Multi-scale temporal evolution and spatial heterogeneity of PM_2.5 and PM₁₀ across 33 monitoring stations in Delhi (2017–2024): diurnal, seasonal, and long-term trends

Delhi experiences some of the highest particulate matter concentrations globally, yet comprehensive assessments integrating long-term temporal trends and spatial heterogeneity using completeness-controlled monitoring data remain limited. This study characterized the multi-scale temporal evolution and spatial variability of PM_2.5 and PM₁₀ across Delhi using hourly observations from 33 monitoring stations during 2017–2024, with an additional descriptive comparison for April–August 2025. Hourly data were obtained from the Central Pollution Control Board Continuous Ambient Air Quality Monitoring System repository. Stations satisfying predefined completeness criteria (≥ 75% annual completeness for both pollutants in at least five years) were retained, yielding a fixed monitoring network for analysis. Long-term monotonic trends were assessed using Mann–Kendall tests and Sen’s slope estimation, supplemented by sensitivity analyses including exclusion of 2020, one-percent trimming of extreme observations, and period comparisons. Annual mean PM_2.5 declined from 134.2 µg/m³ in 2017 to 105.8 µg/m³ in 2024 (21.2% reduction), while PM₁₀ declined from 248.4 to 213.7 µg/m³ (14.0% reduction). Despite these reductions, city-wide monotonic trends were not statistically significant (p = 0.174 for both pollutants). Spatial heterogeneity remained substantial, with approximately 1.67-fold variation in PM_2.5 and 1.84-fold variation in PM₁₀ across stations. Seasonal cycles were pronounced, with minimum concentrations during the monsoon season and maximum concentrations during the post-monsoon period. Inter-station variability declined modestly over time, while April–August 2025 concentrations were slightly lower than corresponding historical averages. Despite observed improvements, particulate matter concentrations remained substantially above National Ambient Air Quality Standards and World Health Organization guideline values. These findings highlight the persistent air-quality burden in Delhi and demonstrate the value of completeness-controlled monitoring frameworks for long-term urban air-quality assessment.