Persistent air pollution regimes and regulatory exceedances of PM10, PM2.5, and O3 in the Monterrey Metropolitan Area, Mexico

Urban metropolitan regions with strong industrial activity exhibit complex air pollution dynamics driven by continuous emissions, atmospheric processes, and rapid urban expansion, challenging conventional interpretations of regulatory compliance and population exposure. This study presents a multiyear spatiotemporal assessment of PM 10 , PM 2.5 , and O 3 across five representative municipalities of the Monterrey Metropolitan Area (MMA), Mexico, during 2019–2023. High-resolution observational data were analyzed using an integrated framework that combined seasonal climatological classification, regulatory exceedance metrics, exceedance persistence indicators, run-length analysis, nonparametric statistical comparisons (Kruskal–Wallis test, α = 0 . 05 ), and diagnostic time-series evaluation. The annual mean concentrations of PM 10 ranged from approximately 35 to 74 μ g m −3 , with an apparent increase in regulatory exceedances after 2021 in rapidly urbanizing municipalities, coinciding with the implementation of stricter daily limits under NOM-025-SSA1-2021. Apodaca exhibited the most critical evolution, with persistence of exceedance P = 0 . 66 in 2023, indicating regulatory non-compliance for more than two-thirds of the year. Santa Catarina, Obispado, Cadereyta, and García also showed sustained PM 10 noncompliance, suggesting widespread, sustained exposure across multiple municipalities. Run-length analysis indicates that PM 10 exceedances are increasingly organized into multi-day sequences, with a substantial fraction of exceedance days embedded within prolonged runs lasting more than one week. Diagnostic time-series evaluation further reveals temporal stability and recurring seasonal structure. In contrast, PM 2.5 and O 3 exhibited lower exceedance frequencies and pronounced seasonal modulation, with no exceedances of current Mexican O 3 standards. These findings demonstrate that air pollution in the MMA is characterized by persistent and spatially heterogeneous exposure regimes rather than episodic exceedance events, highlighting the need for regulatory assessment frameworks that explicitly account for temporal continuity and cumulative exposure.