Boundary-Layer Height Outweighs Wind Speed in Explaining Daily Urban Air Pollution: Unique-Variance Attribution and a City Typology for 10 French Cities

Meteorology drives the day-to-day pollution variation behind short-term health effects, and its misattribution biases exposure estimates and trend normalisation. Wind speed and boundary-layer height (BLH), the two main dispersion controls, are strongly collinear and their relative roles are contested. We quantified the variance unique to each with semi-partial R² for NO₂, O₃, PM₁₀ and PM₂.₅ in France's ten largest cities over 2018–2025, across four frames, from pooled levels to deseasonalized, de-trended anomalies, checked with generalized additive models. Wind and BLH covary strongly (mean r ≈ 0.70, 0.81 on anomalies). On pooled levels, BLH carries four (NO₂, O₃) to nine (particulate matter) times the unique variance of wind, whereas their raw correlations with the primary pollutants differ by a factor of only 1.3 to 1.4. This lead is robust for particulate matter. For O₃, temperature dominates and humidity then exceeds BLH, plausibly a photochemical signal. For NO₂ it is partly seasonal: on anomalies the two become comparable, and wind leads only in cities with coherent regional winds. The contrasts yield a three-type typology, ventilation-controlled, transitional and boundary-layer-controlled, which predicts the meteorological covariate each city needs. A single meteorological adjustment is therefore inadequate across cities, and BLH should become a first-tier predictor.