₂ concentration is recorded in summer (17.24 ± 6.59 µg/m³), likely attributed to emissions from industrial sources and thermal power stations. For PM₂.₅, winter demonstrates the highest surface concentration (148.41 ± 79.61 µg/m³), with the post-monsoon season following (133.80 ± 82.14 µg/m³). A significant novel contribution of this study lies in the application of Concentration Weighted Trajectory (CWT) and cluster analysis via backward trajectory modeling of air parcels at an elevation of 100 m above ground level (AGL) using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. This methodology, applied across the Winter (DJF), Summer (MAM), Monsoon (JJAS), and Post-monsoon (ON) seasons, unveils critical insights into the spatio-temporal patterns and emission source regions for these pollutants. The trajectories reveal that major pollutant transport pathways are predominantly WNW, NW, and NNW. Trans-boundary long-range transport from NW is identified as a significant factor, while regional contributions from Punjab and Haryana are substantial. The study identifies that the winter season is primarily responsible for the highest pollutant concentrations across all pollutants, except SO₂, which exhibits its peak contributing area during the summer season.