Clustering time series analysis of tropospheric NO2 and AOD satellite data in global megacities: exploring patterns linked to energy-related variables

Abstract

Megacities, with their high population density and intensive industrial activity, are among the largest contributors to global air pollution. This study analyzes annual trends of tropospheric nitrogen dioxide (tropoNO₂) and aerosol optical depth (AOD) using satellite observations across 34 megacities from 2005 to 2022. To capture variability and underlying drivers, we applied Non-Negative Matrix Factorization combined with Hierarchical Clustering, linking pollutant dynamics to energy, socioeconomic, and environmental indicators, with particular attention to the global financial crisis (2008–2009) and the COVID-19 pandemic (2020–2021). Key findings include: (i) tropoNO₂ shows stronger correlations with energy consumption indicators than AOD, especially in developed cities, (ii) Cluster analysis revealed three distinct response patterns to global crises: cities with minimal, moderate, or significant emission changes, (iii) Developing cities exhibited greater AOD fluctuations, highlighting vulnerability to economic shocks and weaker regulatory frameworks, (iv) tropoNO₂ displayed heterogeneous behaviors, with developed cities often grouped in clusters showing significant changes, potentially associated with motorized transport emissions. This study highlights the contrasting behaviors of tropoNO₂ and AOD across megacities, shaped by socioeconomic conditions, regulatory strength, and energy dependence. The proposed methodology provides a robust tool for identifying urban emission patterns and their relationship with economic stability, offering valuable insights for environmental mitigation strategies and sustainable urban planning, as well as for understanding and anticipating the future impacts of global crises or structural changes in energy consumption patterns. These findings contribute to advancing healthier and more resilient urban environments on a global scale.