Assessment of spatio-temporal dynamics of tropospheric NO₂ and covariates using the wavelet analysis: a Punjab case...

Nitrogen dioxide (NO₂), a major component of ambient air pollution, has adverse effects on human health, the environment, and even the climate. Therefore, the current study analyzes spatiotemporal patterns, identifies hotspots, and explores the potential sources and driving factors of NO₂ over Punjab from 2018 to 2025 using satellite remote sensing observations. The results indicate an average NO₂ concentration of about 37.37 µmol/m² during the study period. The high monthly concentration is observed in November. Seasonal peak NO₂ is observed in winter, followed by autumn, summer, and spring. The hotspot analysis identifies most of the NO₂ hotspots in northeastern Punjab, including Lahore, Sheikhupura, Kasur, Nankana Sahib, Gujranwala, and Faisalabad, with z-scores ≥ 3.129. The average concentration in these hotspot areas is 92.31 µmol/m² (Lahore), 65.07 µmol/m² (Sheikhupura), 59.11 µmol/m² (Kasur), 54.42 µmol/m² (Nankana Sahib), 50.65 µmol/m² (Gujranwala), and 50.5 µmol/m² (Faisalabad). The road transportation sector is found to be the major contributor to high NO_x emissions. The average emission from this sector is 1.87 × 10^-11 kgm^-2 s^-1. The road traffic sector accounts for ~ 33.88% of total NO_x emissions in Punjab. In addition to local emissions, external transport of NO₂ from India and Afghanistan may also contribute to increasing NO₂ concentration over Punjab. This is indicated by the PSCF and CWT analyses based on the HYSPLIT backward trajectories and NO₂ concentration. Moreover, wavelet analysis also suggests a strong statistically significant association between NO₂ and meteorological factors, landscape features, and nighttime light.