Influence of Synoptic Weather Patterns and Thermal Conditions on Urban Air Pollution Dynamics in North-Central Spain

Urban air pollution is strongly influenced by both local emission sources and large-scale atmospheric circulation patterns. This study examined the influence of synoptic weather patterns and thermal conditions on urban air pollution dynamics in Valladolid, North-Central Spain, using long-term observations of nitrogen dioxide (NO₂), fine particulate matter (PM_2.5), and ozone (O₃) collected between 2006 and 2024. Meteorological variables, synoptic circulation types, thermal stress indices, and air-quality indicators were analysed using descriptive statistics, seasonal analysis, Pearson correlation, regression models, and graphical techniques. The results showed that anticyclonic weather patterns were the dominant synoptic regimes over Valladolid and were frequently associated with pollution accumulation events. Heatwave conditions drove the formation of higher concentrations of NO₂, PM_2.5, and especially O₃, indicating the strong role of atmospheric stability and enhanced photochemical activity during extreme heat events. Seasonal analysis revealed that NO₂ and PM_2.5 concentrations were highest during winter, whereas O₃ reached maximum values during summer, with a peak monthly concentration of 66.85 µg m⁻³ recorded in July. Long-term variability further indicated that meteorological conditions and thermal extremes significantly influenced pollutant behaviour over time. Temperature–pollutant relationships demonstrated that warmer conditions favored ozone formation, while stagnant atmospheric conditions enhanced pollutant accumulation near the surface. The air quality health index (AQHI) also showed higher health risks during warm and stable atmospheric periods. Hence, the study demonstrates the important role of synoptic weather patterns and thermal variability in shaping urban air pollution dynamics in Valladolid. The findings provide useful information for air-quality forecasting, environmental management, and public health planning in medium-sized inland cities under increasing climate variability.