Optimizing the green-grey ratio: Nonlinear effects of urban form on PM2.5 in mid-sized cities

• Green-Grey Ratio defines air quality resilience in mid-sized cities. • Green infrastructure efficacy reveals a thermal tipping point at 19°C. • Intermediate Green-Grey ratios (0.8–1.2) minimize PM2.5 during heat. • Excessive vegetation density can trap pollution by reducing ventilation. • Mid-sized cities require heat-adaptive zoning, not static greening. Urban greening is widely promoted as a strategy to mitigate particulate air pollution and urban heat island, yet its effectiveness varies across climatic conditions. This study examines whether urban greening exhibits a temperature-dependent threshold in influencing PM₂.₅ concentrations in a mid-sized Midwestern city representative of temperate urban regions. Using high-frequency air-quality observations integrated with gridded urban morphology data, we analyze how the balance between green and built infrastructure, captured by a Green–Grey Ratio, interacts with temperature to shape PM₂.₅ variability. Results reveal a nonlinear relationship in which PM₂.₅ concentrations are lowest at intermediate green–grey balances, but only under warm conditions. Below approximately 19–20°C, PM₂.₅ variability is dominated by meteorological factors and combustion indicators, and urban greening shows limited influence. Above this threshold, vegetation and urban form increasingly modulate baseline PM₂.₅ levels and the likelihood of extreme pollution events. Random Forest analysis corroborates this transition, indicating higher relative importance of vegetation metrics during warm periods. The key novelty is demonstrating that greening benefits for PM₂.₅ are non-monotonic and temperature-contingent, with an intermediate Green–Grey “optimum” and a warm-regime tipping point that has direct implications for heat-resilient urban design. These findings suggest that optimizing the balance between green and built infrastructure requires accounting for climatic context and nonlinearity, with implications for urban design and air-quality management in mid-sized cities.